ISUP interworking with ISDN

Interworking with ISDN

ISDN uses a common channel (the D channel) for access signaling; this compliments the common channel network signaling ISUP uses and provides a complete digital signaling path between end users when ISDN is used for network access and ISUP is used throughout the core network. The ISUP/ISDN interworking specifications for ITU-T, ETSI, and Telcordia are found in the following standards:

• ITU-T Q.699—Interworking of Signaling Systems—Interworking Between Digital Subscriber Signaling System No. 1 and Signaling System No. 7
• ETSI EN 300-899-1 Integrated Services Digital Network (ISDN); Signaling System No. 7; Interworking Between ISDN User Part (ISUP) Version 2 and Digital Subscriber Signaling System No. one (DSS1); Part 1: Protocol Specification
• Telcordia GR-444 Switching System Generic Requirements Supporting ISDN Access Using the ISDN User Part

A correlation exists between the ISDN messages from the user premises and the ISUP messages on the network side of the call. Figure 8-18 illustrates this correlation using an ISDN-to-ISDN call over an ISUP facility. Table 8-1 lists the message mapping that occurs between the two protocols for the basic call setup shown in the diagram.

http://www.informit.com/content/images/bok_1587050404/elementLinks/th08fig18.gif

Fig: ISUP-ISDN Interworking

Message Mapping Between ISDN and ISUP

Many of the fields within these messages also have direct mappings. For example, the bearer capability field in the ISDN Setup message maps to the ANSI User Service Info or the IAMs ITU Transmission Medium Requirements field. There are fields that have no direct mapping, such as the NOC Indicators and FCIs in the IAM. Many of the fields that do not have direct mapping contain network-specific information that would not be useful for the ISDN signaling endpoint.

End-to-End Signaling

The ability to perform end-to-end signaling is accomplished using ISDN access signaling and ISUP network signaling. End-to-end signaling is the passing of information across the network that is only pertinent to the two communicating endpoints. Generally, this means that the two phone users are connected across the network. The network itself can be viewed as a communications pipe for the user information.

There are two different methods for end-to-end signaling over ISUP: the Pass Along Method (PAM) and the SCCP Method. As shown in Figure 8-19, PAM exchanges end-to-end signaling by passing along information from one node to the next, based on the physical connection segments. The SCCP method uses a call reference to pass end-to-end data between endpoints without having to pass through each individual hop. PAM is the method that is currently used throughout the network for end-to-end signaling.

ISDN Signaling Indicators in the IAM

The following set of fields in the IAM FCI comprises what is known as the Protocol Control Indicator (PCI):

• End-to-end method indicator
• Interworking indicator
• IAM segmentation indicator
• ISDN User Part indicator

These fields provide information about the protocol communication across the ISUP connection. The Protocol Control Indicator fields are of particular importance to ISDN because they identify whether ISDN signaling can be exchanged across the network. If the Interworking Indicator is set to interworking encountered, it indicates that a non-SS7 connection (such as MF signaling) has been used in a circuit connection. It also indicates that SS7 signaling cannot be exchanged across this connection because it would prevent an ISDN terminal from being able to relay signaling across the network that depended on an SS7 connection all the way.

The ISDN User Part indicator field indicates whether ISUP has been used for every call leg up to the current exchange. If this field is set to ISDN User Part not used all the way, it might not be possible to pass ISDN information across the network.

The ISDN User Part preference indicator field indicates to the receiving node whether the call needs an outgoing ISUP connection.

The preference field might contain the following values:

• ISDN User Part preferred
• ISDN User Part required
• ISDN User Part not required

For calls originating from an ISDN set, the preference field is set to ISDN User Part preferred unless specified otherwise by different services. If it is available during outgoing trunk selection, an ISUP facility is chosen; an ISUP facility is "preferred," but not necessarily required. If an ISUP facility is not available, the call is still set up if a non-ISUP facility is available. If a call is being established that requires the ability to pass service information—such as end-to-end signaling—across the network, the preference field is set to ISDN User Part required. A call with a preference of "required" is not set up unless an ISUP facility is available. For example, setting up a multichannel ISDN video connection would not be possible without end-to-end ISUP signaling.

Although the PCI provides information about the connection across the network, it does not specify the actual protocol of the access signaling. The FCI includes the ISDN access indicator bit to indicate whether the originating terminal is an ISDN set.