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 framing format. Figure 4-3 shows the ESF superframe. As with the SF superframe, it begins with frames, each of which is made up of 24 8-bit time slots with a single frame bit at the beginning. The 24 frames are put together into a single superframe.

Figure 4-3. The ESF superframe

Of the 24 framing bits, only six are needed for synchronization. Every fourth frame contributes a bit to the synchronization pattern, 001011. CSU/DSUs can easily identify the synchronization pattern because it cannot shift onto itself. Synchronization requires 2 kbps from the aggregate T1 capacity, which is only half of the bandwidth required by SF framing.

To provide a path error check, six bits in the frame bit sequence are used for a cyclic redundancy check (CRC). Like the frame-synchronization pattern, the CRC requires another 2 kbps. The CRC is a value that is calculated by using the data payload of the superframe together with the frame bit sequence as an input. After sending a superframe, the sender calculates the CRC and places that value in the six bits in the framing bit sequence of the next superframe. Upon reception, the receiver calculates the CRC and compares it to the CRC received in the subsequent superframe. A difference indicates potential corruption somewhere in the previous superframe.

ESF Signaling

Making the T1 transparent to arbitrary user data required changing the signaling method from altering bits of user data to using a separate signaling channel. The remaining 12 bits of the framing bit sequence are used to create a 4 kbps channel called the facilities data link (FDL). Alarms and performance data are reported over the FDL. Telcos commonly use it to collect circuit performance data. The performance report message format is fully specified in T1.403. The FDL is not limited to performance monitoring, however. It may be used as a DSU-to-DSU communication channel, or adopted for proprietary purposes.

Code words: ESF alarms and instructions

Urgent messages, such as alarm conditions, are reported over the FDL by sending code words. Carriers may also use the FDL to send commands to the customer’s CSU/DSU to activate loopbacks.^[10] When idle, the pattern 01111110 is sent continously. Table 4-3 shows common ESF code words along with their message types. Priority messages override all other code words and LAPD messages.

Table 4-3. ESF code words

Code word	Type	Description
01111110	N/A	Idle code transmitted when no code word or LAPD message is present; also used as LAPD demarcation flag
11111111 00000000	Priority	Yellow alarm/RAI
11111111 01010100	Priority	Loopback retention
11111111 01110000	Command	Line loopback activation
11111111 00011100	Command	Line loopback deactivation
11111111 00101000	Command	Payload loopback activation
11111111 01001100	Command	Payload loopback deactivation
11111111 00100100	Command	Universal loopback deactivation

Alarm conditions are transmitted for at least one second and until the condition causing the alarm is repaired. A 1-second interval is required between successive alarm signals.

Command/response code words are transmitted ten times. When the carrier initiates a line loopback condition for maintenance and testing purposes, the FDL transmits the loopback retention signal to avoid sending any loopback control codes or performance reports back to the carrier.

Other FDL code words can be used to trigger protection switching, which is a controlled switchover to backup transmission facilities that occurs when the primary path fails. FDL code words may also indicate how accurate the clock is on one end of a span.

ESF performance reporting

Non-alarm messages are transmitted using a message-based protocol that resembles the Link Access Procedure, D Channel (LAPD), an ITU protocol originally designed for ISDN signaling messages. Use of the FDL for message-based performance reporting is described in detail in Appendix C.

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^[10] Bellcore specified an unused command (00010010 11111111) to put the remote smart jack into a loopback state.