Local network technology. Basic technologies or network technologies of local area networks. Network operating systems

Rapid development local networks, which has now been further embodied in the 10 Gigabit Ethernet standard and construction technologies wireless networks IEEE 802.11b/a is gaining more and more attention.

Ethernet technology has now become the de facto standard for cable networks.

And although Ethernet technology has not been found in its classical form for a long time, the ideas that were originally laid down in the IEEE 802.3 protocol received their logical continuation in both Fast Ethernet and Gigabit Ethernet technologies.

For the sake of historical justice, we note that technologies such as Token Ring, ARCNET, 100VG-AnyLAN, FDDI and Apple Talk also deserve attention. Well. Let's restore historical justice and remember the technologies of bygone days.

I think there is no need to talk about the rapid progress in the semiconductor industry observed in the last decade. Network equipment suffered the fate of the entire industry: an avalanche-like growth in production, high speeds and minimal prices. In 1995, which is considered a turning point in the history of the Internet, about 50 million new Ethernet ports were sold. A good start for market dominance, which became overwhelming over the next five years.

This price level is not available for specialized telecommunications equipment. The complexity of the device does not play a special role in this case - it is rather a question of quantity. Now this seems quite natural, but ten years ago the unconditional dominance of Ethernet was far from obvious (for example, in industrial networks there is still no clear leader).

However, only in comparison with other methods of building networks can one identify the advantages (or disadvantages) of today's leader.

The most common (but far from the only) deterministic access methods are the polling method and the transfer of rights method. The polling method is of little use in local networks, but is widely used in industry to control technological processes.

The transfer of rights method, on the contrary, is convenient for transferring data between computers.

The principle of operation is to transmit a service message - a token - over a network with a ring logical topology. Receiving a token grants the device the right to access the shared resource. Choice workstation in this case it is limited to only two options. In any case, it must send the token to the next device in line. Moreover, this can be done after delivery of the data to the recipient (if available) or immediately (if there is no information that needs to be transmitted). During the passage of data, the marker is absent in the network, other stations have no transmission capability, and collisions are impossible in principle. For processing

possible errors

, as a result of which the marker may be lost, there is a mechanism for its regeneration.

Random access methods are called non-deterministic. They provide for competition between all network nodes for the right to transmit. Simultaneous transmission attempts by several nodes are possible, resulting in collisions.

The most common method of this type is CSMA/CD (carrier-sense multiple access/collision detection). Before transmitting data, the device listens to the network to make sure no one else is using it. If the transmission medium is being used by someone at this moment, the adapter delays the transmission, but if not, it begins to transmit data.

In the case when two adapters, having detected a free line, start transmitting simultaneously, a collision occurs. When it is detected, both transmissions are interrupted and the devices repeat the transmission after some arbitrary time (of course, after first “listening” to the channel again to see if it is busy). To receive information, a device must receive all packets on the network to determine whether it is the destination.

Ethernet's birthday is May 22, 1973. It was on this day that Robert Metcalfe and David Boggs published a description of the experimental network they had built at the Xerox Research Center. It was based on a thick coaxial cable and provided a data transfer rate of 2.94 Mbit/s. New technology received the name Ethernet (over-the-air network), in honor of the ALOHA University of Hawaii radio network, which used a similar mechanism for dividing the transmission medium (radio air).

By the end of the 70s, Ethernet had a solid theoretical basis.

And in February 1980, Xerox, together with DEC and Intel, presented the IEEE development, which three years later was approved as the 802.3 standard.

Ethernet's non-deterministic method for gaining access to the data transmission medium is carrier sense multiple access with collision detection (CSMA/CD). Simply put, devices share the transmission medium chaotically, randomly. In this case, the algorithm can lead to far from equal resolution of competition between stations for access to the medium. This, in turn, can cause long access delays, especially under congested conditions. In extreme cases, the transmission speed can drop to zero.

Because of this disorganized approach for a long time It was (and still is) believed that Ethernet does not provide high-quality data transmission. It was predicted that it would be replaced first by Token Ring, then by ATM, but in reality everything happened the other way around.

The fact that Ethernet still dominates the market is due to the great changes it has undergone during its 20-year existence. That “gigabit” in full duplex that we now see in networks

• entry level
• , bears little resemblance to the founder of the 10Base 5 family. At the same time, after the introduction of 10Base-T, compatibility is maintained both at the level of device interaction and at the cable infrastructure level.
• Development from simple to complex, growth along with user needs - this is the key to the incredible success of the technology. Judge for yourself:
• 1985 - The second version of the IEEE 802.3 (Ethernet II) specification was released, in which minor changes were made to the packet header structure. A rigid identification of Ethernet devices (MAC addresses) has been formed. An address list has been created where any manufacturer can register a unique range (currently costs only $1,250);
• September 1990 - IEEE approves 10Base-T (twisted pair) technology with a physical star topology and hubs. The CSMA/CD logical topology has not changed. The standard is based on developments by SynOptics Communications under the general name LattisNet;
• 1990 - Kalpana (later it was quickly purchased along with the CPW16 switch developed by the future giant Cisco) offers switching technology based on the refusal to use shared communication lines between all nodes of the segment;
• 1992 - the beginning of the use of switches (swich). Using the address information contained in the packet (MAC address), the switch organizes independent virtual channels between pairs of nodes. Switching effectively transforms the non-deterministic Ethernet model (with contention for bandwidth) into a data-addressed system without the user's attention;
• 1993 - IEEE 802.3x specification, full duplex and connection control for 10Base-T appears, IEEE 802.1p specification adds multicast addressing and an 8-level priority system. Fast Ethernet proposed;
• Fast Ethernet, IEEE 802.3u (100Base-T) standard, was introduced in June 1995.

On this a short history we can finish: Ethernet has taken on quite modern shapes, but the development of technology, of course, has not stopped - we will talk about this a little later.

Undeservedly forgotten ARCNET

ttached Resource Computing Network (ARCNET) is a network architecture developed by Datapoint in the mid-70s. ARCNET has not been adopted as an IEEE standard, but partially complies with IEEE 802.4 as a token-passing network (logical ring). The data packet can be any size ranging from 1 to 507 bytes.

Of all local networks, ARCNET has the most extensive topology capabilities. Ring, common bus, star, tree can be used in the same network. In addition to this, very long segments (up to several kilometers) can be used. The same wide possibilities apply to the transmission medium - both coaxial and fiber optic cables, as well as twisted pair, are suitable.

This inexpensive standard was prevented from dominating the market by its low speed - only 2.5 Mbit/s. When Datapoint developed ARCNET PLUS with transfer speeds of up to 20 Mbit/s in the early 1990s, time had already passed. Fast Ethernet did not leave ARCNET the slightest chance for widespread use.

Nevertheless, in favor of the great (but never realized) potential of this technology, we can say that in some industries (usually process control systems) these networks still exist. Deterministic access, auto-configuration capabilities, and negotiation of exchange rates in the range from 120 Kbit/s to 10 Mbit/s in difficult real production conditions make ARCNET simply irreplaceable.

In addition, ARCNET provides the ability, necessary for control systems, to accurately determine the maximum access time to any device on the network under any load using a simple formula: T = (TDP + TOBSNb)SND, where TDP and TOB are the transmission time of a data packet and one byte, respectively, depending on the selected transmission speed, Nb is the number of data bytes, ND is the number of devices on the network.

Token Ring is a classic example of token passing

oken Ring is another technology that dates back to the 70s. This development of the blue giant - IBM, which is the basis of the IEEE 802.5 standard, had a greater chance of success than many other local networks. Token Ring is a classic token-passing network. The logical topology (and physical in the first versions of the network) is a ring. More modern modifications are built on twisted pair cables in a star topology, and with some reservations are compatible with Ethernet.

The original transmission speed described in IEEE 802.5 was 4 Mbit/s, but a more recent implementation of 16 Mbit/s exists. Because of its more streamlined (deterministic) method of accessing the medium, Token Ring was often promoted in its early stages as a superior replacement for Ethernet.

Despite the existence of a priority access scheme (which was assigned to each station individually), it was not possible to provide a constant bit rate (Constant Bit Rate, CBR) for a very simple reason: applications that could take advantage of these schemes did not exist then. And nowadays there are not much more of them.

Given this circumstance, it was only possible to guarantee that the performance for all stations in the network would decrease equally. But this was not enough to win the competition, and now it is almost impossible to find a really working Token Ring network.

FDDI - the first local network on fiber optics

Fiber Distributed Data Interface (FDDI) technology was developed in 1980 by an ANSI committee. It was the first computer network to use only fiber optic cable as a transmission medium. The reasons that prompted manufacturers to create FDDI were the insufficient speed (no more than 10 Mbit/s) and reliability (lack of redundancy schemes) of local networks at that time. In addition, this was the first (and not very successful) attempt to bring data networks to the “transport” level, competing with SDH.

The FDDI standard stipulates data transmission over a double ring of fiber optic cable at a speed of 100 Mbit/s, which allows you to obtain a reliable (reserved) and fast channel. The distances are quite significant - up to 100 km around the perimeter.

Logically, the network’s operation was based on the transfer of a token.

Additionally, a developed traffic prioritization scheme was provided. At first, workstations were divided into two types: synchronous (having a constant bandwidth) and asynchronous. The latter, in turn, distributed the transmission medium using an eight-level priority system.

Incompatibility with SDH networks did not allow FDDI to occupy any significant niche in the field of transport networks. Today this technology has practically been replaced by ATM. And the high cost left FDDI no chance in the fight with Ethernet for the local niche. Attempts to switch to cheaper copper cable did not help the standard either. CDDI technology, based on the principles of FDDI, but using twisted pair cables as a transmission medium, was not popular and was preserved only in textbooks.

