DATACOM introduction

1. LAN Protocols and TopologiesAn important part of designing a Local Area Network (LAN) is choosing theLAN protocol that operates on the lower two layers of the OSI model. When youselect a LAN protocol, you select a set of characteristics. The characteristicsshould be studied and a decision should be based on the needs of theworkgroups.

IEEE 802.3 Ethernet, including Fast Ethernet and Gigabit Ethernet Token Ring (IBM and IEEE 802.5) Fibre Data Distributed Interface (FDDI) Wireless LAN

Physical and Logical Network TopologiesThese network protocols utilise particular physical media, for example a TokenRing utilises unshielded twisted pair (UTP) in a star formation.

A well as the physical network topology there is also the logical networktopology. In some special cases, a physical network topology will not affect theway the network operates. The actual signal path is called a logical topology.

Media connection devices or media access schemes can be used to pass asignal in a logical path that is different to the physical media path. For example,in a Token Ring network. Token Ring LANs often use copper cable arranged ina star topology with a hub at the centre. The hub does not repeat incomingsignals to all other attached devices, as in a normal star topology. The hub’scircuitry distributes each incoming signal to the next device in a predeterminedlogical ring or round-robin scheme. therefore the physical topology employed isa star, while the logical topology is a ring.

Media Access MethodsThe following media access methods describe rules that govern when networkdevices are allowed to transmit:

Contention, which is based on the premise that media access should beallowed on a first-come, first-served basis. Contention systems aredesigned so that all devices on the network can transmit whenever theywant. An example of this method is carrier sense multiple access/collisiondetection (CSMA/CD)

Token-passing, which involves a small data frame (the token) beingpassed in an orderly fashion from one device to another. A token is aspecial message that temporarily gives media control to the device holdingthe token

Polling, which is an access method that designates one device (called acontroller, primary, or master) as a media access administrator. Thisdevice queries each of the other devices in some predetermined order tosee whether they have information to transmit

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Ethernet 802.3What is Ethernet?The Oxford Dictionary defines it as: ‘A system of communication for local areanetworks by coaxial cable that prevents simultaneous transmission by morethan one station.’

Ethernet was invented at the Xerox Palo Alto Research Centre in the 1970s byDr. Robert M. Metcalfe. It was designed to support research on the "office ofthe future," which included one of the world's first personal workstations, theXerox Alto. The first Ethernet system ran at approximately 3-Mbps and wasknown as "experimental Ethernet." A formalised specification was notdeveloped until the 1980’s when DEC, Intel and Xerox created the DIXstandard. This was the first production of ethernet as we now know it, a systemoperating at a speed of 10 Mbit/s.

The Institute of Electrical and Electronic Engineers (IEEE) Ethernet standardwas first published in 1985 entitled "IEEE 802.3 carrier sense multiple accesswith collision detection (CSMA/CD) Access Method and Physical LayerSpecifications." The IEEE standard has been adopted by the InternationalStandards Organisation (ISO), making it a worldwide networking standard.

Ethernet is a typical network technology that employs the bus topology. TheEthernet system consists of three basic elements:

A physical medium used to carry ethernet signals between computers. A set of media access control rules embedded in each Ethernet interface

that allow multiple computers to fairly arbitrate access to the sharedEthernet channel.

An Ethernet frame which consists of a standardised set of bits used tocarry data over the system.

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To be more specific, Ethernet is one of a family of network architecturesgoverned by a protocol called Carrier Sense Multiple Access/Collision Detect(CSMA/CD).

The basic idea of CSMA/CD is that all the stations on the network areconnected to a common bus. Therefore, all the stations are continuouslylistening to the messages transmitted on the bus, and each station collects themessages that were addressed to its address.

For a station to transmit a message, it first listens to the common media knownas ‘the ether’. If the media is busy, no station introduces any messages.

However if the media is idle any station may transmit its messages.Occasionally, this arbitrary access to the common media can cause collisionsamong messages. Should a station detect that a message it transmittedcollided with another, it would re-transmit the message after a random amountof time.

CSMA/CD Ethernet uses carrier sense multiple access/collision detection (CSMA/CD).This means that any device can gain access to the network at any time:

If a device wants to transmit data, it first listens to the network through anetwork interface for a carrier signal. If it hears nothing, then it will transmit.If a carrier signal is detected, then all interfaces must wait until the carrierceases before transmitting. This process is known as carrier sense.

All interfaces have an equal ability to send frames onto the network. Nointerface has any priority over any other. This is called multiple access.

If two devices want to transmit simultaneously then they will both listen,hear no sound and transmit. A collision will then occur. It is important tonote that no information is lost when a collision occurs. In the event of acollision, the Ethernet interface backs off and must then wait for a randomamount of time for the cable to become idle again before retransmitting theframe. However if the devices begin to transmit at the same time againanother collision will occur. To avoid multiple collisions Ethernet requireseach device to delay after a collision before attempting to retransmit. Ifmultiple collisions are occurring and a frame collides on 16 consecutiveoccasions then the Ethernet frame is discarded. This will only occur if theEthernet channel is overloaded for lengthy periods of time, or is broken insome way.

The main drawback to CSMA/CD is that it is impossible to calculate how long adevice will have to wait before it can transmit.

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Ethernet Access MediaEthernet access media are usually expressed in the form XBaseY, where X isthe transmission speed, and Y can indicate the type of physical medium or themaximum length of the network segment. ‘Base’ stands for basebandsignalling. Baseband is a type of digital transmission in which each mediumcarries only one signal or channel at a time.There are several different Ethernet access media available, including

10Base5 - Thick co-axial cable 10Base2 - Thin co-axial cable 10BaseT - Twisted-pair cable 10BaseF - Fibre-optic cable

The thick co-axial medium was the first medium defined in the earliest Ethernetspecifications. Next came the thin co-axial, followed by the twisted-pair andfibre-optic media types. Of the four specified types, the twisted-pair media typeis the most widely used for making connections to the desktop.

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Extending EthernetEthernet was designed to be easily expandible to meet the networking needs ofa given site. To help extend Ethernet systems, networking vendors sell devicesthat provide multiple Ethernet ports. These devices are known as hubs sincethey provide the central portion, or hub, of a media system.There are two kinds of hub:

Repeater hubs - Each port of a repeater hub links individual Ethernetmedia segments together to create a larger network that operates as asingle Ethernet LAN.

Switching hubs - This kind of hub provides packet switching, typicallybased on bridging ports. Each port of a packet switching hub provides aconnection to an Ethernet media system that operates as a separateEthernet LAN. Unlike a repeater hub whose individual ports combinesegments together to create a single large LAN, a switching hub makes itpossible to divide a set of Ethernet media systems into multiple LANs thatare linked together by way of the packet switching electronics in the hub.

While an individual Ethernet LAN may typically support anywhere from a few upto several dozen computers, the total system of Ethernet LANs linked withpacket switches at a given site may support many hundreds or thousands ofmachines.

Components in an Ethernet systemThe components, as defined in the IEEE standard, required for making anattachment to a 10 Mbit/s media system are:

Physical medium - This is used to carry Ethernet signals amongcomputers. This can be any one of several 10 Mbit/s media types includingthick or thin coaxial cable, twisted-pair cable, and fibre optic cable.

Medium dependant interface (MDI) - This is a piece of hardware used formaking a direct physical and electrical connection to the network cable. Inthe case of thick coaxial Ethernet, the most commonly used MDI is a typeof clamp that is installed directly onto the coaxial cable. For twisted-pairEthernet, the MDI is an eight-pin connector, which is also referred to as anRJ-45 telephone-style jack. The eight-pin jack provides a connection to thefour twisted-pair wires used to carry network signals in the 10 Mbit/stwisted-pair media system.

