Chapter 3

Chapter 3 - LAN Communications Standards 1996, BICSI LAN Design Manual - CD-ROM, Issue 11

Overview

LAN communications definedA fundamental requirement of any LAN is a transmission channel. Once this channel hasbeen put into place, the next step is to use it to allow stations to communicateto transmitdata, exchange files and messages, and to monitor and manage the network.There are three basic configurations for the transmission of data:

A point-to-point connection, where two computers or a computer and a peripheraldevice are directly attached to each other.

A LAN, which is usually a shared data communications facility. Here, a number ofstations can attach to the transmission channel but only one can successfullytransmit at a time.

A WAN (Wide Area Network) which represents communication between LANs overextended geographic distances. Often, the communications channels used forWAN activity are owned by a third party, such as a telephone company.

For all of the above configurations, the interface between the devices must either be thesame, or appropriate hardware/software must be provided to convert the coding, formatting,framing and protocols between the different systems.The focus of this chapter is on how communications are achieved between stations on aLAN.


Communications terminology Asynchronous communications.

Each data character is coded as a string of bits and characters are separated bystart-character bit(s) and stop bit(s).

Synchronous communications.No start and stop bits are used. Groups of bits are separated using a clockingmechanism. That is, the sending and receiving stations are in synch with eachother.

Error detection and correction.Techniques to permit a station to detect corrupted data and initiate aretransmission.

Bandwidth.A measure of data throughput. The information transmission capacity of thetransmission system, typically measured in Megabits per second (Mbps) in LANenvironments.

Packets/Frames/Datagrams.A package of data with header informationtypically, source and destinationaddresses, error correction information, sequence numbers and other information.Sent and received by stations on a LAN.

Bit stream.A series of binary 0s and 1s, representing the message being communicated.


CCITT, continued

Communications standards organizations

American National Standards Institute (ANSI)ANSI is responsible for the development of both the FDDI (Fiber Distributed Data Interface)and the TP-PMD (Twisted-Pair- Physical Medium Dependent) LAN standards. Both of thesetechnologies operate at 100 Mbps, FDDI over optical fiber cabling and TP-PMD over twisted-pair cabling. They are discussed in greater detail in a later chapter.

Comite Consultatif Internationale de Telegraphique etTelephonique (CCITT)More commonly known in English as the Consultative Committee on International Telegraphyand Telephony.The CCITT forms part of the International Telecommunications Union (ITU), based inGeneva, Switzerland. The ITU is a United Nations agency and all UN members belong to theITU.The job of the CCITT is to study, recommend and standardize technical and operationalissues for telecommunications. The activities of the CCITT can be divided into three areas:

Study GroupsTheir focus is to set standards for telecommunications equipment, systems,networks and services.


Plan Committees (World Plan and Regional Plan Committees)Their focus is to develop general plans for a harmonized evolution of networks andservices.

Specialized Autonomous GroupsTheir focus is to produce handbooks, strategies and case studies for supportmainly of developing countries.

The CCITT is more recently referred to as ITU-TSS (International TelecommunicationsUnion-Telecommunications Standardization Section).

CCITT (ITU) recommendationsThese recommendations have a non-binding statusno one is forced to follow them.However, there is an increasing awareness that following such recommendationssimplifies interconnection and interoperability of devices on a worldwide basis.Two well-known series of standards produced by the CCITT include the V-series and theX-series. The V-series covers transmission over telephone networks and has been used toclassify modem devices. The X-series covers Open Systems Interconnection (OSI)standards, discussed later in this chapter.

CCITT, continued


Examples of V-series CCITT standards V.22 1200 bps (bits per second) full-duplex modem standard. V.22bis 2400 bps full-duplex modem standard. V.28 Defines circuits in RS-232 interface. V.32 Asynchronous and synchronous 4800 and 9600 bps standard. V.32bis Asynchronous and synchronous standard up to 14400 bps. V.34 A proposed standard for 28 Kbps transmission rates. V.35 Defines high data rates over combined circuits. V.42 Defines error checking standards. V.42bis Defines modem compression using Lempel-Ziv method.

Examples of X-series CCITT standards X.25 (ISO 7776) Packet-switching network interface. X.200 (ISO 7498) OSI Reference Model. X.400 (ISO 10021) Message handling (e-mail). X.500 (ISO 9594) Directory Services. X.700 (ISO 9595) Common Management Information Protocol (CMIP).


International Organization for Standardization (ISO)Commonly referred to as the International Standards Organization, ISO is the worlds mostinfluential standards organization. Based in Geneva, ISO has developed a seven-layerreference model for computer networking known as the Reference Model of Open SystemsInterconnection (less formally, the OSI model).

OSI ModelThe OSI model is concerned with how systems exchange informationtheirinterconnectionnot with the internal functions performed by a given system. This modeluses a layered approach, where sets of functions are assigned to different layers.A description of the OSI model and the role it plays in the LAN environment is detailedlater in this chapter.


Institute of Electrical and Electronics Engineers (IEEE)The IEEE is a United States-based organization responsible for the development of localarea network architecture standards. It was founded in 1884 for the purpose of promoting themembers, theories and practices in the fields of electrical, and later, electronic and computerengineering.The IEEE Computer Society Local Network Committee has developed several standardsrelated to the LAN environment. They have produced, and continue to update, a set ofstandards defined under IEEE Project 802 (so named because it was formed in February,1980).

IEEE Project 802Project 802 is responsible for providing recommendations regarding how data should betransmitted from one network device to anothercorresponding to the lowest two layers ofthe OSI seven-layer model (the Physical and Data Link layers).Early on in its work on developing LAN standards, it became clear to members that asingle standard, meeting all LAN requirements, would be impossibly complex. For thisreason, Project 802 worked towards developing sets, or families, of standards. Theobjective was to encourage compatibility within a given family of standards, whilepermitting the different families to meet various market requirements.The role Project 802 plays in the LAN environment is detailed later in this chapter.


Communications fundamentals

Protocols

DefinedA protocol is defined as:

A specific set of rules relating to the formatting and timing of datatransmission between two devices.

Protocols are therefore, the rules that govern the communications between two computers.They represent a standard procedure that two data devices accept and use tocommunicate with each other.

Computer communications protocols are much like diplomatic protocols which, forexample, define the proper procedures for communications between two heads of state.They specify who talks to whom, at what point in time, what they can say to each other,and how they must say it.Communication protocols are defined within the context of a layered architectureaprotocol stack such as the OSI seven-layer reference model. In a layered architecture,each layer builds a protocol data unit (PDU) that it sends to the peer, or equivalent, layerin the destination computer. Upper layers of a protocol stack build PDUs, then pass themto lower layers for further packaging.

Protocols, continued


Function of a protocolProtocols for data communications cover such areas as:

Speed of communication. Electrical characteristics. Use of shared resources. Message length. Addressing. Error handling.


