Data Communications Networking Devices - 4th Ed.pdf

4.11 CHANNEL SERVICE UNITS ____________________________________________________ 527a dc component to the signal and the ability to obtain clock recovery from the signalin an all-ones condition. Unfortunately, the disadvantage associated with thissignaling method is that a sequence of spaces is encoded as a period of zero voltageor no signal and repeaters on a span line cannot recover clocking without a signaloccurring every so often.For repeaters to properly recover clocking, a certain number of binary ones mustbe contained within a transmitted signal. The process used to ensure a minimumnumber of ones ¯ows on a T1 or E1 line is called zero suppression and is performedby CSUs. Since North American and European approaches to this problem differ,we will examine the encoding methods used to ensure an appropriate signalcontains a minimum number of binary ones for each location.North AmericaIn North America, AT&T publication 62411 sets the ones density requirement tobe `n' ones in each window of 8 X n ‡ 1) bits, where n varies from 1 to 23. Thismeans that a T1 carrier cannot have more than 15 consecutive zeros n = 1) andthere must be approximately three ones in every 24 consecutive bits n = 2 to 23).Two methods used to provide this minimum ones density include binary 7 zerocode suppression and binary 8 zero substitution.Binary 7 zero code suppressionUnder the binary 7 zero code suppression method a binary one is substituted in bitposition 7 of each time slot if all eight positions are zeros. An example of thismethod of ensuring a minimum ones density is illustrated at the top of Figure4.100a). Although it might appear wiser to select the least signi®cant bit forinversion, this cannot be done since the setting of a frame bit to zero when bitpositions 2 to 8 in the previous time slot were set to zero would result in a string of16 consecutive zeros if the bits in the time slot following the frame bit were zero andbit position 8 was used for substitution. Figure 4.100b) illustrates this worst-casescenario which explains why bit position 7 in each time slot is used for bit valueinversion to ensure a minimum ones density.If a data channel contains all 0s, the data can be corrupted due to B7 zerosuppression. Therefore a data channel normally is restricted to seven usable databits, with one bit in the data channel set to a 1. This prevents the data channel frombeing corrupted, but also limits its data rates to 56 kbps.When one bit is set to a 1 on a DS0 channel, the channel is known as a non-clearchannel. The 56 kbps on a non-clear channel is also known as a DS-A channel.A T1 clear channel is one in which all 64 kbps in each DS0 are usable. On privatemicrowave systems, B7 zero code suppression is normally not required, permittingclear channel capability.Binary 8 zero substitutionThe binary 8 zero substitution B8ZS) technique was developed by BellLaboratories and is now sanctioned by the ITU for use in North America. This