IEEE standards. IEEE Standards IEEE operating principles include

The IEEE 802.X standards cover the two lower layers of the OSI model - physical and channel. This is due to the fact that it is these levels that most reflect the specifics of local networks. The higher levels, starting with the network, have common features for both LAN and WAN.

The specificity of local networks is also reflected in the division of the link layer into two sublevels:

- logical data transmission (Logical Link Control, LLC);

- media access control (MAC).

The LLC layer, acting above the MAC layer, is responsible for establishing a communication channel and for error-free sending and receiving of data messages, and also implements the functions of an interface with an adjacent network layer.

The MAC layer provides shared access to the physical layer, definition of frame boundaries, recognition of frame destination addresses. This level ensures the sharing of a common environment, making it available in accordance with a certain algorithm at the disposal of one or another station on the network. After access to the medium is obtained, it can be used by a higher level - the LLC level, which organizes the transfer of logical data units, information frames, with different levels quality of transport services. Through the LLC layer, the network protocol requests from the link layer the transport operation it needs with the required quality. The IEEE 802 standards have a fairly clear structure, shown in Figure 4.1.

Figure 4.1 - Structure of IEEE 802.X standards

Committee 802 includes the following number of subcommittees:

802.1 - Internetworking - networking;

802.2 - LLC - logical data transfer control;

802.3 - Ethernet with CSMA / CD access method;

802.4 - Token Bus LAN - local area networks with Token Bus access method;

802.5 - local area networks with Token Ring access method;

802.6 - Metropolitan Area Network, MAN - metropolitan area networks;

802.7 - Broadband Advisory Group;

802.8 - Technical Advisory Group on OSI;

802.10 - Network Security - network security;

802.11 - Wireless Networks - wireless networks;

802.12 - Demand Priority Access LAN, l00VG-AnyLAN - local area networks with priority demand access method.

The LLC layer provides the upper layers with three types of procedures:

- LLC1 - service without connection establishment and without confirmation;

- LLC2 - service with connection establishment and confirmation;

- LLC3 is a connectionless but acknowledged service.

Ethernet network was first designed in the 70s. by Dr. Robert Metcalfe as part of the "office of the future" project. At the time, it was a 3 Mbps network. In 1980, Ethernet was standardized by the DECIntelXerox Consortium (DIX) as a 10 Mbps network, and in 1985 it was standardized as 802m by the IEEE. Since then, the new Ethernet technology has inherited features of the basic structure of the original Ethernet design, providing a logical bus topology and Carrier Sense Multiple Access with Collision Detection (CSMA / CD). Different types of Ethernet use different physical topologies (eg star or bus) and different types of cables (eg UTP, coaxial, fiber optic).

There are several different types of Ethernet, which are described in Table 4.1.

Table 4.1

Some types of Ethernet networks and their description

Token-Ring. Token Ring networks use a shared data transmission medium, which consists of lengths of cable connecting all stations on the network in a ring. The ring is treated as a shared resource. The access algorithm is based on the transfer by stations of the right to use the ring in a certain order. The right to use the ring is transmitted using a special format frame called a token or token.

Token Ring networks operate at two bit rates - 4 and 16 Mbps. The first speed is defined in the 802.5 standard, and the second is an evolution of the Token Ring technology. Mixing stations operating at different speeds in one ring is not allowed. Token Ring networks, operating at 16 Mbps, also have improvements in the access algorithm compared to the 4 Mbps standard.

Token Ring technology is more complex than Ethernet. It has the properties of fault tolerance. The Token Ring network defines network control procedures that use a ring-shaped feedback structure - a sent frame is always returned 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 performed manually by the service personnel.

To monitor the network, one of the stations acts as a so-called active monitor, which is selected during ring initialization as the station with the maximum MAC address.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 the active monitor, the latter, in a healthy 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 rest of the network stations begin the procedure for selecting a new active monitor.

FDDI (Fiber Optic Distributed Data Interface) standard - focused on high speed transmission (100 Mbit / s) and on the use of fiber optic cable. At the same time, it has the advantages of noise immunity, maximum secrecy of information transmission and excellent galvanic isolation of subscribers. The high transfer rate allows to solve problems inaccessible to slower networks (transmission of images in real time). Fiber-optic cable solves the problem of transmitting data over a distance of several kilometers without retransmission, which allows building large networks, covering even entire cities and having a low error rate. The FDDI standard was based on the token access method. Ring topology. The network uses two multidirectional fiber-optic cables, one of which is usually in reserve, but this solution also allows the use of full-duplex information transmission (simultaneously in two directions) with twice the effective speed of 200 Mbit / s (with each of the two channels operating at a speed 100 Mbps). A star-ring topology with hubs included in the ring (as in Token-Ring) is also used.

To create a reliable mechanism for transmitting data between two stations, it is necessary to define a protocol that will allow receiving and transmitting various data over communication channels.

HDLC (High-Level Data Link Control) is a high-level data link control protocol. The basic principles of the HDLC protocol: logical connection mode, control of corrupted and lost frames using the sliding window method, frame flow control using the RNR (receiver not ready) and RR (receiver ready) commands.

Today, HDLC on dedicated lines has superseded Point-to-Point Protocol, PPP. The fact is that one of the main functions of the HDLC protocol is the recovery of distorted and lost frames. However, digital channels are popular today, which, even without external procedures for frame recovery, have high quality(BER is 10 -8 - 10 -9). HDLC recovery functions are not required to operate on such a channel. When transmitting over analog dedicated channels, modern modems themselves use protocols of the HDLC family. Therefore, the use of HDLC at the router or bridge level becomes unjustified.

PPP has become the de facto standard for wide area communications to connect remote clients to servers and to form connections between routers on a corporate network. When developing the PPP protocol, the HDLC frame format was taken as a basis and supplemented with its own fields. PPP fields are embedded in the data field of the HDLC frame.