Packets/Frames/Datagrams

DefinedA packet or datagram is a package of data that is transmitted between stations over atransmission channel. Packets are assembled by the protocol layer(s) in the sendingstation and disassembled by the corresponding protocol layer(s) in the receiving station.Once a packet reaches the bottom of the protocol stack, it is broken into one or moreframes for transmission as a bit stream over the communications channel. The frameformat is defined by the LAN technology used.A packet consists of two componentsthe data being exchanged (sometimes referred toas the payload) and headers. Headers include the source and destination addresses andcontrol information. The data being exchanged can be:

Data, such as the contents of files. Messages and commands, such as a request for service. Control codes for managing the communication session, such as codes to indicate

communication errors and the need for retransmission.


Layered architectures

DefinedLayering is a design approach specifying different functions and services at different levelsin a protocol stack. The protocol stack itself defines how communication hardware andsoftware interoperate at various levels. The Open Systems Interconnection seven-layerreference model is an example of such a protocol stack.Protocol stacks have the following characteristics:

Lower layers provide services to upper layers. Each layer provides a set of services. Services are defined by protocols. Service access points (SAPs) are the connection points between layers.


Hardware independence

OverviewA major problem in the early days of LAN computing was the lack of hardware and cablingindependence. That is, the hardware and the cabling system were dependent on the typeof LAN environment.The difficulty was that no one type of hardware and cabling was best suited for allsituations. LAN buyers confirmed the need for these different environments by purchasingdifferent systems for different organizational requirements. Unfortunately, few of thesediverse systems were able to communicate with each other.Rather than focus on a single, general-purpose LAN to satisfy all requirements, standardscommittees work towards achieving hardware independence, defined as the ability ofdifferent components to communicate and work with each other.

ObjectivesThere are certain objectives that should be met in the development of opencommunications standards:

Connectivity It should be possible to connect different hardware and software products to form a

networking system.

Hardware independence,continued


Modularity It should be possible to use a relatively small set of general-purpose building

blocks in a wide variety of network environments.

Ease of implementation It should be possible to quickly and easily install in a variety of configurations to

meet the needs of the users.

Ease of use It should be possible for network users to use the communications facilities without

knowledge of the network structure or implementations.

Reliability It should provide error detection and correction functions.

Ease of modification It should be possible for the network to evolve as user needs changes or as new

technologies become available.Much of the credit for hardware independence and ease of communication betweendevices goes to the International Organization for Standardization (ISO) and the Instituteof Electrical and Electronics Engineers (IEEE). Their work in these areas is discussed inthe following pages.


The Open Systems Interconnection (OSI) model

OverviewThe primary objective of the OSI model is to provide a framework for the development ofstandards for computer-to-computer communications. Specifically, the model details thestandards which allow for the flexible interconnection of systems using data communicationsfacilities. A system can be defined as follows:

One or more computers and the associated software, peripherals,operators, physical processes and transfer means that forms anautonomous whole capable of processing and/or transferringinformation.

This broad definition of a system allows the OSI model to be applied to very simple systemsas well as to complex ones.The model is meant to provide a generalized view of a layered architecture. The model,introduced in 1978, divides the communication process into a hierarchy of seveninterdependent functional layers. Each layer has a built-in interface to the adjacent layer(s).

OSI overview, continued


FIGURE 3.1: THE OSI MODEL

Physical

Data Link

Network

Transport

Session

Presentation

Application

Layer 1

Layer 2

Layer 3

Layer 4

Layer 5

Layer 6

Layer 7


Defining the seven layers of the OSI model

Physical LayerThe physical layer is responsible for the transmission of bit streams across a particularphysical transmission channel. This involves a connection between two devices allowingelectrical signals to be exchanged between them.

Data Link LayerThe data link layer is responsible for providing reliable data transmission from one deviceto another and for shielding higher layers from concerns regarding the physicaltransmission channel. This layer is concerned with the error-free transmission of packetsbetween network devices.

Network LayerThe network layer is concerned with routing data from one network device to another. It isresponsible for establishing, maintaining and terminating the network connection betweenany number of devices and for transferring data along that connection. It is possible tohave only one path for network connection or many possible routes to choose from when aconnection is established between any two devices on a network.

Defining the seven layers of theOSI model, continued


Transport LayerThe transport layer is responsible for providing data transfer between two users at anagreed-upon level of quality. After establishing a connection between two users, thetransport layer is responsible for selecting a particular class of service to be used,monitoring the transmission for billing purposes, ensuring that the appropriate servicequality is maintained, and for generating an alert if this quality has been compromised.

Session LayerThe session layer provides the services used to organize and synchronize a given dialogoccurring between devices and to manage the data exchange. A major purpose of thesession layer is to control when devices in communications with each other can send andreceive data. Among other factors, this is based on whether the devices can send andreceive data concurrently or alternately.

Presentation LayerThe presentation layer is responsible for the presentation of information in a way that ismeaningful to the network devices. Included in the specifications are character codetranslations, data conversions, or data compression and expansion.

Application LayerThe application layer provides a means for similar or dissimilar application processes toexchange information. Included are services used to establish and terminate connectionsbetween devices, and services to monitor and manage the systems being interconnectedas well as the various resources being employed.


Purpose of the OSI modelThe OSI model is concerned with the exchange of information between a pair of opensystems. It is not concerned with the internal functions of the individual systems.The model is concerned with the capability of systems to cooperate in the exchange ofinformation and in the ability to accomplish the tasks governing this exchange.The primary motive in the development of the OSI model was to provide a framework forstandardization. Within the model, one or more protocol standards can be developed at eachlayer. The OSI model defines, in general terms, the functions to be performed at a particularlayer.This model is meant to facilitate the standards-making process in two ways:

The pace at which communications standards are developed is made faster withthe model. The functions of each layer are well-defined, allowing standards to bedeveloped independently and simultaneously for each layer.

The introduction of new standards is simplified. The boundaries between layers arewell-defined, therefore, a new standard in one layer need not affect any of theworkings of the other layers.


Using the OSI model

The hardware levelLayers 1 and 2 (Physical and Data Link) are referred to jointly as the hardware level. Theyprovide the fundamental link between two devices on which other, more sophisticated,services are built.At this level, three protocols are in common useEthernet, Token-bus and Token-ring.These have been defined by the IEEE Project 802 and are further discussed later in thischapter.

The transport levelLayers 3 and 4 (Network and Transport) make up the transport level. This level isresponsible for controlling network communications.Actual communications on a network involve only two types of devicesthe sendingdevices and the receiving devices. A temporary communications linka virtualconnectionis made between these devices when they need to communicate. This virtualconnection is different from the permanent physical connection each device has to thenetwork.Software at this level establishes and manages the temporary connection between thesending device and the receiving device.

