Message Transfer Part
Encyclopedia
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
Q.701,
Q.702,
Q.703,
Q.704 and
Q.705.
Tests for the MTP are specified in the ITU-T
recommendations
Q.781 for MTP2 and in
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. Regional Bell Operating Companies (RBOC) usually follow the Telcordia Technologies (formerly Bellcore) Generic Requirements document GR-246-CORE. In Europe, national MTP protocols are based on ETSI
EN 300-008-1.
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. MTP and SCCP are together referred to as the Network Service Part (NSP).
There is no one-to-one mapping of MTP Levels 1 through 3 onto the OSI model
. Instead, MTP provides the functionality of Layers 1, 2 and part of Layer 3 in the OSI model
. The part of Layer 3 of the OSI model
that MTP does not provide, is provided by SCCP or other Level 4 parts (MTP users).
ITU-T Recommendation Q.2110 or
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
.
ITU-T Recommendation Q.2140 and
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.
MTP2 provides 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 SCTP
by the SIGTRAN
protocol M2UA, described in RFC 3331.
MTP Level 2 is tested using the protocol tester and test specifications described in
Q.755,
Q.755.1,
Q.780 and
Q.781.
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 ITU-T Recommendation Q.2150.0 as provided by MTP3b (Q.2150.1), SSCOP or SSCOPMCE (Q.2150.2) or SCTP
(Q.2150.3). MTP Level 3 functions can also be provided by using the IETF SIGTRAN
M3UA
protocol, described in RFC 4666, in 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 as required and propagates information about route availability through the network.
Access to the signalling network functional level's service interface (as described in Q.701) can be provided over SCTP
by the SIGTRAN
protocol M3UA
, described in RFC 4666.
MTP Level 3 is tested using the protocol tester and test specifications described in
Q.755,
Q.755.1,
Q.780 and
Q.782.
The remaining components of the SS7 stack are all directly, or indirectly, MTP Users. Some examples of parts at Level 4 are SCCP, ISUP
and TUP. The services provided to MTP Level 4 by the MTP (that is, MTP to MTP Users) is described in ITU-T Recommendation Q.701.
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
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
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
recommendations
Q.701,
Q.702,
Q.703,
Q.704 and
Q.705.
Tests for the MTP are specified in the ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
recommendations
Q.781 for MTP2 and in
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
Ansi
Ansi is a village in Kaarma Parish, Saare County, on the island of Saaremaa, Estonia....
T1.111. Regional Bell Operating Companies (RBOC) usually follow the Telcordia Technologies (formerly Bellcore) Generic Requirements document GR-246-CORE. In Europe, national MTP protocols are based on ETSI
EN 300-008-1.
Functional levels
The SS7 stack can be separated into four functional levels:- Level 1 is the Signalling Data Link Functional Level (Data Link Level).
- Level 2 is the Signalling Link Functional Level (Link Level).
- Level 3 is the Signalling Network Functional Level (Network Level).
- Level 4 is the MTP User and consists of SCCP, ISUPISDN User PartThe ISDN User Part or ISUP is part of the Signaling System #7 which is used to set up telephone calls in Public Switched Telephone Networks...
, TUP, or any other MTP User.
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. MTP and SCCP are together referred to as the Network Service Part (NSP).
There is no one-to-one mapping of MTP Levels 1 through 3 onto the OSI model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
. Instead, MTP provides the functionality of Layers 1, 2 and part of Layer 3 in the OSI model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
. The part of Layer 3 of the OSI model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
that MTP does not provide, is provided by SCCP or other Level 4 parts (MTP users).
Signalling Data Link Functional Level
MTP Level 1 is described in ITU-T Recommendation Q.702, and provides the Signalling Data Link functional level for narrowband signalling links. For broadband signalling links,ITU-T Recommendation Q.2110 or
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
E-carrier
In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...
or T-carrier
T-carrier
In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
.
Signalling Link Functional Level
MTP Level 2 is described in ITU-T Recommendation Q.703, and provides the Signalling Link functional level for narrowband signalling links. For broadband signalling links,ITU-T Recommendation Q.2140 and
Q.2210 describe the signalling link function referred to as MTP3b. The signalling link functional level may also be provided using the SIGTRAN
SIGTRAN
SIGTRAN is the name, derived from signaling transport, of the former Internet Engineering Task Force working group that produced specifications for a family of protocols that provide reliable datagram service and user layer adaptations for Signaling System 7 and ISDN communications protocols....
protocol M2PA described in RFC 4165.