The main difference between PPP and other link-layer protocols is that it achieves consistent operation various devices using a negotiation procedure during which various parameters are transmitted, such as line quality, authentication protocol and encapsulated protocols network layer... The negotiation procedure takes place during the connection establishment.

The PPP protocol is based on four principles: negotiated acceptance of connection parameters, multi-protocol support, protocol extensibility, independence from global services.

One of the capabilities of the PPP protocol is the use of several physical lines to form one logical channel, the so-called channel trunking (a common logical channel can consist of channels of different physical nature. For example, one channel can be formed in the telephone network, and the other can be virtual switched channels Frame relay networks). This feature is implemented by an additional protocol called MLPPP (Multi Link PPP).

Main literature: 5.

Further reading: 12.

Control questions:

1. What two sublevels of the data link layer do you know?

2. What types of Ethernet networks do you know?

3. What is the topology of the Token Ring network?

4. What is the FDDI network topology?

5. What is the difference between HDLC and PPP protocols?

In 1980, the IEEE Institute organized the 802 LAN Standardization Committee, which resulted in the adoption of the IEEE 802-x family of standards, which provide guidelines for the design of the lower layers of local area networks. Later, the results of the work of this committee formed the basis for a set of international standards ISO 8802-1 ... 5. These standards were based on the very common proprietary standards for Ethernet, ArcNet, and Token Ring.

In addition to the IEEE, other organizations have participated in the work on the standardization of LAN protocols. So, for networks operating on fiber, the American Institute for Standardization ANSI developed the FDDI standard, which provides a data transfer rate of 100 Mb / s. The work on standardization of protocols is also carried out by the ECMA association, which has adopted the ECMA-80, 81, 82 standards for local network type Ethernet and subsequently the ECMA-89.90 standards for the token transfer method.

IEEE 802.X family standards cover only two lower layers of the seven-layer OSI model - physical and channel. This is due to the fact that it is these levels that most reflect the specifics of local networks. The higher levels, starting with the network, have largely common features for both local and global networks.

The specificity of local networks is also reflected in the division of the link layer into two sublevels, which are often also called layers. The Data Link Layer is divided into two sublevels in local networks:

logical data transmission (Logical Link Control, LLC);

media access control (MAC).

The MAC layer appeared due to the existence of a shared data transmission medium in local networks. It is this level that ensures the correct sharing of the common environment, providing it in accordance with a certain algorithm at the disposal of one or another station on the network. After access to the medium is obtained, it can be used by a higher level - the LLC level, which organizes the transfer of logical data units, information frames, with different levels of quality of transport services. In modern local area networks, several protocols of the MAC layer have become widespread, implementing various algorithms for accessing a shared medium. These protocols fully define the specifics of technologies such as Ethernet, Fast Ethernet, Gigabit Ethernet, Token Ring, FDDI, l00VG-AnyLAN.

The LLC layer is responsible for transferring data frames between nodes with varying degrees of reliability, and also implements the functions of an interface with an adjacent network layer. It is through the LLC layer that the network protocol requests from the link layer the transport operation it needs with the required quality. At the LLC level, there are several modes of operation that differ in the presence or absence of frame recovery procedures at this level in the event of their loss or distortion, that is, differing in the quality of transport services at this level.

The MAC and LLC layer protocols are mutually independent - each MAC layer protocol can be used with any LLC layer protocol, and vice versa.

The IEEE 802 standards have a fairly clear structure, shown in Fig. 62. This structure is the result of a great deal of work done by the 802 to highlight common approaches and common functions across proprietary technologies, and harmonize their description styles. As a result, the link layer was divided into the two mentioned sublevels. The description of each technology is divided into two parts: a description of the MAC layer and a description of the physical layer. As you can see from the figure, almost every technology has several options for the physical layer protocols corresponding to a single MAC layer protocol (in the figure, in order to save space, only Ethernet and Token Ring technologies are shown, but everything said is also true for other technologies, such as ArcNet, FDDI, l00VG-AnyLAN).

Rice. 62. Structure of IEEE 802.X standards

Above the data link layer of all technologies, a common LLC protocol is depicted, which supports several modes of operation, but is independent of the choice of a specific technology. The LLC standard is overseen by the 802.2 subcommittee. Even technologies that are not standardized by 802.2 rely on the LLC protocol defined by the 802.2 standard, such as the ANSI standardized FDDI protocol.

Standing apart are the standards developed by the 802.1 subcommittee. These standards are common to all technologies. In the 802.1 subcommittee, general definitions of local networks and their properties were developed, the relationship between the three layers of the IEEE 802 model and the OSI model was defined. But the most practically important are the 802.1 standards, which describe the interaction of various technologies with each other, as well as standards for building more complex networks based on basic topologies. This group of standards is collectively referred to as internetworking standards. This includes such important standards as the 802. ID standard, which describes the logic of the bridge / switch, the 802.1H standard, which determines the operation of the translating bridge, which can combine Ethernet and FDDI, Ethernet and Token Ring, etc. without a router. Today, a set of standards developed by the 802.1 subcommittee continues to grow. For example, it was recently updated with the important 802.1Q standard, which defines how VLANs are built in switch-based networks.

The 802.3,802.4,802.5 and 802.12 standards describe LAN technologies that have evolved as a result of improvements in proprietary technologies that formed their basis. Thus, the basis of the 802.3 standard was the Ethernet technology developed by Digital, Intel and Xerox (or Ethernet DIX), the 802.4 standard appeared | as a generalization of Datapoint Corporation's ArcNet technology, and the 802.5 standard largely matches IBM's Token Ring technology.

