SS7 SIGNALING UNITS

 

The SS7 protocol stack consists of various layers. Since these layers must communicate with each other and with layers at other points on the network, a common "language" must be used. The SS7 protocol provides the standard that defines the language by requiring signaling points to communicate through signaling units.

 

Signaling units are packets of digital information that follow a specific format. They are composed of various fields, which differ depending on the type of signaling unit and its purpose.

The 3 types of signaling units are:

 

1. Fill In Signal Unit (FISU)

As their name implies, Fill In Signal Units (FISUs) are used to "fill in" periods of time when no other types of signaling units (i.e., MSU or LSSU) are being sent across a link. FISUs keep a link active so that all signaling points utilizing that link know that the link is operational. This allows immediate notification in the event that a link has failed or has been taken offline, even when the link has not been used for transmitting communication data.

 

Another function of a FISU is to act as an acknowledgement for an MSU. When a signaling point receives an MSU, it will send a FISU, which contains a confirmation, back to the originating signaling point. This also assists in detecting link and signaling unit errors.

                            

FISUs go no further up a stack than MTP Level 2. If a problem occurs over a link, MTP Level 2 will notify MTP Level 3, which, in turn, initiates corrective actions.

 

               

2. Link Status Signal Unit (LSSU)

The Link Status Signal Unit (LSSU) facilitates information exchanges between adjacent signaling points about the operational status of the links connecting them. MTP Level 3 uses these exchanges to provide the following link status indications:

• Normal
• Out of Alignment
• Out of Service
• Emergency Status

3. Message Signal Unit (MSU)

Message Signal Units (MSUs) are used by SSPs and SCPs to transmit information required to complete a call (i.e., voice or data communication). MSUs that are sent and distributed to levels above MTP Level 3 use the data for the respective application. For example, MTP Level 3 distributes an ISUP MSU to the ISUP layer. ISUP then extracts the data needed for the respective ISUP application.

 

Various levels exist above MTP Level 3. An MSU is distributed based on a field contained within it, which identifies the sub-type of the MSU. ISUP MSUs are used for call control; SCCP MSUs are used for network management and call processing; TCAP MSUs are used for database querying.

In actuality, a TCAP signaling unit does not really exist. MTP Level 3 recognizes an MSU for TCAP as an SCCP message. SCCP then relays the TCAP part of the message to the TCAP router.

 

TCAP data can then be used to invoke remote procedures in support of Advanced Intelligent Network (AIN) services (e.g., 800 numbers). Nearly everything in the wireless industry and all high-end services offered in the telecommunications industry today are TCAP applications.

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