1 Introduction

ISDN supplementary services allow an AG ISDN application to implement powerful functionality beyond basic call control. These functions include:

An AG ISDN application can access supplementary services using the standard AG ISDN Messaging API. Supplementary services messages are included in standard ACU messages sent to or received from the AG ISDN protocol stack. Each service specification and any associated data is stored in an extended data area appended to the ACU message buffer. Supplementary services can be invoked with any primitive passing over the ACU interface.

One primitive may contain multiple extended data structures. This allows the application to invoke multiple services with one primitive, or to receive multiple indications with one primitive coming up from the ACU interface. For details, see Chapter 2.

ISDN supplementary services are implemented differently under various variants. AG ISDN supports a subset of the supplementary services in two variants:

ETS 300 specifications describe a protocol designed to access network services from an intelligent user terminal. The protocol is not symmetric. It requires two distinct roles, the user side and the network side. AG ISDN supports both sides for basic call control. Supplementary services, at this time, have been implemented only for the user side.

Figure 1 shows a sample network, and the points where an AG ISDN ETS 300 supplementary service application can interface with the network. As shown in the figure, an AG ISDN application written for the ETS 300 variant interfaces with the network on the user side of the S/T reference point of a CEPT E1 PRI ISDN trunk.

Figure 1: Network Diagram Showing Position of ETSI Application

Supplementary services over the T reference point are generally requests by the application for the network to do something on behalf of a subscriber or interface (e.g. transfer a call).

Q.SIG specifications describe interactions between nodes in a private ISDN network (PISN). Each node is a Private ISDN Network Exchange (PINX). These nodes play identical roles in the network. Q.SIG is a symmetric protocol because there is no distinct user side and network side.

Figure 2 shows a sample Q.SIG network, and the reference points where an AG ISDN Q.SIG supplementary service application can join into the network. As shown in the figure, an AG ISDN application written for this variant interfaces with the network at the Q reference point. The role of the application is to implement call control, and control message exchanges at this point.

Figure 2: Network Diagram Showing Position of Q.SIG Application

Figure 3 shows the part of an AG ISDN application that is the Q reference point.

Note: Application code related to local access (analog lines, etc.) is not considered part of the Q reference point.

Figure 3: Q Reference Point

Supplementary services over the Q reference point are generally requests by the application for services from another node in the network, or notifications that the local node has performed various services.

Note: If you are building a Q.SIG application, the PINX node address must be specified in your initial call to isdnStartProtocol. For more information, see Section 3.10.

This section describes the relationship of the AG ISDN supplementary service implementation to ETSI specifications.

The NMS ETS 300 variant implementation is a reference to the Digital Subscriber Signalling System No. One (DSS1) European Telecommunications Standard (ETS) produced by the Signaling Protocols and Switching (SPS) Technical Committee of the European Telecommunications Standards Institute (ETSI). The standard can be catagorized as a basic Q.931 protocol with the addition of ASN.1 facilities to support ISDN supplementary services.

Below is a partial list of the documents describing supplementary services as they are implemented in the NMS ETSI supplementary service package:

Q.SIG specifications are produced by a consortium of standards-producing bodies, including ETSI, the European Computer Manufacturer's Association (ECMA), and the International Organization for Standardization (ISO). The production effort is coordinated by the IPNS Forum.

In the ISO specifications, Q.SIG is referred to as Private Signaling System No. One.

The Q.SIG specifications identify different types of Private ISDN Network Exchanges (PINXs), and different supplementary services. This creates a conformance "matrix."

The Q reference point is different than typical reference points, in that it describes the functions of a part of the network, rather than describing a point of interface to the network. The reference point location also implies that there is no user or network side of a connection. Rather, all Q.SIG signaling is symmetric between adjacent nodes.