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SS7 protocol suite
OSI Layer SS7 Protocols
Application INAP, MAP, IS-41...


Network MTP Level 3 + SCCP
Data link MTP Level 2
Physical MTP Level 1

The ISDN User Part or ISUP is part of the Signaling System #7 which is used to set up telephone calls in Public Switched Telephone Networks. It is specified by the ITU-T as part of the Q.76x series,[1] ANSI (T1.113-YEAR) and Telcordia Technologies formerly Bellcore Generic Requirements document GR-246-CORE.

When a telephone call is set up from one subscriber to another, many telephone exchanges will be involved, possibly across international boundaries. To allow a call to be set up correctly, where ISUP is supported, a switch will signal call-related information like called or calling party number to the next switch in the network using ISUP messages.

The telephone exchanges are connected via E1 or T1 trunks which transport the speech from the calls. These trunks are divided into 64 kbit/s timeslots, and one timeslot can carry exactly one call. Each timeslot between two switches is uniquely identified by a Circuit Identification Code (CIC) that is included in the ISUP messages. The exchange uses this information along with the received signalling information (especially the Called Party Number) to determine which inbound CICs and outbound CICs should be connected together to provide an end to end speech path.

In addition to call related information, ISUP is also used to exchange status information for, and permit management of, the available timeslots. In the case of no outbound CIC being available on a particular exchange, a release message is sent back to the preceding switches in the chain so a new route can be tried.


ISUP variants

Different ISUP variants exist. The main specification task is performed by the ITU-T. In Europe ETSI releases its own ISUP specification which is very close to the ITU-T ISUP.[2] The ITU-T and ETSI ISUP are used for international connections and they are the base for national ISUP variants. Most of the countries have their own ISUP variant to cover national specific requirements. In the USA ANSI specifies the North American ISUP variant which is quite different from the ITU-T ISUP.

ITU-T specification versions

  • 1984 - ISUP Red Book
  • 1988 - ISUP Blue Book
  • 1991 - ISUP Q.767[3]
  • 1992 - ISUP'92 White Book (segmentation, compatibility, new supplementary services)
  • 1997 - ISUP'97 (new procedures, IN CS1, new supplementary services)

According to ITU-T Q.761 section 2.4.1 ISUP interworking ISUP'92 is backwards compatible with ISUP Blue Book and Q.767[3] for basic call procedures and supplementary services except for some procedures (eg. number portability).[4] Additionally the compatibility features introduced in this version ensure forward compatibility with newer versions.

Message types

An ISUP message contains a fixed header containing the circuit identification code and the ISUP message type, followed by a fixed-length part and optional variable-length part that are dependent on the type of message being sent. ISUP messages can be sent using the services of the Message Transfer Part, or, less often, the Signalling Connection Control Part. These messages are transmitted in various stages of call setup and teardown. The most common messages are:[5]

  • Initial Address Message (IAM) — First message sent to inform the partner switch that a call has to be established on the CIC contained in the message. Contains the called and calling number, type of service (speech or data) and many more optional parameters.
  • Subsequent Address Message (SAM) — In case the IAM did not contain the full called number, one or more SAMs may follow containing additional digits.
  • Address Complete Message (ACM) — Message returned from the terminating switch when the subscriber is reached and the phone starts ringing.
  • Answer Message (ANM) — Sent when the subscriber picks up the phone. Normally charging starts at this moment.
  • Release (REL) — Sent to clear the call when a subscriber goes on hook.
  • Release complete (RLC) — Acknowledgement of the release – the timeslot is idle afterwards and can be used again. This is also sent (without a preceding Release message) if the terminating switch determines that the call cannot be completed. The terminating switch also sends a Cause Value to explain the reason for the failure, e.g., "User busy".

Sample call flow

This is a very basic call flow involving only two telecom switches which exchange the ISUP messages. The subscriber interfaces are not covered here and are only listed for a better understanding.

A subscriber     telco switch A    telco switch B     B subscriber
Off hook
Dial digits --->
                            -- IAM -->
                                                -Ringing ->
                            <-- ACM --
                                                       Off hook
                            <-- ANM --
-----------------------    Conversation    -----------------------
On hook
                            -- REL -->                  On hook
                            <-- RLC --

Detailed call flows are provided in ITU-T Recommendation Q.784.1.

Message format

8 7 6 5 4 3 2 1
Routing Label


CIC Least Significant 8 Bits
Padding CIC Most Sig. 4 Bits
Message type
Mandatory fixed part


Mandatory variable part


Optional part


The Signalling Information Field (SIF) for all ISUP Message Signal Units (MSU) contain the following components:[6]

  • Routing Label
  • Circuit Identification Code
  • Message Type
  • Mandatory Fixed Part
  • Mandatory Variable Part
  • Optional Part

The Routing Label indicates the Point Codes of the originating and destination nodes in the network; it also includes the Signalling Link Selection field that is used to select between the multiple routes an MSU could take between two nodes.

The Circuit Identification Code is used to specify which trunk between two switches is used to carry a particular call. Note that some versions of ANSI ISUP permit a CIC with 14 significant bits instead of the 12 that are shown.[7]

The Message Type indicates the ISUP message type. The presence and form of the remaining 3 components are determined by this message type. The Mandatory fixed part, when present, contains the mandatory, fixed-length parameters associated with the message type. The Mandatory variable part, when present, contains the mandatory, variable-length parameters associated with the message type. The Optional part, when present, contains the optional parameters permitted to be included in the message type.

When sent using the services of the Signalling Connection Control Part, ISUP messages passed to SCCP in the User Data parameter (NSDU) consist of only the last 4 components (Message Type, Mandatory fixed part, Mandatory variable part, Optional part""). The routing label and circuit identification code are not included in the user data passed to SCCP.[8]



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