Editing Message Transfer Part (section)

==Functional levels==
The [[Signaling System 7|SS7]] stack can be separated into four functional levels:<ref>[http://www.itu.int/rec/T-REC-Q.700-199303-I/en/ ITU-T Recommendation Q.700], Figure 3/Q.700, ''SS No. 7 functional levels'', p. 8.</ref>
Level 1 through Level 3 comprise the ''MTP'', and Level 4 the ''MTP user''. ''MTP Level 3'' is sometimes abbreviated ''MTP3''; ''MTP Level 2'', ''MTP2''.<ref>See for example, RFC 3331 and RFC 3332 for the usage ''MTP2'' and ''MTP3''.</ref> MTP and [[Signalling Connection Control Part|SCCP]] are together referred to as the ''Network Service Part (NSP)''.<ref>[http://www.itu.int/rec/T-REC-Q.711-200103-I/en/ ITU-T Recommendation Q.711 (03/2001)], Figure 1/Q.711, ''Functional diagram for the SCCP in Signalling System No. 7'', p. 2.</ref>

There is no one-to-one mapping of MTP Levels 1 through 3 onto the [[OSI model]].<ref>[http://www.itu.int/rec/T-REC-Q.700-199303-I/en/ ITU-T Recommendation Q.700 (03/1993)], Clause 4, ''OSI layering and SS No. 7'', p. 9.</ref> Instead, MTP provides the functionality of [[layer 1|layers 1]], [[layer 2|2]] and part of [[layer 3]] in the [[OSI model]].<ref>[http://www.itu.int/rec/T-REC-Q.700-199303-I/en/ ITU-T Recommendation Q.700 (03/1993)], Figure 2/Q.700, ''Architecture of SS No. 7'', p. 7.</ref> The part of layer 3 of the [[OSI model]] that MTP does not provide, is provided by [[Signalling Connection Control Part|SCCP]] or other Level 4 parts (MTP users).<ref>[http://www.itu.int/rec/T-REC-Q.1400-199303-I/en/ ITU-T Recommendation Q.1400 (03/1993)], Clause 2.2, ''OSI layering and SS No. 7'', p. 4.</ref><ref name="q1400fig1">[http://www.itu.int/rec/T-REC-Q.1400-199303-I/en/ ITU-T Recommendation Q.1400 (03/1993)], Figure 1/Q.1400, ''Relationship between SS No. 7 Functional Levels and OSI Layering'', p. 5.</ref>

===Signalling Data Link Functional Level===
''MTP Level 1'' is described in [http://www.itu.int/rec/T-REC-Q.702/en/ ITU-T Recommendation Q.702], and provides the ''Signalling Data Link'' functional level for narrowband signalling links. For broadband signalling links,
[http://www.itu.int/rec/T-REC-Q.2110/en/ ITU-T Recommendation Q.2110] or
[http://www.itu.int/rec/T-REC-Q.2111/en/ Q.2111] describe the signalling data link function.

MTP1 represents the physical layer. That is, the layer that is responsible for the connection of SS7 Signaling Points into the transmission network over which they communicate with each other. Primarily, this involves the conversion of messaging into electrical signal and the maintenance of the physical links through which these pass. In this way, it is analogous to the Layer 1 of ISDN or other, perhaps more familiar, protocols.

MTP1 normally uses a timeslot in an [[E-carrier]] or [[T-carrier]]. The Physical interfaces defined include E-1 (2048&nbsp;kbit/s; 32 64&nbsp;kbit/s channels), DS-1 (1544&nbsp;kbit/s; 24 64&nbsp;kbit/s channels), V.35 (64&nbsp;kbit/s), DS-0 (64&nbsp;kbit/s), and DS-0A (56&nbsp;kbit/s).

===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].

===Signalling Network Functional Level===
''MTP Level 3'' is described in [http://www.itu.int/rec/T-REC-Q.704/en/ ITU-T Recommendation Q.704], and provides the ''Signalling Network'' functional level for narrowband signalling links and, with only minor modifications described in
[http://www.itu.int/rec/T-REC-Q.2210/en/ ITU-T Recommendation Q.2210], for broadband signalling links. The functions of ''MTP Level 3'' may also be replaced with the ''Generic Signalling Transport Service'' described in [http://www.itu.int/rec/T-REC-Q.2150.0/en/ ITU-T Recommendation Q.2150.0] as provided by ''MTP3b'' ([http://www.itu.int/rec/T-REC-Q.2150.1/en/ Q.2150.1]), ''SSCOP or SSCOPMCE'' ([http://www.itu.int/rec/T-REC-Q.2150.2/en/ Q.2150.2]) or [[Stream Control Transmission Protocol|SCTP]] ([http://www.itu.int/rec/T-REC-Q.2150.3/en/ Q.2150.3]). ''MTP Level 3'' functions can also be provided by using the [[IETF]] [[SIGTRAN]] [[M3UA]] protocol, described in RFC 4666, in [[IP Signalling Point|IPSP]] mode.

MTP3 provides routing functionality to transport signaling messages through the SS7 network to the requested endpoint. Each network element in the SS7 network has a unique address, the [[Point Code]] (PC). Message routing is performed according to this address. A distinction is made between a [[Signaling Transfer Point]] (STP) which only performs MTP message routing functionalities and a Signaling End Point (SEP) which uses MTP to communicate with other SEPs (that is, telecom switches). MTP3 is also responsible for network management; when the availability of MTP2 data links changes. MTP3 establishes alternative links and re-routes traffic away from failed links and signaling points and propagates information about route availability through the network. Also controls traffic when congestion occurs.

Access to the signalling network functional level's service interface (as described in [http://www.itu.int/rec/T-REC-Q.701/en/ Q.701]) can be provided over [[Stream Control Transmission Protocol|SCTP]] by the [[SIGTRAN]] protocol [[M3UA]], described in RFC 4666.

MTP Level 3 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.782/en/ Q.782].

===MTP Users===
''Level 4'' consists of ''MTP Users''.
The remaining components of the [[Signaling System 7|SS7]] stack are all directly, or indirectly, ''MTP Users''. Some examples of parts at ''Level 4'' are [[Signalling Connection Control Part|SCCP]], [[ISDN User Part|ISUP]], [[Telephone User Part|TUP]], and, in the UK, [[Interconnect User Part|IUP]].<ref name="q1400fig1"/> The services provided to ''MTP Level 4'' by the ''MTP'' (that is, MTP to MTP Users) is described in [http://www.itu.int/rec/T-REC-Q.701/en/ ITU-T Recommendation Q.701].775148760