Data Communications Networking Devices - 4th Ed.pdf

160 ______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSTE2TE2 type equipment are devices with non-ISDN interfaces,such as RS-232 orthe ITU X or V-series interfaces. This type of equipment must be connectedthrough a TA terminal adapter) functional grouping,which in effect converts anon-ISDN interface R) into an ISDN Sending interface S),performing both aphysical interface conversion and protocol conversion to permit a TE2 terminal tooperate on ISDN.Terminal adaptersDue to the large base of non-ISDN equipment currently in operation,the terminaladapter can be expected to play an important role as the use of this digital networkexpands. The terminal adapter TA) performs a series of functions to convert non-ISDN equipment for use on the ISDN network. First,it must adapt the data rateof the non-ISDN device to either a 64 kbps B channel or a 16 kbps D channeloperating rate. Next,it must perform the conversion of data from the non-ISDNdevice to a format acceptable to ISDN. For example,a non-ISDN device,such asan intelligent modem,might have its AT commands converted into ISDN D-channel signaling information. Other functions performed by TAs include theconversion of electrical,mechanical,functional,and procedural characteristics ofnon-ISDN equipment interfaces to those required by ISDN and the mapping ofnetwork layer data to enable a signaling terminal to be `understood' by ISDNequipment.Since a basic access channel operates at a multiple of most non-ISDN equipmentrates,most terminal adapters include a multiple number of R interface ports. Thisallows,for example,an asynchronous modem connected to a personal computer,afacsimile machine,and a telephone to be connected to a basic access line via the Rinterface. In fact,most commercially available terminal adapters have three or fourR interface ports.Rate adaptionRate adaption is the process during which the data rate of slow-speed devices isincreased to the 64 kbps synchronous data rate of an ISDN B channel. During therate adaption process,the data stream produced by a non-ISDN device is paddedwith dummy bits by the terminal adapter and clocked at a 64 kbps data rate.In 1984,the CCITT approved its rate adaption standard known as the V.110recommendation. Originally,this recommendation was strictly for synchronouslyoperated devices and was modi®ed in 1988 to support asynchronous devices. Theframing speci®ed by the V.110 recommendation is complex,with each 80-bit framecontaining a 17-bit frame alignment pattern,while the actual rate adaption processcan involve between one and three steps,with the actual number of steps dependentupon the operating rate of the terminal and its operating mode. Asynchronousdevices require a three-stage process,while synchronous terminals operating below64 kbps require a two-stage process as indicated in Figure 1.92.