The original proprietary technologies and their modified versions - 802.x standards in some cases existed in parallel for many years. For example, ArcNet technology has not been fully brought into compliance with the 802.4 standard (now it is too late to do this, since somewhere around 1993 the production of ArcNet equipment was phased out). Differences between Token Ring technology and the 802.5 standard also arise periodically, as IBM regularly makes improvements to its technology and the 802.5 committee reflects these improvements in the standard with some delay. The exception is Ethernet technology. The last proprietary Ethernet standard, DIX, was adopted in 1980, and no one has attempted to proprietary Ethernet since then. All innovations in the Ethernet family of technologies come only as a result of the adoption of open standards by the 802.3 committee.

Later standards were initially developed not by one company, but by a group of interested companies, and then submitted to the appropriate IEEE 802 subcommittee for approval. This happened with Fast Ethernet, l00VG-AnyLAN, Gigabit Ethernet technologies. The group of interested companies formed at first a small association, and then, as the work developed, other companies joined it, so that the process of adopting the standard was open in nature.

Today Committee 802 includes the following set of subcommittees, which include both those already mentioned and some others:

802.1 - Internetworking - networking;

802.2 - Logical Link Control, LLC - logical data transmission control;

802.3 - Ethernet with CSMA / CD access method;

802.4 - Token Bus LAN - local area networks with Token Bus access method;

802.5 - Token Ring LAN - local area networks with Token Ring access method;

802.6 - Metropolitan Area Network, MAN - metropolitan area networks;

802.7 - Broadband Technical Advisory Group - Broadband technical advisory group;

802.8 - Fiber Optic Technical Advisory Group - technical advisory group for fiber optic networks;

802.9 - Integrated Voice and data Networks - integrated voice and data networks;

802.10 - Network Security - network security;

802.11 - Wireless Networks - wireless networks;

802.12 - Demand Priority Access LAN, l00VG-AnyLAN - local area networks with priority access on demand method.

Ethernet technology

The Ethernet standard was adopted in 1980. DEC, Intel and Xerox jointly developed and published the Ethernet standard. The number of networks built on the basis of this technology is currently estimated at 5 million, and the number of computers operating in such networks is 50 million.

The basic principle behind Ethernet is a random method of accessing shared media. This medium can be thick or thin. coaxial cable, twisted pair, fiber optic or radio waves (by the way, the first network built on the principle of random access to a shared medium was precisely the radio network).

In the Ethernet standard, the topology of electrical connections is strictly fixed. Computers are connected to a shared environment according to a typical shared bus structure. Using a time-shared bus, any two computers (devices) can exchange data. Access control to the communication line is carried out by special controllers - Ethernet network adapters. Each computer, or more precisely, each network adapter, has a unique address.

Depending on the type of physical medium, the IEEE 802.3 standard has modifications: 10Base-5, 10Base-2, 10Base-T, 10Base-FL, l0Base-FB. For transmission of binary information over cable for all variants of the physical layer of Ethernet technology, providing a throughput of 10 Mbit / s, the Manchester code is used.

All types of Ethernet standards are based on the same method of separating the data transmission medium - the CSMA / CD (Carrier Sense Multiple Access / Collision Detection) access method and provide a transmission speed of 10 Mbit / s. In Russian, this access method is called MDKN / OS (Multiple Access with Media Control and Collision Detection).

The physical specifications of the IEE 802.3 Ethernet technology today include the following transmission media:

10Base-5 - coaxial cable with a diameter of 0.5 "(" thick "coaxial). With a characteristic impedance of 50 Ohm and a maximum segment length of 500 m (without repeaters);

10Base-2 - coaxial cable with a diameter of 0.25 "(" thin "coaxial). With a characteristic impedance of 50 Ohm and a maximum segment length of 185 m (without repeaters);

10Base-T - cable with unshielded twisted pair (UTP - Unshielded Twisted Pair), forming a star topology based on a hub, the distance between the hub and the end node is not more than 100 m.

10Base-F is a fiber optic cable with a topology similar to that of the 10Base-T standard.

The parameters of the physical layer specifications of the Ethernet standard are shown in the table **.

Table **

Ethernet Physical Layer Specification Parameters

Data transmission medium		Maximum segment length, m	Maximum distance between network nodes (when using repeaters), m	Maximum number of stations in a segment	Maximum number of repeaters between any stations on the network
	Thick coaxial cable RG-8 or RG-11; AUI cable
	Thin coaxial cable RG-58A / U or RG-58C / U
	Unshielded twisted pair of categories 3, 4, 5
	Multimode fiber optic cable

The number 10 in the above names denotes the bit rate of these standards - 10 Mbps, and the word Base denotes a transmission method on a single base frequency of 10 MHz (as opposed to methods that use multiple carrier frequencies, which are called Broadband). The last character in the name of the physical layer standard indicates the type of cable.

The CSMA / CD protocol used in Ethernet networks to resolve conflicts when accessing the transmission medium imposes a number of restrictions on the devices and cabling of networks.

A segment (collision domain) cannot contain more than 1024 devices (DTE).

A collision domain is a part of an Ethernet network, all nodes of which recognize a collision, regardless of where in the network the collision occurred. An Ethernet network built on repeaters always forms one collision domain. A collision domain corresponds to one shared environment. Bridges, switches, and routers divide the Ethernet network into multiple collision domains.

In networks based on coaxial cables, additional restrictions are introduced on the number of stations and the length of cables.

The essence of the multiple random access method implemented in Ethernet is as follows. A computer on an Ethernet network can transmit data over the network only if the network is free, that is, if no other computer is currently exchanging data. Therefore, an important part of Ethernet technology is the procedure for determining the availability of the medium.

After the computer has made sure that the network is free, it starts transmitting, while "hijacking" the medium. The time of exclusive use of the shared medium by one node is limited by the transmission time of one frame. A frame is a unit of data that is exchanged between computers on an Ethernet network. The frame has a fixed format and, along with the data field, contains various service information, for example, the recipient's address and the sender's address.