Using the OSI model, continued


Transport-level protocolsSeveral transport level protocols are commonly found in the LAN environment. Each ofthese protocols has its own characteristics, advantages and disadvantages. Two of themore commonly used LAN transport level protocols are the following:

IPX/SPXNovells transport protocol, IPX/SPX (Internetwork Packet Exchange/ Sequenced PacketExchange) is based on Xeroxs XNS (Xerox Network Systems). It was designed at a timewhen few LAN-specific transport protocol existed. Today it is a widely-used protocol inLAN environments.

NetBEUIDeveloped by IBM and Microsoft, NetBEUI (NetBIOS Extended User Interface) wasintroduced in 1984, along with NetBIOS (Network Basic Input and Output System), itscorresponding session layer protocol. Since both of these influential vendors declared thatthey would use NetBIOS/NetBEUI in their future networking products, many other vendorshave adopted these protocols for their networking systems.

Both of these protocols enjoy a wide level of support in the LAN community. This,combined with the independence between the transport and the hardware levels, meansthat either of these protocols can be used to establish communications between stationson dissimilar LANs.

Using the OSI model, continued


The application-to-transport levelThe application-to-network level is the point where an application running on a station canattach to the network joining that station to the others.Some applications, such as gateway communications between a LAN station and amainframe system, are communications-intensive and require point-to-pointcommunications services. A point-to-point connection is an uninterrupted connectionbetween two devices. These applications usually attach to the network at Layer 5(Session).The other application-to-network layer is Layer 6 (Presentation). Nearly all applicationprograms attach to the network through this layer.

The application levelLayer 7 (Application) defines the communications interface for network applicationssoftware designed specifically to be used over a network. Network operating systems usethe services defined at this level, as do many network utility programs, such as electronicmail systems. The network applications in turn, support various user applications.It should be noted that user applications, such as word processing, are not defined usingthe OSI model; only products used for network communications are defined.


Applying the OSI model in the LAN environmentThe OSI model was developed for computer networks in general, not specifically for the LANenvironment. However, the first two layers of the OSI reference model play an important rolein establishing LANcommunicationsthe Physical layerand the Data Link layer. Both ofthese layers are clearly defined byIEEE Project 802, discussed next.The OSI reference model is crucial innetwork-to-networkcommunications internetworking. Itallows for the development ofstandards to enable communicationsbetween two dissimilar networks.The role of the OSI reference modelin internetworking is detailed in alater chapter.

FIGURE 3.2:THE OSI MODEL ANDLAN COMMUNICATIONS

.....................

.....................

.....................

.....................

.....................

Application

Presentation

Session

Transport

Network

Data Link

Physical

Network applicationssoftware

SpecializedNetwork

communicationssoftware

Media Access Control

Physical Link Control

Logical Link Control

OSI Model LAN communications


IEEE Project 802

OverviewIEEE Project 802 was initiated specifically for the LAN environment. The rationale of thisgroup was to develop standards which would be widely accepted. This, in turn, would ensurea high-volume market, encouraging manufacturers to commit the necessary resources todeveloping standardized products. More importantly, it would allow the equipment from thesemanufacturers to communicate with each other.The original series of standards developed under Project 802 were subsequently adopted byANSI in 1985 as American National Standards and by ISO in 1987 with the designation 8802.Project 802 has produced a set of standards for LAN communications. They are designed tobe a subset of the OSI model. This means higher-layer protocols can be developed for variousnetwork services without the need to produce new LAN protocols to accompany them.The relationship between the OSI model and Project 802 follows:

The physical layer of the various Project 802 standards correspond to the physicallayer of the OSI model.

The Data Link layer of the OSI model has been divided into two sublayers byProject 802Medium Access Control (MAC) and Logical Link Control (LLC).LAN architectures are subdivided in this manner because a set of common linkcontrol subfunctions can apply to all LANs, while the medium access controltechnique can differ for each one.


802 standardsThe IEEE 802 is made up of many subcommittees. A brief overview of these committeesfollows.

IEEE 802.1 - High-Level Interface The IEEE committee defining the relationship between the IEEE 802 standards

and the ISO Open Systems Interconnection (OSI) reference model. It provides forLAN management and bridging standards.

IEEE 802.2 - Logical Link Control A Data Link layer standard used with IEEE 802.3, 802.4 and 802.5 standards.

IEEE 802.3 - Carrier Sense Multiple Access with Collision Detection A Physical layer standard specifying a linear bus LAN with a CSMA/CD access

method commonly associated with Ethernet and Fast Ethernet.

IEEE 802.4 - Token-bus A Physical layer standard specifying a token-passing access method on a bus

topology. It is used by the Manufacturing Automation Protocol (MAP), developed byGeneral Motors, and by ARCnet.

802 standards, continued


IEEE 802.5 - Token-ring A Physical layer standard specifying a token-passing access method on a ring

topology.

IEEE 802.6 - Metropolitan Area Network (MAN) Describes a topology known as Distributed Queue Dual Bus (DQDB). This

topology consists of two parallel runs of cable to link devices over a metropolitan(city-sized) area. The transmission medium is usually optical fiber and transmissionspeed in the range of 100 Mbps.

IEEE 802.7 - Broadband Technology The IEEE technical advisory group for broadband LANs. This committee provides

technical advice to other subcommittees on broadband networking techniques.

IEEE 802.8 - Fiber Optics The IEEE technical advisory group for optical fiber LANs. This committee provides

technical advice to other subcommittees on optical fiber networks as alternatives toexisting copper-based networks.

IEEE 802.9 - Integrated Services LAN The IEEE committee working on the integration of voice, data and video traffic over

other 802 LANs.



IEEE 802.10 - LAN Security The IEEE technical advisory group for security. This group is working on the

definition of a standard security model that will operate over a variety of networks.It incorporates both authentication and encryption methods.

IEEE 802.11 - Wireless The IEEE committee working on standards for wireless networks. This group is

working on the standardization of media such as spread-spectrum radio,narrowband radio, infrared and transmission over power lines.

IEEE 802.12 - Demand Priority The IEEE committee charged with the standard for 100 Mbps Ethernet using a

Demand Priority access method, commonly referred to as 100VG-AnyLAN.



FIGURE 3.3: PROJECT 802 LAYERS AND SUBLAYERS

802.3MediumAccess

802.3Physical

802.4MediumAccess

802.4Physical

802.5MediumAccess

802.5Physical

802.1 Bridging

802.2 Logical Link

802.7 Broadband Advisory

802.8 Optical fiber Advisory

OSI layers

Data link

Physical

802.12MediumAccess

802.12Physical


Physical layer overviewThe Physical layer is responsible for establishing, maintaining, and releasing physicalconnections between two network devices and for transmitting bits over the transmissionchannel. Included is:

Encoding data into the correct form for transmission. Generating the signal. Controlling the timing of devices so that they are synchronized with the signal

being transmitted and received.IEEE Project 802 accommodates the three transmission media used at the Physical layertwisted-pair cable, coaxial cable and optical fiber cable. Its specifications, for a given LANarchitecture, include:

The type of cable. The type of transmission. The encoding method. The data rate.


