Connectionless Services and Messages

SCCP provides the transport layer for the connectionless services of TCAP (discussed in the section entitled "Transaction Capabilities Applications Part [TCAP]"). TCAP-based services include 800, 888, 900, calling card, and mobile applications. Together, SCCP and MTP3 transfer non-circuit based messages used in these services. The SCCP also enables the STP to perform GTT on behalf of the end office exchange. The end office exchange views the 800 number as a functional address or, in other words, as a global title address. Because global title addresses are not routed, the SCCP in the end office exchange routes query messages to its home STP.

In this section, connectionless services are based on end office exchanges querying a database to obtain the routing number for an 800 number. The following is an example of how this works in the network.

Together, SCCP and MTP3 transport TCAP 800-based queries to centralized databases. The connectionless messages passed between the SCCP and MTP are called Unitdata Messages (UDTs) and Unitdata Service Messages (UDTSs).

The SCCP sends a UDT to transfer subsystem information, and it sends a UDT to perform the GTT function. UDTs also are used to query and receive responses from databases. Table 4-2 lists parameters used in the UDT message.

Table 4-2. UDT Parameters

Parameter

Type

Length (Octets)

Message Type

M

1

Protocol Class (PRC)

M

1

Called Party Address (CDA)

M

3 minimum

Calling Party Address (CGA)

M

3 minimum

Subsystem Data

M

Variable

Source: ITU-T Q.713 (7/96)

A UDTS is sent to the originating SCCP advising that the receiving SCCP was unable to deliver the UDT to its destination. The return cause parameter indicates why the UDT is being returned. Table 4-3 lists parameters used in the UDTS.

Table 4-3. UDTS

Parameter

Type

Length (Octets)

Message Type

M

1

Return Cause

M

1

CDA

M

3 minimum

CGA

M

3 minimum

Subsystem Data

M

Variable

Source: ITU-T Q.713 (7/96)

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