SIGTRAN: Signaling Transport Protocol Stack

Signaling Transport (SIGTRAN) refers to a protocol stack for the transport of Switched Circuit Network (SCN) signaling protocols (such as SS7/C7 an Q.931) over an IP network. SIGTRAN, an evolution of the PSTN signaling, defines adaptors and a core transport capabilities that blend SS7 and packet protocols to provide users with the best both technologies have to offer. Applications of SIGTRAN include:Internet dial-up remote access, IP telephony interworking with PSTN and other services as identified. The key components in the SIGTRAN achitecture are as follows:

MGC - Media Gateway Controller, responsiblefor mediating call control (between the SGand MG) and controlling access from the IPworld to/from the PSTN.
SG - Signaling Gateway, responsible forinterfacing to the SS7 network and passingsignaling messages to the IP nodes.
MG - Media Gateway, responsible for packetizationof voice traffic and transmitting thetraffic towards the destination.
IP SCP - an IP-enabled Service Control Point(SCP). This exists wholly within the IP network, but is addressable from the SS7 network.
IP Phone - generically referred to as a terminal.

The interfaces pertaining to signaling transport include SG to MGC, SG to SG. Signaling transport may potentially be applied to the MGC to MGC or MG to MGC interfaces as well, depending on requirements for transport of the associated signaling protocol. For interworking with SS7-controlled SCN networks, the SG terminates the SS7 link and transfers the signaling information to the MGC using signaling transport. The MG terminates the interswitch trunk and controls the trunk based on the control signaling it receives from the MGC. For interworking with PSTN (Public Switched Telephone Network), IP networks will need to transport signaling such as Q.931 or SS7 ISUP messages between IP nodes such as a Signaling Gateway and Media Gateway Controller or Media Gateway.

Stream Control Transmission Protocol (SCTP), a core protocol in the SIGTRAN protocol stack, provides transport layer services over IP. Examples of such transport include: transport of signaling between a Signaling Gateway and Media

Gateway or Media Gateway Controller; transport of signaling ("backhaul") from a Media Gateway to a Media Gateway Controller; transport of TCAP between a Signaling Gateway and other IP nodes. SCTP is used by one of the following User Adaptation layer protocols.

SUA: Signalling Connection Control Part User Adaptation Layer
IUA: ISDN Q.921-User Adaptation Layer
M3UA: SS7 Message Transfer Part 3 (MTP3) User Adaptation layer
M2UA: SS7 Message Transfer Part 2 (MTP2) User Adaptation layer
M2PA: MTP2 Peer-to-peer user Adaptation layer
V5UA: V5.2-User Adaptation Layer

For each, the SS7 stack is substituted at one of its well-defined layers with a packet transport replacement. By moving up to higher layers higher in the stack, more of the legacy SS7 concepts can be eliminated and replaced with flexible packet and IP protocol routing capabilities. Because SIGTRAN is an industry standard, it allows customers to interoperate in a multi-vendor environment.

SIGTRAN Architecture