Data Link layer overview

Logical Link Control (LLC) sub-layer recommendationsLogical Link Control (LLC) specifies mechanisms for addressing stations across thetransmission medium and for controlling the exchange of data between two devices. Itincludes provisions for establishing connections, for data transfer and for connectiontermination.The role of this sub-layer is to shield the higher-level layers from the low-level signalingspecifications of each LAN architecture. Project 802 defines only a single standard at thislevelit is a common element in all of the 802 standards.

The logical link control sub-layer is responsible for the following: Initiating control signal interchange. Organizing data flow. Interpreting commands. Generating responses. Carrying out error control and recovery functions.

Data Link layer overview,continued


Media Access Control (MAC) sub-layer recommendationsThis sub-layer of the Data Link layer is responsible for defining the rules allowing networkdevices to share the transmission channel. It defines how the many devices sharing thesingle physical transmission channel gain orderly access to the medium. Four functionsmake up the media access control sub-layer standard:

Medium Access Management The rules or procedures used by network devices to control the sharing of the

transmission medium.

Framing The addition of header and trailer information necessary to identify the beginning

and end of a packet, to synchronize the sender with the receiver, to route thepacket, and to provide for error detection.

Addressing The determination of the appropriate network addresses to identify the devices

involved in sending and receiving a message.

Error Detection The checking done to ensure that a packet has been transmitted and received

correctly.

Data Link layer overview,continued


Although many access control methods exist, Project 802 has standardized three methods: CSMA/CD [introduced in IEEE 802.3]. Token-bus [introduced in IEEE 802.4]. Token-ring [introduced in IEEE 802.5].

Each of these methods is described on the following pages.


IEEE 802.3 CSMA/CD Media Access Control

Definition of CSMA/CDThe IEEE 802.3 standard specifies the CSMA/CD (Carrier Sense Multiple Access withCollision Detection) media access control method. CSMA/CD is the most commonlyemployed access method for LANs using a bus or tree topology. It is the media accesscontrol method used by Ethernet.CSMA/CD operates per the following steps:

A station with a message to transmit listens to the transmission medium to see ifanother station is currently transmitting a message.

If the transmission medium is quietno other station is transmittingthetransmission is sent.

When two or more stations have a message to send, it is possible that theytransmit at precisely the same time, resulting in a collision on the network.

When a collision occurs, all receiving stations ignore the garbled transmission. The transmitting stations stop transmitting as soon as they detect a collision. Each of the transmitting stations waits a random period of time and attempts to

transmit again.

Definition of CSMA/CD, continued


Carrier sensingCarrier sensing is the approach used by CSMA/CD for listening to the transmissionmediumthe carrier to see if it is free. If it is, the encapsulated data frame is passed tothe physical layer for transmission. If the carrier is busy, it continues to be monitored untilit is free.

Collision detectionAfter transmission has begun, monitoring of the transmission medium continues. Whentwo signals collide, their messages get mixed and become unreadable. If this happens, theaffected stations stop transmitting and send out a jamming signal. This jamming signalensures that all other stations on the network are aware a collision has occurred.


CSMA/CD functionsThe IEEE CSMA/CD standard defines a model made up of six functions. Three of thesefunctions are concerned with sending data and the three others are concerned with receivingdata. The receiving functions operate in parallel with the sending functions.

Data encapsulation/decapsulationThe data encapsulation and decapsulation function is performed by the Media AccessControl sublayer. This process is responsible for addressing and error-checking functions.

Data encapsulationData encapsulation is performed by the sending station. It is the act of addinginformationaddresses and error control bytesto the beginning and end of the data unitto be transmitted. This is done after the data packet is received from the Logical LinkControl sublayer. The information added to the packet is needed to perform the followingtasks:

Synchronize the receiving station with the signal. Delimit the beginning and end of the frame. Identify the addresses of both the sending and receiving stations. Detect transmission errors.

CSMA/CD functions, continued


Data decapsulationData decapsulation is performed by the receiving station. When a frame is received, thereceiving station is responsible for performing the following tasks:

Recognizing the destination address and determining if it matches the stationsown address.

Performing an error check. Removal of the control information that was added by the data encapsulation

function in the sending station.

Media access managementThe media access management function is also performed by the Media Access Controlsublayer.In the sending station, the media access management function is responsible fordetermining whether the transmission medium is available. If the channel is available,transmission can begin. Additionally, the management function is responsible fordetermining what action should be taken when a collision is detected and when to try toretransmit.

In the receiving station, the media access management function is responsible forperforming validity checks on a frame before passing it to the data decapsulation function.

CSMA/CD functions, continued


Data encoding/decodingThe data encoding/decoding function is performed by the Physical layer. This function isresponsible for getting the electrical form of the data transmission onto the transmissionmedium.

Data encodingData encoding is performed by the sending station. It is responsible for translating the bitsinto the correct electrical signals to be sent across the transmission medium. WithCSMA/CD, Manchester phase encoding is used to translate the bit stream into electricalsignals.In addition, this function is responsible for listening to the transmission medium and fornotifying the media access management function whether the medium is free or busy or ifa collision has been detected.

Data decodingData decoding is performed by the receiving station. It is responsible for translating theelectrical signal back into a bit stream.


CSMA/CD transmission frameA transmission frame is defined as:

A group of bits in a specific format, with anindicator at each end to signal thebeginning and end of the frame.

The defined format of a frame allows networkequipment to recognize the meaning and purpose of thespecific bits in the frame. A frame is usually a logicaltransmission unit containing control information forerror checking and addressing purposes.The CSMA/CD (IEEE 802.3) frame format is shownbelow.

FIGURE 3.4:CSMA/CD (IEEE 802.3) FRAME FORMAT

Frame CheckSequence

Pad(optional)

Information

Length Count

Source Address

Destination Address

Start FrameDelimiter

Preamble7 bytes

1 byte

2 or 6 bytes

Variable

2 bytes

2 or 6 bytes

0 - n bytes

4 bytes

CSMA/CD transmission frame,continued


The CSMA/CD frame components are responsible for the following tasks:

PreambleThe preamble is responsible for providing the synchronization between the sending andreceiving device.It is a series of 56 bits (7 bytes) of alternating 1s and 0s found at the beginning of theframe.

Start Frame DelimiterThe start frame delimiter follows the preamble. As its name implies, it indicates the startof the data frame. The start frame delimiter is 1 byte in lengthmade up of the following8-bit sequence10101011.

Address FieldsEach of the address fieldsthe destination address and the source addresscan beeither 2 bytes or 6 bytes in length. If universal addressing is used, the addresses must be6 bytes each. But if local addressing is used they may be either 2 or 6 bytes long. Bothdestination and source addresses must be of the same length for all devices on a givennetwork.The destination address field specifies the station(s) to which the data is to be sent. Anaddress referring to a specified group of stations is known as a multicast group addressand an address referring to all of the stations on the network is known as a broadcastaddress.The source address identifies the station making the transmission.