Medium attachment unit (MAU) - This device is called a transceiver in theoriginal DIX Ethernet standard, since it both transmits and receives signalson the medium. The medium dependent interface just mentioned isactually a part of the MAU, and provides the MAU with a direct physicaland electrical connection to the medium.

Attachment unit interface (AUI) - This is called a transceiver cable in theDIX standard. The AUI provides a path for signals and power carriedbetween the Ethernet interface and the MAU. The AUI may be connectedto the Ethernet interface in the computer with a 15-pin connector.

Data terminal equipment (DTE) - The networked device itself is defined asDTE in the IEEE standard. Each DTE attached to an Ethernet is equippedWith an Ethernet interface. The Ethernet interface provides a connection tothe Ethernet media system and contains the electronics and softwareneeded to perform the medium access control functions required to send aframe over the Ethernet channel. Ethernet ports on repeaters do not usean Ethernet interface. A repeater port connects to the Ethernet mediasystem using the same AUI, MAU, and MDI equipment. However, repeaterports operate at the individual bit level for Ethernet signals, moving the

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signals directly from segment to segment. Therefore, repeater ports do notuse Ethernet interfaces to perform their function, since they do not operateat the level of Ethernet frames.

The MAU and MDI are specifically designed for each media type used inEthernet. Coaxial MAUs differ from twisted-pair MAUs, for example, both in thetechnology used for the actual connection to the media (the MDI), as well asthe method used for sending Ethernet signals over the media and for detectingcollisions.

Twisted-pair Media System (10Base-T)The specifications for the twisted-pair media system were published in 1990.This system has since become the most widely used medium for connectionsto the desktop.

The 10BASE-T system operates over two pairs of wires, one pair used forreceive data signals and the other pair used for transmit data signals. The twowires in each pair must be twisted together for the entire length of the segment,a standard technique used to improve the signal carrying characteristics of awire pair. Multiple twisted-pair segments communicate by way of a multiporthub.

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Ethernet Frame FormatEthernet 802.3 transmits information in the form of frames. The frame formatconsists of the following:

PreambleA preamble of 7 bytes, each containing the bit pattern 10101010. TheManchester encoding of this pattern produces a 10 MHz square wave for 5.6ms in order to allow the receiver’s clock to synchronise with the sender.

Start of Frame ByteThe next field is the start of frame byte containing 10101011, to identify thestart of the frame itself.

Addresses The frame contains two addresses:

The destination address The source address

The standard allows 2 and 6 byte addresses, but the parameters defined for 10Mbit/s baseband standard use only the 6 byte addresses. The high order bit ofthe destination address is a 0 for ordinary addresses and 1 for groupaddresses. Group addresses allow multiple stations to listen to singleaddresses. When a frame is sent to a group address, all the stations in thegroup receive it. Sending to a group of stations is called Multicast. The addressconsisting of all 1 bits is reserved for Broadcast. The frame containing all 1s inthe destination field will be delivered to all stations on the network. Theaddressing uses bit 46 (adjacent to the high order bit) to distinguish betweenlocal and global addresses. Local addresses are assigned internally within anetwork by a system administrator, whereas global addresses are assigned by

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the IEEE. This eliminates the duplication of addresses globally.

The length field tells how many bytes are present in the data field, this can be aminimum of 0 to a maximum of 1500. While a data field type of 0 bytes isacceptable, it can cause a problem. When a transceiver detects a collision, ittruncates the current frame, which means the bad bits of frame appear on thecable all of the time. In order to identify garbage from valid frame bits, 802.3states that valid data must be at least 64 bytes long from destination address tochecksum. If the data portion of the frame is less than 46 bytes, the pad field isused to fill out the frame to the minimum size. In addition to this a minimumlength frame is used to prevent a station from completing the transmission of ashort frame before the first bit has even reached the far end of the cable, whereit may collide with other frames.

The final 802.3 field is checksum. It is effectively a 32-bit hash code of the data.This detects error in codes using the cyclic redundancy check (CRC).

Media Access Control (MAC) AddressingComputer network entities need some way to distinquish between differentdevices on the network. This is done through addressing. Physical deviceaddresses are unique hardware addresses typically assigned by hardwarevendors. The hardware vendors use addresses that are allocated to them by astandards organisation. The format of the address depends on the mediaaccess method being used (that is why they are called media access control orMAC addresses.

There are two MAC layer addresses in an Ethernet frame. The first is theaddress of the source device and the second is the intended destinationdevice.

A MAC address is a hardware address that uniquely identifies each node on anetwork. MAC addresses are also known as physical device addresses. The

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addresses are typically burned into network cards in the factory.

MAC addressing is controlled by the OSI data link layer, and therefore is usedto communicate only on the local network. This means that when a message istransmitted to another device on the same network segment, the destinationdevice’s MAC address is used. Therefore, the need for broadcasts is avoided.

When a message is transmitted to a device on another network segment, theMAC address of the nearest router is used, and it is up to the router to send thedata further.

LAN Broadcast TypesIn the LAN environment messages can be transferred in three different ways:

Unicast, which is the communication from one device to another over anetwork. In other words, a point to point communication

Multicast, which is the broadcast of messages to a selected group ofdevices, such as workstations. Multicast is communication between asingle device and multiple members of a device group. For example. aLAN-attached workstation might address a transmission to a number ofother LAN-attached devices

Broadcast, which involves sending a transmission to all stations on theNetwork

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Fast EthernetTo date we have been discussing conventional Ethernet, its access media andone method used to overcome the problem of only a single station havingaccess to the LAN at a time. Now solutions used to speed up the transmissionrates will be examined. Fast Ethernet is one such solution and it has thefollowing features:

It runs at 100 Mbit/s as opposed to 10 Mbit/s of Ethernet. It is backwards compatible. Many organisations want faster networks but

do not want to change all their hardware This would be the case if theywere to use alternative technologies, for example Fibre Distributed DataInterface (FDDI). The most efficient solution is to upgrade to Fast Ethernet.An upgrade to Fast Ethernet means there is no need to change existinghardware and thus the cost of increased bandwidth is reduced to areasonable price.

Since the bit-time is reduced from 100 nanoseconds to 10 nanoseconds, ittakes less time to transmit or receive data thus resulting in faster speeds.

Transmission MediaThere are a number of different transmission media which can be used for FastEthernet. They all use two physical links between nodes, one for transmissionand one for reception.

100Base - TX is a two-pair system for data grade (category 5) unshieldedtwisted pair (UTP) or shielded twisted pair (STP) cabling.

100Base - T4 is a four-pair system for both voice and data grade (category3, 4 or 5) UTP cabling. It is a lower cost alternative. To achieve 100 Mbit/sover lower-quality cable, 100Base - T4 uses four twisted pair lines betweennodes, with data transmission making use of three pairs in one direction ata time.

100Base - FX uses two optical fibres, one for transmission and one forreception.

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Components used in a Fast Ethernet SystemThe figure shows the components defined in the IEEE standard for making anattachment to a 100-Mbps Fast Ethernet media system. These componentsare:

Physical Medium This could be any one of the three 100 Mbit/s media types. A connection ismade to the medium with the medium dependent interface, or MDI. This is an8-pin twisted-pair connector or fibre optic connector in the 100BASE-T system.

Physical Layer Device (PHY)This device performs the same general function as a transceiver in the 10 Mbit/s Ethernet system. It may be a set of integrated circuits inside the Ethernet portof a network device, therefore invisible to the user, or it may be a small boxequipped with an MII cable, like the outboard transceiver and transceiver cableused in 10 Mbit/s Ethernet.

