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6.4 THE V5.1 STANDARD 141Figure 6.4-1. V5.1 protocol stack.ISDN lines are present, and up to two if only PSTN lines are present. C-channels canbe assigned to TS 16, 15, and 31, in that order, and are called C-channel 1, 2, and 3,respectively. The Control C-path must be in TS 16. The other C-paths can be in anyC-channel. Time slots not assigned to C-channels can be provisioned for userinterfaces.6.4.1 V5.1 Data Link Layer (L2) ProtocolThe data link layer protocol used by V5.1 is called LAPV5 and is based on DSS1LAPD (Section 10.2). LAPV5 is subdivided into three sublayers: LAPV5-EF(Envelope Function sublayer), LAPV5-DL (Data Link sublayer), and AN-FR(Frame Relay sublayer). Figure 6.4-1 shows the relationships between the sublayers.LAPV5-EF Sublayer. This sublayer provides a common frame envelope forISDN and other types of messages. One of its main functions is to identify theISDN user port to which an ISDN message is related. For non-ISDN messages itidentifies the protocol type but not the individual user port, which is identified bythe L3addr IE of the Layer 3 protocols.The format of LAPV5-EF frames is shown in Fig. 6.4-2. The class of messagescontained in the Information fields is indicated by the values of the EFaddrFigure 6.4-2. Frame format for the Envelope Function Sublayer. (From Rec. G.964. Reproducedwith the kind permission of ITU.)