CSMA/CD transmission frame,continued


Length CountThis is a 2-byte field indicating the length of the data field that follows. It is needed todetermine the length of the data field in those cases when a pad field is used.

Information FieldThe information field contains the actual data packet to be transmitted. Its length isvariable.

Pad FieldA pad field is used to ensure that the frame meets a minimum length requirement. A framemust contain a minimum number of bytes in order for stations to detect collisionsaccurately.

Frame Check SequenceThe frame check field is used as an error-control mechanism.When the transmitting device assembles a frame, it performs a calculation on the bits inthe frame. The algorithm used to perform this calculation always results in a 4-byte value.The sending device stores this value in the frame check sequence field.

When the destination device receives the frame, it performs the same calculation andcompares the result to that in the frame check sequence field. If the two values are thesame, the transmission is assumed to be correct. If the two values are different, thedestination device can request a retransmission of the frame.


IEEE 802.3 naming conventionA variety of IEEE 802.3 implementations are available. To distinguish between them, anotation has been developed. The notation specifies three characteristics of theimplementation:

The data rate in Mbps. The signaling method used. The approximate maximum cable segment length in hundreds of m.

Some of these IEEE 802.3 implementations and their characteristics are as follows:

1Base-5 The IEEE standard for baseband Ethernet at 1 Mbps over twisted-pair cabling to a

maximum distance of 500 m (1640 ft).

10Base-5 The IEEE standard for baseband Ethernet at 10 Mbps over coaxial trunk and

AUI (Attachment Unit Interface) twisted-pair cable to a maximum distance of500 m (1640 ft).

10Base-2 The IEEE standard for baseband Ethernet at 10 Mbps over thin coaxial cable to a

maximum distance of 185 m (607 ft).

IEEE 802.3 naming convention,continued


10Broad-36 The IEEE standard for broadband Ethernet at 10 Mbps over broadband coaxial

cable to a maximum distance of 3600 m (2.25 mi).

10Base-T The IEEE standard for baseband Ethernet at 10 Mbps over unshielded twisted-pair

following a star horizontal cabling topology, with a maximum distance of100 m (328 ft) from the station to the hub.

10Base-F The IEEE standard for baseband Ethernet at 10 Mbps over optical fiber to a

maximum distance of 2 km (1.25 mi).

100Base-TX The IEEE standard for baseband Ethernet at 100 Mbps over two twisted-pairs

either 2-pair Category 5 UTP or 2-pair STP cabling.

100Base-T4 The IEEE standard for baseband Ethernet at 100 Mbps over four-pair UTP

cablingeither Category 3, 4 or 5.

100Base-FX The IEEE standard for baseband Ethernet at 100 Mbps over a two-fiber

62.5/125 m optical fiber cabling system.


IEEE 802.4 Token Bus Media Access Control

Definition of Token-busThe IEEE 802.4 standard specifies the Token-bus media access control method. It is one oftwo token passing access methods. IEEE 802.4 is based on a physical bus or tree topology.The Token-bus approach requires a station to have possession of a token in order totransmit. The token is passed from station to station in a logical ring.IEEE 802.4 is the basis for LAN architectures often used in factory automation, such as MAPand ARCnet.Token-bus operates in the following manner:

A station having data to transmit must first be in possession of the token. Thetoken gives a device the right to transmit.

When a station receives the token, it broadcasts its transmission for all stations tohear. The station is given a predetermined amount of time to send its message.

When the station has transmitted all of its data unit, or it has run out of time, thetoken is passed to the next station.

If a station runs out of time, it must wait until the next time it possesses thetoken to transmit the rest of the data.

Definition of Token-bus, continued


Devices on the network acknowledge only those transmissions addressed to them. When a token is passed to the next device, the address field is changed. The token

is always passed from station to station in order of decreasing addresses. Whenthe lowest address is reached, the token is sent to the station with the highestnetwork address.

If a station receiving the token has no message to transmit, the token isimmediately passed on to the next station in the hierarchy.

Since possession of the token is needed for a station to transmit, there is nopossibility of a collision. With no possibility of a collision, there is no minimumlength requirement for data packets and only minimal control information isrequired for proper processing.


Token-bus functionsThe Token-bus standard addresses five key areas as part of the Media Access Controlsublayer.

Interface to the Logical Link Control sublayerThe MAC sublayer receives data packets from the LLC sublayer and prepares them fortransmission. It also receives incoming data packets and prepares them to be passed on tothe LLC sublayer.

Token handlingThis function makes provisions for:

Passing tokens from station to station. Recognizing a token when it is received. Prioritizing data packets.

Ring maintenanceRing maintenance functions include initialization of the logical ring during network start-upand modification of the logical ring as stations are connected to or disconnected from thenetwork.

Token-bus functions, continued


Fault detection and recoveryFaults must be detected and, when possible, corrected as soon as possible. Somepossible faults occurring on a Token-bus network include:

Multiple tokens. Lost tokens.

Token pass failures. Stations with unresponsive receivers. Duplicate station addresses.

Sending and receiving dataIn order to send or receive data, control information must be added to and removed fromthe data packet. Therefore, the MAC sublayer is responsible for preparing data packetsand passing them to the Physical layer for transmission. At the receiving end, the packetsmust be taken from the Physical layer and stripped of control information.


Token-bus transmission frameThe Token-bus frame format is shown below:

FIGURE 3.5: TOKEN-BUS FRAME FORMAT

End Delimiter

Frame CheckSequence

Information

Source Address

Destination Address

Frame Control

Start Delimiter

Preamble0 - n bytes

1 byte

1 byte

0 - 819 bytes

2 or 6 bytes

4 bytes

1 byte

2 or 6 bytes

Token-bus transmission frame,continued


The Token-bus frame components are responsible for performing the following tasks:

PreambleThe preamble is responsible for providing the synchronization between the sending andreceiving device.The length of this field and its contents depend on the modulation method being used andthe speed of the network.

Start DelimiterThe start delimiter follows the preamble. As its name implies, it indicates the start of thedata frame. The start frame delimiter is 1 byte in length and contains a signaling patternthat is always different from the datathe actual signaling pattern varies with theencoding scheme used.

Frame Control FieldThis field identifies the type of frame being sentLogical Link Control data frames, tokencontrol frames, Media Access Control management data frames, or special-purpose dataframes.



Address FieldsEach of the address fieldsthe destination address and the source addresscan beeither 2 bytes (16-bit addresses) or 6 bytes (48-bit addresses) in length. If universaladdressing is used, the addresses must be 6 bytes each. But if local addressing is usedthey may be either 2 or 6 bytes long. Both destination and source addresses must be ofthe same length for all devices on a given network.The source address must be for an individual device. The destination address can be anindividual address, a group address or a broadcast address.

