Editing Message Transfer Part

{{primary sources|date=August 2012}}
{{SS7 stack|Deep=}}
The '''Message Transfer Part''' (MTP) is part of the [[Signaling System 7]] (SS7) used for communication in [[Public Switched Telephone Network]]s. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners.

MTP is formally defined primarily in [[ITU-T]] recommendations
[http://www.itu.int/rec/T-REC-Q.701/en/ Q.701],
[http://www.itu.int/rec/T-REC-Q.702/en/ Q.702],
[http://www.itu.int/rec/T-REC-Q.703/en/ Q.703],
[http://www.itu.int/rec/T-REC-Q.704/en/ Q.704] and
[http://www.itu.int/rec/T-REC-Q.705/en/ Q.705].
Tests for the MTP are specified in the [[ITU-T]] recommendations
[http://www.itu.int/rec/T-REC-Q.781/en/ Q.781] for MTP2 and in
[http://www.itu.int/rec/T-REC-Q.782/en/ Q.782] for MTP3. These tests are used to validate the correct implementation of the MTP protocol.

Different countries use different variants of the MTP protocols. In North America, the formal standard followed is [[ANSI]] T1.111. In Europe, national MTP protocols are based on [[ETSI]]
[http://webapp.etsi.org/workprogram/Report_WorkItem.asp?WKI_ID=19130 EN 300-0088-1].

==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

==References==
{{Reflist}}

; ITU-T recommendations
* [http://www.itu.int/rec/T-REC-Q.700/en/ ITU-T Recommendation Q.700], ''Introduction to CCITT Signalling System No. 7''.
* [http://www.itu.int/rec/T-REC-Q.701/en/ ITU-T Recommendation Q.701], ''Functional description of the message transfer part (MTP) of Signalling System No. 7''.
* [http://www.itu.int/rec/T-REC-Q.702/en/ ITU-T Recommendation Q.702], ''Signalling data link''.
* [http://www.itu.int/rec/T-REC-Q.703/en/ ITU-T Recommendation Q.703], ''Signalling link''.
* [http://www.itu.int/rec/T-REC-Q.704/en/ ITU-T Recommendation Q.704], ''Signalling network functions and messages''.
* [http://www.itu.int/rec/T-REC-Q.705/en/ ITU-T Recommendation Q.705], ''Signalling network structure''.
* [http://www.itu.int/rec/T-REC-Q.706/en/ ITU-T Recommendation Q.706], ''Message transfer part signalling performance''.
* [http://www.itu.int/rec/T-REC-Q.707/en/ ITU-T Recommendation Q.707], ''Testing and maintenance''.
* [http://www.itu.int/rec/T-REC-Q.708/en/ ITU-T Recommendation Q.708], ''Assignment procedures for international signalling point codes''.
* [http://www.itu.int/rec/T-REC-Q.709/en/ ITU-T Recommendation Q.709], ''Hypothetical signalling reference connection''.
* [http://www.itu.int/rec/T-REC-Q.710/en/ ITU-T Recommendation Q.710], ''Simplified MTP version for small systems''.
* [http://www.itu.int/rec/T-REC-Q.750/en/ ITU-T Recommendation Q.750], ''Overview of Signalling System No. 7 management''.
* [http://www.itu.int/rec/T-REC-Q.751.1/en/ ITU-T Recommendation Q.751.1], ''Network element management information model for the Message Transfer Part (MTP)''.
* [http://www.itu.int/rec/T-REC-Q.751.3/en/ ITU-T Recommendation Q.751.3], ''Network element information model for MTP accounting''.
* [http://www.itu.int/rec/T-REC-Q.752/en/ ITU-T Recommendation Q.752], ''Monitoring and measurements for Signalling System No. 7 networks''.
* [http://www.itu.int/rec/T-REC-Q.755/en/ ITU-T Recommendation Q.755], ''Signalling System No. 7 protocol tests''.
* [http://www.itu.int/rec/T-REC-Q.755.1/en/ ITU-T Recommendation Q.755.1], ''MTP Protocol Tester''.
* [http://www.itu.int/rec/T-REC-Q.780/en/ ITU-T Recommendation Q.780], ''Signalling System No. 7 test specification — General description''.
* [http://www.itu.int/rec/T-REC-Q.781/en/ ITU-T Recommendation Q.781], ''MTP Level 2 test specification''.
* [http://www.itu.int/rec/T-REC-Q.782/en/ ITU-T Recommendation Q.782], ''MTP Level 3 test specification''.
* [http://www.itu.int/rec/T-REC-Q.1400/en/ ITU-T Recommendation Q.1400], ''Architecture framework for the development of signalling and OA&M protocols using OSI concepts''.
* [http://www.itu.int/rec/T-REC-Q.2110/en/ ITU-T Recommendation Q.2110], ''B-ISDN ATM adaptation layer — Service specific connection oriented protocol (SSCOP)''.
* [http://www.itu.int/rec/T-REC-Q.2111/en/ ITU-T Recommendation Q.2111], ''B-ISDN ATM adaptation layer — Service specific connection oriented protocol in a multilink and connectionless environment (SSCOPMCE)''.
* [http://www.itu.int/rec/T-REC-Q.2140/en/ ITU-T Recommendation Q.2140], ''B-ISDN ATM adaptation layer — Service specific coordination function for signalling at the network node interface (SSCF at NNI)''.
* [http://www.itu.int/rec/T-REC-Q.2150.0/en/ ITU-T Recommendation Q.2150.0], ''Generic signalling transport service''.
* [http://www.itu.int/rec/T-REC-Q.2150.1/en/ ITU-T Recommendation Q.2150.1], ''Signalling transport converter on MTP3 and MTP3b''.
* [http://www.itu.int/rec/T-REC-Q.2150.2/en/ ITU-T Recommendation Q.2150.2], ''Signalling transport converter on SSCOP and SSCOPMCE''.
* [http://www.itu.int/rec/T-REC-Q.2150.3/en/ ITU-T Recommendation Q.2150.3], ''Signalling transport converter on SCTP''.
* [http://www.itu.int/rec/T-REC-Q.2210/en/ ITU-T Recommendation Q.2210], ''Message transfer part level 3 functions and messages using the services of ITU-T Recommendation Q.2140''.

; IETF RFCs

* {{IETF RFC|2719|link=no}}, ''Framework Architecture for Signaling Transport''.
* {{IETF RFC|4165|link=no}}, ''Signaling System 7 (SS7) Message Transfer Part 2 (MTP2) — User Peer-to-Peer Adaptation Layer (M2PA)''.
* {{IETF RFC|3331|link=no}}, ''Signaling System 7 (SS7) Message Transfer Part 2 (MTP2) — User Adaptation Layer (M2UA)''
* {{IETF RFC|4666|link=no}}, ''Signaling System 7 (SS7) Message Transfer Part 3 (MTP3) — User Adaptation Layer (M3UA)''

==External links==
* [https://web.archive.org/web/20130202180444/http://www.cs.utexas.edu/users/vin/Classes/CS386M-Fall04/Readings/ss7_tutorial_pt.pdf] - SS7 Tutorial.

[[Category:Signaling System 7]]