Medium Independent Interface (MII)The MII is an optional set of electronics that provides a way to link the Ethernetmedium access control functions in the network device with the physical layerdevice (PHY) that sends signals onto the network medium. An MII mayoptionally support both 10 Mbit/s and 100 Mbit/s operation, allowing suitablyequipped network devices to connect to both 10BASE-T and 100BASE-Tmedia segments. The MII is designed to make the signalling differences amongthe various media segments transparent to the Ethernet chips in the networkdevice. The MII converts the line signals received from the various mediasegments by the transceiver (PHY) into digital format signals that are thenprovided to the Ethernet chips in the device. The optional MII electronics, andassociated 40-pin female connector and MII cable, makes it possible toconnect a network device to any of several media types, providing maximumflexibility. The MII electronics may be linked to an outboard transceiver througha 40-pin MII connector and a short MII cable. The MII cable for use withoutboard 100 Mbit/s transceivers is specified as a 40-pin cable with a 40-pinplug on one end, equipped with male jack screws that screw into mating femalescrew locks. The cable can be a maximum of 0.5 meters in length (about 19.6inches). It is also possible for the outboard transceiver to be attached directly tothe MII connector on the device with no intervening cable, if the design of thetransceiver allows it.

Data Terminal Equipment (DTE)For a typical station connection the DTE (computer) contains an Ethernetinterface which forms up and sends Ethernet frames that carry data betweencomputers attached to the network. The Ethernet interface is attached to themedia system using a set of equipment that might include an outboard MIIcable and PHY (transceiver) with its associated MDI (twisted-pair RJ45-stylejack or fibre optic connector). The interface or repeater port might also bedesigned to include the PHY electronics internally, in which case all you willsee is the MDI for whatever physical medium the interface or port wasdesigned to support. Each media type in the Fast Ethernet system has a PHYand MDI specifically designed and wired for use on that kind of segment.

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Gigabit EthernetOne of the most important questions network administrators face is how to gethigher bandwidth without disrupting the existing network. Gigabit Ethernetfollows the same form, fit and function as its 10 Mbit/s and 100 Mbit/s Ethernetpredecessors, allowing a straightforward, incremental migration to higher-speednetworking. All three Ethernet speeds use the same IEEE 802.3 frameformat, full-duplex operation and flow control methods. In half-duplex mode,

Gigabit Ethernet employs the same fundamental CSMA/CD access method toresolve contention for the shared media. And, Gigabit Ethernet uses the samemanagement objects defined by the IEEE 802.3 group. Gigabit Ethernet isEthernet, only faster. It is the next step in Ethernet. It runs at a speed of 1000Mbit/s which is a 10-fold increase on Fast Ethernet. Like Fast Ethernet, it is alsobackward compatible. It was originally designed to transport data traffic overpacket-based networks and Gigabit Ethernet is the next logical step.Gigabit Ethernet is attractive for 2 main reasons:

It preserves Ethernet simplicity It supports a very high bandwidth

The demand for gigabit speeds in the backbone is driven by the rapiddeployment of Fast Ethernet to the desktop.

Gigabit Ethernet is a familiar technology so investments in hardware andpersonnel are protected. It is also more sophisticated than Fast Ethernet andtherefore will require more training and support.

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Token Ring 802.5Token Ring is a LAN protocol, which was first developed by IBM. Token Ring isstandardized in IEEE 802.5, published in 1985. The protocol deals with theproblem of collision, which is defined as a state where two stations transmit atthe same time. In order to avoid the situation of collision there was a need tocontrol the access to the network. This kind of control is possible by the use ofa control (permission), called token. The token is passed from one station toanother according to a set of rules (only the machine that has the token at thatparticular time is allowed to transmit on the ring). The ring consists of ringstations and transmission medium. Data travels sequentially from station tostation. Only the station in possession of the token is allowed to transmit data.

Each station repeats the data, checks for errors, and copies the data ifappropriate. When the data is returned to the sending station, it removes it fromthe ring (see also Token Ring Mechanism). The Token Ring protocol supportspriorities in transmission. It is implemented by setting the priority bits in theToken Ring frame.

Token Ring is a first and second layer protocol in the OSI (Open SystemsInterconnection) seven layer model. The first release of Token Ring wascapable of 4 Mbit/s data transmission rate, the transmission rate was improvedlater to 16 Mbit/s. Token Ring operates on many cable types. There are anumber of ring benefits that can be identified.

Token Passing Access MethodIn Token Ring architecture, the sequence in which users gain access iscontrolled. the controlling station, called the active monitor, generates a specialsignalling sequence called a token that controls the right to transmit. This tokenis continuously passed logically, not physically, around the network from onenode to the next. When a station has something to send, it captures the tokenand changes it to a frame, setting its status to busy. It then adds the frameheader, information and trailer fields. the header includes the addresses of therecipient stations. All nodes read the frame as it is passed around the ring todetermine if they are the recipient of the message. If they are, they extract thedata and then retransmit the frame to the next station on the ring. When theframe returns to the originating station, it removes the frame and reissues afree token, which can be used by another station.

Early Token Release (ETR) MechanismA station that wants to transmit waits for a free token. The station transmits aframe and then releases a new token. The next station that wants to transmitwaits for a free token and transmits a frame and then releases a new token intothe ring and so on. The ETR mechanism enables multiple frames on the ring,and therefore the ring is more effective. When working in a large Token Ringperformance is improved, enabling a mixture of stations with and without ETR.

802.5 Frame FormatThe 802.5 frame format consists of the following fields:

The starting delimiter (SD) and ending delimiter (ED) fields show thebeginning and ending of the frame. Each of the fields contains invaliddifferential Manchester patterns (HH and LL) to ascertain them from databytes

The access control (AC) byte contains the token bit, and also the monitor

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bit, priority bits, and reservation bits The Frame Control (FC) byte distinguishes data frames from various

possible control frames The destination address and source address fields contain 2 or 6 byte

addresses. One or the other must be used because a mixture of addressescannot reside on the same cable

Data Unit (DU) The following field is Data, which can be any lengthassuming the frame can still be transmitted within the token-holding time.The Checksum field is used to detect transmission errors

The Frame Status (FS) byte contains the A and C bits. When a framearrives at the interface of a station with the destination address, theinterface turns on the A bit as it passes through. If the interface copies theframe to the station, it also turns on the C bit. A station might fail to copy aframe due to lack of buffer space or other reasons

There are three combinations possible when the sending station drains theframe from the ring. These combinations provide an automaticacknowledgement of each frame. If a frame is rejected but the station ispresent, the sender has the option of a retry in a short time. The A and Cbits reside in the frame status to allowing reliability in that the checksumdoes not include the aforementioned bits:

The ending delimiter holds an E bit which is set if any interface detects anerror. For example, a non Manchester pattern where that is not permitted.In addition to this it also contains a bit used to mark the last frame in alogical sequence. Often described as an end-of–file bit.

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Token Ring ManagementToken Ring provides a high level of network management intelligence and faulttolerance. A Token RIng’s network’s built-in media access control (MAC) offersa total of 19 MAC frames that provide significant network management,including reporting on network errors and adapter card status. The result is ahigh degree of fault tolerance and easier trouble shooting. Some specificmanagement capabilities includes:

Sending out a new token if one is lost Resending messages Ensuring that the token is passed in its proper sequence Managing conjestion Automatically correcting network errors

A process called beaconing automatically detects problems with the ring andattempts to correct them. For example, the beaconing process can force afailing station to leave the ring or isolate a failing network to be bypassed,thereby providing a level of self-healing that is not available in most LANs.

