Data Communications Networking Devices - 4th Ed.pdf

670 _________________________ WIDE AREA NETWORK DATA CONCENTRATION EQUIPMENTThe local end then sends the address by toggling its M lead, in effect, placingdialing pulses on that lead which is used by the remote end to effect the desiredconnection. Once a call is completed, either party will drop its M lead, resulting inthe other side responding by dropping its M lead. Currently, there are ®ve types ofE&M signaling: types I, II, III, IV and V. The major difference between E&Msignaling types relates to the method by which an on-hookcondition isestablishedÐground or open.Table 6.11 lists some of the more common types of voice interface cardssupported by many T- and E-carrier multiplexer vendors. The two-wire and fourwiretransmission only interfaces are designed to support permanent two-wire andfour-wire connections between two points that do not require the passing ofsignaling information. Both types of interfaces are normally used to support a datamodem connection through a T- or E-carrier multiplexer.Table 6.11 Common types of voice interface cards2-wire transmission only4-wire transmission only2-wire E&M4-wire E&M2-wire Foreign Exchange FX)4-wire Foreign Exchange FX)The two-wire and four-wire E&M interfaces usually support the connection ofPBXs and telephone company equipment to a T- or E-carrier multiplexer. Aspreviously mentioned, there are ®ve types of E&M signaling, with each typeapplicable to both two-wire and four-wire operations.The two-wire foreign exchange of®ce interface is designed to support theattachment of a T- or E-carrier multiplexer to a PBX or central of®ce switchingequipment that provides an open or closed foreign exchange termination point.Voice digitization supportIn addition to the method of bandwidth assignment and allocation, a third majorfeature affecting the ef®ciency of T- and E-carrier multiplexers is the type of voicedigitization modules the device supports. Although most T- and E-carrier multiplexerssupport the use of PCM and ADPCM, some vendors also support the use ofadapter cards that contain proprietary voice digitization modules. One example ofthis is adaptive speech interpolation which changes the digitization rate of selectedvoice channels from 32 kbps to 24 kbps as available bandwidth becomes saturated.Although some proprietary techniques may offer advantages in both the ®delity of areconstructed voice signal as well as in the bandwidth required to carry the signal,their use restricts an organization to one vendor's product.Although PCM and ADPCM are supported by most T1 and E1 multiplexers, thetype of ADPCM supported warrants consideration. While ITU standards govern