# ISDN User Part

The ISDN (Integrated Services Digital Network) User Part or ISUP is part of Signaling System No. 7 (SS7), which is used to set up telephone calls in the public switched telephone network (PSTN). It is specified by the ITU-T as part of the Q.76x series.
When a telephone call is set up from one subscriber to another, several telephone exchanges could 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 party number to the next switch in the network using ISUP messages.
The telephone exchanges may be connected via T1 or E1 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. Regardless of what facilities are used to interconnect switches, each circuit 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 signaling information (especially the called party number) to determine which inbound and outbound circuits 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 circuits. In the case of no outbound circuit being available on a particular exchange, a release message is sent back to the preceding switches in the chain.

## ISUP variants

Different ISUP variants exist. ITU-T specifies the variant used in the international network. In Europe ETSI releases its own ISUP specification which is close that of the ITU-T. ITU-T ISUP is used for international connections and is the base for some national ISUP variants. Most countries have their own variation of ISUP to cover national requirements. ANSI specifies variations of ISUP utilized under the North American Numbering Plan; however, some countries under the NANP differ in their support of some procedures (for example, LATA is meaningless within Canada. Also, RBOCs support Telcordia procedures not fully specified by ANSI.) Some countries outside the NANP support ANSI-based variants (e.g. Mexico).
While these variations of ISUP differ in subtle ways, the vast majority of ISUP message type, parameter type, and parameter field code-points, and related fundamental call processing procedures, agree across all variants.

### ITU-T specification versions

* 1980 – ISUP Yellow Book
* 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 (e.g. 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 and the ISUP message type, followed by a mandatory fixed-length parameter part, a mandatory variable-length parameter part, and an optional parameter 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 release. 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 number, type of service (speech or data) and optional parameters.
* Subsequent address message (SAM) — For networks that support overlap dialing procedures, and then in the case that the IAM did not contain the full called number, one or more SAMs follow containing additional digits. This message is not supported by networks that only support en bloc dialing procedures.
* Address complete message (ACM) — Message returned from the terminating switch when the subscriber is reached and the phone starts ringing, or when the call traverses an interworking point and the intermediate trunk is seized.
* Call progress (CPG) — Contains additional information about the progress of a call. Normally sent after the ACM when the status of the call changes from that reported in the ACM.
* Answer message (ANM) — Sent when the subscriber picks up the phone, a resource is connected or answer supervision is returned by an interworking point. Normally charging starts at this moment. It is required that the call be cut through in both directions by this point.
* Connect (CON) — Sent when the call is answered by an automatic terminal. This message replaces the ACM, CPG and ANM for calls that are answered by automatic terminals.
* Release (REL) — Sent to clear the call when a subscriber goes on hook. This is also sent (in direct response to an IAM) if the terminating switch determines that the call cannot be completed. In either case, the terminating switch provides a cause value in the message to explain the reason for the release, e.g., "User busy".
* Release complete (RLC) — Acknowledgment of the release – the circuit is idle afterward 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.
Detailed call flows are provided in ITU-T Recommendation Q.784.1.
Below is a detailed exchange of ISUP messages involving two ISDN telecom switches. The report was from an Alcatel S12 digital switch.

## Cause codes

Release cause codes are used to identify and debug any events occurring in ISDN User Part signaling. Every event in ISUP signaling generates a cause code number. Even for a normal ISUP call, a cause code is generated. There are lot of applications developed based on the cause code from ISUP signaling. Similarly Telecom operators trace for Causecodes to debug any call failures.
Following are the list of cause codes used. Cause codes only defined by number are effectively undefined, and may be used for proprietary solutions.
* Unallocated (unassigned) number
* No route to specific transit network
* No route to destination
* Send special info tone
* Misdialed trunk prefix
* Channel unacceptable
* Call awarded and being delivered in established channel
* Preemption
* Preemption – circuit reserved for reuse
* 10
* 11
* 12
* Suspend
* Resume
* 15
* Normal call clearing
* User busy
* No user responding
* No answer from user (user alerted)
* Subscriber absent
* Call rejected
* Number changed
* Redirect to new destination (unallocated destination number ANSI)
* Unknown business group (ANSI)
* Exchange routing error (ANSI)
* Non-selected user clearing
* Destination out of order
* Invalid number format
* Facility rejected
* Response to STATUS ENQUIRY
* Normal, unspecified
* 32
* 33
* No circuit/channel available
* 35
* 36
* 37
* Network out of order
* Permanent frame mode connection out of service
* Permanent frame mode connection operational
* Temporary failure
* Switching equipment congestion
* Access information discarded
* Requested channel/circuit not available
* Preemption (ANSI)
* Precedence call blocked
* Resources unavailable, unspecified
* 48
* Quality of service unavailable
* Requested facility not subscribed
* Call type incompatible with service request (ANSI)
* 52
* Outgoing calls barred within CUG
* Call blocked due to group restrictions (ANSI)
* Incoming calls barred within CUG
* 56
* Bearer capability not authorized
* Bearer capability not presently available
* 59
* 60
* 61
* Inconsistency in designed outgoing access information and subscriber class
* Service or option not available, unspecified
* 64
* Bearer capability not implemented
* Channel type not implemented
* 67
* 68
* Requested facility not implemented
* Only restricted digital bearer capability is available
* 71
* 72
* 73
* 74
* 75
* 76
* 77
* 78
* Service or option not implemented, unspecified
* 80
* Invalid call reference value
* Identified channel does not exist
* A suspended call exists, but this call identity does not
* Call identity in use
* No call suspended
* Call having the requested call identity has been cleared
* User not member of CUG
* Incompatible destination
* 89
* Non-existing CUG
* Invalid transit network selection
* 92
* 93
* 94
* Invalid message, unspecified
* Mandatory information element is missing
* Message type non-existing or not implemented
* Message incompatible with call state or message type non-existent or not implemented
* Information element non-existent or not implemented
* Invalid information element contents
* Message not compatible with call state
* Recovery on timer expiry
* Parameter non-existent or not implemented - passed on
* 104
* 105
* 106
* 107
* 108
* 109
* Message with unrecognized parameter discarded
* Protocol error, unspecified
* 112
* 113
* 114
* 115
* 116
* 117
* 118
* 119
* 120
* 121
* 122
* 123
* 124
* 125
* 126
* Interworking, unspecified.

## Message format

| 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |
| Routing Label ... ||||||||
| CIC Least Significant 8 Bits ||||||||
| Padding | Padding | Padding | Padding | CIC Most Sig. 4 Bits | CIC Most Sig. 4 Bits | CIC Most Sig. 4 Bits | 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 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.
* 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.