ISDN User Part
The ISDN User Part (ISUP) defines the protocol and
procedures used to set-up, manage, and release trunk circuits that
carry voice and data calls over the public switched telephone
network (PSTN). ISUP is used for both ISDN and non-ISDN calls. Calls
that originate and terminate at the same switch do not use ISUP
Basic ISUP Call Control
Figure 8 depicts the ISUP signaling associated with a
- When a call is placed to an out-of-switch number, the
originating SSP transmits an ISUP initial address message
(IAM) to reserve an idle trunk circuit from the originating switch
to the destination switch (1a). The IAM includes the originating
point code, destination
point code, circuit identification code (circuit "5" in
Fig. 8), dialed digits and, optionally, the calling party number
and name. In the example below, the IAM is routed via the home STP
of the originating switch to the destination switch (1b).
Note that the same signaling link(s) are used for the duration of
the call unless a link failure condition forces a switch to use an
alternate signaling link.
Figure 8. Basic ISUP Signaling
- The destination switch examines the dialed number, determines
that it serves the called party, and that the line is available
for ringing. The destination switch rings the called party line
and transmits an ISUP address complete message (ACM) to the
originating switch (2a) (via its home STP) to indicate that the
remote end of the trunk circuit has been reserved. The STP routes
the ACM to the originating switch (2b) which rings the calling
party's line and connects it to the trunk to complete the voice
circuit from the calling party to the called party.
example shown above, the originating and destination switches are
directly connected with trunks. If the originating and destination
switches are not directly connected with trunks, the originating
switch transmits an IAM to reserve a trunk circuit to an
intermediate switch. The intermediate switch sends an ACM to
acknowledge the circuit reservation request and then transmits an
IAM to reserve a trunk circuit to another switch. This processes
continues until all trunks required to complete the voice circuit
from the originating switch to the destination switch are
- When the called party picks up the phone, the destination
switch terminates the ringing tone and transmits an ISUP answer
message (ANM) to the originating switch via its home STP
(3a). The STP routes the ANM to the originating switch
(3b) which verifies that the calling party's line is
connected to the reserved trunk and, if so, initiates
- If the calling party hangs-up first, the originating switch
sends an ISUP release message (REL) to release the trunk
circuit between the switches (4a). The STP routes the REL
to the destination switch (4b). If the called party hangs
up first, or if the line is busy, the destination switch sends an
REL to the originating switch indicating the release cause (e.g.,
normal release or busy).
- Upon receiving the REL, the destination switch disconnects the
trunk from the called party's line, sets the trunk state to idle,
and transmits an ISUP release complete message (RLC) to the
originating switch (5a) to acknowledge the release of the
remote end of the trunk circuit. When the originating switch
receives (or generates) the RLC (5b), it terminates the
billing cycle and sets the trunk state to idle in preparation for
the next call.
ISUP messages may also be transmitted during the connection phase
of the call (i.e., between the ISUP Answer (ANM) and Release (REL)
ISUP Message Format
ISUP information is carried in the Signaling
Information Field (SIF) of an MSU. The SIF contains the
routing label followed by a 14-bit (ANSI) or 12-bit (ITU)
circuit identification code (CIC). The CIC indicates the
trunk circuit reserved by the originating switch to carry the call.
The CIC is followed by the message type field (e.g., IAM,
ACM, ANM, REL, RLC) which defines the contents of the remainder of
the message (Fig. 9).
Figure 9. ISUP Message Format
Each ISUP message contains a mandatory fixed part
containing mandatory fixed-length parameters. Sometimes the
mandatory fixed part is comprised only of the message type field.
The mandatory fixed part may be followed by the mandatory
variable part and/or the optional part. The mandatory
variable part contains mandatory variable-length parameters. The
optional part contains optional parameters which are identified by a
one-octet parameter code followed by a length indicator ("octets to
follow") field. Optional parameters may occur in any order. If
optional parameters are included, the end of the optional parameters
is indicated by an octet containing all zeros.
Initial Address Message
An Initial Address Message (IAM) is sent in the "forward"
direction by each switch needed to complete the circuit between the
calling party and called party until the circuit connects to the
destination switch. An IAM contains the called party number in the
mandatory variable part and may contain the calling party name and
number in the optional part.
Figure 10. ANSI and ITU-T Initial Address
Message (IAM) Format
with ISUP Part 2...