Overview

The following topics provide an overview of TCAP and how it is used to provide enhanced network services:

• Generic service interface

• Role of TCAP in call control

• TCAP within the SS7 protocol stack

• Transaction and component sublayers

Generic Service Interface

TCAP is designed to be generic to accommodate the needs of a wide variety of different services. This chapter focuses on understanding these generic mechanisms. Chapter 11, "Intelligent Networks (IN)," examines the prominent network services that use TCAP in an effort to understand how services use these generic mechanisms. Some common services that use TCAP include number translation services, such as Enhanced 800 Service (toll-free) and Local Number Portability (LNP). Other examples of TCAP users are Custom Local Area Signaling Services (CLASS), Mobile Wireless, and Advanced Intelligent Network (AIN) services. Figure 10-1 shows how TCAP uses standardized components as the basic building blocks for services across network nodes.

Figure 10-1. Standardized Components Used to Create a Generic Interface

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Most TCAP services can be viewed as a dialogue of questions and answers. A switch needs additional information that is associated with call processing, or with a particular service that causes it to send a TCAP query that requests the needed information. As shown in Figure 10-2, the answer returns in a TCAP response, which provides the necessary information, and normal call processing or feature processing can resume. The query for information can be sent to a Service Control Point (SCP) or to another SSP, depending on the type of service and the information required. The SCP is an SS7-capable database that provides a centralized point of information retrieval. It typically handles number translation services, such as toll-free and LPN; however, SCPs are also used for a number of additional IN/AIN services.

Role of TCAP in Call Control

TCAP is used to provide information to SSPs. This information is often used to enable successful call completion, but TCAP is not involved in the actual call-setup procedures. The protocol's circuit-related portion, such as ISUP and TUP, perform the call setup. This interaction between the service information provided by TCAP and the circuit-related protocol that performs the call setup occurs at the application level, not at the SS7 protocol layer. Within the SSP, the switching software that is responsible for call processing interacts with both the TCAP side of the SS7 stack and the call setup side of the stack (ISUP, TUP) to complete the call.

TCAP Within the SS7 Protocol Stack

As shown in Figure 10-3, TCAP is at level 4 of the SS7 protocol stack. It depends upon the SCCP's transport services because TCAP itself does not contain any transport information. First, SCCP must establish communication between services before TCAP data can be delivered to the application layer. Refer to Chapter 9, "Signaling Connection Control Part (SCCP)," for more information on SCCP's transport services. TCAP interfaces to the application layer protocols above it, such as the ITU Intelligent Network Application Part (INAP), ANSI AIN, and ANSI-41 Mobile Switching to provide service-related information in a generic format. The application layer that passes information down to be encapsulated within TCAP is known as a Transaction Capability User (TC-User). The terms application, service, and TC-User are used interchangeably.

Transaction and Component Sublayers

The TCAP message is composed of two main sections: the transaction sublayer and the component sublayer. A transaction is a set of related TCAP messages that are exchanged between network nodes. The transaction portion identifies the messages that belong to the same transaction using a Transaction Identifier (TRID). The message's component portion contains the actual instructions, or "operations," that are being sent to the remote application. This chapter examines both areas in detail, along with the procedures surrounding their use.