Developed by AT&T and HP - 100VG-AnyLAN

The Quartet Coding scheme, using a 5B/6B redundant code, allowed the use of 4-pair twisted pair category 3, which was then almost more widespread than the modern 5th category. The transition period, in fact, did not affect Russia, where, due to the later start of construction of communication systems, networks were laid everywhere using the 5th category.

In addition to using legacy wiring, each 100VG-AnyLAN hub can be configured to support 802.3 (Ethernet) frames or 802.5 (Token Ring) frames.

The Demand Priority media access method defines a simple two-level priority system - high for multimedia applications and low for everything else.

I must say, this was a serious bid for success. Let down by the high cost, due to the greater complexity and, to a large extent, the technology being closed to replication by third-party manufacturers. Added to this is the already familiar Token Ring lack of real applications that take advantage of the priority system. As a result, 100Base-T managed to permanently and definitively seize leadership in the industry.

Innovative technical ideas a little later found application, first in 100Base-T2 (IEEE 802.3у), and then in “gigabit” Ethernet 1000Base-T.

Apple Talk, Local Talk

Apple Talk is a protocol stack proposed by Apple in the early 80s.

Initially, Apple Talk protocols were used to work with network equipment, collectively called Local Talk (adapters built into Apple computers). The network topology was built as a common bus or “tree”, its maximum length was 300 m, the transmission speed was 230.4 Kbps. The transmission medium is shielded twisted pair. The Local Talk segment could connect up to 32 nodes. Small throughput quickly necessitated the development of adapters for higher bandwidth network environments: Ether Talk, Token Talk and FDDI Talk for Ethernet, Token Ring and FDDI networks, respectively. Thus, Apple Talk went the route of universality to

link level

and can adapt to any physical network implementation.

Another virtually unknown type of network in Russia is UltraNet. It was actively used to work with supercomputer-class computing systems and mainframes, but is currently being actively replaced by Gigabit Ethernet.

UltraNet uses a star topology and is capable of providing information exchange speeds between devices up to 1 Gbit/s. This network is characterized by a very complex physical implementation and very high prices, comparable to supercomputers.

To control UltraNet, PC computers are used, which are connected to a central hub. Additionally, the network may include bridges and routers for connecting to networks built using Ethernet or Token Ring technologies. Can be used as a transmission medium coaxial cable

and optical fiber (for distances up to 30 km).

Industrial and specialized networks It should be noted that data networks are used not only for communication between computers or for telephony. There is also a fairly large niche of industrial and specialized devices. For example, CANBUS technology is quite popular, created to replace thick and expensive wiring harnesses in cars with one common bus. This network does not have a large selection of physical connections, the segment length is limited, and the transmission speed is low (up to 1 Mbit/s). However, CANBUS is a successful combination of quality indicators and low price implementations necessary for small and medium-sized automation. TO

similar systems

can also include ModBus, PROFIBUS, FieldBus.

Today, the interests of CAN controller developers are gradually shifting towards home automation.

In the mid-1980s, the American National Standards Institute (ANSI) and the International Consultative Committee on Telephony and Telegraphy (CCITT) began developing the ATM (Asynchronous Transfer Mode) standards as a set of recommendations for the B-ISDN (Broadband Integrated) network. Services Digital Network). Only in 1991, the efforts of academic science culminated in the creation of the ATM Forum, which still determines the development of technology. The first major project made using this technology in 1994 was the backbone of the famous NSFNET network, which previously used the T3 channel.

The essence of ATM is very simple: you need to mix all types of traffic (voice, video, data), compress it and transmit it over one communication channel. As noted above, this is achieved not through any technical breakthroughs, but rather through numerous compromises. In some ways this is similar to the way we solve differential equations.

Continuous data is divided into intervals that are small enough to perform switching operations.

Naturally, this approach greatly complicated the already difficult task of developers and manufacturers of real equipment and delayed the implementation timeframe unacceptably for the market.

The size of the minimum portion of data (cells - in ATM terminology) is influenced by several factors. On the one hand, increasing the size reduces the speed requirements of the cell processor-switch and increases the efficiency of channel utilization. On the other hand, the smaller the cell, the faster transmission is possible. Indeed, while one cell is being transmitted, the second (even the highest priority) is waiting. Strong mathematics, the mechanism of queues and priorities can slightly smooth out the effect, but not eliminate the cause. After quite a lot of experimentation, in 1989 the cell size was determined to be 53 bytes (5 bytes of service and 48 bytes of data).

The second compromise of ATM is connection-oriented technology. Before a transmission session, a sender-receiver virtual channel is established at the link layer, which cannot be used by other stations, whereas in traditional statistical multiplexing technologies no connection is established, and packets with specified address. To do this, the port number and connection identifier, which is present in the header of each cell, are entered into the switching table. Subsequently, the switch processes incoming cells based on the connection IDs in their headers. Based on this mechanism, it is possible to regulate the throughput, delay and maximum data loss for each connection - that is, to ensure a certain quality of service.

All of these properties plus good compatibility with the SDH hierarchy allowed ATM to relatively quickly become a standard backbone networks data transmission. But with the full implementation of all the capabilities of the technology, big problems arose. As has happened more than once, local networks and client applications

did not support ATM functions, and without this, a powerful technology with great potential turned out to be just an unnecessary conversion between the worlds of IP (essentially Ethernet) and SDH. This was a very unfortunate situation that the ATM community tried to correct. Unfortunately, there were some strategic miscalculations. Despite all the advantages of fiber optics over copper cabling, the high cost of interface cards and switch ports made 155 Mbps ATM extremely expensive for use in this market segment. Having attempted to identify low-speed solutions for, The ATM Forum has become embroiled in a destructive debate over what speed and connection type to target. Manufacturers are divided into two camps: supporters of copper cable with a speed of 25.6 Mbit/s and supporters of optical cable with a speed of 51.82 Mbit/s. After a series of high-profile conflicts (the initial speed chosen was 51.82 Mbit/s), the ATM Forum proclaimed 25 Mbit/s as the standard. But precious time was lost forever. In the technology market, we had to meet not with “classic” Ethernet with its shared transmission medium, but with Fast Ethernet and switched 10Base-T (with the hope of the soon appearance of switched 100Base-T). High price, small number of manufacturers, need for more qualified service, problems with drivers, etc. only made the situation worse. Hopes for penetration into the corporate network segment collapsed, and the rather weak intermediate position of ATM was consolidated for some time. This is its position in the industry today.

ComputerPress 10"2002

Network technologies of local networks

In local networks, as a rule, a shared data transmission medium (mono-channel) is used and the main role is played by protocols of the physical and data link layers, since these levels best reflect the specifics of local networks.

Network technology is an agreed set of standard protocols and software and hardware that implement them, sufficient to build computer network. Network technologies are called core technologies or network architectures.

Network architecture determines the topology and method of access to the data transmission medium, the cable system or data transmission medium, the format of network frames, the type of signal encoding, and the transmission speed. In modern computer networks, such technologies or network architectures as: Ethernet, Token-Ring, ArcNet, FDDI have become widespread.

Network technologies IEEE802.3/Ethernet

Currently, this architecture is the most popular in the world. Popularity is ensured by simple, reliable and inexpensive technologies. A classic Ethernet network uses two types of standard coaxial cable (thick and thin).

However, the version of Ethernet that uses twisted pairs as a transmission medium has become increasingly widespread, since their installation and maintenance are much simpler. Ethernet networks use bus and passive star topologies, and the access method is CSMA/CD.

The IEEE802.3 standard, depending on the type of data transmission medium, has modifications:

 10BASE5 (thick coaxial cable) - provides a data transfer rate of 10 Mbit/s and a segment length of up to 500 m;

 10BASE2 (thin coaxial cable) - provides a data transfer rate of 10 Mbit/s and a segment length of up to 200 m;;

 10BASE-T (unshielded twisted pair) - allows you to create a network using a star topology. The distance from the hub to the end node is up to 100m. The total number of nodes should not exceed 1024;

 10BASE-F (fiber optic cable) - allows you to create a network using a star topology. The distance from the hub to the end node is up to 2000m.
In development of Ethernet technology, high-speed options have been created: IEEE802.3u/Fast Ethernet and IEEE802.3z/Gigabit Ethernet. The main topology used in Fast Ethernet and Gigabit Ethernet networks is passive star.

Fast Ethernet network technology provides a transmission speed of 100 Mbit/s and has three modifications:

 100BASE-T4 - uses unshielded twisted pair (quad twisted pair). The distance from the hub to the end node is up to 100m;

 100BASE-TX - uses two twisted pairs (unshielded and shielded). The distance from the hub to the end node is up to 100m;

 100BASE-FX - uses fiber optic cable (two fibers in a cable). Distance from the hub to the end node is up to 2000m; .

Gigabit Ethernet – provides a transfer speed of 1000 Mbit/s. The following modifications of the standard exist:

 1000BASE-SX - uses fiber optic cable with a light signal wavelength of 850 nm.

 1000BASE-LX - uses fiber optic cable with a light signal wavelength of 1300 nm.

 1000BASE-CX – uses shielded twisted pair cable.

 1000BASE-T – uses quad unshielded twisted pair cable.
Fast Ethernet and Gigabit Ethernet networks are compatible with networks based on the Ethernet standard, so it is easy and simple to connect Ethernet, Fast Ethernet and Gigabit Ethernet segments into a single computer network.

The only drawback of this network is the lack of a guarantee of access time to the medium (and mechanisms providing priority service), which makes the network unpromising for solving real-time technological problems. Specific problems sometimes creates a limit on the maximum data field of ~1500 bytes.

Different encoding schemes are used for different Ethernet speeds, but the access algorithm and frame format remain unchanged, which guarantees software compatibility.

The Ethernet frame has the format shown in Fig.