Information FieldThe information field contains the actual data packet to be transmitted. Its length isvariable. It may contain a Logical Link protocol data unit, token control data, managementdata or special-purpose dataas indicated in the frame control field.

Frame Check SequenceThe frame check field is used as an error control mechanism.When the transmitting device assembles a frame, it performs a calculation on the bits inthe frame. The algorithm used to perform this calculation always results in a 4-byte value.The sending device stores this value in the frame check sequence field.When the destination device receives the frame, it performs the same calculation andcompares the result to that in the frame check sequence field. If the two values are thesame, the transmission is assumed to be correct. If the two values are different, thedestination device can request a retransmission of the frame.



End DelimiterThe end delimiter marks the end of the frame and shows the position of the frame checksequence field. Just as with the start delimiter, the signaling value is always different fromthe data.


Token-bus optionsWhile the general principle behind Token-bus is token passing over a bus or tree topology,the standard actually specifies three Physical layer optionsbroadband, carrierband andoptical fiber.

Broadband Token-busThe definition of broadband states that such a transmission technique allows multipletransmissions to occur simultaneously, with each transmission taking place at a differentfrequency on the cable.Broadband Token-bus is defined by the following characteristics:

It uses 75 coaxial cable as the transmission mediumcoaxial cable is commonlyused for broadband systems.

It follows a tree topologythis is sometimes referred to as a directional bus. It supports data channels with the following bandwidths and transmission speeds:

1.5 MHz bandwidth with a transmission speed of 1 Mbps. 6 MHz bandwidth with a transmission speed of 5 Mbps. 12 MHz bandwidth with a transmission speed of 10 Mbps.

Token-bus options, continued


Carrierband Token-busCarrierband signaling requires that the whole frequency spectrum of the cable be devotedto a single transmission path for the purpose of transmitting analog signals. For thisreason, carrierband is also referred to as single-channel broadband.Carrierband Token-bus is defined by the following characteristics:

It uses 75 coaxial cable as the transmission medium. It follows a traditional bus topology. Since there is only one data channel, electronics are simpler and less expensive

than those used for broadband.

Specifications exist for transmission speeds of 1, 5 and 10 Mbps.

Optical fiber Token-busThe optical fiber specification is the most recent addition to the IEEE 802.4 standard.Optical fiber Token-bus is defined by the following characteristics:

The transmission medium is 2 optical fibersone for transmit and one forreceivethat supports a spectral width of 270 nanometers (nm) with a centerwavelength between 800 and 910 nm.

The optical fiber specification can be used with any topology that is logically abusphysically, it usually follows a star topology where all devices are connectedto a central active or passive hub.

Specifications exist for transmission speeds of 5, 10 and 20 Mbps.


IEEE 802.5 Token-ring Media Access Control

Definition of Token-ringIEEE 802.5 is the second of the token passing access control methods. Token-ring is mostcommonly used in a network structure following both a logical and physical ring topology.The right to transmit is controlled by a token. The method of transmission is as follows:

A token indicating the right to transmit is known as a free token. When a stationreceives a free token it changes the configuration of the token to that of a busytoken.

The busy token is included as part of each data unit transmitted. The station isallowed to transmit data units until a predetermined time is reached.

The data unit travels from station to station around the ring. Each station receiving a data unit checks the address to see if it should process the

information. If the data unit is intended for another station, it is passed on to the next station in

the ring.

Definition of Token-ring, continued


The intended destination station, upon receiving and processing the data unit, setsthree control bits in the data unit before sending it to the next station:

The Address Recognized control bit allows the destination station to indicatethat it identified the data unit as being addressed to it.

The Packet Copied control bit shows that the destination station sent a copy ofthe data unit to the LLC sublayer for processing.

The Error control bit shows that an error condition was detected. This control bitcan be set by any station on the ring, not only by the destination station.

Once the data unit returns to the originating station, it is removed from the network.The station then sends a free token to the next station in the ring.


Token-ring functionsToken-ring tokens use three control bits indicating if the data unit was processed or if anyerror conditions were detected. The different combinations of two of these bits, addressrecognized and packet copied, allow the source station to differentiate between differentconditions. For example, it would be able to see if:

The data unit was recognized by the destination station and processed. The data unit was recognized by the destination station but it was not able to

process it. The data unit was not recognized by the destination station or the destination

station is nonexistent or inactive.Token-ring differs in two other areas with respect to method of operationits manner of faultmanagement and an optional priority scheme.

Fault managementThere are three serious error conditions affecting the operation of Token-ring:

The loss of a token. A constantly busy token. The failure of a station on the ring.

In order to detect and then correct the first two of these conditions, Token-ring designatesone of the stations on the network as an active monitor. All the remaining stations aredesigned as passive monitorsmonitoring the operation of the active monitor.

Token-ring functions, continued


The station designated as the active monitor performs the following duties: It continuously monitors the network. If no token is detected for a predetermined amount of time, the monitor assumes

that the token has been lost and issues a new token.

To check for a constantly busy token, the monitor sets a monitor bit in a busy tokenas it passes. If the busy token returns with the monitor bit still set, the monitor willknow that the sending station did not remove the data unit from the network. Themonitor then changes the data unit to a free token and passes it to the next device.

If the active monitor fails, the passive monitors use a contention procedure to determinewhich one of them becomes the new active monitor.The third serious error condition is that of a failed station. If a station fails, it may not beable to transmit, causing the ring to be broken. To accommodate for such an occurrence, abypass switch in incorporated into each device. Closing this bypass switch removes thestation from the ring and allows data units to circulate properly. When a star-wiredtopology is used, the bypass switches are found in the central hub. This makes physicalfailures in the ring easier to correct.



Optional priority schemeToken-ring access control can be operated on either a nonpriority basis or a priority basis.When operating on a nonpriority basis, a station can transmit data units as soon as itreceives a free token.When operating on a priority basis, three bits in each data unit are used to represent thecurrent priority. A station receiving a free token follows a set procedure as follows:

The priority value of the token is compared to the priority value of the data unit tobe transmitted.

If the priority value of the data unit is equal to or higher than that of the token, thedata is sent.

If the priority value of the data unit is lower, the data is not sent.Each frame also has three reservation bits. These can be used by a station to reserve thetoken for its data transmission. The process to use the reservation bits is as follows:

A station must have a data unit to transmit that has a priority greater than zero. When this station receives and retransmits a frame, it sets the reservation bits in

the token to the same priority value of the data unit. When the original sending station removes the data unit and generates a free token

it checks the reservation bits and the priorities of additional data units it has tosend.



If the reservation bits or the priority of the waiting data have a higher value than thecurrent priority, this original sending station resets the current priority to the highervalue.

When a priority is reset to a higher value, the station saves the previous priorityvalue and it is responsible for eventually restoring the token priority to that originallower value.


Token-ring transmission frameThe Token-ring frame format is shown below.

