ch. 4 - wan, wide area networksframe relay provides a fast packet switching capability that is used...

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Ch. 4 - WAN, Wide Area Networks

1 X.25 - access

2 X.25 - connection

3 X.25 - packet format

4 X.25 - pros and cons

5 Frame Relay

6 Frame Relay - access

7 Frame Relay - frame format

8 Frame Relay - addressing

9 Frame Relay - access rate and CIR

10 Frame Relay - pros and cons

11 ATM

12 ATM - Reference Model

13 ATM - Cell format

14 ATM - Services

15 ATM - Access

16 Classical IP and ARP over ATM

17 Multiprotocol over ATM (MPOA)

18 ATM - Standardization

19 ATM - LANE

20 ATM - Future

21 ATM - Pros and cons


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One of the most common WAN services is X.25. It is a packet switched network based onthe ITU standard X.25. The big difference between X.25 and leased lines is no dedicatedleased connection between the source and the destination exists in X.25.

In a packet switched network, the data is transmitted to the nearest packet switch in thenetwork. Then the packets go from one switch to another until they reach the destinationnode. In X.25, hosts with different bandwidths can talk to each other. Conversion of thebandwidth is integrated in the network. Maximum bandwidth used in an X.25 network is 64kbps per virtual channel. A virtual channel is logical connection which is established betweensource- and destination host before the data can be transferred.

The X.25 network has many functions for secure data transfer, which guarantee that thepackets arrive uncorrupted at the destination.Equipment can be connected to the X.25 network in many different ways. Equipment withsynchronous leased connections can deliver packets to the closest packet switch in thenetwork.

There is also one standard for dial-up synchronous service to an X.25 network and it iscalled X.32. X.32 is often used when you want a temporary connection to a node in X.25network.

In the ISDN standard there is also a specification on how to use the D-channel to connect toan X.25 network.


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Asynchronous terminals can not create the ordinary X.25 packets, so a PAD, which standsfor Packet Assembler Disassembler, must be used to convert the asynchronous data trafficto synchronous and then retransmit the data to the nearest packet switch. This standard iscalled X.28 and it is a dial-up service used to reach X.25 networks.


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When you establish a virtual channel through X.25 a call setup is sent to the network. Thispacket is called ”call request” and contains address information and other things.

The network routes this packet to the destination address, which receives it as an ”incomingcall”. The destination host will then answer this request and send a packet back with theinformation ”Call accepted” included. The network transports the packet back to the initialsource which receives the ”Call Connected” information. The logical channel is nowestablished and the data transmission can start. The normal packet length in X.25 network is128 bytes.


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X.25 is an interface between layer 3, the network layer, and layer 4, the transport layer, inthe OSI model. X.25 offers a network service to higher layers in the OSI model.

On layer 1, the physical layer, X.21 is used. On layer 2, the data link layer, LAPB, whichstands for Link Access Procedure Balanced, is used.

An X.25 packet is composed of a series of fields, as shown in the picture.The fields include data, addressing and control information.

Layer 3, X.25 fields make up an X.25 packet and include a header and user data.


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X.25 was pronounced dead when frame relay networks arrived in early 1990s. Despite suchproclamations, reality has shown X.25 is still going strong. The X.25 market just keeps ongrowing, satisfying customers with low cost and reliable service. The U.S. market for X.25services has continued to grow about 5 percent per year. There are three major factorsresponsible for this:

1. X.25 connection among private networks and LANs

2. Access to on-line services, like gambling and cash dispensing.

3. Dial-in LAN access.

Advantages of X.25 are:

1. X.25 is a worldwide service, offered by many providers.2. Equipment vendors support X.25 in their products.3. Different access methods, both synchronous and asynchronous, with various bandwidthsare supported.

Disadvantages of X.25 are:

1. X.25 is expensive for large traffic volumes.2. Due to large amount of control data the performance is low.


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Frame Relay was originally conceived as a protocol for use over ISDN. Initial standards weredone by ITU and ANSI.

Frame Relay provides a fast packet switching capability that is used between user devicessuch as hosts and routers and network equipment such as switching nodes. User devices areoften referred to as ”data terminal equipment”, while the network equipment is referred to as” data circuit-terminating equipment”.

The network providing the Frame Relay interface can be either a public network or aprivately owned network.

Seen from the user’s point of view, Frame Relay and X.25 are very similar. However, FrameRelay differs significantly from X.25 in its functionality and format. In particular, FrameRelay is a more streamlined protocol, facilitating higher performance and greater efficiency.


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Normally Frame Relay is used together with leased lines. Frame Relay as a dial-up service isseldom used.

The real advantage of using Frame Relay is that you have one physical connection but manylogical connections. If you had leased lines to several connections, without using FrameRelay, you would be forced to have a unique line for every connection.


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The frame used in Frame Relay is shown in this picture.