Ethernet Frame Format (the numbers at the top of the figure indicate the field size in bytes)

Field preamble contains 7 bytes 0xAA and serves to stabilize and synchronize the environment (alternating signals CD1 and CD0 with the final CD0), followed by the field SFD(start frame delimiter = 0xab), which is intended to detect the start of the frame. Field EFD(end frame delimiter) specifies the end of the frame. Checksum field ( CRC- cyclic redundancy check), as well as the preamble, SFD and EFD, are generated and controlled at the hardware level. Some modifications of the protocol do not use the efd field. The fields available to the user are starting from recipient addresses and ending with the field information, inclusive. After crc there is an interpacket gap (IPG - interpacket gap) of 9.6 μsec or more in length. The maximum frame size is 1518 bytes (preamble, SFD and EFD fields are not included). The interface scans all packets traveling along the cable segment to which it is connected, because it is possible to determine whether the received packet is correct and to whom it is addressed only by receiving it in its entirety. The correctness of the packet according to CRC, length and multiplicity of an integer number of bytes is made after checking the destination address.

When the computer is connected to the network directly using a switch, the restriction on the minimum frame length is theoretically removed. But working with shorter frames in this case will become possible only by replacing the network interface with a non-standard one (both for the sender and the recipient)!

If in the frame field protocol/type If the code is less than 1500, then this field characterizes the frame length. Otherwise, it is the protocol code whose packet is encapsulated in the Ethernet frame.

Access to the Ethernet channel is based on the algorithm CSMA/CD (carrier sense multiple access with collision detection).In Ethernet, any station connected to the network can attempt to start transmitting a packet (frame) if the cable segment to which it is connected is free. The interface determines whether a segment is free by the absence of a “carrier” for 9.6 μsec. Since the first bit of the packet does not reach the rest of the network stations simultaneously, it may happen that two or more stations attempt to transmit, especially since delays in repeaters and cables can reach quite large values. Such matches of attempts are called collisions. A collision is recognized by the presence of a signal in the channel, the level of which corresponds to the operation of two or more transceivers simultaneously. When a collision is detected, the station interrupts transmission. The attempt can be resumed after a delay (a multiple of 51.2 μs, but not exceeding 52 ms), the value of which is a pseudo-random variable and is calculated independently by each station (t= RAND(0.2 min(n,10)), where n - contents of the attempt counter, and the number 10 is backofflimit).

Typically, after a collision, time is divided into a number of discrete domains with a length equal to twice the packet's propagation time in the segment (RTT). For the maximum possible RTT, this time is 512 bit cycles. After the first collision, each station waits for 0 or 2 time domains before trying again. After the second collision, each station can wait 0, 1, 2 or 3 time domains, etc. After the nth collision random number lies within the range 0 - (2 n - 1). After 10 collisions, the maximum random shutter speed stops increasing and remains at 1023.

Thus, the longer the cable segment, the longer the average access time.

After waiting, the station increases the attempt counter by one and begins the next transmission. The default retry limit is 16; if the number of retries is reached, the connection is terminated and a corresponding message is displayed. The transmitted long frame helps to “synchronize” the start of packet transmission by several stations. Indeed, during the transmission time, with a noticeable probability, the need for transmission at two or more stations may arise. The moment they detect packet completion, the IPG timers will be enabled. Fortunately, information about the completion of packet transmission does not reach the stations of the segment at the same time. But the delays this entails also mean that the fact that one of the stations has started transmitting a new packet is not immediately known. If several stations are involved in a collision, they can notify the other stations by sending a jam signal (jam - at least 32 bits). The contents of these 32 bits are not regulated. This arrangement makes a repeat collision less likely. The source of a large number of collisions (in addition to information overload) can be the prohibitive total length of the logical cable segment, too many repeaters, a cable break, the absence of a terminator (50-ohm cable termination) or a malfunction of one of the interfaces. But collisions in themselves are not something negative - they are a mechanism that regulates access to the network environment.

In Ethernet, with synchronization, the following algorithms are possible:

A.

1. If the channel is free, the terminal transmits a packet with probability 1.
2. If the channel is busy, the terminal waits for it to become free and then transmits.

B.

1. If the channel is free, the terminal transmits the packet.
2. If the channel is busy, the terminal determines the time of the next transmission attempt. The time of this delay can be specified by some statistical distribution.

IN.

1. If the channel is free, the terminal transmits the packet with probability p, and with probability 1-p it postpones the transmission for t seconds (for example, to the next time domain).
2. When the attempt is repeated with a free channel, the algorithm does not change.
3. If the channel is busy, the terminal waits until the channel is free, after which it acts again according to the algorithm in point 1.

Algorithm A seems attractive at first glance, but it contains the possibility of collisions with a probability of 100%. Algorithms B and C are more robust against this problem.

The effectiveness of the CSMA algorithm depends on how quickly the transmitting side learns about the fact of a collision and interrupts the transmission, because continuation is pointless - the data is already damaged. This time depends on the length of the network segment and delays in the segment equipment. Twice the delay value determines the minimum length of a packet transmitted in such a network. If the packet is shorter, it can be transmitted without the sending party knowing it was damaged by the collision. For modern Ethernet local networks, built on switches and full-duplex connections, this problem is irrelevant

To clarify this statement, consider the case when one of the stations (1) transmits a packet to the most remote computer (2) in a given network segment. Let the signal propagation time to this machine be equal to T. Let us also assume that machine (2) tries to start transmitting just at the moment the packet arrives from station (1). In this case, station (1) learns about the collision only 2T after the start of transmission (the signal propagation time from (1) to (2) plus the collision signal propagation time from (2) to (1)). It should be taken into account that collision registration is an analog process and the transmitting station must “listen” to the signal in the cable during the transmission process, comparing the reading result with what it is transmitting. It is important that the signal encoding scheme allows collision detection. For example, the sum of two signals with level 0 will not allow this to be done. You might think that transmitting a short packet with corruption due to a collision is not such a big deal; delivery control and retransmission can solve the problem.

It should only be taken into account that retransmission in the event of a collision registered by the interface is carried out by the interface itself, and retransmission in the case of response delivery control is performed by the application process, requiring resources central processor workstation.

Double rotation time and collision detection

Clear recognition of collisions by all network stations is a necessary condition for the correct operation of the Ethernet network. If any transmitting station does not recognize the collision and decides that it transmitted the data frame correctly, then this data frame will be lost. Due to the overlap of signals during a collision, the frame information will be distorted, and it will be rejected by the receiving station (possibly due to a checksum mismatch). Most likely, the corrupted information will be retransmitted by some upper-layer protocol, such as a connection-oriented transport or application protocol. But retransmission of a message by protocols upper levels will occur over a significantly longer time interval (sometimes even after several seconds) compared to the microsecond intervals that the Ethernet protocol operates on. Therefore, if collisions are not reliably recognized by Ethernet network nodes, this will lead to a noticeable decrease in the useful throughput of this network.

For reliable collision detection, the following relationship must be satisfied:

T min >=PDV,

where T min is the transmission time of a frame of minimum length, and PDV is the time during which the collision signal manages to propagate to the farthest node in the network. Since in the worst case the signal must travel twice between the stations of the network that are most distant from each other (an undistorted signal passes in one direction, and a signal already distorted by a collision propagates on the way back), this time is called double revolution time (Path Delay Value, PDV).

If this condition is met, the transmitting station must have time to detect the collision caused by its transmitted frame even before it finishes transmitting this frame.

Obviously, the fulfillment of this condition depends, on the one hand, on the length of the minimum frame and network capacity, and on the other hand, on the length cable system network and signal propagation speed in the cable (this speed is slightly different for different types of cable).

All parameters of the Ethernet protocol are selected in such a way that during normal operation of network nodes, collisions are always clearly recognized. When choosing parameters, of course, the above relationship was taken into account, connecting the minimum frame length and the maximum distance between stations in a network segment.

The Ethernet standard assumes that the minimum length of a frame data field is 46 bytes (which, together with service fields, gives a minimum frame length of 64 bytes, and together with the preamble - 72 bytes or 576 bits). From here a limit on the distance between stations can be determined.

So, in 10 Mbit Ethernet, the minimum frame length transmission time is 575 bit intervals, therefore, the double turnaround time should be less than 57.5 μs. The distance that the signal can travel during this time depends on the type of cable and for a thick coaxial cable it is approximately 13,280 m. Considering that during this time the signal must travel along the communication line twice, the distance between two nodes should not be more than 6,635 m In the standard, the value of this distance is chosen significantly less, taking into account other, more stringent restrictions.

One of these restrictions is related to the maximum permissible signal attenuation. To ensure the required signal power when it passes between the most distant stations of a cable segment, the maximum length of a continuous segment of a thick coaxial cable, taking into account the attenuation it introduces, was chosen to be 500 m. Obviously, on a 500 m cable, the conditions for collision recognition will be met with a large margin for frames of any standard length, including 72 bytes (the double turnaround time along a 500 m cable is only 43.3 bit intervals). Therefore, the minimum frame length could be set even shorter. However, technology developers did not reduce the minimum frame length, keeping in mind multi-segment networks that are built from several segments connected by repeaters.

Repeaters increase the power of signals transmitted from segment to segment, as a result, signal attenuation is reduced and a much longer network can be used, consisting of several segments. In coaxial Ethernet implementations, developers have limited maximum amount There are five segments in the network, which in turn limits the total length of the network to 2500 meters. Even in such a multi-segment network, the collision detection condition is still met with a large margin (let us compare the distance of 2500 m obtained from the permissible attenuation condition with the maximum possible distance of 6635 m in terms of signal propagation time calculated above). However, in reality, the time margin is significantly less, since in multi-segment networks the repeaters themselves introduce an additional delay of several tens of bit intervals into the signal propagation. Naturally, a small margin was also made to compensate for deviations in cable and repeater parameters.

As a result of taking into account all these and some other factors, the ratio between the minimum frame length and the maximum possible distance between network stations was carefully selected, which ensures reliable collision recognition. This distance is also called the maximum network diameter.

