Guidelines for Selecting SS7 Protocols

Signaling System Number 7 (SS7) is a sophisticated, integrated signaling protocol designed to increase the efficiency of telephone service delivery. It has become the central nervous system for the delivery of wireline and wireless, and more recently IP services, the latest step in the convergence of communication networks. All networks need signaling to create connections, activate service, and deliver traffic.

 

SS7 Protocol Layers

Signaling System 7 was introduced by AT&T in 1975 and approved by worldwide standard bodies in 1980.

 

SS7 basic functions are as follows:

• MTP (Message Transfer Part) - Provides a reliable transfer and delivery of signaling information across signaling networks.
• SCCP (Signaling Connection Control Point) - Provides additional routing capabilities via SubSystem Numbers (SSNs). It also offers the capability of routing based on dialed digits or global title translation.
• ISUP (ISDN User Part) - Provides the transport of call set-up information between two signaling points.
• IUP (Interconnect User Part) - Supports customer services and network features at the point of interconnect between public networks in the UK.
• TCAP (Transaction Capability Application Part) - Provides the capability of transferring non-circuit-related information between signaling points.
• TUP (Telephone User Part) - Provides the transport of call set-up information between two signaling points only for voice services.
  
  

These basic protocols determine the ability to deploy applications worldwide. Many support additional higher-level protocols such as TIA/EIA-41, GSM MAP, CAP, BSSAP, AIN and INAP.

 

SS7 Protocols and OS Support

Compliance to standards determines the general applicability of the protocol. Strict compliance to the very latest standard revision is seldom possible. Compliance tables are used to precisely define a protocol capability for a given release.

 

Do not overlook the importance of the programming language and operating system supported. These determine the tools and skills needed to develop your applications. An API that is the same architecture for all protocols and is operating system independent allows your applications to grow and move with little or no change.

 

By making maximum use of valuable training, tools, and experience the breadth of operating system and protocol support determines the ease with which future applications and enhancements can be quickly accommodated.

 

API and Application Support

When the application is mission critical, the software that is part of the API and protocol support infrastructure becomes very important. Reliability of the API and protocol support infrastructure must be part of the application design.

 

This is often a larger effort than the application itself. Using fault tolerant hardware does not mitigate the benefits of a complete, robust software infrastructure designed specifically for SS7 requirements.

 

SS7 Level Capabilities

Each protocol level has areas of specification that are determined by the particular implementation of the protocol. Implementation limitations can have serious consequences. Details are very important, and the "broadest" implementations offer the best foundation for future applications and deployments.

 

Country Variants

ISUP, unlike TCAP, has more than 100 country-specific accommodations. These variants ensured compatibility with existing circuit switches as SS7 network signaling was implemented.

 

SS7 Boards

Consideration of the SS7 network physical connection and associated hardware (board) capabilities is as important as selecting the protocols that support your application. The board determines the chassis you use, the number of SS7 links that can be connected, the manner in which maintenance is accomplished (online, offline), and whether the board is tested and delivered with the protocol software. A board that is fully integrated with the SS7 software is most desirable.

 

Evolution to SS7 over IP

Currently the industry is focused on the efforts of the IETF SIGTRAN Working Group that addresses switched circuit network (SCN) signaling, with an emphasis on SS7 over IP. Protocols under development by the IETF SIGTRAN Working Group include:

• SCTP (Streams Control Transmission Protocol) - Provides generic transport for SCN signaling .
• M2PA (MTP 2 Peer-to-peer Adaptation Layer) - Enables SS7 links replacement over IP.
• M2UA (MTP 2 User Adaptation Layer) - Enables SS7 back-hauling from remote end-points over IP.
• M3UA (MTP 3 User Adaptation Layer) - Enables SS7 User Parts (e.g. ISUP and SCCP) to run over IP.
• SUA (SCCP User Adaptation Layer) - Enables SS7 Application Parts (e.g. TCAP) to run over IP.