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MT9079AP Datasheet(PDF) 7 Page - Mitel Networks Corporation |
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MT9079AP Datasheet(HTML) 7 Page - Mitel Networks Corporation |
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7 / 54 page  MT9079 4-243 ST-BUS bit 7 is synonymous with PCM 30 bit 1; bit 6 with bit 2: and so on. See Figure 33. PCM 30 time slot zero is reserved for basic frame alignment, CRC-4 multiframe alignment and the communication of maintenance information. In most configurations time slot 16 is reserved for either channel associated signalling (CAS or ABCD bit signalling) or common channel signalling (CCS). The remaining 30 time slots are called channels and carry either PCM encoded voice frequency signals or digital data signals. Channel alignment and bit numbering is consistent with time slot alignment and bit numbering. However, channels are numbered 1 to 30 and relate to time slots as per Table 1. Basic Frame Alignment Time slot zero of every basic frame is reserved for basic frame alignment and contains either a Frame Alignment Signal (FAS) or a Non-frame Alignment Signal (NFAS). FAS and NFAS occur in time slot zero of consecutive basic frames as can be see in Table 4. Bit two is used to distinguish between a FAS (bit two = 0) and a NFAS (bit two = 1). Basic frame alignment is initiated by a search for the bit sequence 0011011 which appears in the last seven bit positions of the FAS, see Frame Algorithm section. Bit position one of the FAS can be either a CRC-4 remainder bit or an international usage bit. Bits four to eight of the NFAS (i.e., Sa4 - Sa8) are national bits, which telephone authorities used to communicate maintenance, control and status information. A single national bit can also be used as a 4 KHz maintenance channel or data link. Bit three, the ALM bit, is used to indicate the near end basic frame synchronization status to the far end of a link. Bit position one of the NFAS can be either a CRC-4 multiframe alignment signal, an E-bit or an international usage bit. Refer to an approvals laboratory and national standards bodies for specific requirements. CRC-4 Multiframing The primary purpose for CRC-4 multiframing is to provide a verification of the current basic frame alignment, although it can be used for other PCM 30 Timeslots 0 1 2 3 ....15 16 17 18 19 ....31 Voice/Data Channels X 1 2 3 ....15 X 16 17 18 ....30 Table 1 - Time slot to Channel Relationship functions such as bit error rate estimation. The CRC-4 multiframe consists of 16 basic frames numbered 0 to 15, and has a repetition rate of 16 frames X 125 microseconds/frame = 2 msec. CRC-4 multiframe alignment is based on the 001011 bit sequence, which appears in bit position one of the first six NFASs of a CRC-4 multiframe. The CRC-4 multiframe is divided into two submultiframes, numbered 1 and 2, which are each eight basic frames or 2048 bits in length. The CRC-4 frame alignment verification functions as follows. Initially, the CRC-4 operation must be activated and CRC-4 multiframe alignment must be achieved at both ends of the link. At the local end of a link all the bits of every transmit submultiframe are passed through a CRC-4 polynomial (multiplied by X4 then divided by X4 + X + 1), which generates a four bit remainder. This remainder is inserted in bit position one of the four FASs of the following submultiframe before it is transmitted, see Table 4. The submultiframe is then transmitted and at the far end the same process occurs. That is, a CRC-4 remainder is generated for each received submultiframe. These bits are compared with the bits received in position one of the four FASs of the next received submultiframe. This process takes place in both directions of transmission. When more than 914 CRC-4 errors (out of a possible 1000) are counted in a one second interval, the framing algorithm will force a search for a new basic frame alignment. See Frame Algorithm section for more details. The result of the comparison of the received CRC-4 remainder with the locally generated remainder will be transported to the near end by the E-bits. Therefore, if E1 = 0, a CRC-4 error was discovered in a submultiframe one received at the far end; and if E2 = 0, a CRC-4 error was discovered in a submultiframe two received at the far end. No submultiframe sequence numbers or re-transmission capabilities are supported with layer 1 PCM 30 protocol. See CCITT G.704 and G.706 for more details on the operation of CRC-4 and E-bits. CAS Signalling Multiframing The purpose of the signalling multiframing algorithm is to provide a scheme that will allow the association of a specific ABCD signalling nibble with the appropriate PCM 30 channel. Time slot 16 is reserved for the communication of Channel Associated Signalling (CAS) information (i.e., ABCD signalling bits for up to 30 channels). Refer to CCITT |
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