mes in its definition of a superframe, but only information, supervisory control, and other (future) needs, becomes six bits in its framing pattern. Rather than resynchronising every 1.5 available. milliseconds as in the regular format, the ESF only needs to resyn- The ESF format also provides in a 6-bit Cyclic Redundancy chronise every 3.0 milliseconds. The 193rd bits are now being Check (CRC). The check character is used for monitoring the looked at as part of one 24-bit word. Substantial progress in VLSI transmission quality and overall performance of the DS1 facility. technology has enabled the equipment to keep timing more accur- The CRC-6 is generated from the bits of the preceding frame. For ately. This implies that fewer bits are required for this housekeeping the calculation, the framing bits of that frame are considered to function. The result is that 4000bit/s of channel are freed without equal 1. Since the CRC-6 allows detection of apparent degradation losing any functionality or any additional bits. This bandwidth can in transmission quality, problems may be fixed before a total failure be used for direct communication between intelligent network occurs. The CRC-6 detects about 98.4% of single bit or multiple bit monitoring devices and centralised network monitoring computers. errors. The CRC 6 also provides false frame protection. This follows Figure 21.6 depicts the ESF format. The successive 193rd bits are from the fact that if the wrong synchronisation boundary is selected, shown explicitly. There are 6 bits in the frame synchronisation the CRC will not calculate correctly. word, rather than 12. The D bit represents link level data, while the Under ESF new VF signalling capabilities become available by C bit handles error checking and monitoring functions. With the providing two additional robbed signalling bits, allowing repre- extended superframe format all previous functionality, including sentation of up to 16 states. These bits are known as C and D, which Superframe Superframe Superframe fra