Frame Relay Encapsulation and Framing
A Frame Relay-connected router encapsulates each Layer 3 packet inside a Frame Relay header and trailer before it is sent out an access link. The header and trailer are defined by Frame Relay (or more specifically, the Link Access Procedure Frame Bearer Services [LAPF] specification, ITU Q.922-A). The sparse LAPF framing provides error detection with an FCS in the trailer, a DLCI field (discussed in detail later in this chapter), plus a few other header fields. Figure 13-5 diagrams the frame.
Figure 13-5 LAPF Framing
However, routers actually use a longer header than just the standard LAPF header because ...
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