The Ethernet network is designed in such a way that when a frame enters the shared data transmission medium, all network adapters simultaneously begin to receive this frame. All of them parse the destination address located in one of the initial fields of the frame, and if this address matches their own address, the frame is placed in the internal buffer of the network adapter. Thus, the destination computer receives the data intended for it.

Sometimes a situation may arise when two or more stations simultaneously decide that the network is free and begin to transmit information. This situation, called a collision, prevents the correct transmission of data over the network. The Ethernet standard provides an algorithm for detecting and correctly handling collisions. The likelihood of a collision depends on the amount of network traffic.

After detecting a collision, network adapters that tried to transmit their frames stop transmitting and, after a pause of a random length, try to access the medium again and transmit the frame that caused the collision.

For a collision to occur, it is not necessary for several stations to start transmitting absolutely simultaneously, such a situation is unlikely. It is much more likely that a collision occurs due to the fact that one node starts transmitting earlier than the other, but the signals of the first simply do not have time to reach the second node by the time the second node decides to start transmitting its frame. That is, collisions are a consequence of the distributed nature of the network.

According to the Ethernet specifications, a station must be aware of a collision before completing a packet transmission. Since the minimum preamble packet is 576 bits long, collision detection should take less time anyway.

The 9.6 microseconds (IPG - inter packet gap) time interval between the end of one packet and the beginning of the next one makes it possible to clearly distinguish individual packets. When transmitting packets through repeaters, this gap can be reduced. The repeater re-synchronizes the signals (retiming) to eliminate distortion during transmission over the network medium. In general, during recovery, the packet length is increased by including additional sync bits. The increase in packet length occurs at the expense of reducing the IPG.

When a packet passes through several repeaters, the IPG can be greatly reduced. If the gap between packets is too small, the DTE device that received these packets may not have time to process the received packet by the time the next one arrives. Based on this, the length of the worst path in the segment is limited so that the change in the packet length on this path does not exceed 49 bits. To overcome these limitations, segmentation is used - dividing the network into smaller fragments connected by bridges, routers or switches.

The general restrictions for all Simple Ethernet standards are as follows:

Nominal throughput, Mbps ......................................... 10

The maximum number of stations in the network ............................................. ........... 1024

Maximum distance between nodes in the network, m .............................. 2500 (in 10Base-FB 2750)

Maximum number of coaxial segments in the network ............................... 5

To check the compliance of the network with the requirements of the NEE 802.3 standard, you must draw a diagram of the local network, including all devices, indicating the length and type of cable for each connection, and make sure that all of the following requirements are met:

There is no path between two devices on the network with more than 5 repeaters;

There are no more than 1024 stations in the network (repeaters are not counted);

The network contains only IEEE 802.3-compliant components, and host modules, hubs, and transceivers use only AUI, 10Base-T, FOIRL, 10Base-F, 10Base-5, or 10Base-2 cables;

Optical connections have a fairly low attenuation, and the number of connectors meets the requirements of IEE 802.3j;

There are no connections in the network that exceed the maximum allowable length;

Limitations for paths with 3 repeaters. If the longest path contains 3 repeaters, the following requirements must be met:

There should be no optical connections longer than 1000m between repeaters;

There should be no optical connections longer than 400 m between repeaters and DTE;

There should be no 10Base-T connections longer than 100 m.

Restrictions for paths with 4 repeaters. With four repeaters in the longest path, the following requirements must be met:

There should be no optical connections longer than 500m between repeaters;

There should be no 10Base-T connection longer than 100 m;

The network should not have more than 3 coaxial segments with the maximum cable length.

Restrictions for paths with 5 repeaters. If there are 5 repeaters in the longest path, the following restrictions are imposed:

Only optical (FOIRL, 10Base-F) connections or 10Base-T should be used;

There should be no copper or optical connections with end stations longer than 100 m;

The total length of optical connections between repeaters should not exceed 2500 m (2740 m for 10Base-FB);

These assessment methods are simple, but not accurate enough. Some configurations that do not meet the listed requirements turn out to be compliant with the IEEE 802.3 requirements.

To comply with the requirements of ШEE 802.3, the network must simultaneously fulfill two conditions:

Collision detection delay: the length of the path between any two points should not exceed 575 bits;

Inter-Packet Spacing: The change in packet length should not exceed 49 bits.

Networking with a large number stations are possible through a hierarchical connection of hubs, forming a tree structure. Figure 63 shows such a structure, which forms a common collision area - one collision domain.

Rice. 63. Hierarchical connection of Ethernet hubs

Different types of frames are used in Ethernet technology. Table *** shows the four main types of Ethernet frames.

Table *** Simple Ethernet Frame Formats

Let's consider the specific fields of each frame type.

Ethernet II, the first developed for Ethernet networks. The Ethernet 802.3 frame type was created by Novell and is the basic frame type for NetWare networks. Ethernet 802.2, developed by the IEEE 802.3 subcommittee as a result of the standardization of Ethernet networks, the frame contains additional fields.

Ethernet SNAP is an upgrade to the Ethernet 802.2 frame.

The numbers in parentheses indicate the lengths of the frame fields in bytes

R - preamble - is a seven-byte sequence of ones and zeros (101010 ....). This field is intended to synchronize the receiving and transmitting stations;

SFD (Start Frame Delimiter) - sign of the beginning of the frame (10101011);

DA (Destination Address), SA (Source Address) - recipient and sender addresses. They represent the physical addresses of Ethernet network adapters and are unique. The first three bytes of the address are assigned to each manufacturer of Ethenet adapters (for Intel adapters this will be 00AA00h, and for 3Com adapters - 0020AFh), the last 3 bytes are determined by the manufacturer. For broadcast frames, the DA field is set to FFFFFFFFh;

Length ~ the length of the transmitted packet;

Type - the field defines the type of the network layer protocol, the packet of which is carried by this frame (8137h - for the IPX protocol, 0800h - for the IP protocol, 809Bh - for the AppleTalk protocol, etc.).

