The extended superframe format

The DSl format described above was developed in the 1960s andis based on technology then available. The basic challenge of DS1 transmission is that of preserving bit and word synchronism. To do so, 24 eight-bit words (corresponding to samples for 24 telephone conversations) are followed by one bit (the 193rd bit) for frame tracking and management. There are 8000 repetitions of this pattern in one second, which results in an additional 8kbit/s bandwidth loss.

The extended superframe format, denoted as ESF, was announced in 1981 and adds two enhancements. First, the number of frames in a superframe is increased to 24, rather than 12. This provides extra signalling bits for telephone signalling, а С bit and a D bit. A 24 frame pattern makes an additional bandwidth available on the fram­ing bit. This is used to send maintenance data between Tl interfaces.

The ESF has 24 frames in its definition of a superframe, but only six bits in its framing pattern. Rather than resynchronising every 1.5 milliseconds as in the regular format, the ESF only needs to resynchronise every 3.0 milliseconds. The 193rd bits are now being looked at as part of one 24-bit word. Substantial progress in VLSI technology has enabled the equipment to keep timing more accur­ately. This implies that fewer bits are required for this housekeeping function. The result is that 4000bit/s of channel are freed without losing any functionality or any additional bits. This bandwidth can be used for direct communication between intelligent network monitoring devices and centralised network monitoring computers.

Figure 21.6 depicts the ESF format. The successive 193rd bits are shown explicitly. There are 6 bits in the frame synchronisation word, rather than 12. The D bit represents link level data, while the С bit handles error checking and monitoring functions. With the extended superframe format all previous functionality, including the VF signalling rule, remain available. No new bits from the 1.544Mbit/s are taken away from the user.

Since twenty four frames have now to be examined by the equip­ment to establish synchronisation and extract other channel infor­mation, it takes longer to regain lost synchronisation. However, with the newer sophisticated clocking mechanism this should only hap­pen very rarely.

A 4000bit/s data link, also referred to as the Embedded Operations Channel (EOC) or Facilities Data Link (FDL), used for maintenance information, supervisory control, and other (future) needs, becomes available.

The ESF format also provides in a 6-bit Cyclic Redundancy Check (CRC). The check character is used for monitoring the transmission quality and overall performance of the DS1 facility. The CRC-6 is generated from the bits of the preceding frame. For the calculation, the framing bits of that frame are considered to equal 1. Since the CRC-6 allows detection of apparent degradation in transmission quality, problems may be fixed before a total failure occurs. The CRC-6 detects about 98.4% of single bit or multiple bit errors. The CRC 6 also provides false frame protection. This follows from the fact that if the wrong synchronisation boundary is selected, the CRC will not calculate correctly.

Under ESF new VF signalling capabilities become available by providing two additional robbed signalling bits, allowing repre­sentation of up to 16 states. These bits are known as С and D, which complement the A and В bits of the regular superframe format. Out of these, 16 codes options are available for transparency (no robbed signalling bits), two state signalling (A bit only), four state signall­ing (A and В bits), or sixteen state signalling (А, В, С, and D bits). In the transparent signalling option, all eight bits within a frame within the extended superframe are given to the user for data. When the A signalling mode is employed, the LSB of every sixth signall­ing frame is robbed to show the desired bit. When A/B signalling is used, the first and third signalling frames are robbed to carry the A bits, while the second and fourth signalling frames will carry the В bits. If the A/B/C/D signalling is employed, the desired bits are gained by robbing the first, second, third, and fourth frames, respec­tively.

From the above we can conclude that ESF has economically reallocated the 8kbit/s that have long been used to manage the DS1 facilities. 2000 bits are used for framing (6 bits distributed over 24 framing bits and there are 333 such 24-bit words per second). 2000 bits are used for error detection and performance determination (6 bits distributed over 24 framing bits). Finally, 4000 bits are used for telemetry and facility management and/or reconfiguration (12 bits distributed over 24 framing bits).

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