ISDN User Part

The ISDN User Part or ISUP is part of the Signaling System #7 which is used to set up telephone calls in Public Switched Telecom Networks. It is specified by the ITU-T as part of the Q.7xx series.

When a telephone call is set up from one subscriber to another, many telephone exchanges will be involved, possibly across international boundaries. To allow the call to be setup correctly, the switches 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) which is included in the ISUP messages. The exchange uses this information 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 of the available timeslots. In the case of no outbound CIC being available on a particular exchange, a blocking messages is sent back to the previous switch in the chain so a new route can be tried.

Different ISUP variants exist. The main specification task is performed by the ITU-T. In Europe ETSI releases an 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. In the USA ANSI specifies the north American ISUP variant which is quite different from the ITU-T ISUP.

Message types

After the mandatory fixed-length Signaling Information Field, a ISUP message contains a variable-length part that is dependent on the type of message being sent. 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.

• 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.

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 --

Signaling Information Field (ANSI)

0	1	2	3	4	5	6	7
Destination Point Code - Member
Destination Point Code - Cluster
Destination Point Code - Network
Originating Point Code - Member
Originating Point Code - Cluster
Originating Point Code - Network
Padding			Signaling Link Selection
CIC Low-order Octet
Padding		CIC High-order 6 bits
Message type
Variable-length part ...

The Signaling Information Field is required on all ISUP Message Signal Units. It indicates the Point Codes of the originating and destination nodes in the network, along with the circuit identification code (CIC). The CIC is used to specify which trunk between two switches is used to carry a particular call. The Signalling Link Selection field is used when there are multiple routes an MSU could take between two nodes. This table describes the ANSI version of the ISUP protocol, which is used in the United States. Countries in Europe and other parts of the world often use the protocol specified by the ITU, which varies considerably from the ANSI version.