8.7 Clear Channel Capability

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Support for the clear channel capability is another extremely desirable feature for T1 multiplexers. This option allows users to transmit DS0 or DS1 signals unconstrained by the general technical requirement that such signals not contain more than 15 consecutive zeros. This is achieved by implementing bipolar eight zero substitution (B8ZS). This line-coding technique uses bipolar violations to synchronize devices, a solution that does not require the use of extra bits, which means that a T1 circuit can use the full 64-Kbps channel for data, instead of only 56 Kbps. The scheme results in a gain of 192 Kbps of usable bandwidth on a full T1. The clear channel capability also applies to other types of T-carrier circuits, including T3.

This clear channel capability is required for ISDN, but as recently as year-end 2002, not all carriers supported it. The 64-Kbps clear channel capability is required for video applications over ISDN; carriers that provide only 56 Kbps cannot support video applications reliably.

To take advantage of the clear channel capability, the CPE must have the B8ZS capability and conform to ESF. For data applications involving frame relay or dedicated Internet access, the router or IAD will provide these features. Channel banks and multiplexers support B8ZS and ESF as well. For example, with the ESF capability integral to the multiplexer’s public network interface, bit-error rates can be picked up on a dynamic basis over the digital facility. On a static basis, the multiplexer’s transport management system comes into play by comparing error rates to predefined thresholds that are established by the network administrator when setting up the profile for each circuit.

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