Two forms of error correction are provided: the basic method and the preventive cyclic retransmission method. The basic method applies to (a) signaling links using noninter-continental terrestrial transmission means and (b) intercontinental signaling links where one-way propagation is less than 15 msec.
The preventive cycle retransmission method applies to (a) intercontinental signaling links where the one-way delay is equal to or greater than 15 msec and (b) signaling links established via satellite.
In cases where one signaling link with an intercontinental link set is established via satellite, the preventive cycle retransmission method is used for all signaling links of that set.
The basic method is a noncompelled, positive/negative acknowledgment, retransmission error correction system. A signal unit that has been transmitted is retained at the transmitting signaling link terminal until a positive acknowledgment for that signal unit is received. If a negative acknowledgment is received, then the transmission of new signal units is interrupted and those signal units which have been transmitted but not yet positively acknowledged (starting with that indicated by the negative acknowledgment) will be transmitted once, in the order in which they were first transmitted.
The preventive cyclic retransmission method is a noncompelled, positive acknowledgment, cyclic retransmission forward error correction system. A signal unit that has been transmitted is retained at the transmitting signaling unit terminal until a positive acknowledgment for that signaling unit is received. During the period when there are no new signal units to be transmitted, all signal units which have not been positively acknowledged are retransmitted cyclically.
The forced retransmission procedure is defined to ensure that forward error correction occurs in adverse conditions (e.g., degraded BER and/or high-traffic loading). When a predetermined number of retained, unacknowledged signal units exist, the transmission of new signal units is retransmitted cyclically until the number of acknowledged signal units is reduced.
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