Voice Over IP

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. ...