Data Communications Networking Devices - 4th Ed.pdf

4.11 CHANNEL SERVICE UNITS ____________________________________________________ 525registers for 15-minute intervals, and can be stored and retrieved for up to a 24-hour period. In comparison, under ANSI T1.403, parameters are broadcast everysecond and store only three seconds of data. Although AT&T and severalcommunications carriers support both methods, AT&T appears to be migratingtowards the ANSI method even though that method does not maintain historicaldata, nor does it allow user access to registers.Framing patternThe third type of frame bits are used to generate the framing pattern. Here, framebits 4, 8, 12, 16, 20 and 24 are used to generate the F e framing pattern whosecomposition is 001011. These six bits result in a 2 kbps framing pattern. In Table4.42 you will ®nd a summary of the ESF framing pattern.CEPT PCM-30 formatCEPT PCM-30 is a PCM format used for time division multiplexing of 30 voice ordata circuits onto a single twisted pair cable using digital repeaters and is primarilyused in Europe.The standard CEPT frame is 32 channels 8 bits/channel or 256 bits. With 8000samples per second, the CEPT data rate becomes 8000 256, or 2.048 Mbps. Notethat under this format there are no framing bits added to a frame as done under theNorth American T1 format. This is because the framing bits are carried withinspeci®c time slots. Thus, a European CSU must examine bits within a speci®c timeslot.Frame compositionEach CEPT PCM-30 frame consists of 32 time slots to include 30 voice, onealignment and one signaling, with each time slot represented by eight bits. Sinceeach PCM channel is sampled 8000 times per second, the standard CEPT-30 datarate is 32 8 8000, or 2.048 Mbps.Alignment signalAn alignment signal 0011011) is transmitted in bit positions 2 to 8 of time slot 0 inalternating frames. This signal is used to enable each channel to be distinguished atthe receiver. Bit position 1 in time slot zero carries the International bit, whileframes not containing the frame alignment signal are used to carry National andInternational signaling and alarm indication for loss of frame alignment. Figure4.99 illustrates the composition of the CEPT-30 frame and multiframe, where themultiframe consists of 16 frames, numbered from frame 0 to frame 15.To avoid imitation of the frame alignment signal, alternating frames ®x bit 2 to a1 in time slot zero which is the reason why a 1 is entered into that bit position forodd time slot 0 frames.