ISDN User Part
The ISDN User Part (ISUP) provides signaling support for all voice and non-voice services identified as part of ISDN. ISUP includes all of the flexibility found in the message-oriented signaling of ISDN’s Q.931 message format. It also extends beyond simple call establishment and disestablishment to include network management.
ISUP provides all basic services necessary for call setup and management. It also supports supplementary services such as message transport after a circuit has been established.
Although the ITU-T has specified generalized formats of ISUP messages, Recommendation Q.763 refers to a reserved coding space to allow for modifications required by a particular national telecommunications administration. ANSI T1.113 defines the coding and message structure found in North American SS7 networks.
ISUP Call Control Messages
Because ISUP is concerned with call establishment and disestablishment, the message types it uses are similar to those we find in Q.931 for ISDN call signaling.
Initial Address Message
The Initial Address Message (IAM) is the first sent in a call setup sequence. It contains the called and calling numbers, together with other information such as whether the calling number can be passed to the terminating subscriber, the identity of the interexchange carrier (IEC) to which the call should be passed, identity of the trunk to use between two circuit switches, etc. The assumption is that this message will contain all the dialed digits; this is usually referred to as en bloc signaling. However, the standard allows a subset of the dialed digits to be passed in the IAM and defines a Subsequent Address Message (SAM), one or more of which can be sent to convey the remaining digits.
An abbreviated representation of the Signaling Information field (SIF) of the initial address message (IAM) is shown on the visual. The first field in the SIF is the Routing Label, which contains the OPC, DPC and the SLS fields. Other fields are noted below.
- The Circuit Identification Code (CIC) field contains the identity of the trunk connection to which this message applies.
- The Message Type parameter indicates in this case that it is an IAM.
- Following the Message Type is a series of mandatory and optional parameters, some of which are shown on the visual.
- Nature of Connection indicates whether a continuity check is required on the circuit. It also indicates if a satellite link is permitted.
- Forward Call Indicators show whether ISUP is preferred or mandatory all the way.
- Calling Party’s Category indicates the operator’s language (e.g., French, English) and whether this is an ordinary subscriber or a subscriber with priority.
- User Service Information includes information from a Q.931 Bearer Capability Information Element, for example.
- Called Party Number contains the destination address and Calling Party Number contains the originating address together with an indicator of whether the calling number can be displayed at the destination.
- Transit Network Selection contains the Carrier Identification Code if the call must be handed off to an interexchange carrier.
Address Complete Message
The Address Complete Message (ACM) is sent by the terminating SSP to indicate that the subscriber’s phone is ringing.
The Answer Message (ANM) is sent by the terminating SSP to indicate the called subscriber has answered and the talk path should be completed (if it has not been already).
Call Progress (CPG) can be sent by the terminating SSP after it has sent an ACM. For example, this could be used to signal that a call has been forwarded as a result of a busy line.
Release and Release Complete
Release (REL) is sent to signal that one subscriber has disconnected and the circuit should be released immediately. Release Complete (RLC) is sent as an acknowledgement that a circuit has been released.
Continuity Check Request and Continuity
Continuity Check Request (CCR) and Continuity (COT) are sent to request a continuity check on a circuit between two switches and to report the success or failure of the test.
Information Request and Information
Information Request (INR) is sent usually by the terminating SSP to solicit additional call related information. Information (INF) is used to respond to the INR and contains the requested information.
Forward Transfer (FOT) is sent when an operator at the originating end of a call needs to signal the need for an operator near the destination to assist in completing the call.
In the visual, a voice call is being initiated in a PSTN and terminated in a VoIP network; note the signaling messages being exchanged over the SS7 portion of the network. The SS7oIP portion of the system is between the signaling gateway (SG) and the media gateway controller (MGC). The signaling gateway in this environment is transparent to the SS7 network. The initiating SP considers the MGC another signaling point attached to the SS7 network.
The ISUP messages are encapsulated in the M3UA protocol, which provides the same service access points as MTP3 for the ISUP messages. When the signaling gateway receives the initial address message (IAM) from the PSTN, it maps the signal point code found in the IAM to the MGC’s IP address. Once converted, the SG initiates an SCTP connection to the MGC. The MGC responds to the initiation message and then receives the IAM encapsulated in SCTP and M3UA.
The ISUP exchange follows normal processes for the telephone connection setup. Upon completion of the call, the SCTP connection is terminated between the signaling gateway and the media gateway controller.