As the frame transmission rate increases, which occurs in new standards based on the same CSMA/CD access method, such as Fast Ethernet, the maximum distance between network stations decreases in proportion to the increase in transmission rate. In the Fast Ethernet standard it is about 210 m, and in the Gigabit Ethernet standard it would be limited to 25 meters if the developers of the standard had not taken some measures to increase the minimum packet size.

PDV calculation

To simplify calculations, IEEE reference data is typically used to provide propagation delay values ​​for repeaters, transceivers, and various physical media. In table Table 3.5 provides the data necessary to calculate the PDV value for all physical Ethernet network standards. The bit interval is designated bt.

Table 3.5.Data for calculating PDV value

The 802.3 Committee tried to simplify the calculations as much as possible, so the data presented in the table includes several stages of signal propagation. For example, the delays introduced by a repeater consist of the input transceiver delay, the repeater delay, and the output transceiver delay. However, in the table all these delays are represented by one value called the segment base. To avoid the need to add the delays introduced by the cable twice, the table gives double the delay values ​​for each type of cable.

The table also uses concepts such as left segment, right segment and intermediate segment. Let us explain these terms using the example of the network shown in Fig. 3.13. The left segment is the segment in which the signal path begins from the transmitter output (output T x in Fig. 3.10) of the end node. In the example, this is a segment 1 . The signal then passes through intermediate segments 2-5 and reaches the receiver (input R x in Fig. 3.10) of the most distant node of the most distant segment 6, which is called the right one. It is here that, in the worst case, frames collide and a collision occurs, which is what is implied in the table.

Rice. 3.13.Example of an Ethernet network consisting of segments of different physical standards

Each segment has an associated constant delay, called the base, which depends only on the type of segment and on the position of the segment in the signal path (left, intermediate or right). The base of the right segment in which the collision occurs is much larger than the base of the left and intermediate segments.

In addition, each segment is associated with a signal propagation delay along the segment cable, which depends on the segment length and is calculated by multiplying the signal propagation time along one meter of cable (in bit intervals) by the cable length in meters.

The calculation consists of calculating the delays introduced by each cable segment (the signal delay per 1 m of cable given in the table is multiplied by the length of the segment), and then summing these delays with the bases of the left, intermediate and right segments. General value PDV should not exceed 575.

Since the left and right segments have different base latency values, in the case of different types of segments at remote edges of the network, it is necessary to perform calculations twice: once taking a segment of one type as the left segment, and a second time taking a segment of another type. The result can be considered the maximum PDV value. In our example, the extreme network segments belong to the same type - the 10Base-T standard, so double calculation is not required, but if they were segments of different types, then in the first case it would be necessary to take the segment between the station and the hub as the left one 1 , and in the second, consider the segment between the station and the hub to be left 5 .

The network shown in the figure in accordance with the rule of 4 hubs is not correct - in the network between segment nodes 1 and 6 there are 5 hubs, although not all segments are lOBase-FB segments. In addition, the total network length is 2800 m, which violates the 2500 m rule. Let's calculate the PDV value for our example.

Left segment 1 / 15.3 (base) + 100 * 0.113= 26.6.

Intermediate segment 2/ 33,5 + 1000 * 0,1 = 133,5.

Intermediate segment 3/ 24 + 500 * 0,1 = 74,0.

Intermediate segment 4/ 24 + 500 * 0,1 = 74,0.

Intermediate segment 5/ 24 + 600 * 0,1 = 84,0.

Right segment 6 /165 + 100 * 0,113 = 176,3.

The sum of all components gives a PDV value of 568.4.

Since the PDV value is less than the maximum permissible value of 575, this network passes the double signal turnaround time criterion despite the fact that its total length is more than 2500 m and the number of repeaters is more than 4

PW calculation

To recognize the network configuration as correct, it is also necessary to calculate the reduction in the interframe interval by repeaters, that is, the PW value.

To calculate PW, you can also use the values ​​of the maximum values ​​for reducing the interframe interval when passing through repeaters of various physical environments, recommended by IEEE and given in Table. 3.6.

Table 3.6.Reducing the interframe interval by repeaters

In accordance with these data, we will calculate the PVV value for our example.

Left segment 1 10Base-T: reduction of 10.5 bt.

Intermediate segment 2 10Base-FL: 8.

Intermediate segment 3 10Base-FB: 2.

Intermediate segment 4 10Base-FB: 2.

Intermediate segment 5 10Base-FB: 2.

The sum of these values ​​gives a PW value of 24.5, which is less than the 49-bit interval limit.

As a result, the network shown in the example complies with Ethernet standards in all parameters related to both segment lengths and the number of repeaters

Maximum Ethernet Performance

The number of Ethernet frames processed per second is often specified by bridge/switch and router manufacturers as the primary performance characteristic of these devices. In turn, it is interesting to know the net maximum throughput of an Ethernet segment in frames per second in the ideal case when there are no collisions in the network and no additional delays introduced by bridges and routers. This indicator helps to assess the performance requirements of communication devices, since each device port cannot receive more frames per unit of time than the corresponding protocol allows.

For communications equipment, the most difficult mode is processing frames of minimal length. This is explained by the fact that a bridge, switch or router spends approximately the same time processing each frame, associated with viewing the packet forwarding table, forming a new frame (for the router), etc. And the number of frames of the minimum length arriving at the device per unit time, naturally more than frames of any other length. Another performance characteristic of communications equipment - bits per second - is used less frequently, since it does not indicate what size frames the device processed, but on frames maximum size reach high performance, measured in bits per second is much easier.

Using the parameters given in table. 3.1, let's calculate maximum performance Ethernet segment in units such as the number of transmitted frames (packets) of minimum length per second.

NOTEWhen referring to network capacity, the terms frame and packet are usually used interchangeably. Accordingly, the units of performance measurement frames-per-second, fps and packets-per-second, pps are similar.

To calculate the maximum number of frames of minimum length passing over an Ethernet segment, note that the size of a frame of minimum length together with the preamble is 72 bytes or 576 bits (Fig. 3.5.), so its transmission takes 57.5 μs. By adding the interframe interval of 9.6 μs, we obtain that the period of repetition of frames of minimum length is 67.1 μs. Hence, the maximum possible throughput of an Ethernet segment is 14,880 fps.

Rice. 3.5.Towards calculating the throughput of the Ethernet protocol

Naturally, the presence of several nodes in a segment reduces this value due to waiting for access to the medium, as well as due to collisions leading to the need to retransmit frames.

The maximum length frames of Ethernet technology have a field length of 1500 bytes, which together with service information gives 1518 bytes, and with the preamble it amounts to 1526 bytes or 12,208 bits. The maximum possible throughput of an Ethernet segment for maximum length frames is 813 fps. Obviously, when working with large frames, the load on bridges, switches and routers is quite noticeably reduced.

Now let's calculate the maximum useful throughput in bits per second that Ethernet segments have when using frames of different sizes.

Under useful protocol bandwidth refers to the transmission rate of user data carried by the frame data field. This throughput is always less than the nominal bit rate of the Ethernet protocol due to several factors:

· frame service information;

· interframe intervals (IPG);

· waiting for access to the environment.

For frames of minimum length, the useful throughput is:

S P =14880 * 46 *8 = 5.48 Mbit/s.

This is much less than 10 Mbit/s, but it should be noted that frames of the minimum length are used mainly for transmitting receipts, so this speed has nothing to do with the transfer of actual file data.

For frames of maximum length, the usable throughput is:

S P = 813 * 1500 * 8 = 9.76 Mbit/s,

which is very close to the nominal speed of the protocol.

We emphasize once again that such speed can be achieved only in the case when two interacting nodes on an Ethernet network are not interfered with by other nodes, which is extremely rare,

Using medium-sized frames with a data field of 512 bytes, the network throughput will be 9.29 Mbps, which is also quite close to the maximum throughput of 10 Mbps.

ATTENTIONThe ratio of the current network throughput to its maximum throughput is called network utilization factor. In this case, when determining the current throughput, the transmission of any information over the network, both user and service, is taken into account. The coefficient is an important indicator for shared media technologies, since with the random nature of the access method, a high value of the utilization coefficient often indicates low useful network throughput (that is, the rate of transmission of user data) - nodes spend too much time on the procedure for gaining access and retransmitting frames after collisions.

In the absence of collisions and access waits, the network utilization factor depends on the size of the frame data field and has a maximum value of 0.976 when transmitting frames of maximum length. Obviously, in a real Ethernet network, the average network utilization can differ significantly from this value. More complex cases of determining network capacity, taking into account access waiting and handling collisions, will be discussed below.

Ethernet Frame Formats

The Ethernet technology standard, described in IEEE 802.3, describes a single MAC layer frame format. Since the MAC layer frame must contain an LLC layer frame, described in the IEEE 802.2 document, according to IEEE standards, only a single version of the link layer frame can be used in an Ethernet network, the header of which is a combination of the MAC and LLC sublayer headers.

However, in practice, Ethernet networks use frames of 4 different formats (types) at the data link level. This is due to the long history of the development of Ethernet technology, dating back to the period before the adoption of IEEE 802 standards, when the LLC sublayer was not separated from the general protocol and, accordingly, the LLC header was not used.

A consortium of three firms Digital, Intel and Xerox in 1980 submitted to the 802.3 committee their proprietary version of the Ethernet standard (which, of course, described a specific frame format) as a draft international standard, but the 802.3 committee adopted a standard that differed in some details from DIX offers. The differences also concerned the frame format, which gave rise to the existence of two different types of frames in Ethernet networks.

Another frame format emerged as a result of Novell's efforts to speed up its Ethernet protocol stack.

Finally, the fourth frame format was the result of the 802.2 committee's efforts to bring previous frame formats to some common standard.

