Editing Message Transfer Part (section)

===Signalling Link Functional Level===
''MTP Level 2'' is described in [http://www.itu.int/rec/T-REC-Q.703/en/ ITU-T Recommendation Q.703], and provides the ''Signalling Link'' functional level for narrowband signalling links. For broadband signalling links,
[http://www.itu.int/rec/T-REC-Q.2140/en/ ITU-T Recommendation Q.2140] and
[http://www.itu.int/rec/T-REC-Q.2210/en/ Q.2210] describe the signalling link function referred to as ''MTP3b''. The signalling link functional level may also be provided using the [[SIGTRAN]] protocol [[M2PA]] described in RFC 4165.

''MTP Level 2'' ensures accurate end-to-end transmission of a message
across a signaling link.

MTP2 provides flow control, error detection and sequence checking, and retransmits unacknowledged messages. MTP2 uses packets called ''signal units'' to transmit SS7 messages. There are three types of signal units: [[Fill-in Signal Unit]] (FISU), [[Link Status Signal Unit]] (LSSU), [[Message Signal Unit]] (MSU).

Access to the signalling link functional level's service interface can be provided over [[Stream Control Transmission Protocol|SCTP]] by the [[SIGTRAN]] protocol [[M2UA]], described in RFC 3331.

MTP Level 2 is tested using the protocol tester and test specifications described in
[http://www.itu.int/rec/T-REC-Q.755/en/ Q.755],
[http://www.itu.int/rec/T-REC-Q.755.1/en/ Q.755.1],
[http://www.itu.int/rec/T-REC-Q.780/en/ Q.780] and
[http://www.itu.int/rec/T-REC-Q.781/en/ Q.781].