Verizon Wireless’s deployment of IMS (IP Multimedia Subsystem), to support its LTE roll-out with all-IP services, is just about the most advanced project of its kind in the world, so the US carrier’s suppliers and the results of its ‘big bang’ approach are closely watched by more cautious cellcos elsewhere. Being a frontrunner brings business benefits, such as the opportunity to deploy services no rival has. It also carries high risks, and any problem is closely scrutinized and criticized. So when Verizon suffered a major LTE outage earlier in the year, it hit the headlines and brought the aggression of the IMS strategy into question.
The culprit, it emerged, was that undersung plague of the mobile data network, the ‘signalling storm’. This has never attracted the attention, or investment, of the ‘data storm’, but can have just as catastrophic effect on the user experience or even the network’s ability to stay up at all. In turn, that throws a new spotlight on technologies which can help, and which were previously confined to the shadows of the carrier’s network. One of these is the emerging class of Diameter signalling routers, which suppliers like Tekelec, Acme Packet and others claim can protect 4G carriers from signalling meltdown.
In Verizon’s case, the outage was traced back to a software bug in a network element of the IMS core, triggered by heavy signalling and rapidly spreading throughout the system. In the end it shut off all access to 3G and 4G. Meanwhile, another early LTE deployer, Telenor of Norway, suffered an 18-hour outage which was also caused by a signalling overload. The signalling traffic between servers in the mobile network increased far beyond normal levels, the cellco told the regulator, to the extent that they could no longer connect calls to recipients. As users take advantage of the higher bandwidth of LTE to indulge in even heavier use of ‘chatty’ applications like Facebook and some email systems ‘ which constantly poll the network for updates ‘ the problem will only get more acute.
These chatty devices constantly cast off IP sessions and particularly generate network signalling requests. Launching the iPhone’s browser, for example, instantly sets off about 15 different signalling requests. The IP data signalling protocol, Diameter (roughly equivalent to SIP in IP voice), is increasingly used in place of traditional TDM/SS7 protocols, and adds greater performance ‘ but was not necessarily designed with all the pressures of 4G traffic patterns in mind. Diameter is becoming the primary signaling protocol for AAA (authentication, authorization and accounting), mobility management, policy control and policy enforcement, but was only originally intended to handle some of those functions. ‘In a major break from the TDM-based networks of the past, policy management in all-IP networks is critically dependent on how the Diameter signaling environment is architected, protected and managed,’ saysYankee Group analyst Brian Partridge.
The Diameter protocol was developed in 1998 to provide an AAA framework for applications such as remote network access or IP mobility. Advantages over existing protocols like Radius included greater reliability, scalability and security, plus the system was optimized from day one for mobile as well as fixed connections. The 3GPP subsequently adopted Diameter it as the primary signaling protocol for policy exchange in LTE and IMS, where it combines with SIP signalling to support the new class of voice and IP applications.
Apart from the basic requirements of making LTE and IP networks efficient and secure, there is a key commercial incentive for carriers to investigate Diameter solutions ‘ the ability to support LTE roaming. Voice and data roaming over 2G and 3G networks generate huge revenues and one of the critical challenges facing 4G operators is how to adopt the spectral and cost efficiencies of the new IP platforms, without sacrificing roaming income.
However, as the tasks which Diameter handles grow more complex, simple point-to-point signalling between servers and network elements becomes unmanageable as the number of connections can multiple to the point where the server falls over. Instead, the traffic needs to be centrally and intelligently managed through a hub ‘ a platform variously called a Diameter router, Diameter signalling controller (DSC) or (by Acme Packet) a PEC (policy exchange controller). These products act as routing hubs for all Diameter messages, and also provide security, interoperability, routing and reporting functions, creating a secure edge to external Diameter border points. They simplify the mesh of connections between Diameter elements like the Home Subscriber Server, PCRF and Mobility Management Entity; and enable federated service delivery, supporting policy exchange and revenue sharing between a network owner and its MVNOs or over-the-top/cloud providers.
Tekelec certainly seems to have convinced Verizon that such platforms can protect the carrier from more embarrassing outages, signing the highest profile 4G Diameter signal routing deal to date with the US operator in August. Director of product management Jason Emery told ConnectedPlanet that its work with Verizon adds a system-wide approach to conventional Diameter signalling. Elements like congestion management are essential but were not built into the standard, so they are now being supported as added value features in the central routers. IMS vendors like Huawei and Ericsson are sure to start including these in one-stop shop offerings too, but for now the field is clear for specialized suppliers like Tekelec, Acme Packet, Openet, IntelliNet and Traffix.
Traffix CEO Ben Volkow said at the recent 4G World conference that ‘LTE is, door-to-door, creating new signaling requirements for Diameter. There are just more and more boxes and more and more complexity and signalling. With voice over LTE and machine-to-machine coming too, the amount of signaling will only grow. It’s going to be a bombardment. You can’t manage that type of network point-to-point.’
Despite the challenge, the volume demand for Diameter routers is unlikely to be as great as for their SIP-based cousins, the session border controllers, which secure and manage VoIP and other sessions at an increasingly large number of points in the network. Most operators are putting their Diameter routers in the center, though that may change as packet data loads rise. There are different approaches to the nascent technology too ‘ Acme Packet is largely ignoring legacy signalling protocols from pre-IP networks, like TDM and SS7, and playing to its IP strengths; Tekelec, which has roots in those legacy systems, emphasizes the bridge between old and new. That mirrors the larger debate about whether IMS should be implemented from scratch and in a big bang way, or whether it needs to coexist with traditional networks for many years as carriers migrate step-by-step.
Many factors will drive the use of Diameter routers, as well as just raw traffic levels. One is the rise of data plans shared among different devices, says Infonetics Research, which forecasts that the number of mobile broadband devices sold globally on shared data plans will rise by a CAGR of 89% a year from 2011 to 2015. By 2015, over 15% of these gadgets will be on such data plans, but this adds to network complexity and ‘entails a radical change from the way in which customers have been managed to this point’. Shira Levine, directing analyst at the firm, highlights policy management, subscriber data management and Diameter routing as the main techniques for managing and monetizing shared data tariffs. Policy and charging related information crosses networks via the Diameter protocol, and Levine points out: ‘The strain will only get worse as the number of signaling transactions increases exponentially’, increasing the incentive for cellcos to adopt Diameter routers.