SS7 SIGNALING UNITS (Cont.)

Signaling Link Selection (SLS) and Signaling Link Code (SLC) Fields

When an MSU must be sent to another signaling point, MTP Level 3 must inform MTP Level 2 of the link on which to send the MSU. This information is carried in the Signaling Link Selection (SLS) for SCCP and ISUP messages, and in the Signaling Link Code (SLC) field for MTP messages. This significant difference between the SLS and the SLC is that the SLS is a "virtual" link number and the SLC is a "physical" link number.

Note: SLS, SLC, and the relationship between them are discussed further in the "Link Selection" section of this module.

Message Type Field

Whether an MSU is for ISUP or for SCCP, the Message Type field provides the respective router with a unique identification for the MSU. However, for ISUP MSUs, this field also tells the ISUP router what type of ISUP message it is (e.g., Release Message used during a call tear down).

For SCCP, the Message Type field includes information that the SCCP router needs to determine the structure of the rest of the message.

MTP Management Information Field

When it arrives at a signaling point for MTP, the MSU uses information in the MTP Management Information field to perform its network management functions. This field provides data on actions and procedures that must be executed by MTP to:

• Initialize and monitor signaling links
• Perform auto recovery of links
• Maintain routing schemes to remote signaling points

Note: Network Management responsibilities are discussed in greater detail in the "Network Management" section of this module.

Circuit Identification Code (CIC) Field

ISUP's function is call control, which means controlling voice circuits and identifying which circuit to use when setting up and tearing down a call. ISUP gathers this information from the Circuit Identification Code (CIC) field. This field carries the code for the voice/data circuit that will be used for a call between two SSPs.

Message Information Field

In addition to basic call setup and teardown, ISUP provides supplementary services (e.g., forwarding of a call to a secondary number). The Message Information field in an ISUP MSU provides data needed for the ISUP application to perform these services.

Mandatory Fixed/Variable Part

The Mandatory Fixed/Variable Part of an SCCP MSU varies widely, depending upon the purpose of the message. In a basic sense, this field provides parameters that the SCCP router will use.

User Data Field

The User Data field of an SCCP MSU provides actual data needed by SCCP users. For example, it is data from this field that SCCP sends to the TCAP router. The target TCAP application uses this data.

Cyclic Redundancy Check (CRC) Field

The fact that a message is received does not necessarily mean that it has avoided corruption during transmission. MTP Level 2 uses the Cyclic Redundancy Check (CRC) field to verify that the message passed across the link error-free.

Before a message is sent, the transmitting signaling point calculates a check sum value from the message fields, and then inserts this value in the CRC field. Upon receiving the MSU, the destination point performs the same calculation and expects to produce the same value found in the CRC field. If the calculated result is different, the received message is considered corrupt. It is discarded and a re-transmission is requested.

Note: This field is also sometimes called the "Check Sum" or "Check Bit."

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