Sigtran Protocol – SS7 over IP

Sigtran is the traditional ss7 signaling over the IP network.  The protocol standard is defined in the IETF spec. The SS7 network is fast and reliable and it is a circuit-switched network. Having dedicated resources for voice calls. It has links, which act as a stream of messages.

This enables multiple streams to work in parallel. Facilitates low latency in the ss7 network and multiple paths to the peer node. A user of the ss7 link gets immediate link status (congestion, link down, etc.). Network deploys nodes in mated pairs for redundancy. There is a separate wire for each ss7 link. The protocol specification in RFC 4666.

Sigtran stack:

Similar to other protocols, Sigtran also has a protocol stack.  The lowest layer is the IP layer and the top layer can be any ss7 application layer(e.g INAP, CAP, MAP). In the Sigtran stack, there are Sigtran layers (M3UA, M2PA, M2UA, etc.). Each layer provides services to the upper layer and uses the services of SCTP for network transport.

Sigtran Stack
Sigtran Stack

The Internet is another network which was grown faster and faster with time. Growth was at a hardware level, from metal cables to Fiber optics.

This enabled the fast transfer of IP packets, but still missing other requirements to fulfill to become useful for telecom networks.

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

Single Streaming: TCP/IP uses a single stream in a TCP connection. The stream is the sequence of bytes.  If one byte is corrupted, all bytes stop until corrupted bytes will not be re-transmitted again by the sender.  Imagine there are multiple concurrent calls going on a TCP stream. The problem in one call will create problems in the remaining other calls.

No Message Boundaries:  TCP flow is a stream of bytes, the sender and receiver have to manage message boundaries. Telecom network has coverage all over 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 the IP network fails (e.g cable is removed), there is no immediate indication to the sender or receiver.  This protocol doesn’t have a path health check mechanism. If TCP has to use then it’s the responsibility of the TCP application to check the health of a connection.

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

To overcome all shortcomings of TCP protocol for telecom.  SCTP or stream-controlled transmission protocol was standardized.  This protocol has the following features.

Multi Streaming: A SCTP connection may have multiple streams. While sending a message, the user of SCTP can specify the stream to send. This enables parallel processing of calls without disturbing other calls in case of failure of a single call. While connection setup both endpoints negotiate on incoming and outgoing streams.

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

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

Multi-Homing:  A SCTP endpoint includes a list of IP addresses and a port. This makes a connection to keep active, even in case one IP network fails.

Sigtran Network

 Signalling over IP for SS7 protocol messages, Sigtran standard defines new protocols stack. This brings new nodes into a network. The End node in the Sigtran network is called ASP or application server process. This is similar to SSP or SCP in an SS7 network.  This node runs the actual ss7 application.

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

Sigtran protocol stack:

Sigtran protocol stack has three layers.

Adaptation Layer (M3UA, SUA, M2UA, M2PA):

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

SCTP Layer:

It is a transport layer that provides services to the adaptation layer and uses the services of the IP layer. Does the four-way handshake with peer SCTP for connection setup.

IP Layer:

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

User Adaptation Layers In Sigtran:

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

M3UA or Mtp3 User Adaptation Layer: 

A peer of MTP3 in the Sigtran protocol is M3UA. The user of M3UA is SCCP.

SUA or SCCP User Adaptation Layer: 

The peer of SCCP in Sigtran protocol is SUA. The user of SCCP is TCAP.

M2UA or MTP2 User Adaptation Layer: 

A data link level Sigtran layer. The user of M2ua is the ss7 MTP3 layer. The signalling gateway provides services of MTP2. If an M2UA application server wants to send a message to a remote application hosted by an SS7 node. M2ua asp connects to the SG and sends messages to the remote ss7 node via SG.  The layer can also have IPSP mode too, similar to other layers.

M2UA provides the same primitives to the MTP3 layer. On the network side, it uses SCTP services.

M2PA or Mtp2 Peer Adaptation Layer:

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


Sigtran vs ss7

There are many differences between Sigtran and ss7.  Both have their own transport and protocol stacks. The following tables list the Sigtran vs ss7, key differences.

SS7 Sigtran
A legacy protocol in telecommunication. The media and Signaling path both uses the ss7 protocol. Relatively new protocol in telecommunication. Only signalling uses the Sigtran protocol.
Uses TDM based E1 or T1 links for transport of ss7 messages. Uses an IP network for ss7 signalling messages.
Special hardware is required for ss7 links. This hardware is an 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 an Ethernet card, which is available on all computers.
Protocol Standards defined by ITU-T. Protocol Standards are defined by IETF.
The complex implementation of the network layer. Easy implementation of the network layer.
High cost because ss7 requires additional hardware. Low in cost and uses a transitional IP network interface.