network protocols handbook.pdf

152Protocols GuideWAN - ATM ProtocolsProtocol NameATM Layer: AsynchronousTransfer Mode LayerProtocol DescriptionThe ATM layer, the layer 2 in the ATM reference model, providesan interface between the ATM adaption layer (AAL) andthe physical layer. This layer is responsible for relaying cellsfrom the AAL to the physical layer, such as SONET, for transmissionand from the physical layer to the AAL for use at the endsystems. It determines where the incoming cells should be forwardedto, resets the corresponding connection identifiers andforwards the cells to the next link, buffers incoming and outgoingcells and handles various traffic management functions, suchas cell loss priority marking, congestion indication and genericflow control access. It also monitors the transmission rate andconformance to the service contract (traffic policing).The fields in the ATM header define the functionality of the ATMlayer. The format of the header for ATM cells has two differentforms, one for use at the user-to-network interface (UNI) andthe other for use internal to the network, the network-to-nodeinterface (NNI).Protocol StructureUNI Structure:8 7 6 5 4 3 2 1GFCVPIVCIVPIVCIVCI PT (3 Bit) CLPHECInformation (48 Bytes)• GFC— generic flow control, used to limit the amountof data entering the network during periods of congestion.• VPI — Virtual Path Identifier.• VCI— Virtual Channel Identifier. The VPI and the virtualchannel identifier (VCI) together form the routingfield, also called VPCI, which associates eachcell with a particular channel or circuit. The VCI isa single-channel identifier. The VPI allows groupingof VCs with different VCIs and allows the group tobe switched together as an entity. However, the VPIsand VCIs have significance only on the local link; thecontents of the routing field will generally change asthe cell traverses from link to link. These fields, inUNI, can support up to 16 million desk (user) to networksessions.• PT —Payload Type.• CLP—Cell Loss Priority.• HEC—Header Error Control.NNI Structure:8 7 6 5 4 3 2 1VPIVPIVCIVCIVCI PT (3 Bit) CLPHECInformation (48 Bytes)• VPI — Virtual Path Identifier.• VCI— Virtual Channel Identifier. See above for VPCIdetail definition. These fields, which allows 268 millionsNNI sessions, represent the network-to-noderouting information within the ATM cell.• PT —Payload Type.• CLP—Cell Loss Priority.• HEC—Header Error Control.Related protocolsATM, SONET, AAL0-AAL5, LAN Emulation (LANE), CES, UNI,NNI and Q.2931Sponsor SourceThe ATM protocols is based on standards developed by theITU.http://www.atmforum.com/standards/approved.htmlATM Forum approved specificationsReferencehttp://www.atmforum.com/standards/approved.htmlATM Forum approved specificationshttp://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/atm.htmATM Overview