SS7 SCCP SERVICES

 

When MTP Level 3 sends an MSU from SCCP across the network, it checks the SLS field to determine which link it will use to send the MSU. SCCP sets the SLS, depending on the class of service an application (e.g., TCAP, SCCP) tells it to use for an MSU.

SCCP provides 2 classes of service:

• Connectionless
• Connection Oriented

The significant difference between these classes is that "connectionless" provides unidirectional data transmission, and "connection oriented" provides bidirectional data exchanges between SCCP users.

 

Connectionless

The 2 types of connectionless service are:

• Class 0 - Basic (Unsequenced) Connectionless
• Class 1 - Sequenced Connectionless

These services operate in the same basic manner. They both cause MTP Level 3 to send MSUs independently to their destinations, without needing to establish a bidirectional connection.

When an application that uses SCCP requests a connectionless service, the MSU is sent to the destined SCCP user at the remote point. If required, additional MSUs may be sent.

What is the difference between Class 0 and Class 1 service?

 

Class 0 is the basic, unsequenced delivery of MSUs to a remote point. Class 1 is the sequenced delivery of MSUs.

 

With Class 1 service, an SCCP user can request the in-sequence delivery of a series of MSUs to a specific remote point. This causes SCCP to set and keep the same SLS value for all MSUs sent by a specific application. If the SLS is kept the same, MTP Level 3 will send all MSUs from that application to the destined SCCP user through the same physical link. MTP Level 3 "knows" which link to use each time because of its SLS-SLC map.

 

With Class 0 service, the SLS is incremented by SCCP. This causes MTP Level 3 to send out all MSUs from the same application on whichever link is to be used next.

Note: The SLS-SLC map is discussed in detail in the "Link Selection" section of this module.

 

Connection Oriented

The two types of connection oriented service are:

• Class 2 - Basic Connection Oriented
• Class 3 - Flow Control Connection Oriented

With connection oriented services, the signaling connection between SCCP users may be established on a temporary or a permanent basis. While in use, the connection requires the same link to be used between the points until the event is complete. Therefore, the SLS set by SCCP will not be incremented, forcing MTP Level 3 to use the same link (i.e., SLC paired to the designated SLS).

The Class 2 SCCP service, Basic Connection Oriented, establishes a connection between two SCCP users to allow the in-sequence, segmented delivery of MSUs. Since the service allows in-sequence delivery, SCCP sets and keeps the same SLS value for all MSUs the application sends using the connection. When the SLS is kept the same, MTP Level 3 uses the same SLC to deliver MSUs between the SCCP users.

Note: The selection of links using SLS and SLC is covered in the "Link Selection" section of this tutorial.

 

With Class 2 service, SCCP is also allowed to "segment" MSUs, if necessary. This decreases the excessive amount of time that may be required to send some types of MSUs. Segmenting means breaking large MSUs (i.e., greater than 255 bytes) into one or more data-grams. The segmented MSUs are then sent to the remote SCCP and re-assembled before delivery to the user.

 

<< Back                                                                    Continue to SS7 TCAP Transactions >>