ISUP Message Format

The User Data portion of the MTP3 Signaling Information Field contains the ISUP message, identified by a Service Indicator of 5 in the MTP3 SIO field. Each ISUP message follows a standard format that includes the following information:

• CIC— The Circuit Identification Code for the circuit to which the message is related.

• Message Type— The ISUP Message Type for the message (for example, an IAM, ACM, and so on).

• Mandatory Fixed Part— Required message parameters that are of fixed length.

• Mandatory Variable Part— Required message parameters that are of variable length. Each variable parameter has the following form:

  - Length of Parameter

  - Parameter Contents

Because the parameter is not a fixed length, a field is included to specify the actual length.

• Optional Part— Optional fields that can be included in the message, but are not mandatory. Each optional parameter has the following form:

  - Parameter Name

  - Length of Parameter

  - Parameter Contents

Figure 8-10 shows the ISUP message structure, as described here. This message structure provides a great deal of flexibility for constructing new messages. Each message type defines the mandatory parameters that are necessary for constructing a message. The mandatory fixed variables do not contain length information because the ISUP standards specify them to be a fixed length. Because the mandatory variable parameters are of variable lengths, pointers immediately follow the mandatory fixed part to point to the beginning of each variable parameter. The pointer value is simply the number of octets from the pointer field to the variable parameter length field.

In addition to the mandatory fields, each message can include optional fields. The last of the pointer fields is a pointer to the optional part. Optional fields allow information to be included or omitted as needed on a per-message basis. The optional fields differ based on variables such as the call type or the supplementary services involved. For example, the Calling Party Number (CgPN) field is an optional parameter of the IAM, but is usually included to provide such services as Caller ID and Call Screening.

A single message can include many optional parameters. The optional part pointer field only points to the first parameter. Because the message might or might not include the parameters, and because the parameters can appear in any order, the first octet includes the name of each parameter in order to identify it. The parameter length follows the name to indicate how many octets the parameter contents include. When the parameter name is coded as zero, it signals the end of the optional parameters. During parsing of an incoming ISUP message, optional parameters are processed until the end of optional parameters marker is reached. If the message does not have any optional parameters, the pointer to the optional part is coded to zero.

Basic Call Message Formats

Here, we examine the six messages shown in the basic call setup because they comprise the core message set for basic call setup and release, and are therefore used frequently. There are slight variations in the messages used based on the individual network. For example, Europe uses the SAM frequently and the COT message more rarely. In North America, SAM is not used at all, but COT is used more often. This section considers the following messages:

• Initial Address Message (IAM)

• Subsequent Address Message (SAM–ITU Networks only)

• Continuity Message (COT)

• Address Complete Message (ACM)

• Answer Message (ANM)

• Release Message (REL)

• Release Complete Message (RLC)

The following sections show only the mandatory fields for each message. Keep in mind that many optional parameters can also be included. In each of the figures, the fixed mandatory fields with sub-fields have been expanded to show what they are. For the sake of brevity in the figures, the variable subfields have not been expanded. All of the ISUP Message formats and parameters are documented in ITU-T Q.763. ANSI T1.113 documents the North American version of the messages.

Initial Address Message (IAM)

The IAM contains the information needed to set up a call. For a basic call, it is the first message sent and is typically the largest message in terms of size. Figure 8-11 shows the mandatory fields that the message includes. In addition to the mandatory fields, the ITU-T Q.764 lists more than 50 optional parameters that can be included in the IAM. The mandatory parameters for ITU and ANSI are the same, with the exception of the Transmission Medium Requirements parameter. In ANSI networks, the User Service Info field is used instead.

As shown in Figure 8-11, the Nature of Connection Indicators (NOC) pass information about the bearer circuit connection to the receiving node. The indicators consist of the following subfields:

• Satellite Indicator— Specifies whether one or more satellites have been used for the circuit connection that is being set up. This information is useful when setting up calls to prevent an excessive number of satellite hops, which can reduce the quality of calls.

