V.120

V.110 is the main subject for this part of the chapter, but a few words on V.120 are in order for those readers using ISDN interfaces. In recognition that the V Series will be in existence for a considerable period and in view of the need for ISDN devices (terminal equipment type 1 or TE1) to interwork with non-ISDN devices (TE2), the ITU-T has published the V.120 recommendation. V.120 supports an ISDN interface with a DTE and its associated physical layer interface. The DTE must operate with the V Series interfaces. V.120 also supports the multiplexing of multiple user data links onto the ISDN S/T interface. V.120 uses a link-level protocol based on the modification of Link Access Procedure for the D Channel, LAPD, published as Q.921. LAPD is similar to LAPM and LAPB, discussed earlier in this chapter.

V.120 describes the use of a TA for the ISDN-to-V Series DTE interworking. However, this TA performs more functions than the TA we examined with V.110. It must perform the following services:

• Electrical and mechanical interfaces conversions

• Adaptation of bit transfer rate (as in V.110)

• End-to-end synchronization of traffic

• Call management between the two end-users

Three modes of operations are supported with the V.120 terminal adapter: (1) asynchronous, (2) synchronous, and (3) transparent. The reader is probably aware that asynchronous mode terminals (async TE2s) use start/stop bits and parity checks. The TA accepts the asynchronous stream from the user device and removes the start/stop bits. As an option, parity may be checked by the TA. In either case, the user data characters are placed in a frame for transmission to a peer entity. The peer entity is another TA or a TE1.