Ring Management OperationThe mechanism of the network operation is considered to be the mechanism inthe steady state, but before this can take place the ring must be set up. Also if anew station wishes to join an operational ring it must first go through aninitialization procedure to ensure that it does not interfere with the correctfunctioning of the current active ring. Also, during normal operation it isnecessary for each active station on the ring to monitor its correct operationand if something is not working well it must take some action to try to re-establisha correctly functioning ring. Those functions and others are meant topreserve the correct ring operation are called ring management.

There are two types of stations in the ring: Active monitor (AM) - Any station on the ring can be AM but there is only

one AM station per ring. The AM is chosen during a process called ‘claim

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token process’ after the Active Monitor is chosen all other stations become‘standby monitors’ (SBM). It has the following features:• It is the ring manager• It maintains the master clock• It ensures proper ring delay (24 bit delay in the ring)• It initiates ‘neighbour notification’ every 7 seconds• It monitors token and frame transmission, detects lost tokens and

frames by setting the monitor bit, purging the ring

Standby monitor (SBM) - All other stations on the ring are SBM stations.The SM duties are to detect AM failures and to start monitor contentionprocess, and also to participate in the neighbour notification process.

MAC Frame TypesMedia access control (MAC) frame types involved with the ring managementfunction include the following:

Ring poll - Active monitor sends ring poll every 7 sec. This process is usedto learn the ring configuration. The ring poll routine is: active monitor sendsan active monitor present (AMP) frame. Each downstream station sends astandby monitor resent (SMP) frame. Each downstream node learns itsnext active upstream neighbour (NAUN).

Ring purge -This takes place when token is lost. The purge frame is sentbefore the active monitor initiates a new token. The active monitorbroadcasts ring purge frame to all stations if 10ms elapsed. The Purgeframe resets the stations to normal repeat mode and cancels or restartsappropriate timers.

Claim token process -This is how the new active monitor is elected. It isinitiated when the active\standby monitor detects loss of signal, or a newstation attaches and finds no active monitor.

Beacon - When a station detects a failure of token claiming following ahardware error it transmits a beacon frame. When the next activeupstream neighbour (NAUN) receives 8 beacon frames, it removes itselffrom the ring. It then performs a lobe media test and duplicate addresstest. if there’s a lobe error the station remains out of the ring and the ringcontinues its normal operation and a token claim process takes place.

Ring insertion - When a new station wants to enter the ring it performs aring insertion routine. First it makes a lobe media check – checks the lobeconnections. In the second step it attaches the ring and searches for anactive monitor if there is no active monitor for 18s, it initiates claim tokenprocess. In the third step it transmits a duplicate address test (DAT) frame,each active station checks the content of the frame to see that the newstation address is different from its own, if it is not, the station sets a flag inthe frame to indicate the error. After the frame has circulate back to thesource station the later checks the error flag and acts according to it. Ifthere is no error the new station continues the init procedure by sendingstandby monitor present (SMP) frame.

Hardware error - These are permanent faults, usually concerns hardware(equipment) that stops the ring’s normal operation.

MSAU and HubsA multistation access unit (MSAU) is a device that multiple workstations areconnected to in order to communicate on a Token Ring network. Sometimes anMSAU is referred to as a hub.

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Fibre Distributed Data InterfaceFibre Distributed Data Interface (FDDI) is a high-performance optical fibre LAN,running at 100 Mbit/s. It is used as a backbone to connect copperwire LANsand also for high-speed office networks.The main advantages of FDDI are:

It is faster than most Ethernet networks (100 Mbit/s) It has a greater maximum cable length than Ethernet (1000 metres) It can support more workstations than other network types (1000 per ring) It has good resistance to electromagnetic interference It has improved fault tolerance

Background As the application of LANs has grown, so the demands on them in terms ofdata throughput and reliability have also grown. There are more and moreapplications today that demand high bandwidth, in addition to the growing needfor transmission of documents that incorporate images and other types ofmedia which increases the demand on the capacity of the LAN.

Most early LAN installations comprised a single LAN segment. However, sincethe number of stations that can be attached to a single LAN segment and itsphysical length are limited, LAN installations comprising multiple linkedsegments have evolved. This trend has steadily continued, and today mostlarge LAN installations are of this type.Earlier types of LANs, such as Ethernet and Token Ring, operate at bit ratesranging from 1 to 16 Mbit/s. Two examples of newer and high speed LANswhich accommodate up-to-date demands are DQDB and FDDI.In addition to asynchronous data (which is generated at random time intervals),FDDI rings can also support the transmission of synchronous data (for

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example, digitized voice). The FDDI LAN standard is defined in ISO 9314.

FDDI Architecture (dual-ring)The FDDI LAN standard is defined in ISO 9314.FDDI physical parameters and architecture are:

Media: 1300nm, optical fibres Transmission Method: Baseband Data Rate: 100Mbit/s Topology: physical ring of trees, logical ring Maximum distance between adjacent stations: 2 km Total max. ring length: 100 km Max. number of attached stations: 1000

FDDI uses two counter rotating rings in the same way as a Token Ring LAN toenhance reliability: one is referred to as the primary ring and the other as thesecondary ring. The secondary ring can be used either as an additionaltransmission path or purely as a backup in the event of a break occurring in theprimary ring.

Single Attachment/Dual Attachment StationThere are 4 types of stations (DTEs or concentrators):

Dual attached station (DAS), which is connected to both rings. Single attached station (SAS), which is attached only to the primary ring. Dual attached concentrator (DAC), which is connected to both rings and

provides connection for additional stations and concentrators. It is actuallythe root of a tree

Single attached concentrator (SAC), which is connected only to theprimary ring (through a tree).

In practice, most user stations are attached to the ring via wiring concentrators,since then only a single pair of fibres is needed and the connection cost islower. The basic fibre is dual core with polarized duplex connectors at eachend. This means that each end of the cable has a different physical key so thatit can only be connected into a matching socket (to prevent faulty interchangingof wires which can cause a total break- down of the network). Special couplingunits are used to isolate (bypass) a station when its power is lost (either activeor passive fibre devices). Stations detecting a cable break will go into wrap

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mode (use the secondary ring as backup so both rings are connected to form asingle ring).

Physical InterfaceAs opposed to a basic Token Ring network, in which at any instant there is asingle active ring monitor which supplies the master clock for the ring, in FDDIthis approach is not suitable because of the high data rates. Instead, each ringinterface has its own local clock, and outgoing data is transmitted using thisclock.

All data to be transmitted is encoded prior to transmission using a 4 of 5 groupcode This means that for each 4 bits of data a corresponding 5 bit code word orsymbol is generated by the encoder. Some of these symbols (combinations)are used for link control functions (such as indicating the start and end of eachtransmitted frame or token). In general, the meaning and use of FDDI frame (ortoken) fields is the same as with the basic Token Ring, but because of the useof symbols rather than bits there are some differences in the structure of eachfield.

Ring OperationFDDI's ring operation is basically very similar to the Token Ring early releaseoperation.The main steps that take place in a normal frame transmission are as following:1. Sending station waits for token.

2. Sending station captures and strips token, and then transmits frames.

3. Sending station issues token at the end of transmission.

4. Destination station copies the transmitted frame and sets the A and C bits(address recognized and frame copied indicators).

5. Sending station removes the data from the ring by stripping the sent (andacknowledged) frame.

6. The first bytes of the frame are not stripped, and continue to circulate onthe ring (as a fragment). Each repeating station strips one byte from thefragment, and a transmitting station completely strips it.