Differences in frame formats may result in incompatibility between equipment and network software, designed to work with only one Ethernet frame standard. However, today almost all network adapters, network adapter drivers, bridges/switches and routers can work with all Ethernet technology frame formats used in practice, and frame type recognition is performed automatically.

Below is a description of all four types of Ethernet frames (here, a frame refers to the entire set of fields that relate to the data link layer, that is, the fields of the MAC and LLC layers). The same frame type can have different names, so below for each frame type are several of the most common names:

· 802.3/LLC frame (802.3/802.2 frame or Novell 802.2 frame);

· Raw 802.3 frame (or Novell 802.3 frame);

· Ethernet DIX frame (or Ethernet II frame);

· Ethernet SNAP frame.

The formats of all these four types of Ethernet frames are shown in Fig. 3.6.

conclusions

· Ethernet is the most common local network technology today. In a broad sense, Ethernet is an entire family of technologies that includes various proprietary and standard variants, of which the most famous are the proprietary DIX Ethernet variant, 10-Mbit variants of the IEEE 802.3 standard, as well as the new high-speed Fast Ethernet and Gigabit Ethernet technologies. Almost all types of Ethernet technologies use the same method of separating the data transmission medium - the CSMA/CD random access method, which defines the appearance of the technology as a whole.

· In a narrow sense, Ethernet is a 10-megabit technology described in the IEEE 802.3 standard.

· An important phenomenon in Ethernet networks is collision - a situation when two stations simultaneously try to transmit a data frame over a common medium. The presence of collisions is an inherent property of Ethernet networks, resulting from the random access method adopted. The ability to clearly recognize collisions is due to the right choice network parameters, in particular maintaining the relationship between the minimum frame length and the maximum possible network diameter.

· On network performance characteristics great importance has a network utilization rate that reflects its congestion. When this coefficient is above 50%, the useful network throughput drops sharply: due to an increase in the intensity of collisions, as well as an increase in the waiting time for access to the medium.

· The maximum possible throughput of an Ethernet segment in frames per second is achieved when transmitting frames of the minimum length and is 14,880 frames/s. At the same time, the useful network throughput is only 5.48 Mbit/s, which is only slightly more than half the nominal throughput - 10 Mbit/s.

· The maximum usable throughput of an Ethernet network is 9.75 Mbps, which corresponds to a maximum frame length of 1518 bytes transmitted over the network at 513 fps.

· In the absence of collisions and access waits utilization rate network depends on the size of the frame data field and has a maximum value of 0.96.

· Ethernet technology supports 4 different frame types that share a common host address format. There are formal characteristics by which network adapters automatically recognize the type of frame.

· Depending on the type of physical medium, the IEEE 802.3 standard defines various specifications: 10Base-5, 10Base-2, 10Base-T, FOIRL, 10Base-FL, 10Base-FB. For each specification, the cable type, the maximum lengths of continuous cable sections are determined, as well as the rules for using repeaters to increase the network diameter: the “5-4-3” rule for coaxial network options, and the “4-hub” rule for twisted pair and fiber optics.

· For a "mixed" network consisting of different types of physical segments, it is useful to calculate the total network length and the allowable number of repeaters. The IEEE 802.3 Committee provides input data for these calculations that indicate the delays introduced by repeaters of various physical media specifications, network adapters, and cable segments.

Network technologies IEEE802.5/Token-Ring

Token Ring networks, like Ethernet networks, are characterized by a shared data transmission medium, which in this case consists of cable segments connecting all network stations into a ring. The ring is considered as a common shared resource, and access to it requires not a random algorithm, as in Ethernet networks, but a deterministic one, based on transferring the right to use the ring to stations in a certain order. This right is conveyed through the frame special format, called marker or token.

Token Ring networks operate at two bit rates - 4 and 16 Mbit/s. Mixing stations operating at different speeds in one ring is not allowed. Token Ring networks operating at 16 Mbps have some improvements in the access algorithm compared to the 4 Mbps standard.

Token Ring technology is a more complex technology than Ethernet. It has fault tolerance properties. The Token Ring network defines network operation control procedures that use feedback of a ring-shaped structure - the sent frame always returns to the sending station. In some cases, detected errors in the network operation are eliminated automatically, for example, a lost token can be restored. In other cases, errors are only recorded, and their elimination is carried out manually by maintenance personnel.

To control the network, one of the stations acts as a so-called active monitor. The active monitor is selected during ring initialization as the station with maximum value MAC addresses If the active monitor fails, the ring initialization procedure is repeated and a new active monitor is selected. In order for the network to detect the failure of an active monitor, the latter, in a working state, generates a special frame of its presence every 3 seconds. If this frame does not appear on the network for more than 7 seconds, then the remaining stations on the network begin the procedure for electing a new active monitor.

Token Ring Frame Formats

There are three different frame formats in Token Ring:

· marker;

· data frame;

· interrupt sequence

Physical layer of Token Ring technology

The IBM Token Ring standard initially provided for the construction of connections in the network using hubs called MAU (Multistation Access Unit) or MSAU (Multi-Station Access Unit), that is, multiple access devices (Fig. 3.15). The Token Ring network can include up to 260 nodes.

Rice. 3.15.Physical configuration of the Token Ring network

A Token Ring hub can be active or passive. A passive hub simply interconnects ports so that stations connected to those ports form a ring. The passive MSAU does not perform signal amplification or resynchronization. Such a device can be considered a simple crossover unit with one exception - MSAU provides bypass of a port when the computer connected to this port is turned off. This function is necessary to ensure ring connectivity regardless of the state of the connected computers. Typically, port bypass is accomplished by relay circuits that are powered by DC from the network adapter, and when the network adapter is turned off, the normally closed relay contacts connect the port input to its output.

An active hub performs signal regeneration functions and is therefore sometimes called a repeater, as in the Ethernet standard.

The question arises - if the hub is a passive device, then how is high-quality transmission of signals over long distances, which occurs when several hundred computers are connected to a network, ensured? The answer is that in this case each network adapter takes on the role of a signal amplifier, and the role of a resynchronization unit is performed by the network adapter of the active ring monitor. Each Token Ring network adapter has a repeater unit that can regenerate and resynchronize signals, but only the active monitor repeater unit performs the latter function in the ring.

The resynchronization unit consists of a 30-bit buffer that receives Manchester signals with intervals slightly distorted during the round trip. With the maximum number of stations in the ring (260), the variation in the delay of bit circulation around the ring can reach 3-bit intervals. An active monitor “inserts” its buffer into the ring and synchronizes the bit signals, outputting them at the required frequency.

IN general case The Token Ring network has a combined star-ring configuration. End nodes are connected to the MSAU in a star topology, and the MSAUs themselves are combined through special Ring In (RI) and Ring Out (RO) ports to form a backbone physical ring.

All stations in the ring must operate at the same speed - either 4 Mbit/s or 16 Mbit/s. The cables connecting the station to the hub are called lobe cables, and the cables connecting the hubs are called trunk cables.

Token Ring technology can be used to connect end stations and hubs Various types cables: STP Type I, UTP Type 3, UTP Type 6, as well as fiber optic cable.

When using shielded twisted pair STP Type 1 from the IBM cable system range, up to 260 stations can be combined into a ring with a drop cable length of up to 100 meters, and when using unshielded twisted pair, the maximum number of stations is reduced to 72 with a drop cable length of up to 45 meters.

The distance between passive MSAUs can be up to 100 m when using STP Type 1 cable and 45 m when using UTP Type 3 cable. Between active MSAUs, the maximum distance increases respectively to 730 m or 365 m depending on the cable type.

Maximum length The Token Ring is 4000 m. Restrictions on the maximum ring length and the number of stations in the ring in Token Ring technology are not as strict as in Ethernet technology. Here, these restrictions are largely related to the time the marker turns around the ring (but not only - there are other considerations that dictate the choice of restrictions). So, if the ring consists of 260 stations, then with a marker holding time of 10 ms, the marker will return to the active monitor in the worst case in 2.6 s, and this time is exactly the marker rotation control timeout. Basically, all timeout values ​​in network adapters Token Ring network nodes are configurable, so you can build a Token Ring network with big amount stations and with a longer ring length.

conclusions

· Token Ring technology is developed primarily by IBM and also has IEEE 802.5 status, which reflects the most important improvements being made to IBM technology.

· Token Ring networks use a token access method, which guarantees that each station can access the shared ring within the token rotation time. Because of this property, this method is sometimes called deterministic.

· The access method is based on priorities: 0 (lowest) to 7 (highest). The station itself determines the priority of the current frame and can capture the ring only if there are no higher priority frames in the ring.

· Token Ring networks operate at two speeds: 4 and 16 Mbps and can use shielded twisted pair, unshielded twisted pair, and fiber optic cable as the physical media. The maximum number of stations in the ring is 260, and the maximum length of the ring is 4 km.

· Token Ring technology has elements of fault tolerance. Due to feedback ring one of the stations - the active monitor - continuously monitors the presence of the marker, as well as the rotation time of the marker and data frames. If the ring does not operate correctly, the procedure for its reinitialization is launched, and if this does not help, then the beaconing procedure is used to localize the faulty section of the cable or the faulty station.

· The maximum data field size of a Token Ring frame depends on the speed of the ring. For a speed of 4 Mbit/s it is about 5000 bytes, and at a speed of 16 Mbit/s it is about 16 KB. The minimum size of the frame data field is not defined, that is, it can be equal to 0.

· In the Token Ring network, stations are connected into a ring using hubs called MSAUs. The MSAU passive hub acts as a crossover panel that connects the output of the previous station in the ring to the input of the next one. The maximum distance from the station to the MSAU is 100 m for STP and 45 m for UTP.

· An active monitor also acts as a repeater in the ring - it resynchronizes signals passing through the ring.

· The ring can be built on the basis of an active MSAU hub, which in this case is called a repeater.

