SS7 SIGNALING UNITS (Cont.)

 

MSU Fields

A Message Signaling Unit (MSU) is composed of six (6) fields (plus a "spare"). Each field contains specific data for use by the various levels in an SS7 stack. Data is provided in a digital format (i.e., through patterns of 1s and 0s).

The order in which these fields are transmitted from one signaling point to another is:

 

Flag

Every MSU has a flag that indicates the beginning of a new signal unit and the end of a previous one. MTP Level 2 recognizes a flag by its unique bit pattern (0111 1110). The flag's bit pattern is not repeated anywhere else in an MSU.

 

 

 

Sequencing Control Field

The Sequencing Control field actually comprises four (4) fields, which are used to ensure that an MSU reaches the destined signaling point:

• Forward Sequence Number (FSN)
• Backward Sequence Number (BSN)
• Forward Indicator Bit (FIB)
• Backward Indicator Bit (BIB)

MTP Level 2 reads these fields. Without them, a signaling point would not know whether an MSU was lost during transmission. The signaling point would also be unable to communicate with the other point to request re-transmissions.

 

Forwarding Sequence Number

The Forward Sequence Number (FSN) is set by the transmitting signaling point to provide a unique identifier for an MSU. The Backward Sequence Number (BSN) is set by the signaling point that responds to the receipt of an MSU. The BSN is equal to the FSN used in the acknowledged MSU.

 

The Forward Indicator Bit (FIB) and the Backward Indicator Bit (BIB) are used to indicate a lost MSU(s). The transmitting signaling point sets the FIB, which remains constant. If the signaling point that receives the MSU returns an acknowledgement with a BIB that does not match the utilized FIB, the transmitting signaling point will know it must re-transmit an MSU(s).

 

More detail on the Sequencing Control field is provided in the "Message Errors" section of this module.

 

 

Length Indicator Field

MTP L2 uses the Length Indicator (LI) field to determine the type of signaling unit being received (i.e., LSSU, FISU, or MSU). MTP L2 must identify the type of signaling unit it is receiving to determine whether any further action should be taken in response.

 

 

 

Service Information Octet Field

When MTP Level 2 identifies an MSU, it passes the MSU to MTP Level 3. MTP Level 3 must identify the MSU type, using the Service Information Octet (SIO) field to determine whether the MSU is for the SCCP, ISUP, or MTP at that signaling point.

 

 

 

Signaling Information Field

The Signaling Information Field (SIF) tells MTP L3 whether an MSU is destined for that signaling point or another. The SIF also carries the data that will be used by MTP, ISUP, SCCP, or TCAP to perform various procedures and functions.

 

 

Some components of the SIF are the same, regardless of whether an MSU is for MTP, ISUP, SCCP, or TCAP. Others, however, are only used in certain types of MSUs.

 

Note: TCAP is a user of SCCP; therefore, TCAP information is transmitted in the SIF of an SCCP message.

 

 

Destination Point Code (DPC) Field

When MTP Level 3 receives an MSU, it must know whether the message is destined for that signaling point. The Destination Point Code (DPC) is used to determine this. If the number in this field matches the point code of the local signaling point, then MTP Level 3 will know that it is for local use.

MTP Level 3 must also know the MSU's origin so that responses can be sent to that originating signaling point. The Origination Point Code (OPC) provides this information and contains the point code of the signaling end point (i.e., SSP or SCP) that generated the MSU.

 

Note: More detail on how point codes are used to route messages is provided in the "Inbound Messages" section of this module.

 

 

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