Unlike the basic Token Ring, which is based on the use of priority andreservation bits, the priority operation of the FDDI ring uses a principle that isbased on a parameter known as the token rotation time (TRT).

An example of the importance of the use of good priority mechanism on FDDI isthe need to provide high priority to the transmission of synchronous data, sincelate arrival of such data would render it useless.

FDDI FrameFormatFDDI data frames are similar to 802.5 data frames. The start delimiter (SD) andend delimiter (ED) fields mark the frames limits. The frame control (FC) fieldhighlights what kind of frame it is be it data, control etc. The frame status (FS)byte holds the acknowledgement bits, very much the same as 802.5. The otherfields and their tasks are comparable to 802.5.

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FDDI handles regular asynchronous frames but it also allows specialsynchronous frames for circuit-switched PCM or ISDN data. The synchronousframes are produced every 125 ms by a master station to provide the 8000samples per second needed by PCM systems. Each of these frames has aheader, 16 bytes of non circuit-switched data and up to 96 bytes of circuitswitched data. Thus for example up to 96 PCM channels per frame.

The number 96 was chosen because it allows four T1 channels (4 x 24) at1.544 Mbit/s or three ITU-T E1 channels (3 x 32) at 2.048 Mbit/s to fit in a frameallowing it to be used all over the world. One single synchronous frame every125 ms consumes 6.144 Mbit/s of bandwidth for the 96 circuit-switchedchannels. A maximum of 16 synchronous frames every 125ms allows up to1536 PCM channels and uses up 98.3 Mbit/s.

When a station has acquired one or more timeslots in a synchronous frame,those slots are reserved for it until they are explicitly released. The totalbandwidth not utilised by the synchronous frames is allocated on demand. A bitmask in each of the frames is used to indicate which slots are available for thetask in hand. The non synchronous traffic is divided into priority classes, withthe higher priorities getting first shot at the remaining bandwidth.

The FDDI media access control (MAC) protocol uses the three following timers:

FDDI also has a priority algorithm similar to that of the 802.4. It determineswhich priority classes may transmit during a specific token pass. If the token isahead of schedule all the priorities may transmit, but if it is behind scheduleonly highest priorities can transmit.

Wireless LANA wireless local area network (LAN) is a flexible data communications systemimplemented as an extension to, or as an alternative for, a wired LAN. Usingradio frequency (RF) technology, wireless LANs transmit and receive data overthe air, minimizing the need for wired connections. Thus, wireless LANs

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combine data connectivity with user mobility.Wireless LANs have gained strong popularity in a number of vertical markets,including the health-care, retail, manufacturing, warehousing, and academia.These industries have profited from the productivity gains of using hand-heldterminals and notebook computers to transmit real-time information tocentralized hosts for processing. Today wireless LANs are becoming morewidely recognized as a general-purpose connectivity alternative for a broadrange of business customers.Wireless LANs can be deployed to transmit data, voice and video withinindividual buildings, across campuses, and over metropolitan areas. Some ofthe computer and communications industries' leading vendors are introducingpersonal digital assistants (PDAs), modems, wireless microprocessors andother devices and applications in support of wireless communications.

Advantages of Wireless LANsWith wireless LANs, users can access shared information without looking for aplace to plug in, and network managers can set up or augment networkswithout installing or moving wires. Wireless LANs offer the followingproductivity, convenience, and cost advantages over traditional wired networks:

Mobility - Wireless LAN systems can provide LAN users with access toreal-time information anywhere in their organization. This mobility supportsproductivity and service opportunities not possible with wired networks

Installation speed and simplicity: Installing a wireless LAN system can befast and easy and can eliminate the need to pull cable through walls andceilings

Installation flexibility - Wireless technology allows the network to go wherewire cannot go

Reduced cost-of-ownership: While the initial investment required forwireless LAN hardware can be higher than the cost of wired LANhardware, overall installation expenses and life-cycle costs can besignificantly lower. Long-term cost benefits are greatest in dynamicenvironments requiring frequent moves and changes.

Scalability - Wireless LAN systems can be configured in a variety oftopologies to meet the needs of specific applications and installations.Configurations are easily changed and range from peer-to-peer networkssuitable for a small number of users to full infrastructure networks ofthousands of users that enable roaming over a broad area

Types of Wireless LANsWireless LANs usually have two types of realisation:

Ad-hoc - Several mobile nodes (for example, notebook computers) mayget together in a small area (for example, in a conference room) andestablish peer-to-peer communications among themselves without thehelp of any infrastructure such as wired/wireless backbone. Since a smallcoverage area does not imply insured communication, there is no realreliability. Despite the possibility of ad-hoc networking, most applicationswill require communications with services located in a pre-existinginfrastructure

Infrastructured - Such an infrastructure is typically a higher-speed wired (orwireless) backbone. Therefore we can divide typical network traffic into twodirections: uplink (into the backbone) and downlink (from the backbone).The contact points to the backbone are called access points. The accesspoints can be either base stations for wired infrastuctures or wirelessbridges for wireless infrastructures. Repeaters may also be used for

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enlarging the coverage area of communication

Downlink TrafficDue to the limited bandwidth of wireless LANs, a common channel is typicallyused for communication between an access point and mobile nodes. Downlinkis achieved by broadcasting on this common channel. More precisely, theaccess point broadcasts packets to all mobile nodes even if there is only onedestination. Downlink activity may constitute up to 75 or 80 percent of the totaltraffic in wireless LANs because that nodes on modern LANs often operate in aclient-server mode. For instance there might be a high performanceworkstation or PC acting as a file server. A request for file transfer on the uplinkmay result in a huge file on the downlink.

Uplink Traffic The uplink protocol is the core task for the MAC design of wireless LANs. Torecognize and register new mobile nodes that join the network in any time andplace, a kind of random access protocol is needed.

Thus uplink traffic needs a multiple access protocol to organize thetransmissions from mobile nodes. In the next section ‘Expected features ofwireless LANs’ it is explained why multiple access is more difficult for wirelessLANs than for wired LANs.

Datacom modes, Circuit and Packet SwitchingIn data communications, data can be transmitted by a variety of methods.These methods include

Circuit switching

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Packet switching

Circuit SwitchingIn a circuit switched network, a dedicated communications path is establishedbetween two terminals through the nodes of the network. This path isestablished at the start of the call on request from the calling subscribersubscriber and is cleared down at the end of the call. Clear down is signalled byeither party. Charging for the call is based on duration and distance.

In the Public Switched Telephone Network (PSTN), which is the most commonexample of a circuit switching network, the nodes are the local, transit andinternational switches. The communications path is a connected sequence ofphysical links between these nodes. On each link a logical channel is dedicatedto the connection. Typically, in the PSTN, this is a 64 kbit/s time slot on a PulseCode Modulation (PCM) transmission system. Data generated by the source istransmitted along the dedicated path as rapidly as possible to the destination.

Packet SwitchingA different approach is used in a packet switching network. In this case it is notnecessary to dedicate transmission capacity along a path through the network.Rather, data is sent out in a sequence of small chunks, called packets. Eachpacket is passed through the network from node to node along some pathleading from the source to the destination. Packet switching networks arecommonly used for terminal- to-computer and computer-to-computercommunications.

Data Transmission ProtocolsA protocol is a specific set of rules, procedures or conventions relating to theformat and timing of data transmission between two devices. It is a standardprocedure that two data devices must accept and use to be able to understandeach other. The protocols for data communications cover such things asframing, error handling, transparency and line control.

