IMS TEST AND MONITORING - Figuring out the standards
Ensuring the effectiveness and reliability of complex next generation networks is a 
major test and measurement challenge. Nico Bradlee looks for solutions
Almost without exception the world's major service providers are building flat hierarchical next generation networks (NGNs), capable of carrying voice, data and video traffic. They are creating a single core, access independent network, promising lower opex and enabling cost effective, efficient service development and delivery.
Easy on paper but not so easy to realise the promised capex and opex savings, speedy service launches and business agility. Unlike traditional PSTNs where equipment handles specific tasks, the IP multimedia subsystem (IMS) is a complex functional architecture in which devices receive a multitude of signals. Ensuring QoS and guaranteeing reliability in such a complex network is a test and measurement (T&M), nightmare. Top on the list of operators' priorities are equipment interoperability, protocol definitions, capacity and roaming, which the industry is working to resolve.
According to Frost & Sullivan, the global T&M equipment market earned revenues of $27.4 million in 2007 which is expected to rise to $1.2 billion in 2013. Ronald Gruia, principal analyst, Frost & Sullivan, suggests a change in thinking is needed: operators must reconsider capacity requirements and new ways of testing if they are to avoid surprises.
In the IMS environment there are exponentially more protocols and interfaces with networks and devices - legacy, fixed and wireless. Numerous functions interwork with others and the number of signaling messages are an order of magnitude higher than in traditional networks. The situation is further complicated by a multi-vendor environment in which each function can be provided by different suppliers and, although conforming to standards, equipment may include proprietary features. The advantage is that operators can buy best-of-breed components and, providing they work together and conform to specifications, telcos can add functionality without investing in new platforms or changing the whole network architecture.
Like many new standards, IMS is somewhat fluid and open to interpretation. Although standards have been approved, they are often incomplete, are still evolving or may be ambiguous. Further, each of the different IMS standards organisations, which include 3GPP, ETSI, TISPAN and IETF, publishes regular updates. Vendors interpret standards according to the needs of their customers and may introduce new innovations which they refer to standards bodies for inclusion in future releases. "IMS standards don't define interoperability but interfaces and functions which may be misinterpreted or differently interpreted by vendors," explains Dan Teichman, Senior Product Marketing Manager, voice service assurance at Empirix.
The many IP protocols have advanced very rapidly but standards are still evolving so there is considerable flexibility and variation. "This is a new and exciting area," says Mike Erickson, Senior Product Marketing Manager at Tektronix Communications, "but it is very difficult to test and accommodate error scenarios which grow exponentially with the flexibility provided in the protocol.
"Rapid technology changes and variety make it difficult for people to become experts and it is no longer possible for customers to build their own T&M tools," continues Erickson. "However, new T&M systems are more intelligent, automated, easier to use and capable of testing the different types of access networks interfacing with the common core. Operators must be able to measure QOS and ensure calls can be set up end-to-end with a given quality - this facility must be built into the series of test tools used both in pre-deployment and in live networks."
IMS networks must be tested end-to-end: from the access to the core, including the myriad network elements, functions and connections/interfaces between them. While the types of tests vary little from those currently used in traditional networks, their number is exponentially higher. "Tests break down into functional tests; capacity testing to ensure network components can handle both sustained traffic levels and surges; media testing - confirming multimedia traffic is transmitted reliably through the network; trouble shooting and 24x7 network monitoring to identify anomalies and flag up problems," says Erickson. "The difference is that in relatively closed PSTNs, four to five basic protocols are being considered compared to hundreds in more open VoIP and IMS networks."
No single vendor or operator has the facilities to conduct comprehensive interoperability, roaming, capacity or other tests to ensure equipment conforms to different iterations of IMS or to test the multiple interfaces with devices, gateways and protocols typical in NGNs. The MultiService Forum, a global association of service and system providers, test equipment vendors and users, recently concluded its GMI 2008 comprehensive IMS tests of over 225 network components from 22 participating vendors. Five host labs on three continents were networked together creating a model of the telecoms world. Roger Ward, MSF President says: "The results showed the overall architecture is complex and the choice of implementation significantly impacts interoperability. IMS protocols are generally mature and products interoperate across service provider environments. Most of the problems encountered were related to routing and configuration rather than protocols. IMS demonstrated the ability to provide a platform for convergence of a wide range of innovative services such as IPTV."
These essentially positive results support the need for continuous testing and monitoring before and during implementation, the results of which can be fed back into vendors' test and measurement teams for product development.
"Building products to emulate IMS functions means operators can buy equipment from multiple vendors, emulate and test functions before implementation and without having to build big test labs," says Teichman. "In IMS networks, T&M is not confined to infrastructure: the huge variety of user interfaces must be tested before implementation to avoid network service outages and QOS problems. While they have to test more functional interfaces, most traditional tests are still valid: although the methodology may be the same, the complexity is higher as many more tests are required to get the same information."
Operators face scalability issues as the number of VoIP users increases. The question, suggests Tony Vo, Senior Product Manager at Spirent, is whether IMS can support thousands of users. "Test solutions must generate high loads of calls. All tests are focused around SIP so tests must emulate different applications. GMI 2008 verified the issues and companies can now develop solutions. However, from a T&M perspective, no one solution can solve all problems."
Nico Bradlee is a freelance business and communications journalist
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