network provides can be speeded up by increasing data rate, eliminating error-recovery procedures, and reducing processing time. One source states that a frame relay frame takes some 20 msec to reach the distant end (statistically), where an X.25 packet of similar size takes in excess of 200 msec on terrestrial circuits inside CONUS (CONUS stands for contiguous United States). Another advantage of frame relay over a conventional static TDM connection is that it uses virtual connections. Data traffic is often bursty and normally would require much 3 If bandwidth permits. We cannot increase bandwidth as the popular press insinuates. We can make better use of bandwidth. With data transmission, particularly as the bit rate increases, other bandwidth constraints, besides amplitude response, may become the limiting factor on bit rate. Typically this may be group delay (envelope delay distortion), bandwidth coherence. 12.5 SPEEDING UP THE NETWORK: FRAME RELAY Router Frame Frame Router Frame Frame Frame Frame FRS = frame relay switch Router Figure 12.7 A typical frame relay network. larger bandwidths to support the short data messages and much of the time that bandwidth would remain idle. Virtual connections of frame relay only use the required bandwidth for the period of the burst or usage. This is one reason why frame relay is used SO widely to interconnect LANs over a wide area network (WAN). Figure 12.7 shows a typical frame relay network. 12.5.2 The Genesis of Frame Relay Frame relay extends only through the data link layer (i.e., OSI layer 2). It has derived its data-link layer protocol from the ISDN D-channel LAPD. 4 We discussed LAPD in Section 12.4.8. Frame relay's importance has taken on such a magnitude (it was developed in North America) that the ITU-T organization formulated Rec. I.122, Framework for Frame Mode Bearer Services (Ref. 13) and I.233, Frame Mode Bearer Services (Ref. 14). Even the term LAPD, although modified in many cases for frame relay application, continues to be used. Frame relay has become