SS7/Sigtran, ss7 protocol tutorial, ss7 tutorial

SS7:

SS7 network have following three types of nodes:

1. Signaling Switching Point (SSP):
                These are node which gives access to radio , BTS , BSC , RNC etc. on one side and on other side these nodes are connected to wired ss7 network. These node are e.g MSC, VLR, SGSN etc. These nodes provides roaming information to the HPLMN of mobile subscriber and provide MO, MT , SMS and call functionalities.

2.  Signaling Transfer points (STP):
                These are SS7 nodes which provides routing functionality to the SS7 messages. These are like route in IP network.  All routing is done based on global title or point code. The global title is derived from IMSI or MSISDN. For routing the STP provides global title translation for routing of SS7 messages to the correct destinations.

3. Signaling Control Point (SCP):
               These are nodes , having access to database and queried by other network elements for providing services to a mobile subscriber. One of the example is prepaid charging in this there is a node called Camel Gateway , this uses INAP or Camel Protocol for prepaid charging. This SCP is deployed in home network of a mobile subscriber. One a call is initiated the Camel SCP is queried for services ( if have balance etc.) , if SCP allows then a call is made.

A SS7 protocol stack have following layers.

1. MTP1 , this is the physical layer.
2. MTP2 : This is data link layer. This layers transfers error free MSUs , to directly connected nodes.
3. MTP3 : This is the network layer , routes the end to end messages based on opc and dpc.
4. SCCP: This is the user of MTP3 , it routes messages to different applications running on a node. This does the global title translations.
5. TCAP: This is user of SCCP layers. This layer maintains the transaction on behalf of its users.

Sigtran:

 Sigtran is SS7 signaling over IP network.  SCTP a transport protocol used for SIGTRAN which highly reliable.  Before SCTP , there was protocol TCP which was not good fit for telecom.

SCTP:
       SCTP have following features and TCP comparison

.      1. Multi Homing , a SCTP end point can have multiple IP address and single port. While TCP  is single IP and single port in an end point.
       2. SCTP connections have multiple streams while TCP have a single streams. This avoids to data            delay when one packet is faulty and re transmission takes time.
       3. SCTP can be used as packet (UDP style) or connection based (TCP style).
       4. SCTP have heart beats , so get notify the connection state immediately to SCTP user.
       5. SCTP uses Four-Way handshake to avoid TCP type attacks.
     

   
  User Adaptation Layers:
       
       Sigtran Provides the concept of user adaptation layers. In this SS7 layer is replaced by its equivalent sigtran layer.
     M3UA :
        This is MTP3 user adaptation layer. In this , the MTP3 layer in SS7 is replaced by M3UA layer.
     SUA:
        This is SCCP user adaptation layer. In this , the SCCP layer in SS7 is replaced by SUA layer.
      M2UA:
        This is MTP2 user adaptation layer. In this , the MTP2 layer in SS7 is replaced by M2UA layer.
   M2PA:
        This is used to connect to Mtp3 layers over an IP link instead of E1/T1.

SCTP Four Way Handshake:

•  SCTP Client requests to SCTP layer for new connection.
•  SCTP layer sends INIT request to the peer SCTP layer running on server.in If SCTP server listening on destination port and Ip of INIT message and SCTP association configured on server, Server sends INIT-ACK , to the SCTP client. 
• Client sends COOKIE-ECHO to the SCTP server , and server responds back with COOKIE-ACK
• On client app connect returns and on server side COMM UP Indication is received. 
• To close the association any end may send SHUTDOWN message to peer and peer will respond with SHUTDOWN COMPLETE.

M3UA Procedures :

One Association is setup at SCTP level, after that M3UA procedures are started.
      
ASP mode:

•  ASP sends ASP Up message to the SG.
• SG responds with ASP Up Ack message to ASP.
• SG sends NOTIFY message to the ASP as AS state Inactive.
• ASP sends ASP Active message to SG.
• SG sends ASP Active Ack to the ASP.
• SG send NOTIFY as AS state AS active.

SCCP Layer :

SCCP is the transport layer in SS7. An application over ss7 stack is identified by SSN. The SSN status is maintained by SCCP layer. The SCCP layer does the Global Title Translation.