AT&T DS1/DMi/ISDN-PRI Bipolar Violations, Communication Protocols and 1s-Density Requirement

1-20INTRODUCTION

Bipolar Violations

As noted earlier, the DS1 bit stream is transmitted as a series of pulses. Successive pulses, regardless of the number of intervening spaces (0s), are of opposite polarity. A bipolar violation is the occurrence of two consecutive identical pulses, that is, when two positive or two negative pulses are received in a row, regardless of the number of intervening 0s.

Usually, bipolar violations are caused by noise hits on the DS1 bit stream. For B8ZS, strings of eight 0s are encoded into special sequences that include bipolar violations. Some network-interface equipment, primarily most network channel-terminating equipment, or NCTE (also called customer- service units or CSUs), and network transmission equipment (network high-speed multiplexer), will remove bipolar violations. Therefore, if an application requires B8ZS line coding, then the end-to- end transmission facilities must support B8ZS. Otherwise, the B8ZS encoding will be destroyed. Additional NCTE information is provided in chapter 3, DS1 Transmission and Cabling, and chapter 7, Administration Issues, Options, and Requirements.

A Generic 2 DS1 interface does not process bipolar violations, because they are removed by most NCTEs.

Communication Protocols and 1s-Density Requirement

As mentioned earlier, there are other methods (communication protocols) used to prevent strings of 0s from occurring in the DS1 bit stream. One such protocol is used with System 75 and System 85 as described next.

System 75 and System 85 digital ports interface to data modules. These data modules encode user data consistent with the DMI specification. The DCP is specifically designed to prevent generation of the all-0s octet when using DMI modes 1-3 and, therefore, either the ZCS or B8ZS line-coding formats may be used. But since the ZCS format has no special equipment requirements, ZCS is the preferred format.

Table 1-2,Data-Module Capabilities, summarizes the capabilities of the data modules that can be used on Generic 2 for communications over an ISDN-PRI link. For further details on each data module, refer to About This Document for a list of related data module documents. For complete definitions of the four DMI modes (0 through 3), refer to Digital Multiplexed Interface (DMI) Technical Specification, Issue 3.2, November 1989 (555-025-204). Ask for the most recent version.