MTP2 provides 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 SCTP
Stream Control Transmission Protocol
In computer networking, the Stream Control Transmission Protocol is a Transport Layer protocol, serving in a similar role to the popular protocols Transmission Control Protocol and User Datagram Protocol...
by the SIGTRAN
SIGTRAN
SIGTRAN is the name, derived from signaling transport, of the former Internet Engineering Task Force working group that produced specifications for a family of protocols that provide reliable datagram service and user layer adaptations for Signaling System 7 and ISDN communications protocols....
protocol M2UA, described in RFC 3331.
MTP Level 2 is tested using the protocol tester and test specifications described in
Q.755,
Q.755.1,
Q.780 and
Q.781.
Signalling Network Functional Level
MTP Level 3 is described in ITU-T Recommendation Q.704, and provides the Signalling Network functional level for narrowband signalling links and, with only minor modifications described inITU-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 ITU-T Recommendation Q.2150.0 as provided by MTP3b (Q.2150.1), SSCOP or SSCOPMCE (Q.2150.2) or SCTP
Stream Control Transmission Protocol
In computer networking, the Stream Control Transmission Protocol is a Transport Layer protocol, serving in a similar role to the popular protocols Transmission Control Protocol and User Datagram Protocol...
(Q.2150.3). MTP Level 3 functions can also be provided by using the IETF SIGTRAN
SIGTRAN
SIGTRAN is the name, derived from signaling transport, of the former Internet Engineering Task Force working group that produced specifications for a family of protocols that provide reliable datagram service and user layer adaptations for Signaling System 7 and ISDN communications protocols....
M3UA
M3UA
M3UA stands for MTP Level 3 User Adaptation Layer as defined by the IETF SIGTRAN working group in RFC 4666 . M3UA enables the SS7 protocol's User Parts to run over IP instead of telephony equipment like ISDN and PSTN...
protocol, described in RFC 4666, in IPSP mode.
MTP3 provides routing functionality to transport signaling messages through the SS7 network to the requested endpoint
Endpoint
An endpoint or end point is a mark of termination or completion.* In geometry, an endpoint is a point at which a line segment or ray terminates* Endpoint , the conclusion of a chemical reaction, particularly for titration...
. Each network element in the SS7 network has a unique address, the Point Code
Point Code
An SS7 point code is similar to an IP address in an IP network. It is a unique address for a node , used in MTP layer 3 to identify the destination of a message signal unit ....
(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 as required and propagates information about route availability through the network.
Access to the signalling network functional level's service interface (as described in Q.701) can be provided over SCTP
Stream Control Transmission Protocol
In computer networking, the Stream Control Transmission Protocol is a Transport Layer protocol, serving in a similar role to the popular protocols Transmission Control Protocol and User Datagram Protocol...
by the SIGTRAN
SIGTRAN
SIGTRAN is the name, derived from signaling transport, of the former Internet Engineering Task Force working group that produced specifications for a family of protocols that provide reliable datagram service and user layer adaptations for Signaling System 7 and ISDN communications protocols....
protocol M3UA
M3UA
M3UA stands for MTP Level 3 User Adaptation Layer as defined by the IETF SIGTRAN working group in RFC 4666 . M3UA enables the SS7 protocol's User Parts to run over IP instead of telephony equipment like ISDN and PSTN...
, described in RFC 4666.
MTP Level 3 is tested using the protocol tester and test specifications described in
Q.755,
Q.755.1,
Q.780 and
Q.782.
MTP Users
Level 4 consists of MTP Users.The remaining components of the SS7 stack are all directly, or indirectly, MTP Users. Some examples of parts at Level 4 are SCCP, ISUP
ISDN User Part
The ISDN User Part or ISUP is part of the Signaling System #7 which is used to set up telephone calls in Public Switched Telephone Networks...
and TUP. The services provided to MTP Level 4 by the MTP (that is, MTP to MTP Users) is described in ITU-T Recommendation Q.701.
External links
- SS7 Tutorial - SS7 Tutorial, including MTP description.
- Telcordia GR-246-CORE, - Telcordia Technologies Specification of Signalling System Number 7