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MT9079 Datasheet(PDF) 13 Page - Mitel Networks Corporation |
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MT9079 Datasheet(HTML) 13 Page - Mitel Networks Corporation |
13 / 54 page MT9079 4-249 1) by programming the SOFF7-0 bits to select the desired throughput delay, which is indicated by the phase status word bits RxTS4-0 and RxBC2-0. 2) by controlling the position of the F0i pulse with respect to the received time slot zero position. The phase status word bits RxTS4-0 and RxBC2-0 will also indicate the delay in this application. With RDLY=1, the elastic buffer may underflow or overflow. This is indicated by the RSLIP and RSLPD status bits. If RSLPD=0, the elastic buffer has overflowed and a bit was lost; if RSLPD=1, a underflow condition occurred and a bit was repeated. Framing Algorithm The MT9079 contains three distinct, but interdependent, framing algorithms. These algorithms are for basic frame alignment, signalling multiframe alignment and CRC-4 multiframe alignment. Figure 6 is a state diagram that illustrates these functions and how they interact. After power-up the basic frame alignment framer will search for a frame alignment signal (FAS) in the PCM 30 receive bit stream. Once the FAS is detected, the corresponding bit two of the non-frame alignment signal (NFAS) is checked. If bit two of the NFAS is zero a new search for basic frame alignment is initiated. If bit two of the NFAS is one and the next FAS is correct, the algorithm declares that basic frame synchronization has been found (i.e., SYNC is low). Once basic frame alignment is acquired the signalling and CRC-4 multiframe searches will be initiated. The signalling multiframe algorithm will align to the first multiframe alignment signal pattern (MFAS = 0000) it receives in the most significant nibble of channel 16 (MFSYNC = 0). Signalling multiframing will be lost when two consecutive multiframes are received in error. The CRC-4 multiframe alignment signal is a 001011 bit sequence that appears in PCM 30 bit position one of the NFAS in frames 1, 3, 5, 7, 9 and 11 (see Table 4). In order to achieved CRC-4 synchronization two consecutive CRC-4 multiframe alignment signals must be received without error (CRCSYN = 0). See Figure 6 for a more detailed description of the framing functions. The MT9079 framing algorithm supports automatic interworking of interfaces with and without CRC-4 processing capabilities. That is, if an interface with CRC-4 capability, achieves valid basic frame alignment, but does not achieve CRC-4 multiframe alignment by the end of a predefined period, the distant end is considered to be a non-CRC-4 interface. When the distant end is a non-CRC-4 interface, the near end automatically suspends receive CRC-4 functions, continues to transmit CRC-4 data to the distant end with its E-bits set to zero, and provides a status indication. Naturally, if the distant end initially achieves CRC-4 synchronization, CRC-4 processing will be carried out by both ends. This feature is selected when control bit AUTC = 0. See Figure 6 for more details. Notes for Figure 6: 1) The basic frame alignment, signalling multiframe alignment, and CRC-4 multiframe alignment functions operate in parallel and are independent. 2) The receive channel associated signalling bits and signalling multiframe alignment bit will be frozen when multiframe alignment is lost. 3) Manual re-framing of the receive basic frame alignment and signalling multiframe alignment functions can be performed at any time. 4) The transmit RAI bit will be one until basic frame alignment is established, then it will be zero. 5) E-bits can be optionally set to zero until the equipment interworking relationship is established. When this has been determined one of the following will take place: a) CRC-to-non-CRC operation - E-bits = 0, b) CRC-to-CRC operation - E-bits as per G.704 and I.431. 6) All manual re-frames and new basic frame alignment searches start after the current frame alignment signal position. 7) After basic frame alignment has been achieved, loss of frame alignment will occur any time three consecutive incorrect FAS or NFAS are received. Loss of basic frame alignment will reset the complete framing algorithm. |
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