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.
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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.
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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
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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.
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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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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).
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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.
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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).
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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.
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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.
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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.
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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.
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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.
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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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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