CCITT SIGNALING SYSTEM NO. 7
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 acknowl-
edgment, 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.
Flow control is initiated when congestion is detected at the receiving end of the signaling
link. The congested receiving end of the link notifies the remote transmitting end of the
condition by means of an appropriate link status signal and it withholds acknowledgments
of all incoming message signal units. When congestion abates, acknowledgments of all
incoming signal units are resumed. When congestion exists, the remote transmitting end
is periodically notified of this condition. The remote transmitting end will indicate that
the link has failed if the congestion continues too long.
Basic Signal Unit Format
Signaling and other information originating from a user part is transferred over the sig-
naling link by means of signal units. There are three types of signal units used in SS
1. Message signal unit (MSU)
2. Link status signal unit (LSSU)
3. Fill-in signal unit (FISU)
These units are differentiated by means of the length indicator. MSUs are retransmitted
in case of error; LSSUs and FISUs are not. The MSU carries signaling information; the
LSSU provides link status information; and the FISU is used during the link idle state--it
The signaling information field is variable in length and carries the signaling informa-
tion generated by the user part. All other fields are fixed in length. Figure 14.5 illustrates
the basic formats of the three types of signal units. As shown in the figure, the mes-
sage transfer control information encompasses eight fixed-length fields in the signal unit
that contains information for error control and message alignment. These eight fields are
described in the following. In Figure 14.5 we start from right to left, which is the direction
The opening flag indicates the start of a signal unit. The opening flag of one signal unit
is normally the closing flag of the previous signal unit. The flag bit pattern is 01111110.
The forward sequence number (FSN) is the sequence number of the signal unit in which it
is carried. The backward sequence number (BSN) is the sequence number of a signal unit
being acknowledged. The value of the FSN is obtained by incrementing (modulo 128) the