The most important data link control protocol is High-level Data Link Control(HDLC). Not only is HDLC widely used, but it is the basis for many otherimportant data link control protocols, which use the same or similar formats andthe same mechanisms as employed in HDLC.

HDLC HDLC is a group of protocols or rules for transmitting data between networkpoints (sometimes called nodes).In HDLC, data is organized into a unit (called a frame) and sent across anetwork to a destination that verifies its successful arrival. The HDLC protocolalso manages the flow or pacing at which data is sent.

HDLC is one of the most commonly-used protocols in Layer 2 of the industrycommunication reference model of Open Systems Interconnection (OSI). Layer1 is the detailed physical level that involves actually generating and receivingthe electronic signals. Layer 3, the higher level, has knowledge about thenetwork, including access to router tables. These tables indicate where toforward or send data. To send data, programming in layer 3 creates a frame

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that usually contains source and destination network addresses. HDLC (layer2) encapsulates the layer 3 frame, adding data link control information to anew, larger frame.

Now an ISO standard, HDLC is based on IBM's Synchronous Data Link Control(SDLC) protocol, which is widely used by IBM's customer base in mainframecomputer environments. In HDLC, the protocol that is essentially SDLC isknown as Normal Response Mode (NRM). In Normal Response Mode, aprimary station (usually at the mainframe computer) sends data to secondarystations that may be local or may be at remote locations on dedicated leasedlines in what is called a multidrop or multipoint network. This is a non publicclosed network. Variations of HDLC are also used for the public networks thatuse the X.25 communications protocol and for Frame Relay, a protocol used inboth Local Area Networks (LAN) and Wide Area Networks (WAN), public andprivate.

In the X.25 version of HDLC, the data frame contains a packet. (An X.25network is one in which packets of data are moved to their destination alongroutes determined by network conditions, as perceived by routers, andreassembled in the right order at the ultimate destination.) The X.25 version ofHDLC uses peer-to-peer communication with both ends able to initiatecommunication on duplex links. This mode of HDLC is known as Link AccessProcedure Balanced (LAPB).

ModemsThe name modem comes from the terms modulator and demodulator. This isbasically the function of a modem to modulate and demodulate the signal.

The Data Terminating Equipment (DTE), for example personal computers,

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produce a digital signal, while the most widely available transmission medium,the PSTN, provides analogue access. To enable the transmission of digitalsignals across the PSTN we must modulate the digital signals to analogue formin the transmit direction, and demodulate analogue signals to digital in thereceive direction - the main function of the modem. The modem is oftenreferred to as the Data Communications Equipment (DCE).

Operation of ModemsModems are basically of two different types of operation:

Dial-up modems Leased-line modems

Dial-up Modems Dial-up modems can be either manual answer or auto-answer.Manual answer involves establishing voice communication and both partiesagreeing to manually switch the connection over from voice to modemoperation.

With auto answer modems the B-subscriber’s number can be dialled from thekeyboard. Once the receiving modem (B-subscriber) detects the ringing signal,both modems establish a data link using a suitable protocol. Auto-answermodems can be switched to either manual or automatic mode. Manual mode issimilar to manual answer.

Leased-Line ModemsThese modems are designed specifically for use on leased lines (a dedicatedlink end to end).

Standards The ITU-T standards dealing with data communications (including modems)over the telephone network are referred to as the V series recommendations.ITU-T uses the term “bis” to designate the second in a family of relatedstandards and “ter” to designate the third in a family.

Modulation This is the process of varying some characteristic of the electrical carrier signal(generated in the modem) as the information to be transmitted (digital signalfrom the DTE) on that carrier wave varies. The following modulation methodsare the most commonly used in modems:

Frequency shift keying (FSK) Differential phase shift keying (DPSK) Quadrature amplitude modulation (QAM) QAM/Trellis Coding Modulation (QAM/TCM) Pulse code modulation (PCM)

Modulation Chart:

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AT Commands and RegistersModems store their operating characteristics in non-volatile memory. Thesecharacteristics can be altered (by commands) to change the way the modemfunctions.

Commands are accepted by the modem while it is in Command Mode, which isit is normal state when idle. Commands may be sent to a modem from a PCrunning communication software or any other terminal device.All commands sent to the modem must begin with AT and are input by pressingthe “Enter” key. the AT prefix is known as the Attention Characters. Allcommands may be typed in upper or lower case, but not mixed.

Registers Modems contain registers. A typical modem would contain 29 registers (S0 -S28). These registers have the following functions:

To change operating characteristics To obtain information about the modem To test your modem

Using the “Sr = n” command you can change the value of the modem’sregisters.

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LAPM ProtocolLink access procedure for modems (LAPM) is a HDLC-based error-controlprotocol specified in the V.42 recommendation. LAPM uses the automaticrepeat request (ARR) method, whereby a request for retransmission of anfaulty data frame is automatically made by the receiving device.The principal characteristics of LAPM are:

Error detection through the use of a cyclic redundancy check. Error correction through the use of automatic retransmission of data. Synchronous transmission through the conversion of start-stop data. An initial handshake in start-stop format which minimizes disruption to the

DTEs. Interworking in the non-error-correcting mode with V-Series DCEs that

include asynchronous-to-synchronous conversion according toRecommendation V.14.

LAPM Frame FormatAll DCE-to-DCE communications are accomplished by transmitting frames.Frames are made up of a number of fields:Flag field All frames are delimited by the unique bit pattern "01111110", known as a flag.The flag preceding the address field is defined as the opening flag. The flagfollowing the frame check sequence field is defined as the closing flag. Theclosing flag of one frame may also serve as the opening flag of the next frame.

Transparency is maintained by the transmitters examining the frame contentbetween the opening and closing flags and inserting a "0" bit after allsequences of five contiguous "1" bits. The receiver examines the frame contentbetween the opening and closing flags and discards any "0" bit that directlyfollows five contiguous "1" bits.

Address field The primary purpose of the address field is to identify an error-correctedconnection and the error-correcting entity associated with it.Control field The control field is used to distinguish between different frame types.Information Field Depending on the frame type, an information field may also be present in the frame.

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Frame Check Sequence (FCS) field This field uses a Cyclic Redundancy Check (CRC) polynomial to guard against bit errors.

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Point-to-Point ProtocolThe Point-to-Point Protocol (PPP) is designed for simple links which transportpackets between two peers. These links provide full-duplex simultaneousoperation, and are assumed to deliver packets in order. PPP provides acommon solution for easy connection of a wide variety of hosts, bridges androuters.

PPP is a complete specification for transmitting datagrams between datacommunications equipment from different manufacturers over dial-up anddedicated serial point-to-point links. It is a recommended standard of theInternet Advisory Board (IAB) and is contained in a number of request forcomments (RFCs) produced by the PPP Protocol Working Group.

Traditionally, interoperability across serial links was restricted to equipmentsupplied by the same manufacturer. Now, PPP allows for multi-vendorinteroperability.

PPP was first proposed as a standard in 1990 to replace an older de factostandard known as Serial Line Internet Protocol (SLIP), which requires links tobe established and torn down manually. However, unlike SLIP which onlysupports IP, PPP is not limited in protocol support. PPP provides the flexibilityto add support for other protocols through software upgrades. PPP can alsosimultaneously transmit multiple protocols across a single serial link, limitingthe need to set up a separate link for each protocol. PPP is also ideal forinterconnecting dissimilar devices such as hosts, bridges, and routers overserial links. For example, a standalone TCP/IP host can communicate with arouter across a serial PPP link.

PPP ComponentsThe three main components of PPP are the:

Encapsulation scheme Link Control Protocol (LCP) Network Control Protocol (NCP)

These components are responsible for creating the frame, controlling the link,and managing the network layer protocol respectively.