FCS (Frame Check Sequence) - checksum of all fields of the frame (except for the preamble fields, the sign of the beginning of the frame and the checksum itself). If the length of the transmitted data packet is less than the minimum value, then the Ethernet adapter will automatically pad it up to 46 bytes. This process is called padding. Severe restrictions on the minimum packet length were introduced to ensure the normal operation of the collision detection mechanism.

DSAP (Destination Service Access Point) - the type of the network layer protocol of the receiving station (E0h - for IPX), SSAP (Source Service Access Point) - the type of the network layer protocol of the sending station,

Control - segment number; used when splitting long IP packets into smaller segments; for IPX packets, this field is always 03h (Bullet Datagram Exchange).

The Organizational Unit Identifier (OUI) and ID fields identify the SNAP Protocol ID type of upper layer.

Each station begins to receive a frame from the preamble P. Then it compares the value of the DA address with its own address. If the addresses are the same, or a broadcast frame has arrived, or a special processing program is specified, the frame is copied to the station buffer. If not, then the frame is ignored.

The frame type is identified by the network adapter according to the following algorithm:

If the SA field is followed by a value older than 05DCh, then this is an EthernetII frame,

If the FFFFh identifier is written behind the Length field, then this is an Ethernet 802.3 frame,

If the Length is followed by AAh, it is an Ethernet SNAP frame; otherwise, it is an Ethernet 802.2 frame.

The IEEE (Institute of Electrical and Electronics Engineers) is a professional organization (USA) that defines standards related to networking and other aspects of electronic communications. The IEEE 802.X group describes networking specifications and contains standards, guidelines, and white papers for networking and telecommunications.

IEEE publications are the result of the work of various technical, research and working groups.

The IEEE recommendations are mainly related to the lower 2 layers of the OSI model - physical and channel. These recommendations divide the data link layer into 2 sublayers - lower - MAC (media access control) and upper - LLC (logical link control).

Part of the IEEE standards (802.1 - 802.11) has been adapted by ISO (8801-1 - 8802-11, respectively), receiving the status of international standards. The literature, however, is much more likely to refer to parental standards rather than international ones (IEEE 802.3 rather than ISO / IEC 8802-3).

Below is the short description IEEE 802.X standards:

802.1 - sets standards for network management at the MAC layer, including the Spanning Tree algorithm. This algorithm is used to ensure uniqueness of the path (no loops) in multi-connected networks based on bridges and switches, with the possibility of replacing it with an alternative path in case of failure. The documents also contain specifications for network management and interworking.

802.2 - defines the operation of the LLC sublayer on the data link layer of the OSI model. LLC provides an interface between media access methods and the network layer. LLC functions transparent to higher layers include framing, addressing, error control. This sublayer is used in the 802.3 Ethernet specification but is not included in the Ethernet II specification.

802.3 - describes the physical layer and MAC sublayer for baseband networks using a bus topology and CSMA / CD access method. This standard was developed jointly with companies Digital, Intel, Xerox and is very close to the Ethernet standard. However, the Ethernet II and IEEE 802.3 standards are not completely identical and special measures are required to ensure the compatibility of the heterogeneous nodes. 802.3 also includes Fast Ethernet technologies (100BaseTx, 100BaseFx, 100BaseFl).

802.5 - describes the physical layer and MAC sublayer for networks with ring topology and token passing. IBM Token Ring 4/16 Mbps networks conform to this standard.

802.8 - TAG report on optical networks. This document discusses the use of optical cables in 802.3 - 802.6 networks, as well as guidelines for installing optical cable systems.

802.9 - report working group on the integration of voice and data (IVD). The document specifies the architecture and device interfaces for the simultaneous transmission of data and voice over a single line. The 802.9 standard, adopted in 1993, is compatible with ISDN and uses the LLC sublayer defined in 802.2 and also supports UTP (Unshielded Twisted Pair) cabling systems.

802.10 - This report from the LAN Security Working Group addresses communications, encryption, network management, and security in OSI-compliant network architectures.

802.11 is the name of the working group dealing with the 100BaseVG Ethernet 100BaseVG specifications. The 802.3 committee also proposed specifications for 100 Mbps Ethernet.

Note that the work of the 802.2 committee served as the basis for several standards (802.3 - 802.6, 802.12). The individual committees (802.7 - 802.11) perform primarily informational functions for committees associated with network architectures.

Note also that different 802.X committees have different transmission bit ordering. For example, 802.3 (CSMA / CD) specifies the LSB order in which the least significant bit (least significant bit) is transmitted first, 802.5 (token ring) uses the reverse order - MSB, like ANSI X3T9.5 - the committee responsible for the architectural specifications of FDDI. These two transmission order options are known as "little-endian" (canonical) and "big-endian" (non-canonical), respectively. This difference in transmission order is significant for bridges and routers that link different networks.

Network protocols.

A network protocol is a format for describing transmitted messages and rules according to which information is exchanged between two or more systems.

TCP / IP (Transmission Control Protocol / Internet Protocol) Also known as the Internet Protocol Suite. This protocol stack is used in the Internet family of networks and for interconnecting heterogeneous networks.

IPX / SPX - Internet Packet eXchange / Sequenced Packet eXchange. IPX is used as the primary protocol in Novell NetWare networks for exchanging data between nodes on the network and applications running on different nodes. The SPX protocol contains an extended set of commands compared to IPX to provide more advanced capabilities at the transport layer. SPX provides guaranteed package delivery.