· The Token Ring network can be built on the basis of several rings separated by bridges that route frames based on the “from the source” principle, for which a special field with the route of the rings is added to the Token Ring frame.

Network technologies IEEE802.4/ArcNet

The ArcNet network uses a “bus” and a “passive star” as its topology. Supports shielded and unshielded twisted pair and fiber optic cable. The ArcNet network uses a delegation method to access the media. The ArcNet network is one of the oldest networks and has been very popular. Among the main advantages of the ArcNet network are high reliability, low cost of adapters and flexibility. The main disadvantage of the network is low speed information transmission (2.5 Mbit/s). The maximum number of subscribers is 255. The maximum network length is 6000 meters.

Network technology FDDI (Fiber Distributed Data Interface)

FDDI–a standardized specification for network architecture for high-speed data transmission over fiber optic lines. Transfer speed – 100 Mbit/s. This technology is largely based on the Token-Ring architecture and uses deterministic token access to the data transmission medium. The maximum length of the network ring is 100 km. The maximum number of network subscribers is 500. The FDDI network is a very highly reliable network, which is created on the basis of two fiber optic rings that form the main and backup data transmission paths between nodes.

Main characteristics of the technology

FDDI technology is largely based on Token Ring technology, developing and improving its basic ideas. The developers of FDDI technology set themselves the following goals as their highest priority:

· increase the bit rate of data transfer to 100 Mbit/s;

· increase the fault tolerance of the network through standard procedures for restoring it after various types of failures - cable damage, incorrect operation of a node, hub, high level of interference on the line, etc.;

· make the most of potential network bandwidth for both asynchronous and synchronous (latency-sensitive) traffic.

The FDDI network is built on the basis of two fiber optic rings, which form the main and backup data transmission paths between network nodes. Having two rings is the primary way to increase fault tolerance in an FDDI network, and nodes that want to take advantage of this increased reliability potential must be connected to both rings.

In normal network operation mode, data passes through all nodes and all cable sections of the Primary ring only; this mode is called the Thru- “end-to-end” or “transit”. The Secondary ring is not used in this mode.

In the event of some type of failure where part of the primary ring cannot transmit data (for example, a broken cable or node failure), the primary ring is combined with the secondary ring (Fig. 3.16), again forming a single ring. This mode of network operation is called Wrap, that is, the "folding" or "folding" of the rings. The collapse operation is performed using FDDI hubs and/or network adapters. To simplify this procedure, data on the primary ring is always transmitted in one direction (in the diagrams this direction is shown counterclockwise), and on the secondary ring in the opposite direction (shown clockwise). Therefore, when a common ring of two rings is formed, the transmitters of the stations still remain connected to the receivers of neighboring stations, which allows information to be correctly transmitted and received by neighboring stations.

Rice. 3.16.Reconfiguration of FDDI rings upon failure

FDDI standards place a lot of emphasis on various procedures, which allow you to determine the presence of a failure in the network, and then make the necessary reconfiguration. The FDDI network can fully restore its functionality in the event of single failures of its elements. When there are multiple failures, the network splits into several unconnected networks. FDDI technology complements the failure detection mechanisms of Token Ring technology with mechanisms for reconfiguring the data transmission path in the network, based on the presence of redundant links provided by the second ring.

Rings in FDDI networks are considered as a common shared data transmission medium, so a special access method is defined for it. This method is very close to the access method of Token Ring networks and is also called the token ring method.

The differences in the access method are that the token holding time in the FDDI network is not a constant value, as in the Token Ring network. This time depends on the load on the ring - with a small load it increases, and with large overloads it can decrease to zero. These access method changes only affect asynchronous traffic, which is not critical to small delays in frame transmission. For synchronous traffic, the token hold time is still a fixed value. A frame priority mechanism similar to that adopted in Token Ring technology is absent in FDDI technology. The technology developers decided that dividing traffic into 8 priority levels is redundant and it is sufficient to divide the traffic into two classes - asynchronous and synchronous, the latter of which is always serviced, even when the ring is overloaded.

Otherwise, frame forwarding between ring stations at the MAC level is fully compliant with Token Ring technology. FDDI stations use an early token release algorithm, similar to Token Ring networks with a speed of 16 Mbps.

MAC level addresses are in a standard format for IEEE 802 technologies. The FDDI frame format is close to the Token Ring frame format; the main differences are the absence of priority fields. Signs of address recognition, frame copying and errors allow you to preserve the procedures for processing frames available in Token Ring networks by the sending station, intermediate stations and the receiving station.

In Fig. Figure 3.17 shows the correspondence of the protocol structure of FDDI technology to the seven-layer OSI model. FDDI defines the physical layer protocol and the media access sublayer (MAC) protocol of the data link layer. Like many other local area network technologies, FDDI technology uses the LLC data link control sublayer protocol defined in the IEEE 802.2 standard. Thus, although FDDI technology was developed and standardized by ANSI and not by IEEE, it fits entirely within the framework of the 802 standards.

Rice. 3.17.Structure of FDDI technology protocols

Distinctive feature FDDI technology is the station control level - Station Management (SMT). It is the SMT layer that performs all the functions of managing and monitoring all other layers of the FDDI protocol stack. Each node in the FDDI network takes part in managing the ring. Therefore, all nodes exchange special SMT frames to manage the network.

Fault tolerance of FDDI networks is ensured by protocols of other layers: with the help of the physical layer, network failures for physical reasons, for example, due to a broken cable, are eliminated, and with the help of the MAC layer, logical network failures are eliminated, for example, the loss of the required internal path for transmitting a token and data frames between hub ports .

conclusions

· FDDI technology was the first to use fiber optic cable in local area networks and operate at 100 Mbps.

· There is significant continuity between Token Ring and FDDI technologies: both are characterized by a ring topology and a token access method.

· FDDI technology is the most fault-tolerant local network technology. In case of single failures of the cable system or station, the network, due to the “folding” of the double ring into a single one, remains fully operational.

· The FDDI token access method operates differently for synchronous and asynchronous frames (the frame type is determined by the station). To transmit a synchronous frame, a station can always capture an incoming token for a fixed time. To transmit an asynchronous frame, a station can capture a token only if the token has completed a rotation around the ring quickly enough, which indicates that there is no ring congestion. This access method, firstly, gives preference to synchronous frames, and secondly, regulates the ring load, slowing down the transmission of non-urgent asynchronous frames.

· FDDI technology uses fiber optic cables and Category 5 UTP as the physical medium (this physical layer option is called TP-PMD).

· The maximum number of dual connection stations in a ring is 500, the maximum diameter of a double ring is 100 km. The maximum distances between adjacent nodes for multimode cable are 2 km, for twisted pair UPT category 5-100 m, and for single-mode optical fiber depend on its quality

Computer networks are divided into three main classes:

1. Local computer networks (LAN – LocalAreaNetwork) are networks that connect computers located geographically in one place. A local network unites computers located physically close to each other (in the same room or building).

2. Regional computer networks (MAN - MetropolitanAreaNetwork) are networks that connect several local computer networks located within the same territory (city, region or region, for example, the Far East).

3. Wide Area Networks (WAN - WideAreaNetwork) are networks that unite many local, regional networks and

computers of individual users located at any distance from each other (Internet, FIDO).

Currently, the following standards for building local area networks are used:

Arcnet;(IEEE 802.4)

Token Ring;(802.5)

Ethernet.(802.3)

Let's look at each of them in more detail.

IEEE 802.4 ARCNET technology (or ARCnet, from the English Attached Resource Computer NETwork) is a LAN technology, the purpose of which is similar to the purpose of Ethernet or Token ring. ARCNET was the first technology for creating networks of microcomputers and became very popular in the 1980s for enterprise automation. Designed for organizing a LAN in a “star” network topology.

The basis of communication equipment is:

switch

passive/active hub

Switching equipment has an advantage, as it allows the formation of network domains. Active hubs are used when the workstation is far away (they restore the signal shape and amplify it). Passive - when small. The network uses an assigned access principle for workstations, that is, the station that has received the so-called software token from the server has the right to transmit. That is, deterministic network traffic is implemented.

Advantages of the approach:

Notes: Messages sent by workstations form a queue on the server. If the queue service time significantly (more than 2 times) exceeds the maximum packet delivery time between the two most remote stations, then it is considered that the network capacity has reached its maximum limit. In this case, further expansion of the network is impossible and the installation of a second server is required.

Limit technical characteristics:

The minimum distance between workstations connected to the same cable is 0.9 m.

The maximum network length along the longest route is 6 km.

Limitations are associated with hardware delay in information transmission with a large number of switching elements.

The maximum distance between the passive hub and the workstation is 30 m.

The maximum distance between the active and passive hub is 30 m.

Between active hub and active hub - 600 m.

Advantages:

Low cost of network equipment and the ability to create extended networks.

Flaws:

Low data transfer speed. Following the spread of Ethernet as a LAN technology, ARCNET found application in embedded systems.

The non-profit organization ARCNET Trade Association (ATA) is engaged in support of ARCNET technology (in particular, the distribution of specifications).

Technology - The ArcNET architecture is represented by two main topologies: bus and star. The transmission medium is RG-62 coaxial cable with a characteristic impedance of 93 Ohms, crimped onto BNC plugs with the appropriate termination diameter (different from 10Base-2 (“thin” Ethernet) plugs).

Network equipment consists of network adapters and hubs. Network adapters can be for bus topology, for star topology and universal. Hubs can be active or passive. Passive hubs are used to create star sections of the network. Active hubs can be for bus, star and mixed topologies. Ports for bus topology are not physically compatible with ports for star topology, although they have the same physical connection(BNC socket).

In the case of a bus topology, workstations and servers are connected to each other using T-connectors (the same as in 10Base-2 (“thin” Ethernet) connected to network adapters and hubs and connected by coaxial cable. The extreme points of the segment are terminated with tips with a resistance of 93 Ohms. The number of devices on one bus is limited. The minimum distance between connectors is 0.9 meters and must be a multiple of this value. To facilitate cutting, marks can be applied to the cable. Individual buses can be combined using bus hubs.

