SS7/IP Interworking Tutorial
-Transporting MTP Over IP

For MTP messages transported over SS7 or IP networks, the following requirements are specified by the International Telecommunication Union:

• MTP Level 3 peer-to-peer procedures require a response time within 0.5 sec (500 milliseconds) to 1.2 seconds (1200 ms).
  
  
• no more than 1 in 10 million messages will be lost due to transport failure.
  
  
• no more than 1 in 10,000 million (US terminology: 1 in 10 billion) messages will be delivered out-of sequence (including duplicated messages) due to transport failure.
  
  
• no more than 1 in 10,000 million (US: 1 in 10 billion) messages will contain an error that is undetected by the transport protocol or 1 in 1,000 million (US: 1 in 1 billion) for ANSI (American National Standard Institute) specifications.
  
• availability of any signaling route set (the complete set of allowed signaling paths from a given signaling point towards a specific destination) is 99.9998% or better (downtime of approximately 10 minutes/year or less).
  
  
• the message length (payload accepted) is 272 bytes for narrowband SS7 and 4091 bytes for broadband SS7.

To achieve the functional and performance requirements for MTP, the IETF sigtran Working Group has recommended three new protocols: M2UA, M2PA, and M3UA. Each protocol is described below.

M2UA: MTP2 User Adaptation Layer

M2UA is a protocol defined by the IETF sigtran Working Group for transporting SS7 MTP Level 2 user (i.e. MTP Level 3) signaling messages over IP using the Stream Control Transmission Protocol (SCTP). The M2UA protocol layer provides the equivalent set of services to its users as MTP Level 2 provides to MTP Level 3.

M2UA is used between the Signaling Gateway and Media Gateway Controller in VoIP networks. The signaling gateway receives SS7 messages over an MTP Level 1 and Level 2 interface from a signaling end point (SCP or SSP) or signal transfer point (STP) in the public switched telephone networks. The signaling gateway terminates the SS7 link at MTP Level 2 and transports MTP Level 3 and above to a Media Gateway Controller or other IP endpoint using M2UA over SCTP/IP.

The signaling gateway maintains the availability state of all media gateway controllers to manage signaling traffic flows across active SCTP associations.

M2PA: MTP2 User Peer-to-Peer Adaptation Layer

Like M2UA, M2PA is a sigtran protocol for transporting SS7 MTP Level 2 user part signaling messages (i.e. MTP Level 3) over IP using the Stream Control Transmission Protocol (SCTP). Unlike M2UA, M2PA is used to support full MTP Level 3 message handling and network management between any two SS7 nodes communicating over an IP network. IP signaling points function as traditional SS7 nodes using the IP network instead of the SS7 network. Each switched circuit or IP signaling point has an SS7 point code. The M2PA protocol layer provides the same set of services as MTP Level 2 provides to MTP Level 3.

M2PA can be used between a signaling gateway and a media gateway controller, between a signaling gateway and an IP signaling point, and between two IP signaling points. Signaling points may use M2PA over IP or MTP Level 2 over standard SS7 links to send and receive MTP Level 3 messages.

M2PA facilitates the integration of SS7 and IP networks by enabling nodes in switched circuit networks to access IP telephony databases and other nodes in IP networks using SS7 signaling. Conversely, M2PA allows IP telephony applications to access SS7 databases, such as local number portability, calling card, freephone, and mobile subscriber databases. In addition, using M2UA over IP may result in cost advantages if traditional SS7 links are replaced by IP connections.

In summary, M2PA and M2UA differ in the following ways:

• M2PA: the signaling gateway is an SS7 node with a point code;
  M2UA: the signaling gateway is not an SS7 node and has no point code.
  
  
• M2PA: the connection between the signaling gateway and IP signaling points is an SS7 link;
  M2UA: the connection between the signaling gateway and the media gateway controller is not an SS7 link. Rather, it is an extension of MTP from the signaling gateway to the media gateway controller.
  
  
• M2PA: the signaling gateway can have upper SS7 layers, such as SCCP;
  M2UA: the signaling gateway has no upper SS7 layers as it has no MTP Level 3.
  
  
• M2PA: relies on MTP Level 3 for management procedures;
  M2UA: uses M2UA management procedures.
  
  
• M2PA: IP signaling points processes MTP Level 3 and MTP Level 2 primitives;
  M2UA: the media gateway controller transports MTP Level 3 and MTP Level 2 primitives to the signaling gateway's MTP Level 2 for processing.

M3UA: MTP Level 3 User Adaptation Layer

M3UA is a protocol defined by the IETF sigtran Working Group for transporting MTP Level 3 user part signaling messages (e.g., ISUP, TUP, and SCCP) over IP using the Stream Control Transmission Protocol (SCTP). TCAP or RANAP messages, as SCCP user protocols, may be carried by SCCP using M3UA or by a different sigtran protocol called SUA, as described below.

M3UA is used between a signaling gateway and a media gateway controller or IP telephony database. The signaling gateway receives SS7 signaling using MTP as transport over a standard SS7 link. The signaling gateway terminates MTP-2 and MTP-3 and delivers ISUP, TUP, SCCP and/or any other MTP-3 user messages, as well as certain MTP network management events, over SCTP associations to media gateway controllers or IP telephony databases.

The ISUP and/or SCCP layer at an IP signaling point is unaware that the expected MTP-3 services are not provided locally, but rather by the remote signaling gateway. Similarly, the MTP-3 layer at a signaling gateway may be unaware that its local users are actually remote parts over M3UA. Conceptually, M3UA extends access to MTP-3 services at the signaling gateway to remote IP endpoints. If an IP endpoint is connected to more than one signaling gateway, the M3UA layer at the IP endpoint maintains the status of configured SS7 destinations and route messages according to the availability and congestion status of the routes to these destinations via each signaling gateway.

M3UA does not impose a 272-octet signaling information field (SIF) length limit as specified by SS7 MTP Level 2. Larger information blocks can be accommodated directly by M3UA/SCTP without the need for an upper layer segmentation/re-assembly procedure as specified by the SCCP and ISUP standards. However, a signaling gateway will enforce the maximum 272-octet limit when connected to a SS7 network that does not support the transfer of larger information blocks to the destination. For broadband MTP networks, the signaling gateway will fragment ISUP or SCCP messages larger than 272 octets as required.

At the signaling gateway, the M3UA layer provides interworking with MTP-3 management functions to support seamless operation of signaling between the SS7 and IP networks. For example, the signaling gateway indicates to remote MTP-3 users at IP endpoints when an SS7 signaling point is reachable or unreachable or when SS7 network congestion or restrictions occur. The M3UA layer at an IP endpoint keeps the state of the routes to remote SS7 destinations and may request the state of remote SS7 destinations from the M3UA layer at the signaling gateway. The M3UA layer at an IP endpoint may also indicate to the signaling gateway that M3UA at an IP endpoint is congested.

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