Packet Switch) Frame Relay Interface Frame Relay Network Node Frame Relay Network Frame Relay Network Node Frame Relay Interface Figure 7.13 An example of a Frame Relay interface Chapter 7 Wide Area Networking Frame Relay User Data Frame Check Header Sequence Flag = 1 octet DLCI = data link connection identifier (address) Header = 2 octets C/R = command/response indication User data = 1-4,096 octets EA = extended address FCS = 2 octets FECN = forward explicit congestion notification BECN = backward explicit congestion notification DE = discard eligibility Figure 7.14 Frame Relay frame format (LAPD) tells the receiver, "I'm experiencing delays getting to you, SO anticipate those delays. Don't time-out the session." FECN is rarely used. BECN tells the transmit- ter, "Whoa! We've got delays ahead. Throttle back or slow down on your introduc- tion of data, or we'll end up losing those frames because of congestion." In other words, BECN indicates that all discard-eligible traffic is about to get dumped. You use the discard eligibility field to mark a frame as being either discard eligible or not and to control what occurs between voice and data in, for instance, a period of congestion. Frame Relay enables you to control the traffic flow a bit, and you can determine whether to drop a frame. But notice that there is no place in the frame for defining latency requirements or loss tolerances-the stricter QoS traffic mea- surements. Nonetheless, the switches will read the DLCIs to determine how to properly forward the frame. In a Frame Relay network, the customer environment includes the full com- plement of information resources that the customer wants to use on the network. Next, the CPE-which could be a router, bridge, FRAD, mux, or switch-contains the interface that formats packets into the Frame Relay frames. From the CPE, an access line (called the User-to-Network Interface [UNI]) connects to the Frame Relay provider switch. That UNI could be a leased line, such as 56Kbps/64Kbps or T-1/E-1, an ISDN line, or an analog