NetBEUI - NetBIOS Extended User Interface. Transport protocol used by Microsoft LAN Manager, Windows for Workgroups, Windows NT, and other network operating systems.

Network Adapter (Network Interface Card, NIC) is a computer peripheral device that directly interacts with a data transmission medium, which directly or through other communication equipment connects it with other computers. This device solves the problem of reliable exchange of binary data, represented by the corresponding electromagnetic signals, via external communication lines. As with any computer controller, a network adapter operates under the control of an operating system driver, and the distribution of functions between the network adapter and the driver can vary from implementation to implementation.

Network hub– Various types of topology have found application in local area networks. Along with the widespread "bus" topologies "passive star" and "tree" are used. All types of topologies can use repeaters and passive hubs to connect different network segments. The main requirement for these topologies is the absence of loops (closed loops).

If networks based on the 10BASE-2 or 10BASE-5 specifications are small, then it is quite possible to do without hubs. But hubs must be used for the 10BASE-T specification, which has a passive star topology.

Hubs with an additional fiber-optic port can be used to connect remote groups to the network. There are three types of implementation of such a port:

plug-in slide-in microtransceiver,

a hinged microtransceiver plugged into the AUI socket,

permanent optical port.

Optical concentrators are used as the central device of a distributed network with a large number of individual remote workstations and small workgroups. The ports of such a hub act as amplifiers and perform full packet regeneration. There are hubs with a fixed number of pluggable segments, but some hub types are modular in design, allowing flexibility to adapt to existing conditions. Most often, hubs and repeaters are self-contained units with separate power supplies.

For Fast Ethernet technology, two classes of hubs are defined:

1) Hubs of the first class convert the signals coming from the segments into digital form. And only after that they are transferred to all other segments. This allows you to connect to such hubs segments made according to different specifications: 100BASE-TX, 100BASE-T4 or 100BASE-FX.

2) Hubs of the second class produce simple repetition of signals without conversion. Only one type of segment can be connected to such a hub.

Traffic

Traffic characteristics

Two characteristics of traffic can be distinguished - the unit of data and the way these units are packed. The data unit can be: bit, byte, octet, message, block. They are packed into files, packages, frames, cells. They can also be transferred without packaging.

Speed ​​is measured in units of data per unit of time. For example, packets per second, bytes per second, transactions per minute, and so on. Speed ​​also determines the time it takes a unit of data to transfer over the network.

The actual size of the data transmitted over the network consists of the data itself and the necessary information frame, which constitutes the transmission overhead. Many technologies place limits on the minimum and maximum packet sizes. For example, for X.25 technology, the maximum packet size is 4096 bytes, and for Frame Relay technology, the maximum frame size is 8096 bytes.

There are four of the most General characteristics traffic:

"Explosiveness",

tolerance for delays,

response time,

capacity and bandwidth.

These characteristics, taking into account routing, priorities, connections, etc., determine the nature of the operation of applications on the network.

"Explosive" characterizes the frequency at which traffic is sent by the user. The more often the user sends his data to the network, the bigger it is. A user who sends data regularly, at the same rate, reduces the "explosiveness" indicator to almost zero. This indicator can be determined by the ratio of the maximum (peak) traffic value to the average. For example, if the maximum amount of data sent during peak hours is 100 Mbps and the average amount is 10 kbps, the explosive rate would be 10.

Latency Tolerance measures how applications respond to all kinds of network latency. For example, applications that process financial transactions in real time do not tolerate latency. Long delays can cause these applications to malfunction.

Applications vary greatly in terms of acceptable latency. There are applications that work in real time (video conferencing) - there the latency should be extremely small. On the other hand, there are applications that tolerate delays of several minutes or even hours (email and file transfer). In fig. 2.3 shows how the total response time of the system is made up.

Rice. 2.3. Total network response time

The concepts of network capacity and bandwidth are related, but in essence they are not the same thing. Network capacity is the actual amount of resources available to the user on a particular data transmission path. Network bandwidth is determined by the total amount of data that can be transferred per unit of time. Network capacity differs from network bandwidth due to overhead costs that vary depending on how the network is used. Table 2.1 contains traffic characteristics for various applications.

There are no users or developers who are not concerned about the optimal network infrastructure being created. At the same time, the main question is:

Will the network be performing satisfactorily for some time after its implementation?

Table 2.1 Traffic characteristics of different applications

Application/ Characteristic	Traffic load	Tolerance for delays	Response time	Throughput ability,
Email			Regulated
File transfer			Regulated
CAD / CAM systems			Close to RV
Transaction processing			Close to RV
LAN communication			Real time
Server access			Real time
High quality audio			Real time

Most of the problems arise when trying to "collect" many single-functional networks into one flexible multiservice network. It is even more difficult to get a network that would be able to solve absolutely all problems, at least in the foreseeable future. Network professionals understand that an organization's business functions are constantly changing. The organization is improving its structure, working groups are formed and disappear, production is re-profiled, etc. In turn, web-based applications are changing. Custom workstations now provide services for message processing, video information, telephony, etc.

In this regard, when creating a network with combined functions, it is necessary to ensure the required level of service for each application. Otherwise, users will be forced to abandon the multiservice network in favor of the old dedicated network. As the current state of the Internet shows, treating all traffic on an equal footing presents serious problems, especially when bandwidth is limited. Some applications require fast network responsiveness. Therefore, it became necessary to guarantee response time, network bandwidth and similar parameters. This technology was developed and called Quality of Service (QoS). QoS uses scheduling and schedule prioritization to ensure that high priority graphics are guaranteed better transmission conditions over the network backbone, regardless of the bandwidth requirements of less critical applications. QoS technology can be used to determine the cost of multiservice network services. Quality of Service (QoS) relates the cost of network services to network performance.

