31 Days Before Your CCNA Exam

338 31 Days Before Your CCNA ExamFrame Relay ComponentsFrame Relay provides a means for statistically multiplexing many logical data conversations, referredto as virtual circuits (VC), over a single physical transmission link by assigning connection identifiersto each pair of DTE devices. The service provider switching equipment constructs a switching tablethat maps the connection identifier to outbound ports. When a frame is received, the switching deviceanalyzes the connection identifier and delivers the frame to the associated outbound port. The completepath to the destination is established prior to the transmission of the first frame. Figure 2-2 illustratesa Frame Relay connection and identifies the many components within Frame Relay.Figure 2-2Frame Relay ComponentsPVCRouter ADLCI: 100LMI100 = Active400 = ActiveRouter BDLCI: 200DLCI: 400Local AccessLoop = T1Local AccessLoop = 64 KbpsPVCDLCI: 500Local AccessLoop = 64 KbpsRouter CThe following terms are used frequently in Frame Relay discussions:■■■■■Local access rate: The rate at which data travels into or out of the network, regardless ofother settings.Virtual circuit (VC): Logical circuit, uniquely identified by a data-link connection identifier(DLCI), that is created to ensure bidirectional communication from one DTE device to another.Permanent virtual circuit (PVC): Provides permanently established connections that areused for frequent and consistent data transfers.Switched virtual circuit (SVC): Provides temporary connections that are used in situationsthat require only sporadic data transfer between DTE devices across the Frame Relay network.Data-link connection identifier (DLCI): Contains a 10-bit number in the address field of theFrame Relay frame header that identifies the VC. DLCIs have local significance because theidentifier references the point between the local router and the local Frame Relay switch towhich the DLCI is connected. Therefore, devices at opposite ends of a connection can usedifferent DLCI values to refer to the same virtual connection.