• Continuity Indicator— Designates whether to perform a continuity check on the circuit being set up.

• Echo Control Device Indicator— Specifies whether echo suppression is used on the circuit. Echo suppression is used to increase the quality of voice calls by reducing echo, but it can damage data and fax calls because it subtracts a portion of the voice-band signal.

The Forward Call Indicators (FCI) contain information that specifies both the preferences about call setup in the forward direction and the conditions encountered so far in setting up the call. They include the following subfields:

• National/International Call Indicator— Indicates whether the call is coming in as National or International. International calls are specified by ITU international procedures, and national calls are processed according to national ISUP variant standards.

• End-to-End Method Indicator— Indicates the method used for signaling end-to-end information. SCCP and pass-along are the two end-to-end methods that are used. The pass-along method traverses each node in the connection to deliver information to the correct node. The SCCP method uses connectionless signaling to send information directly to its destination.

• Interworking Indicator— Indicates whether the connection has encountered interworking with non-SS7 facilities (for example, MF trunks). Interworking with non-SS7 facilities can limit or prohibit the capability of supplementary services or certain call types that require SS7 signaling.

• End-to-End Information Indicator— Indicates whether any end-to-end information is available.

• ISDN User Part Indicator— Indicates whether ISUP has been used for every leg of the connection. Note that this is not the same as the Interworking Indicator. It is possible to have an SS7-signaled circuit, but not use ISUP (for example, TUP signaling); however, if interworking has been encountered, this indicator is set to ISDN User Part not used all the way.

• ISDN User Part Preference Indicator— Specifies whether an ISUP facility is required or preferred when choosing an outgoing circuit. Some supplementary services or call types are not possible over non-ISUP facilities. If ISUP is required but not available, the call is released because the requested facility's preference cannot be met. If the preference indicator is set to preferred, an ISUP facility is chosen, if available; however, the call is still set up as long as a facility is available, even if it is not ISUP.

• ISDN Access Indicator— Indicates whether the originating access is ISDN or non-ISDN. ISDN provides a much richer interface to services that is not available on plain analog lines. This indicator suggests that the ISDN interface is available so that end-to-end signaling, backward requests for information, and so on can be carried out.

• SCCP Method Indicator— Indicates which method, if any, is used for SCCP end-to-end signaling. SCCP might use connection-oriented, connectionless, or both.

The Calling Party's Category specifies a general category into which the calling party is classified—such as an ordinary calling subscriber, operator, payphone, or test call.

The Transmission Medium Requirement (TMR) is not applicable to ANSI networks and is only supported in ITU-T networks. It contains the requirements for the bearer circuit capabilities (speech, 3.1-kHz audio, 64-Kb unrestricted, and so forth) that are needed for the call being set up. For example, a video conference might require a 384-Kbs unrestricted circuit to guarantee an acceptable level of video quality.

User Service Information (USI) is used in ANSI networks instead of the ITU-T specified TMR. It contains the requirements for the bearer circuit capabilities (speech, 3.1-kHz audio, and 64-Kbs unrestricted) along with additional information such as layer 1 codec, circuit, or packet transfer mode and other bearer-related specifics.

The Called Party Number (CdPN) is the destination number that the calling party dials. The CdPN contains the following fields:

• Odd/Even Indicator— Indicates an odd or even number of digits in the CdPN.

• Nature of Address Indicator— Indicates the type of number (for example, National Significant Number or International). The receiving switch uses this indicator during translations to apply the number's proper dial plan.

The Internal Network Number Indicator (INN), which is not used for ANSI, specifies whether routing to an internal network number is permitted. This field is used to block routing to specific numbers that should not be directly accessible from outside of the network. For example, if a premium rate number is translated to an internal number, the subscriber is blocked from dialing the internal number to ensure that the appropriate premium rate charges are collected.

• Numbering Plan Indicator— Specifies the type of number plan used. The E.164 ISDN numbering plan is commonly used for voice calls.

