Data Communications Networking Devices - 4th Ed.pdf

154 ______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSFigure 1.88ISDN basic access channel formatBranch Exchange PABX). The ISDN basic access channel format is illustrated inFigure 1.88.As indicated in Figure 1.88,basic access consists of framing F),two bearer B)channels and a data D) channel that are multiplexed by time onto a commontwisted-pair wiring media. Each bearer channel can carry one pulse code modulationPCM) voice conversation or data at a transmission rate of 64 kbps.PCM is a voice digitization technique which results in a data rate of 64 kbpsbeing used to represent a voice conversation. Since there are two B channels thisenables basic access to provide the end-user with the capability to simultaneouslytransmit data and conduct a voice conversation on one telephone line or to be inconversation with one person and receive a second telephone call. In the case of thelatter situation,assuming the end-user has an appropriate telephone instrument,he or she could place one person on hold and answer the second call.The basic access frameThe actual manner by which basic access framing is accomplished is signi®cantlymore complex than previously illustrated in Figure 1.88. In actuality,a variety offraming,echo,activate and DC balancing bits are spread out within a 48-bit basicrate interface frame along with B and D channel bits that transport data. The 48-bit frame actually carries only 36 data bits,with the remaining 12 bits representingline overhead. Since the frame is repeated 4000 times per second this results in aline operating rate of 48 bits/frame 4000 frames/s,or 192 kbps,with the actualdata transfer rate becoming 36 bits/frame 4000 frames/s,or 144 kbps.There are two frame formats used for ISDN basic access. One format is usedwhen frames are transmitted from the network to ISDN terminal equipment.Since the frame ¯ows from the Network Termination NT) to the terminalequipment TE) it is referred to as a basic access NT frame. The second type ofISDN basic access frame ¯ows from the TE to the NT and,as you might expect,isreferred to as a basic access TE frame. Figure 1.89 illustrates the format of the twobasic-access frames.In examining Figure 1.89 note that both NT and TE frames commence with aframing bit for frame alignment or synchronization. In an NT frame the L bits areused to electrically balance the entire frame. In comparison,in the TE frame the Lbits are used to balance each octet of B channel information and each individual Dchannel bit. Through electrical balancing a situation where an excess number ofbinary zeros could occur that would prevent receiver synchronization with data isavoided. In addition,frame balancing limits DC voltage buildup and enablesdevices to communicate at greater distances from one another.The A bit in the NT frame is used to activate or deactivate terminal equipmentTE),enabling the TE to be placed in a low power consumption mode when there