When using a star topology, active and passive hubs are used. The passive hub is a resistive divider-matcher that allows you to connect four cables. All cables in this

In this case, they are connected on a point-to-point basis, without forming buses. There should not be more than two passive hubs connected between two active devices. The minimum length of any network cable is 0.9 meters and must be a multiple of this value. There is a limitation on the cable length between active and passive ports, between two passive ports, and between two active ports.

With a mixed topology, active hubs are used that support both types of connections.

On network adapters of workstations and servers, using jumpers or DIP switches, a unique network address is set, permission to use a BIOS expansion chip that allows remote boot of the workstation (can be diskless), connection type (bus or star topology), connection of a built-in terminator ( the last two points are optional). The limit on the number of workstations is 255 (according to the width of the network address register). If two devices have the same network address, both lose their functionality, but this collision does not affect the operation of the network as a whole.

In a bus topology, a broken cable or terminator leads to network inoperability for all devices connected to the segment that includes this cable (that is, from terminator to terminator). With a star topology, a break in any cable leads to the failure of the segment that is disconnected from the file server by this cable.

The logical architecture of ArcNET is a token ring. Since this architecture, in principle, does not allow collisions, with a relatively large number of hosts (in practice, 25-30 workstations were tested), the performance of the ArcNET network turned out to be higher than 10Base-2, with a four times lower speed in the environment (2.5 versus 10 Mbit/s ).

802.5 Token Ring technology is a local area network (LAN) ring technology with “token access” - a local network protocol that is located at the data link layer (DLL) of the OSI model. It uses a special three-byte frame called a marker that moves around the ring. Possession of a marker gives the owner the right to transmit information on the medium. Token ring network frames travel in a loop. Stations on a Token ring local area network (LAN) are logically organized in a ring topology with data transferred sequentially from one ring station to another with a control token circulating around the control access ring. This token passing mechanism is shared by ARCNET, the token bus, and FDDI, and has theoretical advantages over stochastic CSMA/CD Ethernet.

Token passing Token ring and IEEE 802.5 are prime examples of token passing networks. Token passing networks move a small block of data called a token along the network. Possession of this token guarantees the right to transfer. If the node receiving the token does not have information to send, it simply forwards the token to the next endpoint. Each station can hold a marker for a certain maximum time (default - 10 ms).

This technology offers a solution to the problem of collisions that arise when operating a local network. In Ethernet technology, such collisions occur when information is simultaneously transmitted by several workstations located within the same segment, that is, using a common physical data channel.

If the station holding the token has information to transmit, it captures the token, changes one bit of it (resulting in the token becoming a "beginning of data block" sequence), completes it with the information it wants to transmit, and sends that information to the next station ring network. When a block of information circulates around the ring, there is no token on the network (unless the ring provides early token release), so other stations wishing to transmit information are forced to wait. Therefore, there can be no collisions in Token Ring networks. If early token release is ensured, then a new token can be released after the transmission of the data block is completed.

The information block circulates around the ring until it reaches the intended destination station, which copies the information for further processing. The information block continues to circulate around the ring; it is permanently deleted after reaching the station that sent the block. The sending station can check the returned block to ensure that it was viewed and then copied by the destination station.

Scope of Application Unlike CSMA/CD networks (eg Ethernet), token passing networks are deterministic networks. This means that it is possible to calculate the maximum time that will pass before any end station can transmit. This characteristic, as well as some reliability characteristics, make the Token Ring network ideal for applications where latency must be predictable and network stability is important. Examples of such applications are the environment of automated stations in factories.

It is used as a cheaper technology and has become widespread wherever there are critical applications for which it is not so much speed that is important as reliable delivery of information. Currently, Ethernet is not inferior to Token Ring in reliability and is significantly higher in performance.

Modifications of Token Ring There are 2 modifications for transmission speeds: 4 Mbit/s and 16 Mbit/s. Token Ring uses 16 Mbps

early marker release technology. The essence of this technology is that a station that has “captured” a token, upon completion of data transmission, generates a free token and launches it into the network. Attempts to introduce 100 Mbit/s technology were not crowned with commercial success. Token Ring technology is not currently supported.

Technology 802.3 Ethernet from English. ether “ether”) is a packet technology for transmitting data primarily on local computer networks.

Ethernet standards define wire connections and electrical signals on physical level, frame format and media access control protocols - at the data link layer of the OSI model. Ethernet is primarily described by IEEE group 802.3 standards. Ethernet became the most common LAN technology in the mid-1990s, displacing legacy technologies such as Arcnet, FDDI and Token ring.

When performing work on creating a local network, you need to consider the following:

* Creating a local network and setting up equipment for access to the Internet;

* The choice of equipment should be based on technical specifications, capable of meeting the requirements for data transfer speed;

* The equipment must be safe, protected from injury to people electric shock;

* Each workstation must have a network cable;

* Possible availability of wi-fi throughout the office;

* The location of workplaces must meet the requirements of equipment placement standards in educational institutions;

* The costs of creating a local network must be economically justified;

* Local network reliability.

INTRODUCTION………………………………………………………………………………..3

1 ETHERNET AND FAST ETHERNET NETWORKS……………………………5

2 TOKEN-RING NETWORK…………………………………………………….9

3 ARCNET NETWORK………………………………………………………….14

4 FDDI NETWORK………………………………………………………………………………18

5 100VG-AnyLAN NETWORK…………………………………………………………….23

6 ULTRA-SPEED NETWORKS…………………………………………………….25

7 WIRELESS NETWORKS……………………………………………………….31

CONCLUSION…………………………………………………………….36

LIST OF SOURCES USED………………………39

INTRODUCTION

Since the advent of the first local networks, several hundred different network technologies have been developed, but only a few have become noticeably widespread. This is primarily due to high level standardization of networking principles and their support by well-known companies. However, standard networks do not always have record characteristics and provide the most optimal modes exchange. But the large production volumes of their equipment and, consequently, its low cost give them enormous advantages. It is also important that manufacturers software also primarily focus on the most common networks. Therefore, a user who chooses standard networks has a full guarantee of compatibility of equipment and programs.

The purpose of this course work is to consider existing local network technologies, their characteristics and advantages or disadvantages over each other.

I chose the topic of local network technologies because, in my opinion, this topic is especially relevant now, when mobility, speed and convenience are valued all over the world, with as little time wasted as possible.

Currently, reducing the number of types of networks used has become a trend. The fact is that increasing the transmission speed in local networks to 100 and even 1000 Mbit/s requires the use of the most advanced technologies and expensive scientific research. Naturally, only the largest companies that support their standard networks and their more advanced varieties can afford this. In addition, a large number of consumers have already installed some kind of network and do not want to immediately and completely replace network equipment. It is unlikely that fundamentally new standards will be adopted in the near future.

The market offers standard local networks of all possible topologies, so users have a choice. Standard networks provide wide range permissible sizes network, number of subscribers and, no less important, prices for equipment. But making a choice is still not easy. Indeed, unlike software, which is not difficult to replace, hardware usually lasts for many years; its replacement leads not only to significant costs and the need to re-wire cables, but also to a revision of the organization's computer system. In this regard, errors in the choice of equipment are usually much more expensive than errors in the choice of software.

1 ETHERNET AND FAST ETHERNET NETWORKS

The most widespread among standard networks is the Ethernet network. It first appeared in 1972 (developed by the famous company Xerox). The network turned out to be quite successful, and as a result, in 1980 it was supported by such largest companies, like DEC and Intel). Through their efforts, in 1985, the Ethernet network became an international standard; it was adopted by the largest international standards organizations: IEEE Committee 802 (Institute of Electrical and Electronic Engineers) and ECMA (European Computer Manufacturers Association).

The standard is called IEEE 802.3 (read in English as “eight oh two dot three”). It defines multiple access to a mono bus type channel with collision detection and transmission control. Some other networks also met this standard, since its level of detail is low. As a result, IEEE 802.3 networks were often incompatible with each other in both design and electrical characteristics. However, recently the IEEE 802.3 standard has been considered the standard for the Ethernet network.

Main characteristics of the original IEEE 802.3 standard:

• topology – bus;
• transmission medium – coaxial cable;
• transmission speed – 10 Mbit/s;
• maximum network length – 5 km;
• maximum number of subscribers – up to 1024;
• network segment length – up to 500 m;
• number of subscribers on one segment – ​​up to 100;
• access method – CSMA/CD;
• Narrowband transmission, that is, without modulation (mono channel).

Strictly speaking, there are minor differences between the IEEE 802.3 and Ethernet standards, but they are usually ignored.

The Ethernet network is now the most popular in the world (more than 90% of the market), and presumably it will remain so in the coming years. This was greatly facilitated by the fact that from the very beginning the characteristics, parameters, and protocols of the network were open, as a result of which a huge number of manufacturers around the world began to produce Ethernet equipment that was fully compatible with each other.

The classic Ethernet network used 50-ohm coaxial cable of two types (thick and thin). However, recently (since the early 90s), the most widely used version of Ethernet is that using twisted pairs as a transmission medium. A standard has also been defined for use in fiber optic cable networks. Additions have been made to the original IEEE 802.3 standard to accommodate these changes. In 1995, an additional standard appeared for a faster version of Ethernet operating at a speed of 100 Mbit/s (the so-called Fast Ethernet, IEEE 802.3u standard), using twisted pair or fiber optic cable as the transmission medium. In 1997, a version with a speed of 1000 Mbit/s (Gigabit Ethernet, IEEE 802.3z standard) also appeared.

In addition to the standard bus topology, passive star and passive tree topologies are increasingly being used.

