Development Of Four New Features Of Router's Frame Relay Module

Frame relay is a high-performance, packet-switched protocol thatoperates at the physical and data link layers of the OSI referencemodel. Frame relay is mainly used in WAN and can support severalkinds of services (such as LAN connection and file transmission). Thispaper mainly describes the background, related protocols and thedesign of the four features (functions) in the network device's FrameRelay module. The four features are PPP in Frame Relay,MultilinkFrame Relay,IP Head Compression and Data Compression in FrameRelay.The first one is PPP in Frame Relay. Frame Relay as amulti-protocol switching network, can support the encapsulation ofother kinds of data link layer protocols. PPP is the main protocol ofdata link layer. In order to establish communications over apoint-to-point link, each end of the PPP link has to pass through linkEstablishment phase, optional Authentication phase and network-layerprotocol phase. The authentication phase is not mandatory, but cansupport authenticating and accounting, which is used here. WhenFrame Relay is configured as a point-to-point circuit,PPP can useFrame Relay as a framing mechanism, ignoring its other features. PPPtreats Frame Relay framing as a bit-synchronous link. RFC1973describes the encapsulation format of PPP packet.The second is Multilink Frame Relay. Usually a single WAN lineis used when WAN is designed. When the bandwidth of the line whichthe communications service providers support sometimes can't meetthe requirements and it's too expensive and difficult to upgrade thesingle line, multilink lines solution can be used. By combing multiplephysical interfaces into a Bundle,it can design a Frame Relay interfacethat has more bandwidth than is available from any single physicalinterface. Multilink Frame Relay provides a cost-effective solutionwhen not enough bandwidth a single link can support. When a linkfails,the Bundle continues to support the Frame Relay service bytransmitting across the remaining Bundle Links. FRF.16.1 describesMultilink Frame Relay. End of Bundle is a virtual net device interfacewhich should be designed. The virtual net device interface is treated asa normal net device interface,but only encapsulated by Frame Relayprotocol. And each end of Bundle Link needs to be negotiated with theother end of Bundle Link until joining into the Bundle.Data Compression reduces the size of data frames to betransmitted over a network link. Reducing the size of a frame reducesthe time required to transmit the frame across the network. DataCompression provides a coding scheme at each end of a transmissionlink that allows characters to be removed from the frames of data atthe sending side of the link and then replaced correctly at the receivingside. Because the condensed frames take up less bandwidth, we cantransmit greater volumes at a time. FRF.9 describes the encapsulationof the Data Compression protocol over Frame Relay,the DataCompression protocol format and Operation. It addresses DataCompression on both permanent virtual connections and switchedvirtual connections. The default compression algorithm is ANSIX3.241-1994 ( American National Standard Data CompressionMethod,Adaptive Coding with Sliding Window of InformationInterchange).The concept of IP Header here includes IP, TCP, UDP/RTPheaders and etc. Header Compression relies on many fields beingconstant or changing seldom in consecutive packets belonging to thesame packet stream. Fields that do not change between packets neednot be transmitted at all. Fields that change often with small and/orpredictable values, e.g., TCP sequence numbers, can be encodedincrementally so that the number of bits needed for these fieldsdecrease significantly. Only fields that change often and randomly, e.g.,checksums or authentication data, need to be transmitted in everyheader. In FRF.20,a Frame Relay IP Header Compression ControlProtocol(FRIHCP) is defined which negotiates the use of IP HeaderCompression in each direction of the virtual connection. FRIHCP usesa simple handshake (using configure-request and configure-ackpackets only) to enable the header compression algorithms andnegotiate their parameters. The algorithms are described in RFC 2507(IP Header Compression) and RFC 2508(Compressing IP/UDP/RTPHeaders for Low-Speed Serial Links).