AT&T DS1/DMi/ISDN-PRI manual 10ADMINISTRATION Options and Requirements System

7-10ADMINISTRATION OPTIONS AND REQUIREMENTS — SYSTEM 85

Field 9 V1-V4

Determines which line-coding format will be used to forcibly ensure that the data meets T1-carrier ones-density requirement. The two choices are:

●Zero code suppression (ZCS)

●Bipolar 8 zero code suppression (B8ZS)

For a 56K-bps call over a robbed-bit facility, use the ACCUNET switched digital service.

The ZCS line-coding format scans each byte for the all-zeroes octet and on detecting this bit sequence, substitutes a one for the zero in bit position two. The ZCS format is the most common and it is used widely. The ZCS format can be used (without any consideration of the communications protocol) to transmit seven-bit characters/data at rates up through 56K-bps via modems and multiplexers that do bit stuffing. If the data communications protocol is based on the high-level data link control (HDLC) protocol and data is transmitted as inverted HDLC, then the ZCS option can be used for data rates up through 64K-bps.

System 85 and System 75 data modules use the DCP protocol. The DCP protocol is based on the DMI specification (which uses HDLC as a building platform) and therefore meet these conditions.

The DS1/T1 facilities that use RBS and are optioned for ZCS maintain the ones- density requirement by converting any all-zeroes octet to a string of seven zeros and a one. This does not significantly affect voice and voice-band data since it is in analog (pulse-code modulation of PCM) form. For DS1/T1 facilities that use 24th- channel signaling and are optioned for ZCS, the data communications protocol/communications equipment used must prevent the all-zeroes octet from occurring; otherwise, the ZCS method will forcibly alter the data (causing errors) to guarantee proper ones-density.

The B8ZS line coding format substitutes a unique code (bipolar violation code) for any eight consecutive zeros. This bipolar violation code is detected at the receiving end and converted back to the original string of eight zeros. The B8ZS encoding method permits data transmission at rates up through 64K-bps without consideration of the clear channel transmission protocol.

Several different types of network digital facilities may be linked together to complete the end-to-end connection. Typically, they will be multiplex-derived facilities. Usually, the multiplexer will contain a bipolar violation monitor and removal circuit that corrects all bipolar violations (alters B8ZS data) and also produces an all 1s if a loss of input signal occurs. This bipolar violation monitor and removal feature is currently an inherent part of the MX3, M13, MX2, and M1C multiplexers, as well as most vendors multiplexers.