T1: A Survival Guide

The Extended Superframe (ESF)

SF-framed links have two notable drawbacks. First, a yellow alarm is transmitted by setting the second bit to zero in all of the time slots in a frame. When the yellow alarm is present, no data is received. Unfortunately, setting the second bit position to zero is something that can happen frequently in user data. Altering the bits is not acceptable with data transmission, so the only solution is to use one time slot for yellow-alarm prevention and to set the second bit to one. Although this prevents a false yellow alarm, it sacrifices one DS0 worth of bandwidth. Secondly, the error-detection mechanism with SF links is quite limited. Bipolar violations are a line error check, which means that they can flag potential problems in the local copper portion of the T1 span. Errors may be introduced anywhere along the span, however. Any corruption introduced at the central office, or in the high-speed optical components, cannot be detected by T1 equipment and must be detected by higher-layer protocols. What is needed is a path error check, which verifies data integrity across its entire path from one end to another, no matter what type of transport is used.

In response to these limitations, AT&T developed the extended superframe (ESF), which was introduced on the D5 channel bank in 1982. Advances in electronics made it possible to use a smaller proportion of the frame bit sequence for synchronization and devote it to solving the problems of the SF ...