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4. Simultaneous Ring: The Monster That’s Eating Your Network
Presented by Mark Lindsey, ECG.
Simultaneous Ring may be the most beloved of features of VoIP. In the IP Telephony papers of the 1990s, call forking and simultaneous ring tantalize the reader with the obvious elegance of IP as a replacement -- simply by sending another packet. And today, now that VoIP has won, customers love the ability to have all their phones ring at once. Simultaneous ring goes well beyond a "find me / follow me service. Call Centers delight in the ability to ring every agent's phone at once. And with the advent of Shared-Line Appearance, the functionality of traditional electrically-bridged analog phones is available, courtesy of simultaneous ring.
But these benefits come at a disproportionate cost. There are two principle factors making Simultaneous Ring more trouble than was expected in many networks.
First, Simultaneous ring creates a burst of load in signaling devices, such as the session border controllers at the Service Provider or Customer's edge. Instead of sending one call from the VoIP core down to a customer, they send a blast of calls as fast as possible. Then when one of those customers answers, the core must send another blast of cancellations. And when combined with "Busy Lamp Field" line-state monitoring, the blast of messages could double. Since telecom networks must be built to support the peak burst of traffic, these blasts may decimate a carrier's total capacity.
Second, Simultaneous Ring also strains the common assumptions of call signaling load and the networks built to support VoIP. Many networks have only a single class of prioritized traffic, and both signaling and media is prioritized into that one class. But if the burst of signaling is great enough, Simultaneous Ring could actually saturate the bottleneck link, causing excess jitter or packet loss in the priority queue.
Network designers and operators must be educated on the true costs. In this presentation, we elucidate the signaling behavior and model its expense for today's networks. When equipped with knowledge that Simultaneous Ring is disproportionately expensive, implementors can plan for its use more carefully.
And the tools available for operators must evolve. The common practice of blasting numerous replicas of SIP signaling from the core, across an access network, to many endpoints near one another is preposterously inefficient. Yet VoIP Carriers cannot fix this without the help of software developers and vendors. We suggest a number of lines of research where this problem may be solved so simultaneous ring can be used safely and effectively, without the absurd costs.