SS7 INBOUND MESSAGES

 

When a signaling point receives a message, the message must either be distributed up the stack or, in the case of an STP, routed outbound to another signaling point. When handling an inbound message, MTP Level 2 will read the LI field to determine message type. If the message is an MSU, it is sent upward to MTP Level 3. MTP Level 3 then reads the SIO and SIF fields to determine for which service the MSU will be used and whether it resides at that signaling point.

 

Length Indicator Field

Each type has a different "length." Signaling unit length is the number of octets (i.e., 8-bit bytes) between the Length Indicator (LI) and Cyclic Redundancy Check (CRC) fields. This length value is carried in the LI field of a signaling unit.

 

When a signaling unit arrives, MTP Level 2 checks the value in the LI field to determine its type. For example, if the value in the LI field is either "1" or "2," MTP Level 2 knows the signaling unit is an LSSU.

The 3 types of signaling units and their corresponding LI values are:

• Fill In Signal Unit (FISU) = 0
• Link Status Signal Unit (LSSU) = 1 or 2
• Message Signal Unit (MSU) = 3 through 63

 

Service Information Octet Field

If it identifies a signaling unit as an MSU, MTP Level 2 sends the unit to MTP Level 3. It is at this level that MSU service type is determined (i.e., MTP, SCCP, TUP, or ISUP).

 

MTP Level 3 uses the value in the SIO field to determine which level of the stack will be using the MSU. For example, if the value in the SIO field is "5," then the MSU is for ISUP.

Values for the different MSU service types are:

• 0, 1, 2 = MTP
• 3 = SCCP
• 4 = TUP (no longer widely used)
• 5 = ISUP

 

Signaling Information Field

If MTP Level 2 identifies a signaling unit as an MSU, it sends the unit to MTP Level 3. MTP Level 3 checks the destination point code (DPC), located in the MSU's SIF. If the DPC matches the point code of the local signal point, the message is distributed to the appropriate layer in the stack. If the DPC does not match, the MSU is routed out on the appropriate link to the destined signaling point.

 

More About Point Codes

All types of signaling units carry an Origination Point Code (OPC) and a Destination Point Code (DPC). Simply defined, point codes are unique identification numbers for signaling points. When it places its OPC in a message, a signaling point functions as a "return address"; the DPC functions as the "mailing address."

 

Point code format in an ANSI network is different from that used in a CITTU network.

Formats are as follows:

 

NNNN (CITTU) - e.g., 1234

Digits in a CITTU point code tell MTP Level 3 the following about a signaling point's location:

• Zone
• Area/Network
• Signaling point ID

NNN-NNN-NNN (ANSI) - e.g., 123-456-789

Digits in an ANSI point code tell MTP Level 3 the following about the location of a signaling point:

• Network ID number
• Network Cluster number
• Network Member number

 

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