However, the question arises: what kind of QoS technology should a network technician choose? There are several options: priority queuing in routers, using RSVP, using ATM QoS, etc. But it should be noted that you can always abandon the technology of quality of service. This can be done, for example, by introducing “forceful” methods of bandwidth allocation and not using these methods where they are not needed. To select a specific QoS technology, it is necessary to analyze the QoS requirements of users and consider possible alternatives.

Traffic of different applications

Recently, there has been a growing trend towards the introduction of telephony services, group work on documents, processing messages, video, etc. into applications. This trend determines the requirements for a network backbone, which, by combining a LAN, MAN and WAN backbone, should have multi-service basis and transmit any types of traffic with the required quality.

You can conditionally divide traffic into three categories, which differ from each other in terms of transmission delay:

DReal time traffic... This category includes traffic with audio and video information that does not allow transmission delays. The delay is usually less than 0.1 s, including the processing time at the end station. In addition, the delay should have small fluctuations in time (the jitter effect should be reduced to zero). It should be noted that when information is compressed, traffic in this category becomes very sensitive to transmission errors. At the same time, due to the requirement of a low delay, the errors that arise cannot be corrected by means of repeated sending;

UTransaction traffic. This category requires a delay of up to 1 s. Increasing this limit causes users to interrupt their work and wait for a response, because only after receiving a response can they continue to send their data. Therefore, large delays lead to a decrease in labor productivity. In addition, the variation in the latency values ​​leads to discomfort in operation. In some cases, exceeding the allowable latency will cause the work session to fail and user applications will need to restart it;

About Data traffic. This traffic category can operate with almost any latency, up to a few seconds. A feature of such traffic is increased sensitivity to available bandwidth, but not to latency. An increase in throughput results in a decrease in transmission time. Large data applications are designed primarily to take over the available network bandwidth. Rare exceptions are video streaming applications. For them, both bandwidth and minimization of latency are important.

Within each considered category, graphics are classified according to their assigned priorities. Traffic with higher priority is given priority in processing. An example of priority traffic would be an order transaction.

The introduction of priorities is inevitable when the network resources are insufficient. Priorities can be used to distinguish groups, applications, and individual users within groups.

Audio and video transmission is sensitive to changes in delay or, in other words, to jitter. For example, exceeding the permissible jitter threshold can lead to quite noticeable distortions of images, the need to duplicate video frames, etc. Audio transmission is also sensitive to jitter, since it is difficult for a person to perceive unexpected pauses in a subscriber's speech.

Studies have shown that in the case of transmission of low-quality audio information over the network, the maximum signal delay should be in the range from 100 to 150 ms. In the case of image transmission, this parameter should not exceed 30 ms. Table 2.2 defines the range of acceptable delays in the transmission of audio information.

Table 2.2 Impact of delays on the perception of a voice signal

In addition, since audio and video streams travel through different devices that handle jitter based traffic based on different algorithms, the picture and voice can quickly become out of sync (as is the case with bad movies). The jitter effect can be combated by using a buffer memory on the receiving side. But it should be remembered that the volume of the buffer can reach significant sizes, and this leads to both an increase in the cost of equipment and the opposite effect - an increase in latency due to overhead when processing information in a large buffer.

Program description Net Cracker .

Net Cracker Professional is designed to simulate all types of computer networks, as well as simulate processes in the created networks. When simulating processes in the created network projects, the program allows you to generate reports on the results of the simulation.

The project construction methodology includes the following steps:

The network equipment that will be used to build the network is entered into the project window. If necessary, network adapters from the list are added to workstations and / or servers. It is possible to configure workstations and servers, which is performed by clicking on them with the right mouse button.

In the “Link devices” mode, network equipment and computers are connected.

In order to be able to set traffic to servers necessarily the corresponding general software (SW) is installed (in the list of equipment select the option Network and Enterprise Software).

The default support for traffic types by common software is shown in Table 2.3.

Table 2.3. Default traffic support.

General software	Supported traffic
	File client-server
Small office database server	Data base client-server; SQL
HTTP - server

If the selected generic software does not support a specific type of traffic, then the configuration is carried out as follows:

right-click on the server in the project window;

select an option Configuration in the context menu;

select the common software installed on the server in the configuration window and press the key Plug- in Setup;

select tab Traffic;

set the necessary flags of traffic types;

press the OK key;

close the configuration window.

In the same configuration window, on the tab Server you can set the parameters of the server's response to incoming requests.

4. After selecting the type of traffic, you need to specify the traffic between computers. To do this, press the “Set Traffic” button on the toolbar, then alternately left-click the client station and the server with which the client will exchange data. Traffic can also be set between clients. The traffic direction is determined from the first click to the second. Traffic properties can be changed using the “Global” => ”Data Flow” menu item, including adding and removing network traffic.

5. When selecting a computer or network segment, you must specify the types of displayed statistics in accordance with the task option. To do this, select the “Statistics” item in the drop-down menu, and in the window that appears, check the boxes in which form to display statistics. Statistics can be displayed as a chart, number, graph or voice. Then click OK.

6. In the case of a multi-level project, when during the construction of a network one fragment of the network top level is shown in detail at the lower level (for example, when you want to show the connections between buildings and show the structure of the network inside the building), you should select the expandable fragment, press the right mouse button, and select the => Expand item in the drop-down menu. After that, you can continue to draw the mesh on a new sheet.

7. The simulation process is started with the “Start” button.

After the end of the simulation process, the reports are displayed as follows: select the menu item “Tools” => “Reports” => “Wizard” => “Statistical” => depending on the task. The report can also be obtained without using the services of the wizard, but simply by selecting the appropriate item in the “Reports” submenu. The resulting report can be printed or saved as a file.

The resulting network drawing can be printed using the File => Print menu.

