CCNA Cisco Certified Network Associate Study Guide - FTP Server

Frame Relay 489
Frame Relay provides a communications interface between DTE (Data
Terminal Equipment) and DCE (Data Circuit-Terminating Equipment, such
as packet switches) devices. DTE consists of terminals, PCs, routers, and
bridges—customer-owned end-node and internetworking devices. DCE consists
of carrier-owned internetworking devices.
Popular opinion maintains that Frame Relay is more efficient and faster
than X.25 because it assumes error checking will be done through higherlayer
protocols and application services.
Frame Relay provides connection-oriented, Data Link layer communication
via virtual circuits just as X.25 does. These virtual circuits are logical
connections created between two DTEs across a packet-switched network,
which is identified by a DLCI, or Data Link Connection Identifier. (We’ll get
to DLCIs in a bit.) Also, like X.25, Frame Relay uses both PVCs (Permanent
Virtual Circuits) and SVCs (Switched Virtual Circuits), although most
Frame Relay networks use only PVCs. This virtual circuit provides the complete
path to the destination network prior to the sending of the first frame.
Frame Relay Terminology
To understand the terminology used in Frame Relay networks, first you need
to know how the technology works. Figure 10.4 is labeled with the various
terms used to describe different parts of a Frame Relay network.
FIGURE 10.4 Frame Relay technology and terms
CSU/DSU Demarc Demarc CSU/DSU
Router
DLCI 16
Hub or
switch
Frame actually transverses this.
CO
PVC
Routers see this
Router
DLCI 17
Hub or
switch
User Server
Users only see this.