What is Sigtran

SIGTRAN (SIGnaling TRANsport) :-
        It is a set of protocols defined by IETF to transport SS7 messages over IP networks. It allows IP networks to inter-work with the Public Switched Telephone Network (PSTN) and vice versa.
        The telco switch sends SS7 signals to a signaling gateway (SG) that converts them into SIGTRAN packets, which travel over IP to the next signaling gateway or to a softswitch if the destination is not another PSTN.
    
        Sigtran Protocol Suite is made up of a new Transport Layer -- the Stream Control Transmission Protocol (SCTP) & a set of User Adaption (UA) layers, which mimic the services of lower layers of SS7 & ISDN.

Why SIGTRAN :

• Ordered, Reliable Transfer.
• Redundancy in case of Link Failure.
• Low Loss & Delay.

The key components in the SIGTRAN architecture are :-

• MGC–Media Gateway Controller, responsible for mediating call control (between the SG (Signaling Gateway) and MG (Media Gateway)) and controlling access from the IP world to/from the PSTN.
• SG–Signaling Gateway, responsible for interfacing to the SS7 network and passing signaling messages to the IP nodes.
• MG–Media Gateway, responsible for packetization of voice traffic and transmitting the traffic 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.”
  

 SIGTRAN protocol stack consists of 3 components :

• A standard IP layer.
• A common signaling transport protocol, Stream Control Transmission Protocol (SCTP)
• An Adaptation layer, like - M2PA, M2UA, M3UA, and SUA.  

SCTP :- Stream Control Transmission Protocol
         SCTP is designed to transport SS7 signaling messages over IP networks. It operates directly on top of IP at the same level as TCP. SCTP's basic service is connection oriented reliable transfer of messages between peer SCTP users. Its aim of designing, is to address the Shortcomings of TCP. SCTP is a general purpose protocol, a replacement for TCP.

SCTP has the following set of features :-

• It is a Unicast Protocol - data exchange is between two known endpoints.
• It defines timers of much shorter duration than TCP.
• SCTP uses periodic heart-beat messages to confirm the status of each end point.
  
• It provide reliable transport of data - detecting when data is corrupt or out of sequence, and performing repair as necessary.
• It is Rate-Adaptive, responding to network congestion
• It support Multi-Homing - Each SCTP endpoint may be known by multiple IP addresses. Routing to one address is independent of all others, & if one route fails, another will be used.
• It uses an initialization process, based on cookies, to prevent denial-of-service attacks.
• It supports Bundling, where a single SCTP message may carries multiple "Chunks" of data, each of which contains a whole signaling message.
• It support fragmentation, where a single signaling message may be split into multiple SCTP messages in order to be accommodated within a underlying PDU.
• It is message-oriented, defining structured frame of data, on the other hand, TCP imposes no structured on the transmitted stream of bytes.
  
• It has a multi-streaming capacity, data is split into multiple streams, each with independent sequenced delivery, TCP has no such feature.

Sigtran Adaptation Layers serves common purposes like :-

• To carry upper layer Signaling Protocol over a reliable IP-based transport.
• To provide same class of services offered at the interface of PSTN equivalent.
• Transparent.
• To remove as much need for the lower SS7 layers as possible.

Sigtran currently defines SIX adaption layers :-

1. M2UA :- It provides the services of MTP2 in a Client-Server Situation, such as SG to MGC. Its user would be MTP3.
2. M2PA :- It provides the services of MTP2 in a Peer-to-Peer Situation, such as SG to SG Connections. Its user would be MTP3.
3. M3UA :- It provides the services of MTP3 in both a Client-Server Situation (SG to MGC) & Peer-to-Peer Architecture, Its user would be SCCP and/or ISUP.
4. SUA :- It provides the services of SCCP in a Peer-to-Peer Situation, such as SG to IP SCP Connections. Its user would be TCAP.
5. IUA :- It provides the services of the ISDN Data Link Layer (LAPD), Its user would be an ISDN Layer 3 (Q.931) entity.
6. V5UA :- It provides the services of the V.5.2 Protocol.

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