Classic Ethernet network topology

The maximum cable length of the network as a whole (maximum signal path) can theoretically reach 6.5 kilometers, but practically does not exceed 3.5 kilometers.

A Fast Ethernet network does not have a physical bus topology; only a passive star or passive tree is used. In addition, Fast Ethernet has much more stringent requirements for the maximum network length. After all, with a 10-fold increase in transmission speed and preservation of the packet format, its minimum length becomes ten times shorter. Thus, the permissible value of double signal transmission time through the network is reduced by 10 times (5.12 μs versus 51.2 μs in Ethernet).

The standard Manchester code is used to transmit information on an Ethernet network.

Access to the Ethernet network is carried out using the random CSMA/CD method, ensuring equality of subscribers. The network uses packets of variable length with structure.

For an Ethernet network operating at a speed of 10 Mbit/s, the standard defines four main types of network segments, focused on different information transmission media:

• 10BASE5 (thick coaxial cable);
• 10BASE2 (thin coaxial cable);
• 10BASE-T (twisted pair);
• 10BASE-FL (fiber optic cable).

The name of the segment includes three elements: the number “10” means a transmission speed of 10 Mbit/s, the word BASE means transmission in the base frequency band (that is, without modulating a high-frequency signal), and the last element is the permissible length of the segment: “5” – 500 meters, “2” – 200 meters (more precisely, 185 meters) or type of communication line: “T” – twisted pair (from the English “twisted-pair”), “F” – fiber optic cable (from the English “fiber optic”).

Similarly, for an Ethernet network operating at a speed of 100 Mbit/s (Fast Ethernet), the standard defines three types of segments, differing in the types of transmission media:

• 100BASE-T4 (quad twisted pair);
• 100BASE-TX (dual twisted pair);
• 100BASE-FX (fiber optic cable).

Here, the number “100” means a transmission speed of 100 Mbit/s, the letter “T” means twisted pair, and the letter “F” means fiber optic cable. The types 100BASE-TX and 100BASE-FX are sometimes combined under the name 100BASE-X, and 100BASE-T4 and 100BASE-TX are called 100BASE-T.

The development of Ethernet technology is moving further and further away from the original standard. The use of new transmission media and switches makes it possible to significantly increase the size of the network. Elimination of the Manchester code (in Fast Ethernet and Gigabit Ethernet networks) provides increased data transfer speeds and reduced cable requirements. Refusal of the CSMA/CD control method (with full-duplex exchange mode) makes it possible to dramatically increase operating efficiency and remove restrictions on network length. However, all new varieties of network are also called Ethernet network.

2 TOKEN-RING NETWORK

The Token-Ring network was proposed by IBM in 1985 (the first version appeared in 1980). It was intended to network all types of computers produced by IBM. The mere fact that it is supported by IBM largest producer computer equipment, indicates that she needs to pay special attention. But equally important is that Token-Ring is currently the international standard IEEE 802.5 (although there are minor differences between Token-Ring and IEEE 802.5). This puts this network on the same status level as Ethernet.

Token-Ring was developed as a reliable alternative to Ethernet. And although Ethernet is now replacing all other networks, Token-Ring cannot be considered hopelessly outdated. More than 10 million computers around the world are connected by this network.

IBM has done everything to ensure the widest possible distribution of its network: detailed documentation has been released up to circuit diagrams adapters. As a result, many companies, for example, 3COM, Novell, Western Digital, Proteon and others began producing adapters. By the way, the NetBIOS concept was developed specifically for this network, as well as for another network, the IBM PC Network. If in the previously created PC network Network programs NetBIOS were stored in the built-in read-only memory of the adapter, but the Token-Ring network already used a program emulating NetBIOS. This made it possible to respond more flexibly to hardware features and maintain compatibility with higher-level programs.

What is network technology? Why is it needed? What is it used for? Answers to these, as well as a number of other questions, will be given within the framework of this article.

Several important parameters

1. Data transfer rate. This characteristic determines how much information (measured in most cases in bits) can be transmitted through the network in a certain period of time.
2. Frame format. Information that is transmitted through the network is combined into information packets. They are called frames.
3. Signal coding type. In this case, it is decided how to encrypt information in electrical impulses.
4. Transmission medium. This designation is used for the material, as a rule, it is a cable through which the flow of information passes, which is subsequently displayed on monitor screens.
5. Network topology. This is a schematic construction of a structure through which information is transmitted. As a rule, a tire, a star and a ring are used.
6. Access method.

The set of all these parameters determines the network technology, what it is, what devices it uses and its characteristics. As you can guess, there are a great many of them.

general information

But what is network technology? After all, the definition of this concept was never given! So, network technology is a coordinated set of standard protocols and software and hardware that implement them in a volume sufficient to build a local computer network. This determines how the data transmission medium will be accessed. Alternatively, you can also find the name “basic technologies”. It is not possible to consider them all within the framework of the article due to the large number, so attention will be paid to the most popular: Ethernet, Token-Ring, ArcNet and FDDI. What are they?

Ethernet

At the moment it is the most popular network technology all over the world. If the cable fails, then the probability that it is the one being used is close to one hundred percent. Ethernet can be safely included in the best network information technologies, due to its low cost, high speed and quality of communication. The most famous type is IEEE802.3/Ethernet. But based on it, two very interesting options were developed. The first (IEEE802.3u/Fast Ethernet) allows for a transmission speed of 100 Mbit/second. This option has three modifications. They differ from each other in the material used for the cable, the length of the active segment and the specific scope of the transmission range. But fluctuations occur in the style of “plus or minus 100 Mbit/second”. Another option is IEEE802.3z/Gigabit Ethernet. Its transmission capacity is 1000 Mbit/s. This variation has four modifications.

Token-Ring

Network information technologies of this type are used to create a shared data transmission medium, which is ultimately formed as the union of all nodes into one ring. This technology is based on a star-ring topology. The first one is the main one, and the second one is the additional one. To gain access to the network, the token method is used. The maximum length of the ring can be 4 thousand meters, and the number of nodes can be 260 pieces. The data transfer rate does not exceed 16 Mbit/second.

ArcNet

This option uses a bus and passive star topology. Moreover, it can be built on unshielded twisted pair and fiber optic cable. ArcNet is a true old-timer in the world of networking technologies. The network length can reach 6000 meters, and the maximum number of subscribers is 255. It should be noted that the main disadvantage of this approach is its low data transfer rate, which is only 2.5 Mbit/second. But this network technology is still widely used. This is due to its high reliability, low cost of adapters and flexibility. Networks and network technologies, built according to other principles, may have higher speed indicators, but precisely because ArcNet provides high data yield, this allows us not to discount it. An important advantage of this option is that the access method is used through delegation of authority.

FDDI

Network Computer techologies of this type are standardized specifications for high-speed data transmission architecture using fiber optic lines. FDDI has been significantly influenced by ArcNet and Token-Ring. Therefore, this network technology can be considered as an improved data transmission mechanism based on existing developments. The ring of this network can reach a length of one hundred kilometers. Despite the considerable distance, the maximum number of subscribers who can connect to it is only 500 nodes. It should be noted that FDDI is considered highly reliable due to the presence of a primary and backup data path. Adding to its popularity is the ability to quickly transfer data - approximately 100 Mbit/second.

Technical aspect

Having considered what the basics of network technologies are and what they are used, now let’s pay attention to how everything works. Initially, it should be noted that the previously discussed options are exclusively local means of connecting electronic computers. But there are also global networks. There are about two hundred of them in the world. How do modern network technologies work? To do this, let's look at the current construction principle. So, there are computers that are united into one network. Conventionally, they are divided into subscriber (main) and auxiliary. The former are engaged in all information and computing work. What the network resources will be depends on them. Auxiliary ones are engaged in the transformation of information and its transmission through communication channels. Due to the fact that they have to process a significant amount of data, servers boast increased power. But the final recipient of any information is still ordinary host computers, which are most often represented personal computers. Network information technologies can use the following types of servers:

1. Network. Engaged in the transfer of information.
2. Terminal. Ensures the functioning of a multi-user system.
3. Databases. Involved in processing database queries in multi-user systems.

Circuit Switching Networks

They are created by physically connecting clients for the time that messages will be transmitted. What does this look like in practice? In such cases, a direct connection is created to send and receive information from point A to point B. It includes the channels of one of many (usually) message delivery options. And the created connection for successful transfer must remain unchanged throughout the session. But in this case, quite strong disadvantages appear. So, you have to wait a relatively long time for a connection. This is accompanied by high data transmission costs and low channel utilization. Therefore, the use of network technologies of this type is not common.

Message Switching Networks

In this case, all information is transmitted in small portions. A direct connection is not established in such cases. Data transmission is carried out over the first free available channel. And so on until the message is transmitted to its recipient. At the same time, servers are constantly engaged in receiving information, collecting it, checking it and establishing a route. And then the message is passed on. Among the advantages it should be noted low price transfers. But in this case, there are still problems such as low speed and the impossibility of dialogue between computers in real time.

Packet switching networks

This is the most advanced and popular method today. The development of network technologies has led to the fact that information is now exchanged through short information packets of a fixed structure. What are they? Packets are parts of messages that meet a certain standard. Their short length helps prevent network blocking. Thanks to this, the queue at the switching nodes is reduced. Connections are fast, error rates are kept low, and significant gains are made in terms of network reliability and efficiency. It should also be noted that there are different configurations of this approach to construction. So, if a network provides switching of messages, packets and channels, then it is called integral, that is, it can be decomposed. Some of the resources can be used exclusively. Thus, some channels can be used to transmit direct messages. They are created for the duration of data transfer between different networks. When the session for sending information ends, they break up into independent trunk channels. When using packet technology, it is important to configure and coordinate a large number of clients, communication lines, servers and a number of other devices. Establishing rules known as protocols helps with this. They are part of the network operating system used and are implemented at the hardware and software levels.