The ISDN User Part or ISUP is part of the Signaling System #7 which is used to set up telephone call

Telephone call

A telephone call is a connection over a telephone network between the calling party and the called party.-Information transmission:A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile...

s in Public Switched Telephone Networks. It is specified by the ITU-T

ITU-T

The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....

 as part of the Q.76x series, ANSI

American National Standards Institute

The American National Standards Institute is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organization also coordinates U.S. standards with international...

 (T1.113-YEAR) and Telcordia Technologies

Telcordia Technologies

Telcordia Technologies, formerly Bell Communications Research, Inc. or Bellcore, is a telecommunications research and development company based in the United States created as part of the 1982 Modification of Final Judgment that broke up American Telephone & Telegraph...

 formerly Bellcore Generic Requirements document GR-246-CORE.

When a telephone

Telephone

The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

 call is set up from one subscriber to another, many telephone exchange

Telephone exchange

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

s will be involved, possibly across international boundaries. To allow a call to be set up correctly, where ISUP is supported, a switch

Telephone exchange

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

 will signal call-related information like called

Called party

The called party is a person who answers a telephone call. The person who initiates a telephone call is the calling party....

 or calling party

Calling party

The calling party is a person who initiates a telephone call over the public switched telephone network, usually by dialing a telephone number....

 number to the next switch

Telephone exchange

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

 in the network using ISUP messages.

The telephone exchange

Telephone exchange

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

s are connected via E1

E-carrier

In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...

 or T1

T-carrier

In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....

 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)

Circuit identification code

The ISDN Services User Part Circuit Identification Code is part of the Signaling System #7 which is used to set up telephone calls in Public Switched Telephone Networks. as part of the Initial Address Message ....

 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.
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. ANSI

Ansi

Ansi is a village in Kaarma Parish, Saare County, on the island of Saaremaa, Estonia....

 specifies the ISUP variant utilized in the USA and Canada.

ITU-T specification versions

• 1984 - ISUP Red Book
• 1988 - ISUP Blue Book
• 1991 - ISUP Q.767
• 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 for basic call procedures and supplementary services except for some procedures (eg. number portability). 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

Circuit identification code

The ISDN Services User Part Circuit Identification Code is part of the Signaling System #7 which is used to set up telephone calls in Public Switched Telephone Networks. as part of the Initial Address Message ....

 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

Message Transfer Part

The Message Transfer Part is part of the Signaling System 7 used for communication in Public Switched Telephone Networks. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners....

, 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:

• 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
  Release (ISUP)
  Introduction to ISUP Release :Release codes are usually used to identify and debug any events occurring in ISUP signaling. Every event in ISUP signaling generates a release code number. Even for a normal ISUP call a release code is generated.There are lot of applications developed based on the...
  
   (REL) — Sent to clear the call when a subscriber goes on hook. 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".

• Release complete (RLC) — Acknowledgement of the release – the timeslot is idle afterwards and can be used again.

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:

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

The Routing Label indicates the Point Code

Point Code

An SS7 point code is similar to an IP address in an IP network. It is a unique address for a node , used in MTP layer 3 to identify the destination of a message signal unit ....

s of the originating and destination nodes in the network; it also includes the Signalling Link Selection

Message Transfer Part

The Message Transfer Part is part of the Signaling System 7 used for communication in Public Switched Telephone Networks. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners....

 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

Telephone exchange

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

 is used to carry a particular call. Note that some versions of ANSI

Ansi

Ansi is a village in Kaarma Parish, Saare County, on the island of Saaremaa, Estonia....

 ISUP permit a CIC with 14 significant bits instead of the 12 that are shown.

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

Message Transfer Part

The Message Transfer Part is part of the Signaling System 7 used for communication in Public Switched Telephone Networks. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners....

 and circuit identification code

Circuit identification code

The ISDN Services User Part Circuit Identification Code is part of the Signaling System #7 which is used to set up telephone calls in Public Switched Telephone Networks. as part of the Initial Address Message ....

are not included in the user data passed to SCCP.