FIGURE 3.6: TOKEN-RING FRAME FORMAT

Frame Status1 byte

Ending Delimiter

Frame CheckSequence

Information

Source Address

Destination Address

Frame Control

Access Control

Starting Delimiter1 byte

1 byte

1 byte

Variable

2 or 6 bytes

4 bytes

1 byte

2 or 6 bytes

Token-ring transmission frame,continued


The Token-ring frame components are responsible for performing the following tasks:

Starting DelimiterThe starting delimiter indicates the start of the data frame. It uses a unique signal patternthat does not correspond to either a 0 or 1 bit. These are known as nondata values andensure that no data sequence will ever be mistaken for a delimiter.

Access Control FieldThis field identifies whether the frame is a data frame or a token. It contains a bit used toidentify a constantly busy token, a priority bit and reservations bits.

Frame Control FieldThis field identifies the frame type and for certain types of control frames, the function it isto perform.

Address FieldsEach of the address fieldsthe destination address and the source addresscan beeither 2 bytes (16-bit addresses) or 6 bytes (48-bit addresses) in length. If universaladdressing is used, the addresses must be 6 bytes each. But if local addressing is usedthey may be either 2 or 6 bytes long. Both destination and source addresses must be ofthe same length for all devices on a given network.The source address must be for an individual device. The destination address can be anindividual address, a group address or a broadcast address.

Token-ring transmission frame,continued


Information FieldThe information field contains the actual data packet to be transmitted. This can be eithera protocol data unit being passed from the logical link control sublayer or controlinformation supplied by the media access control sublayer. Its length is variableanywhere from 0 to 17800 bytes in length.

Frame Check SequenceThe frame check field is used as an error control mechanism.When the transmitting device assembles a frame, it performs a calculation on the bits inthe frame. The algorithm used to perform this calculation always results in a 4 byte value.The sending device stores this value in the frame check sequence field.When the destination device receives the frame, it performs the same calculation andcompares the result to that in the frame check sequence field. If the two values are thesame, the transmission is assumed to be correct. If the two values are different, thedestination station can request a retransmission of the frame.

Ending DelimiterThis identifies the end of the frame by containing nondata values. It also contains bits usedto identify whether or not it is the last frame in a multiframe transmission and if an errorhas been detected by any station.

Frame Status FieldThe frame status field contains the address recognized and frame copied control bits.


Overview .................................................................................. 1LAN communications defined....................................................... 1Communications terminology ....................................................... 2

Communications standards organizations ........................ 3American National Standards Institute (ANSI) .......................... 3Comite Consultatif Internationale deTelegraphique et Telephonique (CCITT) ...................................... 3

CCITT (ITU) recommendations ........................................................ 4Examples of V-series CCITT standards ............................................................. 5Examples of X-series CCITT standards ............................................................. 5

International Organization for Standardization (ISO) ............... 6OSI Model ......................................................................................... 6

Institute of Electrical and Electronics Engineers (IEEE) .......... 7IEEE Project 802 ............................................................................... 7

TA

BLE

O

F CO

NTEN

TS TA

BLE

O

F CO

NTEN

TS CH

APTER

3 - LAN CO

MM

UNICATIONS

STA

ND

AR

DS


Communications fundamentals ........................................... 8Protocols ........................................................................................... 8

Defined .............................................................................................. 8Function of a protocol ........................................................................ 9

Packets/Frames/Datagrams ......................................................... 10Defined ............................................................................................ 10

Layered architectures ................................................................... 11Defined ............................................................................................. 11

Hardware independence .............................................................. 12Overview ......................................................................................... 12Objectives ........................................................................................ 12

Connectivity ...................................................................................................... 12Modularity ......................................................................................................... 13Ease of implementation .................................................................................... 13Ease of use ....................................................................................................... 13Reliability .......................................................................................................... 13Ease of modification ......................................................................................... 13

TA

BLE

O

F CO

NTEN

TS TA

BLE

O

F CO

NTEN

TS C

HA

PTER 3 - LAN C

OM

MUNICATIO

NS S

TAN

DA

RDS


The Open Systems Interconnection (OSI) model ............ 14Overview.......................................................................................... 14Defining the seven layers of the OSI model ............................. 16

Physical Layer .................................................................................................. 16Data Link Layer................................................................................................. 16Network Layer................................................................................................... 16Transport Layer................................................................................................. 17Session Layer ................................................................................................... 17Presentation Layer ............................................................................................ 17Application Layer .............................................................................................. 17

Purpose of the OSI model ............................................................ 18Using the OSI model ..................................................................... 19

The hardware level .......................................................................... 19The transport level .......................................................................... 19

Transport-level protocols............................................................... 20IPX/SPX ............................................................................................................ 20NetBEUI ............................................................................................................ 20

The application-to-transport level ................................................... 21The application level ....................................................................... 21

Applying the OSI model in the LAN environment ................... 22

TA

BLE

O

F CO

NTEN

TS TA

BLE

O

F CO

NTEN

TS C

HA

PTER 3 - LAN C

OM

MUNICATIO

NS S

TAN

DA

RDS


IEEE Project 802 ................................................................... 23Overview.......................................................................................... 23802 standards ................................................................................. 24

IEEE 802.1 - High-Level Interface .................................................................... 24IEEE 802.2 - Logical Link Control ..................................................................... 24IEEE 802.3 - Carrier Sense Multiple Access with Collision Detection ............. 24IEEE 802.4 - Token-bus .................................................................................... 24IEEE 802.5 - Token-ring .................................................................................... 25IEEE 802.6 - Metropolitan Area Network (MAN) .............................................. 25IEEE 802.7 - Broadband Technology ................................................................ 25IEEE 802.8 - Fiber Optics ................................................................................. 25IEEE 802.9 - Integrated Services LAN ............................................................. 25IEEE 802.10 - LAN Security ............................................................................. 26IEEE 802.11 - Wireless ..................................................................................... 26IEEE 802.12 - Demand Priority ......................................................................... 26

Physical layer overview................................................................ 28Data Link layer overview .............................................................. 29

Logical Link Control (LLC) sub-layer recommendations ................. 29Media Access Control (MAC) sub-layer recommendations ............ 30

Medium Access Management ........................................................................... 30Framing ............................................................................................................. 30Addressing ........................................................................................................ 30Error Detection.................................................................................................. 30

TA

BLE

O

F CO

NTEN

TS TA

BLE

O

F CO

NTEN

TS C

HA

PTER 3 - LAN C

OM

MUNICATIO

NS S

TAN

DA

RDS


IEEE 802.3 CSMA/CD Media Access Control .................... 32Definition of CSMA/CD ................................................................. 32

Carrier sensing ................................................................................ 33Collision detection ........................................................................... 33

CSMA/CD functions ...................................................................... 34Data encapsulation/decapsulation .................................................. 34

Data encapsulation........................................................................ 34Data decapsulation........................................................................ 35