• Address Signals— The actual digits that comprise the called number. This includes digits 0–9 and the overdecadic digits (A–F), however, the overdecadic digits are not supported in all networks. Each digit is coded as a four-bit field.

Subsequent Address Message (SAM–ITU Networks Only)

Shown in Figure 8-12, the SAM is used to send subsequent address signals (digits) when using overlap signaling for call setup. It has one mandatory variable parameter: the subsequent number. One or more SAMs can be sent after an IAM to carry subsequent digits for call setup that are part of a destination's complete telephony number.

Continuity Message (COT)

As shown in Figure 8-13, the COT message contains the results of a continuity test. It has only one field: the Continuity Indicators. This field uses a single bit to indicate whether a continuity test passed or failed. The test's originator sends the message to the far end of the circuit that is being tested.

Address Complete Message (ACM)

As shown in Figure 8-14, a destination node sends the ACM to indicate that a complete CdPN has been received. When enbloc signaling is used to set up the call, the ACM is sent after receiving the IAM; when overlap signaling is used, it is sent after the last SAM is received. In addition to indicating the successful reception of the CdPN, the ACM sends Backward Call Indicators (BCI) to signal information about the call setup. It is not mandatory for an ACM to be sent when setting up a call. It is permissible to send an ANM after receiving an IAM; this is sometimes referred to as "fast answer."

Many of the fields contained in the Backward Call Indicators are the same as those in the Forward Call Indicators (FCI), which are contained in the IAM. While the FCI signals the call indicators in the forward direction to provide information on the call setup to the terminating access (and intermediate nodes), the BCI signals similar information in the backward direction to the originator.

Here we discuss only the fields that are unique to the BCI. The remaining fields are the same as those we discussed for the FCI, except that they are representative of the call from the terminating end. For example, the ISDN Access Indicator specifies whether the "terminator" is ISDN.

• Charge Indicator— Indicates whether a call should be charged as determined by the charging exchange.

• Called Party's Status Indicator— Indicates whether the subscriber is free.

• Called Party's Category Indicator— Indicates the general category of the called party, an ordinary subscriber, or payphone.

• Holding Indicator— Indicates whether holding is required. This indicator can be used for special services, such as Operator Signaling Services or Malicious Call Tracing, to indicate that the incoming connection should be held. No specification for ANSI networks exists.

Answer Message (ANM)

The ANM is sent to the previous exchange when the called party answers (off-hook). Although it might contain many optional parameters, the ANM does not contain any mandatory fields other than the message type.

Release Message (REL)

As shown in Figure 8-15, the REL message indicates that the circuit is being released. When a RLC has been received in response, the circuit can be returned to the idle state for reuse. The REL message can be sent in either direction. It contains a single mandatory Cause Indicators field to indicate why the circuit is being released.

Cause Indicators specify the cause information associated with the circuit being released. The Cause Indicators contain the general location in the network (such as local, remote, or transit) in which the circuit was released. The Coding Standard indicates which standard is used for decoding the Cause Value (such as ANSI, ITU). ANSI and ITU define some cause values differently, and ANSI also has additional values the ITU does not include.

The Cause Value contains an integer that represents the reason the circuit is being released. This value can be further decomposed into a class and a value. The most significant three bits of the Cause Value field represent the class. Each class is a general category of causes; for example, binary values of 000 and 001 are normal event class, and a value of 010 is resource unavailable. So, a cause value of 1 (unallocated number) is in the normal event class and a cause value of 34 (no circuit available) is in the resource unavailable class. Appendix M, "Cause Values," contains a complete list of the ITU and ANSI cause values.

The Diagnostics field is only applicable to certain cause values. It provides further information pertaining to the circuit release (for example, Transit Network Identity, Called Party Number [CdPN]) for those cause values.

Release Complete Message (RLC)

The RLC message is sent to acknowledge a REL message. Upon receipt of an RLC, a circuit can return to the idle state.