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Encapsulation Standard encapsulation schemes exist for the transmission of datagrams overmost Local Area Networks (LANs) such as Ethernet, Token Ring, ARCnet, andFDDI. In the past, the only Wide Area Network (WAN) encapsulation schemethat provided a standard for the transmission of datagrams was X.25. Theintroduction of new WAN schemes, such as Frame Relay, expanded the varietyof encapsulation schemes available. However, the majority of LAN-to-LANtraffic is still carried over dedicated leased lines. The introduction of PPP allowsthese existing proprietary leased lines the opportunity to convert to a newencapsulation scheme that gives the user the true interoperability thattraditionally could only be found on LANs.

PPP is a full-duplex, bit-oriented protocol that can run over synchronous orasynchronous links. PPP uses a variant of HDLC as the basis forencapsulation. Links may be dedicated or circuit-switched, and PPP can workover copper, fiber optic, microwave, or satellite leased lines. PPP provides dataerror detection while higher layer protocols are responsible for error recovery.There are three parts to PPP encapsulation and these are:

Protocol field Information field Padding

Protocol Field The protocol field is one or two octets, and its value identifies the datagramencapsulated in the information field of the packet.

Information Field The information field is zero or more octets. The information field contains the datagram for the protocol specified in the protocol field.The maximum length of the information field, including padding is termed themaximum receive unit (MRU), which defaults to 1500 octets. By negotiation,consenting PPP implementations may use other values for the MRU.

Padding On transmission, the information field may be padded with an arbitrary numberof octets up to the MRU. It is the responsibility of each protocol to distinguishpadding octets from real information.

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LCP The second component of PPP is the LCP protocol, which operates at the datalink layer to manage communication functions.

In order to establish communications over a point-to-point link, each end of thePPP link must first send LCP packets. LCP, using a four phase process,establishes the link between two PPP peers and negotiates configurationoptions. Only phases one (link establishment and negotiation) and four (linkready) are necessary to establish communications. Phases two(authentication) and three (link quality determination) are optional andcompletely dependent on the PPP implementations at both ends of the link.

LCP ManagementFunctionsThe LCP management functions do the following:

Determine encapsulation format options Negotiate optimal packet size Terminate the link Authenticate the identity of the peer on the link (optional) Negotiate PPP multilink data compression (optional) Link quality monitoring (optional)

In addition to providing procedures for establishing, configuring, testing, andterminating data link connections, LCP also negotiates other non-default LCPoptions such as the MRU.

NCP NCPs are a series of independently-defined protocols that encapsulate networklayer protocols such as TCP/IP, DECnet, AppleTalk, IPX, XNS, and OSI. EachNCP has individual requirements for addressing and advertising connectivityfor its network layer protocol. Each NCP is defined in a separate RFC. Futureprotocols can be supported by defining new NCPs.

After the link has been established (LCP packets have been exchanged), PPPmust send NCP packets to choose and configure one or more network-layerprotocol. Once each of the chosen network-layer protocols has beenconfigured, datagrams from each can be sent.

The link will remain configured for communications until explicit LCP or NCPpackets close the link down, or until some external event occurs (an inactivitytimer expiry or network administrator intervention).

Link OperationIn the process of configuring, maintaining and terminating the point-to-pointlink, the PPP link goes through several distinct phases:

Link dead (physical layer not ready) Link establishment Authentication Network-layer protocol phase Link termination

Link Dead When an external event, such as carrier detection or network administratorconfiguration, indicates that the physical layer is ready to be used, PPP

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proceeds to the link establishment phase.

Link Establishment PhaseThe LCP is used to establish the connection through an exchange ofconfiguration packets:

Send Configure Request (SCR) Send Configure Ack (SCA) Send Configure Nak (SCN)

This exchange is complete, and the LCP opened state entered, once an SCApacket has been both sent and received.All configuration options are assumed to be at default values unless altered bythe configuration exchange.

Authentication PhaseOn some links it may be desirable to require a peer to authenticate itself beforeallowing network-layer protocol packets to be exchanged.By default authentication is not mandatory. If an implementation desires thatthe peer authenticate with some specific authentication protocol, then it mustrequest the use of the authentication protocol during the link establishmentphase.

Network Layer Protocol PhaseOnce PPP has finished the previous phases, each network-layer protocol, suchas IP, IPX or AppleTalk, must be separately configured by the appropriateNCP.

After an NCP has reached the opened state, PPP carries the correspondingnetwork layer packets. During this phase, link traffic consists of any possiblecombination of LCP, NCP and network-layer packets.

Link Termination PhasePPP can terminate the link at any time. This might happen because of loss ofcarrier, authentication failure or link-quality determination. LCP is used to closethe link through an exchange of terminate packets.

PAP Packet-level Procedure (PAP) is a protocol for the transfer of packets betweenan X.25 DTE and an X.25 DCE. PAP is a full-duplex protocol that supports datasequencing, flow control, accountability, and error detection and recovery.

CHAP Challenge Handshake Application Protocol (CHAP) is an authenticationmethod that can be used, for example when connecting to an Internet serviceprovider (ISP). CHAP allows you to log in to your provider automatically,without the need for a terminal screen. It is more secure than PAP.

CHAP is used to periodically verify the identity of the peer using a three-wayhandshake. This is done upon initial link establishment, and may be repeatedanytime after the link has been established.

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PPP Over ISDNSince the ISDN B-channel is by definition a point-to-point circuit link, PPP iswell suited for use over these links.PPP treats ISDN channels as bit- or octet-oriented synchronous links. Theselinks must be full-duplex, but may be either dedicated or circuit-switched. PPPpresents an octet interface to the physical layer. The octet stream is appliedprimarily at the R or T reference points.

Transmission RatePPP does not impose any restrictions regarding transmission rate.

PPP ConfigurationWith a multitude of encapsulation, LCP, and NCP options available for PPP,configuration of the link could be a major concern. However, configuration canbe simplified in bridges or routers by having a default configuration whichautomatically allows peers across a link to negotiate each option. Having sucha default configuration makes PPP one of the easiest data link protocols toconfigure. PPP eliminates manual, time-consuming configuration. It is virtuallyplug and play.

PPP operability TestingInter-Setting up a PPP connection between two peers involves four phases:

LCP negotiation LCP steady state NCP negotiation Network layer protocol data flow

When interconnecting equipment from different manufacturers, problems canoccur at any of these four stages. In general, the closer two peers get towardsthe fourth stage, the more compatible they are. However, even reaching thefourth stage doesn’t guarantee proper operation since basic differences infunctionality can prevent them from communicating.

XDSL33

Although digital transmission over existing copper pairs has been widely usedfor fax communication and Internet access, it falls far below the capabilities ofthe transmission medium because fax only uses the same frequencies asordinary voice communications, as do voice band modems used for Internetaccess. At best, it can achieve about 56 kbit/s, Proper digital communicationcan achieve 160 kbit/s (ISDN) over almost all copper pairs and can also bedone at lower cost than with voice band communication because greatersophistication is required by modems to operate high rates over the narrowbandwidth of a voice-band link.

Similar techniques of echo cancellation and equalization are used both formodems and for digital transmission, but the highest speed voice-bandmodems also use more sophisticated encoding and decoding to maximize thedata rate over voice channels.

Digital transmission over copper pairs was developed to support basic rateISDN operation of two B-channels at 64 Kbit/s each and one D-channel of 16Kbit/s plus the associated synchronization and additional overheads. Todayhigher bit rates are available for transmission over copper pairs. DSLtechnology provides symmetric and asymmetric bandwidth configurations tosupport both one way and two way high bandwidth requirements.