The flag fields delimit the beginning and end of the frame. After the first flag there are twobytes of address information. Ten bits of these two bytes make up the actual circuit ID,called the DLCI, which stands for Data Link Connection Identifier. After the address infothe data follows. The frame ends with check sum and the end flag. A frame can contain up to1600 bytes.


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The 10-bit DLCI value is the heart of the Frame Relay header. It identifies the logicalconnection that is multiplexed into the physical channel. In the basic mode of addressing,DLCIs have local significance; that is, the end devices at two different ends of a connectionmay use a different DLCI to refer to that same connection.If host A wants to communicate with host B it sends packets to the router at A with the IPaddress of host B. The router looks in its routing table and sees that this address belongs to aFrame Relay network and that the destination is to be reached through the Frame Relayaddress DLCI 204.The Frame Relay network is predefined by the network operator so that the DLCI 204 atthe source end matches DLCI 50 at the destination end.The packets from A reach the router at B and are delivered to the host B.


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Frame Relay is offered at access speeds of 56 kbps up to 2 Mbps. Compare this to typical56 kbps or 64 kbps normally used in X.25. The Frame Relay Forum is working on plans topush Frame Relay's maximum speed to 45 Mbps. Frame Relay services offer flexiblebandwidth over the public network. This flexibility is provided through two parameters:

CIR, which stands for Committed Information Rate AR, which stands Access Rate

The CIR establishes the minimum amount of bandwidth that will be available. If the publicnetwork gets overloaded, various mechanisms come into play to ensure the CIR is available.If, on the other hand, the network has free capacity you are able to use higher bandwidthsthan CIR.The actual bandwidth that is used in the physical connection is called access rate.


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Frame Relay services emerged in 1991. Since then almost every major long distance and localTelephone Company, as well as resellers and others, have started to offer frame Relay.Revenues have doubled every year according to some estimates. End users who wouldotherwise chose private networks are now buying frame Relay.

One of the questions often raised about Frame Relay is the competition with the ATMtechnology. Frame Relay is more mature than ATM and much cheaper. It will still take sometime before ATM is an alternative for the majority of users.

Within the next years ATM will become more available, and it may ultimately replace FrameRelay. However, the death of frame Relay will not come about rapidly.

The advantages of Frame Relay are

1. High bandwidth2. High capacity3. Handles much more transferred data than X.25 due to less ”overhead”4. One physical connection but many logical connections

The disadvantages of Frame Relay are

1. An error is detected later than in X.252. Not as fast as ATM.


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ATM which stands for Asynchronous Transfer Mode is a ”De facto standard” developedby the ATM Forum.

ATM is a method of communication, which can be used as the basis for both LAN andWAN technologies.

ATM specifications are being written to ensure that ATM smoothly integrates numerousexisting network technologies, at several levels, for instance, Frame Relay, Ethernet andTCP/IP.

ATM is a switched-based technology. By providing connectivity through a switch (insteadof a shared bus) several benefits are provided:

1. Dedicated bandwidth per connection.2. User defined quality of service.3. Flexible access speeds.

Equipment, Services and Applications for ATM are available today and are being used inlive networks.

In the picture two LANs are connected through an ATM network. This ATM networkcould be either a WAN or a LAN. It is more common to use ATM as a WAN servicebecause of high costs. The reason for using ATM in LAN is not high speed, because youcould get higher speeds with fast or Gigabit Ethernet. The real reason is when you want touse your LAN for different kinds of services, like voice, video and data.


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ATM is not only used for data communication but also for telecommunication.

ATM is available at various speeds. The most commonly used are 25, 155 and 622 Mbps.


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The physical layer provides for transmission of cells over a physical medium. This physicallayer consists of two sublayers: the PMD, Physical Medium Dependent sublayer, and theTC, Transmission Convergence sublayer. The PMD sublayer handles timing andsynchronization and TC handles error control and packs ATM cells into appropriate framessuitable for the physical medium.ATM layer is responsible for handling connections and also for cell redistribution.AAL, which stands for ATM Adaptation Layer, adapts the different classes of applicationsto the ATM layer. This is necessary in order for ATM to support many kinds of serviceswith different traffic characteristics, for instance continuous or bursty, and different systemrequirements like real time voice and video applications or normal data traffic.There are four types of AAL:

AAL-1: ATM Adaptation Layer Type 1 offers support for constant bit rate and time-dependent traffic such as voice and video.

AAL-2: ATM Adaptation Layer Type 2 is a placeholder for variable bit rate videotransmission, which means that it can be used by applications which do not have highrequirements for real time handling.

AAL-3/4: ATM Adaptation Layer Type 3/4 is used for variable bit rate, delay-tolerant datatraffic requiring some sequencing and/or error detection support. AAL-3/4 was originallytwo AAL types, connection-oriented and connectionless, which have been combined.