Notes:

In all variants, the cable lengths are taken arbitrarily (the lengths must not exceed the values ​​admitted by the standard).

For networks with FDDI topology, there are no MSAU devices in the database. Therefore, for this topology, select “Generic LAN’s” => FDDI in the database (schematic drawing of FDDI).

Server type devices remote access can be found in the Routers and Bridges device database => Access Server => open any manufacturer => find a suitable device there. You can then connect either modems or DSU / CSU devices to it.

Building a multi-level (hierarchical) project must start from the topmost level (root), expanding the sublevels through context menu(Expand) the selected object of the current level.

The background image of the terrain map (Map) is selected during setup: menu Sites => Site Setup => Background.

The Institute of Electrical and Electronics Engineers IEEE802 specifications define standards for the physical components of a network. These components are - Network Card(Network Interface Card - NIC) and network media (network media), which belong to the physical and data link layers of the OSI model. The IEEE 802 specifications define the adapter's channel access mechanism and data transmission mechanism. IEEE802 standards subdivide the data link layer into sublevels:

Logical Link Control (LLC) - logical link control sublayer;

Media Access Control (MAC) is a sublayer of device access control.

The IEEE 802 specifications are divided into twelve standards:

The 802.1 (Internetworking) standard defines the mechanisms for managing a network at the MAC layer. Section 802.1 provides basic concepts and definitions, general characteristics and requirements for local area networks, and routing behavior at the link layer, where logical addresses must be mapped to their physical addresses and vice versa.

The 802.2 (Logical Link Control) standard defines the operation of the LLC sublayer on the link layer of the OSI model. LLC provides an interface between media access methods and the network layer.

802.3 (Ethernet Carrier Sense Multiple Access with Collision Detection - CSMA / CD LANs Ethernet) describes the physical layer and MAC sublayer for networks using bus topology and carrier snooping and discovery sharing. conflicts. The prototype of this method is the Ethernet access method (10BaseT, 10Base2, 10Base5). CSMA / CD access method. 802.3 also includes Fast Ethernet technologies (100BaseTx, 100BaseFx).

This access method is used in public bus networks (which include the radio networks that originated this method). All computers on such a network have direct access to a common bus, so it can be used to transfer data between any two nodes on the network. Simplicity of wiring is one of the factors behind the success of the Ethernet standard. The cable to which all stations are connected operates in multiply access (MA) mode.

The CSMA / CD access method defines the basic timing and logical relationships that guarantee the correct operation of all stations in the network.

All data transmitted over the network is placed in frames of a certain structure and supplied with a unique address of the destination station. The frame is then transmitted over the cable. All stations connected to the cable can recognize the fact of frame transmission, and the station that recognizes its own address in the frame headers writes its contents into its internal buffer, processes the received data and sends a response frame over the cable. The source station address is also included in the original frame, so the receiving station knows who to send the reply to.

The 802.4 standard (Token Bus LAN) defines the method of access to the bus with the transfer of a token, the prototype is ArcNet.

ArcNet devices are connected in a bus or star topology. ArcNet adapters support the Token Bus access method and provide 2.5 Mbps performance. This method provides the following rules:

All devices connected to the network can transmit data only after receiving permission to transfer (token);

At any given time, only one station in the network has this right;

A frame transmitted by one station is simultaneously analyzed by all other stations on the network.

ArcNet networks use an asynchronous method of data transfer (Ethernet and Token Ring networks use a synchronous method), i.e. the transfer of each byte in ArcNet is performed by an ISU (Information Symbol Unit) message, consisting of three service start / stop bits and eight data bits.

The 802.5 (Token Ring LAN) standard describes a token-passing ring access method, the prototype is Token Ring.

Token Ring networks, like Ethernet networks, use a shared data transmission medium, which consists of lengths of cable connecting all stations on the network in a ring. The ring is considered as a common shared resource, and for access to it, not a random algorithm is used, as in Ethernet networks, but a deterministic one, based on the transfer of the right to use the ring by stations in a certain order. The right to use the ring is transmitted using a special format frame called a token or token.

The 802.6 (Metropolitan Area Network) standard describes recommendations for regional networks.

The Broadband Technical Advisory Group (802.7) provides guidelines for broadband networking technologies, media, interface, and equipment.

The 802.8 standard (Fiber Technical Advisory Group) contains a discussion of the use of optical cables in 802.3 - 802.6 networks, as well as recommendations for fiber optic networking technologies, media, interface and equipment, the prototype is an FDDI (Fiber Distributed Data Interface) network ...

The FDDI standard uses fiber optic cable and marker-assisted access. The FDDI network is built on the basis of two fiber-optic rings, which form the main and backup data transmission paths between the network nodes. The use of two rings is the main way to improve resiliency in an FDDI network, and nodes that want to use it must be connected to both rings. Network speed up to 100 Mb / s. This technology allows switching up to 500 nodes at a distance of 100 km.

The 802.9 standard (Integrated Voice and Data Network) defines the architecture and interfaces of devices for simultaneous data and voice transmission over the same line, and also contains recommendations for hybrid networks that combine voice and data traffic in one and the same the same network environment.

802.10 (Network Security) addresses communications, encryption, network management, and security in OSI-compatible network architectures.

The 802.11 (Wireless Network) standard describes guidelines for using wireless networks.

100VG technology is a combination of Ethernet and Token-Ring with a transmission rate of 100 Mbps, operating on unshielded twisted pairs Oh. The 100Base-VG project has improved the access method to meet the needs of multimedia applications. The 100VG specification provides support for fiber optic cabling. 100VG technology uses an access method - request priority access. In this case, the network nodes are granted the right of equal access. The hub polls each port and checks for a transfer request, and then resolves the request according to priority. There are two levels of priority, high and low.