Frame Relay - for Faster and More Efficient Data Communications ...

Fig. 9A typical Frame Relay network with DTEs, networknodes and public Frame Relay servicesFig. 10Frame Relay networks use permanent virtualconnections, which means that a physical connectioncan provide several logical channels forsimultaneous usedata transmission protocol focusing onhigh speed and requiring a minimum oferror-correcting and flow-controlling functions.The standard, which is described in bothCCITT and ANSI recommendations, definessignal and data transmission at linklevel, OSI level 2, in the interface betweenuser equipment and network, Fig. 8. (Cf.X.25, which comprises the first three OSIlevels and uses a frame at level 2 for thetransmission of a packet at level 3.) Thestandardisation work is briefly described inBox 1.How Frame Relay worksA Frame Relay network is made up of networknodes and user equipment, DTE.s(Date Terminal Equipment) connected tothe network. The DTE, e.g., a personalcomputer, a gateway, a router or a hostcomputer, is provided with the interfacedefined for Frame Relay, Fig. 9.The sending DTE transmits frames to thenetwork. Each of these frames contains anidentification code (Data Link ConnectionIdentifier, DLCI). All network nodes alongthe path to the final destination contain informationindicating the outgoing channelto which a frame with a specific identificationcode is to be sent. The path betweenthe sending and receiving DTEs has beenpredefined by the network operator. Thistype of connection - a Permanent VirtualConnection - is so far the only one definedin the standard. Virtual connections are notincluded in today's Frame Relay function,but future versions of the protocol are expectedto allow such connections too.The network node routes to the right destinationsthe frames sent from a DTE. Thenetwork node reads the identification codeof the incoming frame and sends the frame(without changing it) on the outgoing channelindicated in the node's routing table.This outgoing channel can either be a connectionto another network node (in whichcase the procedure described above is repeated)or a connection directly to the terminatingDTE. However, the way theframes are handled internally in the networkis not defined in the standard.As in X.25 switching, the use of severalidentification codes permits several parallelsessions in different directions to coexiston one physical connection, Fig. 10.In this way, a DTE can communicate simultaneouslywith different destinationsover the same physical connection to thenetwork. This is necessary if the DTE is acommunications port in an LAN, but it is alsoan attractive solution in cases where theDTE is a personal computer or workstationthat uses several simultaneously activewindows.Handling is simple because the protocoldoes not include any error-correctingmechanism. Speeds of several Mbit/s canbe used without requiring unreasonableprocessing capacity for link handling.Today's version of the standard stipulatesa maximum link transmission speed ofERICSSON REVIEW No. 1-2, 1992