Signaling & Network Control

1. NETW 704 Signaling & Network Control Message Transfer Part 2 (MTP2) Dr. Eng. Amr T. Abdel-Hamid Winter 2008

2. MTP Level 2 • Delimitation of signal units • Alignment of signal units • Signaling link error detection • Signaling link error correction by retransmission • Signaling link initial alignment • Error monitoring and reporting • Link flow control

3. MTP Messages • An SS7 message is called a signal unit (SU). There are three kinds of signal units: Fill-In Signal Units (FISUs), Link Status Signal Units (LSSUs) and Message Signal Units (MSUs)

5. MTP Messages • FISUs are transmitted continuously on a signaling link in both directions unless other signal units (MSUs or LSSUs) are present. FISUs carry basic Level 2 information only (i.e., acknowledgment of signal unit receipt by a remote signaling point). Because a CRC checksum is calculated for each FISU, signaling link quality is checked continuously by both signaling points at either end of the link. • LSSUs carry one or two octets (8-bit bytes) of link status information between signaling points at either end of a link. • MSUs carry all call control, database query and response, network management and network maintenance data in the signaling information field (SIF). MSUs have a routing label, which allows an originating signaling point to send information to a destination signaling point across the network.

6. L1 Feild

7. Flag Field • The flag indicates the beginning of a new signal unit and implies the end of the previous signal unit. • The binary value of the flag is 0111 1110. Before transmitting a signal unit. • MTP Level 2 removes "false flags" by adding a zero-bit after any sequence of five one-bits. Upon receiving a signal unit and stripping the flag. • MTP Level 2 removes any zero-bit following a sequence of five one-bits to restore the original contents of the message. Duplicate flags are removed between signal units. BIT STUFFING

8. MTP 2 Fields • BSN (Backward Sequence Number) : The BSN is used to acknowledge the receipt of signal units by the remote signaling point. The BSN contains the sequence number of the signal unit being acknowledged. • BIB (Backward Indicator Bit) The BIB indicates a negative acknowledgment by the remote signaling point when toggled. • FSN (Forward Sequence Number) The FSN contains the sequence number of the signal unit. • FIB (Forward Indicator Bit) The FIB is used in error recovery like the BIB. Toggled to indicate the retransmission of an SU that was received in error by the remote SP.

9. SIO (Service Information Octet) • The SIO field in an MSU contains the 4-bit subservice field followed by the 4-bit service indicator • The subservice field contains the network indicator (e.g., national or international) and the message priority (zero to three with three being the highest priority). Message priority is considered only under congestion conditions, not to control the order in which messages are transmitted. Low priority messages may be discarded during periods of congestion. • Signaling link test messages receive a higher priority than call setup messages. • The service indicator specifies the MTP user.

10. MTP 2 Fields • SIF (Signaling Information Field) • The SIF in an MSU contains the routing label and signaling information (e.g., SCCP, TCAP and ISUP message data). LSSUs and FISUs contain neither a routing label nor an SIO as they are sent between two directly connected signaling points. • CRC (Cyclic Redundancy Check) • The CRC value is used to detect and correct data transmission errors. For more information.

11. Signal Unit Alignment • Loss of alignment occurs when a nonpermissible bit pattern is received or when an SU greater than the maximum SU size is received. • MTP2 constantly processes the data stream, searching for flags that delineate the SUs. The maximum number of consecutive 1s that should be found in the bit stream is six (as part of the flag), because the transmitting end performs 0 bit insertion. If seven or more consecutive 1s are detected, this signifies a loss of alignment. • The SU length should be in multiples of octets (8 bits). The minimum size of an SU is six octets (FISU), and the maximum size is 279 octets (MSU). If an SU is outside these parameters, this is considered a loss of alignment, and the SU is discarded.

12. Error Detection • The error detection method is performed by a 16-bit CRC on each signal unit. These 16 bits are called check bits (CK bits). • The process uses the ITU-T Recommendation V.41 generator polynomial X16 + X12 + X5 + 1. • The polynomial that is used is optimized to detect error bursts. • The receiving SP performs the same calculation in an identical manner. Finally, the two results are compared; if an inconsistency exists, the SU is discarded, and the error is noted by adding to the Signal Unit Error Rate Monitor (SUERM). In this case, the error correction procedure is applied.

13. Error Correction • Two methods of error correction are available: • Basic error correction (BEC) • Preventive cyclic retransmission (PCR) method.

14. Basic Error Correction • Basic error correction is accomplished using a backwards retransmission mechanism, in which the sender retransmits the corrupt (or missing) MSU and all subsequent MSUs. • This method uses both negative and positive acknowledgments. Positive acknowledgments (ACKs) indicate the correct reception of an MSU, and negative acknowledgments (NACKs) are used as explicit requests for retransmission. Only MSUs are acknowledged and resent, if corrupt, to minimize retransmissions. FISUs and LSSUs are neither acknowledged nor resent if corrupt; however, the error occurrences are noted for error rate monitoring purposes.

15. Example

16. Preventive Cyclic Retransmission • The preventive cyclic retransmission (PCR) method is a positive acknowledgment, cyclic retransmission, forward error correction system. This means that no negative acknowledgments are used and that the system relies on the absence of a positive acknowledgment to indicate the corruption of SUs.