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MT9092AP Datasheet(PDF) 10 Page - Mitel Networks Corporation |
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MT9092AP Datasheet(HTML) 10 Page - Mitel Networks Corporation |
10 / 42 page MT9092 7-12 to determine if the packet is of minimum valid length. These fields are transferred transparently through the FIFO's. Data Transparency (Zero insertion/deletion) Transparency ensures that the contents of a data packet do not imitate a flag, go-ahead, frame abort or idle channel. The contents of a transmitted frame, between the flags, is examined on a bit-by-bit basis and a 0 bit is inserted after all sequences of five contiguous 1 bits (including the last five bits of the FCS). Upon receiving five contiguous 1s within a frame the receiver deletes the following 0 bit. Invalid Frames A frame is invalid if one of the following four conditions exists. Inserted zeros are not part of a valid bit count: 1. If the FCS pattern generated from the received data does not match the 'F0B8' pattern then the last data byte of the packet is written to the receive FIFO with a 'bad packet' indication. 2. A short frame exists if there are less than 25 bits between the flags. Short frames are ignored by the receiver and nothing is written into the receive FIFO. 3. Packets which are at least 25 bits in length but less than 32 bits (between the flags) are also invalid. In this case the data is written to the FIFO but the last byte is tagged with a 'bad packet' indication. 4. If a frame abort sequence is detected the packet is invalid. Some or all of the current packet will reside in the receive FIFO, assuming the packet length before the abort sequence was at least 26 bits long. Frame Abort The transmitter will abort a current packet by substituting a zero followed by seven contiguous 1s in place of the normal data. The receiver will abort upon reception of seven contiguous 1s occurring between the flags of a packet which contains at least 26 bits. Note that should the last receive byte before the frame abort end with contiguous 1s, these are included in the seven 1s required for a receiver abort. This means that the location of the abort sequence in the receiver may occur before the location of the abort sequence in the originally transmitted packet. If this happens, then the last data written to the receive FIFO will not correspond exactly with the last byte received before the frame abort. Interframe Time Fill and Link Channel States When the HDLC transmitter is not sending packets it will wait in one of two states. Interframe Time Fill: This is a continuous series of flags occurring between frames indicating that the channel is active but that no data is being sent. Idle: An idle channel occurs when at least fifteen contiguous 1s are transmitted or received. In both cases the transmitter will exit the wait state when data is loaded into the transmit FIFO. Go-Ahead A go-ahead is defined as the pattern ‘011111110’ (contiguous 7F’s) and is the occurrence of a frame abort sequence followed by a zero, outside of the boundaries of a normal packet. Being able to distinguish a proper (in packet) frame abort sequence from one occurring outside of a packet allows a higher level of signalling protocol which is not part of the HDLC specifications. Transmitter Following initialization and enabling, via the HTxEN bit (address 03h), the transmitter is in the Idle Channel State (Mark Idle). Interframe time fill may be selected by setting the Mark Idle bit (address 03h) high. The transmitter remains in its programmed state until data is written to the Tx FIFO. The transmitter will then proceed as follows: 1) If the transmitter is in the idle state the present byte of ones will be completely transmitted before the opening flag and packet data is sent. 2) If the transmitter is in the interframe time fill state the flag currently being transmitted will be used as the opening flag followed by the packet data. To assist in loading multiple packets into the transmit FIFO the last packet byte is tagged with either EOP (to indicate the end of the current packet) or FA. Control Register 1 (address 03h) bits EOP (end of |
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