-Transmission direction -
(c) Message signal unit (MSU)
Fig. 7.15. Signalling units.
The common fields used in all the signalling units are described below :
Flag. The flag indicates the beginning of a new signal unit and implies the end of the previous signal unit (if any). The binary value of the flag is 0111 1110. The same sequence may occur in messages and wrongly interpreted as flags. By the technique known as bit stuffing and unstuffing, false flag can be prevented. By this technique before transmitting a signal unit, MTP 2 addes a zero bit after any sequence of five one bits. Upon receiving, MTP 2 removes any zero bit following a sequence of five one bits to restore the contents of the messages.
Backward Sequence Number (BSN). The BSN is used to acknowledge the receipt of signal units by the remote signalling point. The BSN contains the sequence number of the signal unit being acknowledged.
Backward Indicator Bit (BIB). A negative acknowledgement is indicated by inverting the BIB bit, which remains unchanged for all subsequent positive acknowledgement.
Forward Sequence Number (FSN) and Forward Indicator Bit (FIB). The FSN
contains the sequence number of the signal unit. The FSN identifies the SU uniquely using modulo 128 count. The FIB is used in error recovery like the BIB. When a signal unit is ready for transmission, the signalling point increments the FSN by 1. The cyclic redundancy check (CRC) checksum value is calculated and appended to the forward message.
Upon receiving the message, the remote signalling point checks the CRC and copies the value of the FSN into the BSN of the next available message scheduled for transmission back to the initiating signalling point. If the CRC is correct, the backward message is transmitted. If the CRC is incorrect, the remote signalling point indicates negative acknowledgement by toggling BIB prior to sending the backward message. When the originating signalling point receives a negative acknowledgement, it retransmits all forward messages, beginning with the corrupted message, with the FIB toggled.
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