Signaling System No. 7 (SS7/C7) - Protocol, Architecture and Services (Full Book)
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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:
Because the parameter is not a fixed length, a field is included to specify the actual length.
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.
Figure 8-10. ISUP Message Format
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:
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.
Figure 8-11. IAM Message Format
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:
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:
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 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.
Subsequent Address Message (SAMITU 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.
Figure 8-12. SAM Message Format
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.
Figure 8-13. COT Message Format
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."
Figure 8-14. ACM Message Format
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.
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.
Figure 8-15. REL Message Format
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)
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