CCITT SIGNALING SYSTEM NO. 7
The SIO directs the signaling message to the proper layer 4 entity, whether SCCP or
user part. This is called message distribution.
The subservice indicator contains the network bits C and D and two spare bits, A and
B. The network indicator is used by signaling message-handling functions determining
the relevant version of the user part. If the network indicator is set at 00 or 01, the two
spare bits, coded 00, are available for possible future needs. If these two bits are coded
10 or 11, the two spare bits are for national use, such as message priority as an optional
flow procedure. The network indicator provides discrimination between international and
national usage (bits D and C).
The routing label forms part of every signaling message:
To select the proper signaling route
To identify the particular transaction by the user part (the call) to which the mes-
The label format is shown in Figure 14.10. The DPC is the destination point code (14 bits)
which indicates the signaling point for which the message is intended. The originating
point code (OPC) indicates the source signaling point. The circuit identification code
(CIC) indicates the one circuit (speech circuit in the TUP case) among those directly
interconnecting the destination and originating points.
For the OPC and DPC, unambiguous identification of signaling points is carried
out by means of an allocated code. Separate code plans are used for the international
and national networks. The CIC, as shown in Figure 14.10, is applicable only to the
TUP. ITU-T (CCITT) Rec. Q.704 shows a signaling link selection (SLS) field fol-
lowing (to the left) the OPC. The SLS is 4 bits long and is used for load sharing.
The ISDN user part address structure is capable of handling E.164 addresses in the
calling and in the called number and is also capable of redirecting address informa-
Telephone User Part (TUP)
The core of the signaling information is carried in the SIF (see Figure 14.10). The TUP
label was described briefly in Section 184.108.40.206. Several signal message formats and codes
are described in the paragraphs below. These follow the TUP label.
One typical message of the TUP is the initial address message (IAM). Its format is
shown in Figure 14.11. A brief description is given of each subfield, providing further
insight on how SS No. 7 operates.
Common to all signaling messages are the subfields H0 and H1. These are the heading
codes, each consisting of 4 bits, giving 16 code possibilities in pure binary coding. H0
identifies the specific message group to follow. "Message group" means the type of
message. Some samples of message groups are as follows:
Message Group Type
Forward address messages
Forward setup messages
Backward setup messages
Unsuccessful backward setup messages
Call supervision messages