AAL-5: ATM Adaptation Layer Type 5 is used for ordinary LAN traffic having variable bitrate, delay-tolerant connection-oriented data traffic requiring minimal sequencing or errordetection support.


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Above ATM Adaptation Layer , there are higher layers in form of different networkprotocols such as IP and IPX.


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Information to be sent using ATM, is segmented into fixed length cells. These cells are thentransported to and re-assembled at the destination. The ATM cell has a fixed length of 53bytes. The length being fixed allows the information to be transported in a predictablemanner. This predictability accommodates different traffic types on the same network. Sincethe switches know that the cells are always the same lengths they don’t need to count thenumber of bytes or look at the length information field in the header data. This makes ATMvery fast.

The cell is broken into two main sections, the Header and the Payload. The Payload, whichis 48 bytes, is the portion, which carries the actual information. The Header, which is 5bytes, carries the addressing information.


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There are three commonly used ATM services.

1. LAN Emulation, used for emulation of standard LANs like Ethernet and Token Ring.

2. Classical IP and ARP over ATM, used for handling TCP/IP traffic over ATM networks.

3. Multiprotocol over ATM, is a mechanism for handling ISO layer 3 protocols transparanetover ATM.


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When information needs to be sent, the sender negotiates a "requested path" with thenetwork for a connection to the destination. When setting up this connection, the senderspecifies the type, speed and other attributes of the call, which determine the end-to-endquality of service. An analogy for this negotiation of qualities would be similar todetermining a method of delivery using US mail. One can choose to send 1st class, overnightor budget delivery.


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Classical IP and ARP over ATM, is a way of making the TCP/IP traffic run over ATM inthe most efficient manner possible. ATM IP and ARP over ATM service is used by thehost and the router in the picture. Both the host and the router must usually be equiped withan ATM network interface card.


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The ATM Forum's Multiprotocol over ATM (MPOA) specification is the industry's firststandard based solution that allows transport of ”Layer 3” protocols transparent over ATMnetworks. At the same time MPOA takes the advantage of the specific benefits that ATMoffers, like lower latency, high performance and quality of service.


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ATM protocols are organized by the ATM Forum. After agreement by members of theATM Forum, standards are presented to the International Telecommunications Union(ITU).The ATM Forum is an international non-profit organization formed with the objective ofaccelerating the use of ATM products and services. In addition, the Forum promotesindustry cooperation and awareness.The ATM Forum consists of a worldwide Technical Committee, three MarketingCommittees for North America, Europe and Asia-Pacific as well as the User Committee,through which ATM end-users participate.There are also RFC standards from the Internet Architecture Board, IAB which describehow to use ATM together with TCP/IP.Examples of ATM standard RFC’s are shown in the picture:


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If you want to connect two LANs with each other through an ATM network there are twopossibilities. The first one is to adapt the communication software on LANs for ATM. Thesecond is to adapt the ATM network so you don’t have to change anything on the LANs.This is called LANE, which stands for LAN Emulation. The LANE protocol creates anemulated LAN by making an ATM network look and behave like an Ethernet or Token Ring.

LANE works as a bridging protocol at layer 2 of the OSI model. It doesn’t use CSMA/CDor token passing for media access control. Instead it uses its own solution, but seen from theLAN side the LANE solution is totally transparent.


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What will the future hold for ATM?

1. ATM will provide a single network for all types of traffic like voice, data, video and soon. This will give improved efficiency and manageability since you only have to manage onenetwork instead of several.

2. Because ATM is not based on a specific type of physical transport, it is compatible withcurrently deployed physical networks. ATM can be carried by twisted pair, coax and fiberoptics.

3. The bandwidths used by ATM are not so important to LANs because there are muchcheaper alternatives. But ATM will have a big impact on WANs, because it is still veryexpensive to get high bandwidths with alternative WAN technologies.

4. One important part of ATM is that the user can define and pay for quality of service,which means that he pays more for traffic with higher priority.

5. ATM has been designed from the onset to be scalable and flexible in: geographic distance,number of users and bandwidths. As of today, the speeds range from megabits to gigabits.This flexibility and scalability assures that ATM will be around for a long time.


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Everybody is talking about ATM today. The most unique aspect of ATM is the possibilityfor users to define and buy priority for their traffic. ATM treats every kind of service as 53-byte data to be delivered. This means that you have one network for everything. ATM willdramatically increase the bandwidths for WANs. Since every cell has the same length theswitches can handle them with high speed. Another advantage of ATM is that it works bothon LANs and WANs.

What are the disadvantages of ATM? Primarily the price, ATM products are very expensiveand the market is waiting for more standards. While the vendors wait for standards theydevelop their own solutions, which gives poor interoperability between different products.

ch. 4 - wan, wide area networksframe relay provides a fast packet switching capability that is used...

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