Sigtran Protocol

Sigtran is the traditional ss7 signaling over IP network.  The protocol standard is define in IETF spec. The SS7 Network is fast and reliable and its a circuit switched network. Having dedicated resources. It have links, which acts as stream of messages. This makes multiple streams to work in parallel. Enables low latency in ss7 network and multiple paths to peer node. User of ss7 link gets immediate link status (congestion, link down etc.). Network deploys nodes in mated pairs for redundancy. There is a separate wires for ss7 links. The protocol specification in rfc 4666.

Sigtran Stack
Sigtran Stack

Internet is another network which was growing faster and faster with the time. Growth was at hardware level , from metal cables to fiber optics. This enabled fast transfer of IP packets, but still missing other requirements to fulfill to become useful for telecom network.

TCP or Transmission Control Protocol, was the only connection oriented protocol for setting up a virtual circuit over IP network. It is reliable , but still was not fit for telecom because of following short comings.

Single Streaming: TCP/IP uses single stream in a tcp connection. Stream is the sequence of bytes.  If one byte is corrupted, all bytes stopped util corrupted bytes will not re-transmitted again by the sender.  Imagine there there are ten concurrent calls going on a TCP stream. Problem in one call, will create problem in remaining nine calls.

No Message Boundaries:  TCP flow is stream of bytes, the sender and received has to manage message boundaries. Telecom network have coverage to all the globe. Both ends need to work on a common protocol to talk. Then creating boundaries was required to set a new protocol if TCP has to use.

No Asynchronous State Indication:  After a TCP/IP connection setup. If IP network fails (e.g cable is removed), there is no immediate indication to the sender or receiver.  This protocol don't have a path health check mechanism. If TCP has to use then its responsibility of TCP application to check health of a connection.

Single Homed:  A TCP connection have a pair of IP address and port. If one IP interface or path  failed, all communication on the connection fails.

To overcoming all short comings of TCP protocol for telecom.  SCTP or stream controlled transmission protocol was standardized.  This protocol have following features.

Multi Streaming: A SCTP connection many have multiple streams. While sending message, user of SCTP can specify the stream to send. This enable parallel processing of calls without disturbing other call in failure of a call. While connection setup both end points negotiates on incoming and outgoing streams.

Packet Oriented: Uses sending packets (like UDP protocol style) in connection oriented mode.

Heart Beat: SCTP Protocol uses heartbeat mechanism for monitoring the health of a connection. User receives connection status whenever there is change in connection state.

Multi Homing:  A sctp end point includes list of ip addresses  and a port. This makes a connection to keep active, even in case of one IP network fails.

Sigtran Network

With signaling over IP , sigtran standard define new protocols stack. This brings new nodes into network. The End node in sigtran  network is called ASP or application server process. This is similar to SSP or SCP in SS7 network.  This nodes runs the actual ss7 application.

HLR is the example of ASP in m3ua. It terminates the SS7 traffic over IP network. An asp may be connected directly to another ASP or it may connect to a signaling gateway for reaching other nodes.  The signaling gateway or SG, is similar to STP in SS7 Network.  It have Sigtran and SS7 transport support. When a new node over m3ua needs to connect to the telecom network, it connects via SG.


Sigtran vs ss7

There are many differences in sigtran and ss7.  Both have its own transport and protocol stacks. Following tables lists the sigtran vs ss7, key differences.

                     SS7                        SIGTRAN 
A legacy protocol in telecommunication. Media and Signaling uses ss7 protocol. Relatively new protocol in telecommunication. Only signaling uses sigtran protocol.
Uses TDM based E1 or T1 links for transport of ss7 messages. Uses IP network for ss7 signaling messages.
Special hardware is reqired for ss7 links. This hardware is a ss7 card. Which implements MTP2 and MTP1. Dialogic, Digium and Adax are the few vendors for ss7 cards. No special hardware is required. The sigtran uses Ethernet card, which is available in all computers.
Protocol Standards defined by ITU-T. Protocol Standards are defined by IETF.
Complex implementation of network layer. Easy implementation of network layer.
High is cost and required additional hardware. Low in cost and uses transitional ip network.

Sigtran protocol stack:

Sigtran protocol stack have three layers.

Adaptation Layer (m3ua, sua, m2ua, m2pa):

This layer uses the services of SCTP protocol and provides the services to a SS7 layer. Starts association with peer node for setting a SCTP connection. Does signaling for bringing AS and ASP up at adaptation layer.

SCTP Layer:

Its a transport layer, provides services to adaptation layer and uses services of IP layer. Does the four way handshake with peer sctp for connection setup.

IP Layer:

Provides services to SCTP and used services of data link layer. Provides routing of IP packets having SCTP as payload.

User Adaptation Layers In Sigtran:

Sigtran is only for transport purpose. The user application should not change if a ss7 node connectivity changes from SS7 to sigtran. To make this possible , in sigtran protocols there are adaptation layers. A SS7 layer is the user of adaptation layer and adaptation layer uses the service of SCTP protocol.  Basis of SS7 layer , sigtran layers are standardize.

M3UA or Mtp3 User Adaptation Layer: 

Peer of MTP3 in sigtran protocol is M3UA. User of M3UA is SCCP.

SUA or SCCP User Adaptation Layer: 

Peer of SCCP in sigtran protocol is SUA. User of SCCP is TCAP.

M2UA or MTP2 User Adaptation Layer: 

Peer of MTP2 in sigtran protocol M2UA. User of M3UA is MTP2.

M2PA or Mtp2 Peer Adaptation Layer:

M2PA is the replacement of MTP2 layer in sigtran or IP. Two MTP3 layers can directly connect to each other over m2pa.  Wgile M2Ua uses the services of MTP2 from SG.



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