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DEFINITIONS: Time Division Multiplex (TDM): The digital stream or channel is capable of carrying a standard 64 Kbps voice channel. Twenty four, 24 voice channels are multiplexed to create an aggregate of 1.536 Mbps. Time Division Multiplex allows a channel to use a slot 1/24 of the time. Each of these time slots is fixed and available to each of the 24 channels. Pulse Code Modulation (PCM): The digital line code of a Time Division Multiplex scheme. In which an analog voice is sampled 8000 times per second, and each sample contains an 8-bit word. Therefore, the digital equivalent of the analog voice wave form is 8000 samples per second x 8 bits per sample = 64000 bits per second, or 64 Kbps. Extended Super Frame (ESF): ESF uses the 8 kbps of overhead normally assigned for the framing format, therefore does not affect the data payload. ESF extends the superframe from 12 consecutive frames of information to a repetitive 24 frames of information. The 8 kbps overhead assigned for framing now is subdivided into three functions. Table 14.7 summarizes how the overhead is shared to supported ESF. ESF used only 6 bits (2 Kbps) for synchronization, 6 additional bits (2 Kbps) for error detection by means of cyclic redundancy checking (CRC-6). This leaves 12 bits of the framing bits for a facility data link communications channel (4 kbps). _____________________________________________________________ Original SF Format Revised ESF Format 8 Kbps for framing only 2 Kbps for framing 2 Kbps for error detection 4 Kbps for facility data link ______________________________________________________________ Framing: The purpose of bit pattern is framing synchronization. This is important because its loss can impact performance through incorrect synchronization which can cause incorrect channel interface (channel 1 of the end equipment can connect to channel 3 of the C.O. channel bank). Errors might cause a loss of frame and disrupt the circuit. The far-end or intermediate equipment may be thrown into a loss of frame condition and thereby loss of data. While the devices along the circuit are reframing data, throughput is disrupted for a period of time. Depending on the equipment in use, this loss could be substantial (old equipment reframing time can last as long as 200 ms, while newer equipment may takes only 10 to 20 ms). Cyclic Redundant Check (CRC): The use of CRC by the CSU is to check the entire ESF 24 frames of information (4632 bits) for accuracy. The CSU performs a mathematic computation on the 4632 data bits using a prime number polynomial which results in a 6-bit pattern. In the next ESF (24 frames), the results of this mathematics is transmitted to the distant end CSU. The far-end CSU uses the same mathematic computation on the arriving ESF frames and check the received results, if the two results are identical, no error has taken place, if not, an error has occurred. The CRC allows for an error detection efficiency of 98.4% or simply stated, it detects 63 out of 64 errors.
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