Configurations where equal bandwidth is provided in both directions, that isfrom the user to the service network and from the service network to the user,are termed symmetric. Configurations where there is a higher bandwidth in onedirection, normally from the service network to the user, that in the otherdirection are termed asymmetric.

The term XDSL covers a number of similar, but competing, forms of thistechnology known as Digital Subscriber Line (DSL). These are listed below:

Asymmetric Digital Subscriber Line (ADSL) Symmetrical Digital Subscriber Line (SDSL) High-bit-rate Digital Subscriber Line (HDSL) Rate adaptive Digital Subscriber Line (RADSL) Very-high-digital-rate Digital Subscriber Line (VDSL)

ADSL allows high-bit-rate transmission from the exchange to the customer (8Mbit/s downstream) and a lower-bit-rate transmission from the customer to theexchange (1 Mbit/s upstream). Asymmetric refers to the different bit-ratebetween downstream and upstream traffic. ADSL is targeted for providingbroadband services to residential users. The characteristic feature of ADSLbased services is that a lot of information is sent by the service provider to thecustomer (downstream) and a small amount of information is sent by thecustomer to the service provider (upstream). ADSL can be used over distancesup to 3 km. In Europe ~90% of all subscriber lines are shorter than 3 km.

ADSL Lite or g.liteADSL lite or g.lite is a sub-rated ADSL solution, with reduced digital signalprocessing requirements than full-rate ADSL systems. Under the name g.lite,the ADSL Lite has a downstream data rate of 1.5 Mbit/s or less. It has a similarreach to full rate ADSL systems. ADSL Lite is seen as the key to massdeployment of ADSL services, because the adaptation of this technology intocheap off the shelf modems is ideal for consumer use.There are three different modulating techniques presently used to support DSL.These are carrierless amplitude phase modulation (CAP), 2B1Q and discrete

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multi-tone modulation (DMT).

Carrierless Amplitude/Phase (CAP) ModulationCAP ADSL offers 7.5 Mbit/s downstream with only 1 Mbit/s upstream.Compared to DMT it is slightly inferior and DMT is now the official ANSI, ETSIand ITU-T standard for ADSL. One twisted copper pair supports POTS on the0-4 KHz range. CAP based DSL technology uses frequencies sufficientlyabove the POTS “voice band” to provide bandwidth for low-speed upstreamand high speed downstream channels.

2B1Q 2B1Q represents a straightforward signal type that has two bits per baudarranged as a quartenary or four level pulse amplitude modulation scheme. Itessentially transmits data at twice the frequency of the signal.

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Discrete Multitone ModulationThe previous figure shows the frequency ranges used for transmission overcopper lines. The lower part of the frequency range, up to 4 kHz, is used by thePlain Ordinary Telephony Service (POTS). ADSL technology enables the useof higher frequencies for broadband services.

ADSL is based on a technology known as Discrete Multitone Technology(DMT). DMT divides the frequency range between 0 MHz and 1.1 MHz in 288subchannels or tones, 32 subchannels for upstream traffic and 256subchannels for downstream traffic. The tone spacing is approximately 4.3kHz. The lower part (0-4 kHz) of the frequency range is used for POTS, andfrom 26 kHz up to 1.1 MHz there are 249 channels used for the ATM traffic.

Quadrature Amplitude Modulation (QAM) is used as the modulation scheme ineach of the subchannels. Therefore each subchannel can bear a moderateamount of data.

The video and data information is transmitted as ATM cells over the copperwires. POTS uses baseband signalling (Baseband on twisted pair wires.Baseband is a one-channel signal, a flow of ones and zeros). The telephony isseparated from the ATM cell flow at the customers premises and the DSLAM(Digital Subscriber Line Access Module) by means of a filter or splitter.

The ANSI (American National Standards Institute) standard specifies twooptions, one with Frequency Division Multiplexing where the lower subchannelsare used in the upstream direction and the rest is used in the downstreamdirection. The other option uses echo cancellation where the samesubchannels are used for upstream and downstream traffic. These are alsoreferred to as category I and category 2 modems, respectively. Category 1modems provide a bandwidth of 6 Mbit/s in the downstream direction and up to

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640 Kbit/s in the upstream direction.

Category 2 modems provide a bandwidth of up to 8 Mbit/s in the downstreamdirection and up to 1 Mbit/s in the upstream direction.

External disturbances and attenuation can impact on the transmission quality ofthe subchannels. One of the advantages with DMT is that it adjusts thebandwidth on each channel individually, according to a signal/noise ratio. Thisis termed Rate Adaptive. In other words, there will be a higher bit rate(bandwidth) where the noise (disturbances) is low, and a lower bit rate, ornone, where the noise is high. This adjustment occurs when the ADSL line isbeing initialised before it is taken into service.

Rate Adaptive ADSL (RADSL)RADSL is a simple extension of ADSL used to encompass and support a widevariety of data rates depending on the line’s transmission characteristics. Thisis advantageous in situations where the line is no longer or has a lower quality,and a lower data rate is acceptable. The DMT ADSL solution described abovehas the RADSL extension.

VDSL transmission can be used at the end of an optical fibre link for the finaldrop to the customer over a copper pair. In fibber-to-the-curb (FTTC) systems,the VDSL tail may be up to 500 meters long, and rates of 25 to 51 Mbit/s areproposed. In fibre-to-the-cabinet (FTTCab) systems, the tail may be over akilometer, and rates of 25 Mbit/s are being considered.

VDSL uses DMT, especially because of its adoption for ADSL by ANSI. As forADSL, the performance of the DMT for VDSL can be improved by bitinterleaving and forward error correction.

The spectrum for VDSL transmission extends to 10 MHz for practical systems,as compared to about 1 MHz for ADSL transmission. However, it starts at ahigher frequency of about 1 MHz, to reduce the interaction with othertransmission systems at lower frequencies and to simplify the filterspecification. Power levels for VDSL need to be lower than for ADSL becausecopper pairs radiate more at higher frequencies, generating greaterelectromagnetic interference.

High Speed Digital Subscriber Loop (HDSL)Digital transmission at 1.544 and 2.048 Mbit/s was developed beforetransmission at 144 kbit/s for basic rate ISDN. The digital subscriber linetransmission techniques subsequently developed for ISDN U-interfaces haveled to innovations in the transmission at higher rates because these higher ratetransmission systems have used older techniques. The application of the newapproaches ISDN transmission to higher transmission rates has led to thedevelopment of HDSL technology.

The initial emphasis on HDSL technology is reflected in the Europeandevelopments for 2.048-Mbit/s operation. The traditional systems for 1.544Mbit/s transmission used two copper pairs - a transmit pair and a receive pair.

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The corresponding HDSL systems also use two pairs, but use a dual duplexapproach at 784 kbit/s on each copper pair.When this technology is adapted to the 2.048 Mbit/s rate, three pairs areneeded (that is, a triple duplex approach). The alternative is to use dual duplexat a higher rate with a corresponding reduction in line length.

SDSL HDSL systems are now available that use 1 single copper pair at 2.048 Mbit/s.These are termed SDSL, Single Rate Digital Subscriber Line. Typicalapplications for this technology is providing T1 (1.5 Mbit/s) and E1 (2.048 Mbit/s) PCM lines.

The initial specifications for HDSL transmission in the United States called fordual duplex operation with a 2B1Q line code. The choice of line code is basedon speed of implementation, since 2B1Q is the U.S. standard for basic digitaltransmission. In Europe, 2B1Q was also agreed on.

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