Embedding Signalling System #7 into the next generation of systems
In the buzzword-driven world of telecommunications, Signalling System #7 (SS7) is being drowned out by the market-hyped promises of next generation networks like IP Multimedia Subsystem (IMS) and 4th Generation (4G) Wireless (3G Long-Term Evolution, Mobile WiMAX, Ultra Mobile Broadband, etc).
In spite of its apparent non-popularity, however, SS7 remains – today, and for the foreseeable future – the heartbeat of the today’s revenue generating telecommunications networks.
Globally, the deployed base of SS7 equipment far eclipses that of any system based on newer and, admittedly, more exciting Internet Protocol (IP) technology. Moreover, new applications like Short Messaging Service (SMS) continue to be delivered on SS7 networks. The advent of SMS caused a spike in SS7 demand, as well as propelling the SS7-over-IP (ie SIGTRAN) market, which is being used to offload SMS traffic onto more efficient IP networks.
The continued demand for SS7 technology is also being driven by the requirement to embed SS7 interworking into next generation platforms. Carriers must provide their customers with service continuity throughout their transition from legacy Circuit Switched (CS) networks to the emerging Packet Switched (PS) domain. Meanwhile, operators also seek to prolong revenue streams from existing investments by leveraging SS7-based Intelligent Network (IN) services and databases directly into their respective NGN environment. In addition carriers also want to transition to cost-saving IP core networks in a homogenous fashion as they accelerate deployment and ultimately shorten time-to-market to reduce network equipment and management expenses.
However, embedding SS7 interworking into existing or new platforms presents a unique set of challenges and demands for operators and a skill set rapidly diminishing in the telecommunications marketplace. Among the challenges are protocol interworking, scalable performance and cost control. The technology vendor or professional who manages to overcome these challenges will successfully tap into the unsung SS7 network opportunity.
The first challenge, protocol interworking, exists because of the myriad of SS7 application protocols and the flux of standardised methods for porting these protocols to the Voice over IP (VoIP) environment, primarily based on Session Initiation Protocol (SIP). SS7 application protocols like ISDN User Part (ISUP) and Intelligent Network Application Part (INAP), not only suffer from disparate requirements levied by global standards bodies, like ANSI, ETSI, and ITU, they also include divergent country variations and Telecommunications Equipment Manufacturer (TEM)-specific extensions. For example, the Trillium ISUP product includes support for fourteen different variants and country-specific extensions
To further complicate things, the interworking between these country variations and SIP is often not standardised or is being addressed by competing standards. To map SS7 to SIP signalling requires bridging the legacy circuit switched call model to the session management model in SIP. The SS7 call model assumes a dedicated circuit between callers, with the majority of intelligence embedded in the network. In contrast, SIP signalling has extended beyond a call model to support all kinds of communications activities, namely voice, video, conferencing, etc., and follows a peer-to-peer approach with the intelligence moved from the core to the edge of the network.
Mapping the divergent SS7 and SIP state machines is achieved in two ways. First, there are standards that define the encapsulation of ISUP messaging in SIP networks. These include SIP for Telephones (SIP-T) from the IETF (RFC 3372) and SIP with encapsulated ISUP (SIP-I) defined by the ANSI (T1.679) and ITU (Q.1912.5) organisations. These standards extend SIP to carry ISUP messages across the SIP network as attachments to the SIP messages.
For VoIP networks, SIP-T has been widely accepted, while the SIP-I method is being advocated within the wireless domain, specifically by the 3GPP and 3GPP2 standards organisations. Encapsulation is needed since the underlying theory driving SIP pushes service intelligence to the peers so SIP signalling can not innately support certain legacy SS7 services; this includes applications which require a very specific sequence of ISUP messages. Finally, encapsulation simplifies regulatory compliance since one retains the ability to deliver ISUP services as opposed to trying to meet complex regulatory requirements with unproven SIP services.
The second approach to mapping the divergent SS7 and SIP state machines is through custom interworking functions. These are necessary due to a lack of standards as well as a desire to interwork SIP with other SS7 application protocols to meet specific requirements beyond standard call control. For example, the IP Multimedia – Service Switching Function (IM-SSF) in IMS is a specialised SIP application server that bridges Mobile Application Protocol (MAP) and CAMEL Application Protocol (CAP)-based services into IMS. The transfer of MAP and CAP signalling to SIP is loosely defined in the IMS standards and will be developed primarily based on the types of services to be bridged.
Another major challenge facing engineers looking to integrate SS7 network interworking into next generation platforms is scalable performance. Carriers are increasingly seeking integrated, multi-protocol signalling gateway functionality rather than a stand-alone black box signalling gateway. Integrating the interworking capability into a similarly functioned node, for example a converged Media Gateway/Signalling Gateway platform as illustrated in Fig. 1, is attractive due to the reduction of cost and overall complexity. Each additional node in the carrier network adds significant operational cost and management overhead, which is why a black box approach to multi-protocol interworking has not been successful. However, embedding the interworking functionality into an existing platform forces the platform developer to meet the performance demands of a multi-protocol signalling gateway utilising only shared compute resources.
The best way to overcome the performance scalability issue is to develop a software architecture that is capable of load sharing across processing entities as well as scaling within emerging multi-core processors themselves. Developing multi-threaded non-blocking software that scales well in multi-core processors allows the platform developer to initially dedicate a single compute blade, or a couple of cores within a processor, to the signalling gateway function. As demand increases, having the capability to spread the load across multiple computer blades within the chassis, each featuring multi-threaded signalling gateways, is a truly scalable and cost-efficient method for meeting carrier requirements.
Controlling the cost of specialised SS7 solutions is essential to remain competitive in the margin-compressed telecommunications market. Typically, the cost of SS7 solutions is significantly higher than IP technology, due to the complexity and specialisation of SS7. The most effective way to control cost is to take advantage of standards-based hardware components and ensure software portability of the signalling gateway function.
If the platform developer bases the solution on standardised hardware, like an AdvancedTCA blade or an Advanced Mezzanine Card (AMC), it may be easily re-used across any of the organisations’s other ATCA platforms. A re-usable ATCA or AMC-based signalling gateway function may then be rapidly integrated into next generation systems if and when a carrier demands this capability. Ensuring software portability is also critical to the integration of the signalling gateway within proprietary platform solutions. As standards-based hardware is in the early adoption stage, the majority of telecommunications platforms are still based on proprietary solutions. The combination of standards-based hardware plus software portability ensures an agile response in the face of rapidly changing carrier demands and reduces system cost based on the wide array of competing vendors.
SS7 solutions represent a continued market opportunity for technology vendors and professionals who have successfully maintained a core competency in SS7, while evolving toward next generation VoIP, IMS, and 4G wireless. The latest network innovations will fail if there is not a simple and cost-effective bridge to existing Public Switched Telephone Network (PSTN) and Global System for Mobile Communications (GSM) wireless networks. The majority of subscribers rely on services provided over these legacy CS technologies powered by SS7.
SS7 interworking solutions that solve the challenges of non-standard protocol conversion and performance scalability, and are based on open standards components, offer an optimised path to embed multi-protocol signalling gateway functionality into Next Generation Network (NGN) platforms.
Todd Mersch is Product Marketing Manager with Continuous Computing, Continuous Computing, San Diego, CA, USA. www.ccpu.com
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