Media access management ............................................................. 35Data encoding/decoding .................................................................. 36

Data encoding ............................................................................... 36Data decoding ............................................................................... 36

CSMA/CD transmission frame .................................................... 37Preamble ........................................................................................................... 38Start Frame Delimiter ....................................................................................... 38Address Fields .................................................................................................. 38Length Count .....................................................................................................39Information Field ............................................................................................... 39Pad Field ........................................................................................................... 39Frame Check Sequence ................................................................................... 39

IEEE 802.3 naming convention ................................................... 401Base-5 ............................................................................................................. 4010Base-5 ........................................................................................................... 4010Base-2 ........................................................................................................... 4010Broad-36 ........................................................................................................ 4110Base-T ........................................................................................................... 4110Base-F ........................................................................................................... 41100Base-TX ...................................................................................................... 41100Base-T4 ....................................................................................................... 41100Base-FX ...................................................................................................... 41

TA

BLE

O

F CO

NTEN

TS TA

BLE

O

F CO

NTEN

TS C

HA

PTER 3 - LAN C

OM

MUNICATIO

NS S

TAN

DA

RDS


IEEE 802.4 Token Bus Media Access Control .................. 42Definition of Token-bus ................................................................ 42Token-bus functions ..................................................................... 44

Interface to the Logical Link Control sublayer ................................ 44Token handling ................................................................................ 44Ring maintenance ........................................................................... 44Fault detection and recovery ........................................................... 45Sending and receiving data ............................................................. 45

Token-bus transmission frame ................................................... 46Preamble ........................................................................................................... 47Start Delimiter ................................................................................................... 47Frame Control Field .......................................................................................... 47Address Fields .................................................................................................. 48Information Field ............................................................................................... 48Frame Check Sequence ................................................................................... 48End Delimiter .................................................................................................... 49

Token-bus options ......................................................................... 50Broadband Token-bus ..................................................................... 50Carrierband Token-bus .................................................................... 51Optical fiber Token-bus ................................................................... 51

TA

BLE

O

F CO

NTEN

TS TA

BLE

O

F CO

NTEN

TS C

HA

PTER 3 - LAN C

OM

MUNICATIO

NS S

TAN

DA

RDS


TA

BLE

O

F CO

NTEN

TS TA

BLE

O

F CO

NTEN

TS IEEE 802.5 Token-ring Media Access Control .................. 52

Definition of Token-ring ................................................................ 52Token-ring functions ..................................................................... 54

Fault management .......................................................................... 54Optional priority scheme ................................................................. 56

Token-ring transmission frame ................................................... 58Starting Delimiter .............................................................................................. 59Access Control Field......................................................................................... 59Frame Control Field .......................................................................................... 59Address Fields .................................................................................................. 59Information Field ............................................................................................... 60Frame Check Sequence ................................................................................... 60Ending Delimiter................................................................................................ 60Frame Status Field............................................................................................ 60

CH

APTER

3 - LAN CO

MM

UNICATIONS

STA

ND

AR

DS


Figure 3.1: The OSI model .................................................... 15

Figure 3.2: The OSI model and LAN communications ....... 22

Figure 3.3: Project 802 layers and sublayers ..................... 27Figure 3.4: CSMA/CD (IEEE 802.3) frame format .............. 37Figure 3.5: Token-bus frame format ..................................... 46

Figure 3.6: Token-ring frame format ..................................... 58

FI

GUR

ES FI

GUR

ES FI

GUR

ES FI

GUR

ES CH

APTER

3 - LAN CO

MM

UNICATIONS

STA

ND

AR

DS

ContentsFiguresThe OSI modelThe OSI model and LAN communicationsProject 802 layers and sublayersCSMA/CD (IEEE 802.3) frame format Token-bus frame formatToken-ring frame format

Overview LAN communications defined Communications terminology

Communications standards organizations American National Standards Institute (ANSI) Comite Consultatif Internationale de Telegraphique et Telephonique (CCITT) CCITT (ITU) recommendations Examples of V-series CCITT standards Examples of X-series CCITT standards

International Organization for Standardization (ISO) OSI Model

Institute of Electrical and Electronics Engineers (IEEE) IEEE Project 802

Communications fundamentals Protocols Defined Function of a protocol

Packets/Frames/Datagrams Defined

Layered architectures Defined

Hardware independence Overview Objectives Connectivity Modularity Ease of implementation Ease of use Reliability Ease of modification

The Open Systems Interconnection (OSI) model Overview Defining the seven layers of the OSI model Physical Layer Data Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer

Purpose of the OSI model Using the OSI model The hardware level The transport level Transport-level protocols IPX/SPX NetBEUI

The application-to-transport level The application level

Applying the OSI model in the LAN environment

IEEE Project 802 Overview 802 standards IEEE 802.1 - High-Level Interface IEEE 802.2 - Logical Link Control IEEE 802.3 - Carrier Sense Multiple Access with Collision Detection IEEE 802.4 - Token-bus IEEE 802.5 - Token-ring IEEE 802.6 - Metropolitan Area Network (MAN) IEEE 802.7 - Broadband Technology IEEE 802.8 - Fiber Optics IEEE 802.9 - Integrated Services LAN IEEE 802.10 - LAN Security IEEE 802.11 - Wireless IEEE 802.12 - Demand Priority

Physical layer overview Data Link layer overview Logical Link Control (LLC) sub-layer recommendations Media Access Control (MAC) sub-layer recommendations Medium Access Management Framing Addressing Error Detection

IEEE 802.3 CSMA/CD Media Access Control Definition of CSMA/CD Carrier sensing Collision detection

CSMA/CD functions Data encapsulation/decapsulation Data encapsulation Data decapsulation Media access management Data encoding/decoding Data encoding Data decoding

CSMA/CD transmission frame Preamble Start Frame Delimiter Address Fields Length Count Information Field Pad Field Frame Check Sequence

IEEE 802.3 naming convention 1Base-5 10Base-5 10Base-2 10Broad-36 10Base-T 10Base-F 100Base-TX 100Base-T4 100Base-FX

IEEE 802.4 Token Bus Media Access Control Definition of Token-bus Token-bus functions Interface to the Logical Link Control sublayer Token handling Ring maintenance Fault detection and recovery Sending and receiving data

Token-bus transmission frame Preamble Start Delimiter Frame Control Field Address Fields Information Field Frame Check Sequence End Delimiter

Token-bus options Broadband Token-bus Carrierband Token-bus Optical fiber Token-bus

IEEE 802.5 Token-ring Media Access Control Definition of Token-ring Token-ring functions Fault management Optional priority scheme

Token-ring transmission frame Starting Delimiter Access Control Field Frame Control Field Address Fields Information Field Frame Check Sequence Ending Delimiter Frame Status Field

Chapter 3

Documents

lan communications standards

communications channels

transmission of data

data character

bicsi lan design manual

corrupted data

package of data

transmission channel