SS7 MESSAGE ERRORS

 

The core responsibilities of MTP Level 2 are to ensure that signaling units arrive at the signaling points they are destined for, and are read correctly. The following actions are performed by MTP Level 2 to meet these responsibilities.

 

Zero-Bit Insertion

The bit pattern for a signaling unit's flag is 0111 1110. While this pattern is not repeated in the actual data within a signaling unit, several series of 1 bits may be misinterpreted as flags. Therefore, a preventive measure is taken to ensure that a signaling point can discern data from flags. This preventive measure is known as "zero-bit insertion," or "bit stuffing."

 

Before a signaling unit is sent from a signaling point, MTP Level 2 will view the sequence of bits within it. After every sequence of five 1-bits, a 0-bit will be inserted. For example, if the sequence 11111111... occurs, the sequence will be 111110111... after zero-bit insertion.

 

Upon receiving an inbound signaling unit and removing the flag, bit sequence and removes the zeros that occur after every sequence of 1-bits.

 

Basic Error Recovery

When a signaling point receives an MSU, it will send a "positive acknowledgement" (in the form of a FISU) to the originating point, indicating that the MSU was received without error. This acknowledgement relies on the use of the following fields in the initial MSU and acknowledgement FISU:

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

Note: An acknowledgement may also be sent in the form of an MSU. It does not have to be an FISU.

 

When it receives an MSU, MTP Level 2 will examine the FSN. The value in this field identifies the MSU and should be the next sequential number from the previous MSU (received from the originating signaling point). For example, if an MSU was previously received and its FSN value was 82, the FSN of the next MSU should be 83.

As long as the FSN values continue in sequence, no error notification will be generated for lost MSUs. If this is the case, the receiving signaling point sends a FISU back to the originating point. Its BSN value will be equal to the FSN value found in the last received MSU.

 

What happens if the FSNs are out of sequence? How does the receiving signaling point tell the originating point that MSUs are missing? Does it simply solve the issue if no positive acknowledgement is received?

 

In such cases, FIB and BIB fields are important. In each MSU sent to a signaling point will be an FIB value of "1" or "0." In the FISU returned by the receiving point, the BIB will match the FIB, as long as the MSUs' FSNs are in sequence.

 

For example, if the FSN in the last MSU received was 82, and the next is 85, then the receiving point knows MSUs 83 and 84 were lost. MTP Level 2 then determines the FIB value, which is set to 0. The receiving point returns an FISU with a BSN of 82 (not 85) and a BIB value of 1. When it receives the FISU, the originating point will view it as a "negative acknowledgement" since the BIB value has been "flipped." Only a receiving signaling point will change the FIB/BIB value, as this is the means by which basic error recovery is initiated.

 

When it receives a negative acknowledgement (indicated by an FIB/BIB mismatch between an MSU and its corresponding FISU), a signaling point takes reactive recovery measures. It views the BSN value in the FISU and retransmits the lost MSUs. In the current example, it means that MSUs 83, 84, and 85 would be re-transmitted.

This re-transmission of messages is the "basic error recovery." Once a recovery has been successful, the originating signaling point continues to use the last BIB value the receiving point has set as its FIB value.

 

 

Preventive Cyclic Retransmission

Preventive Cyclic Retransmission is a proactive error recovery method. It is often used when long delays are expected (e.g., when transmitting over a satellite link) between MSU transmission and acknowledgement receipt. Its purpose is to increase MSU delivery reliability.

 

With this type of recovery method, a signaling point cyclically retransmits MSUs that it has sent to another point but for which it still awaits acknowledgement FISUs. For example, if the MSUs with FSNs 46 through 48 were transmitted, then they are now transmitted, in sequence, until acknowledgement is received. If it receives the same MSU multiple times, the receiving point simply discards redundant messages.

 

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