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Circuits-over-Packets: IMS is Further Delayed


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November 21, 2008

Circuits-over-Packets: IMS is Further Delayed

By Brough Turner, Brough Turner is Chief Strategy Officer of Dialogic.

(This article originally appeared in the October issue of Internet Telephony magazine.)
The 3GPP’s IP Multimedia Subsystem (IMS) is the ultimate next-generation network with which others (CableLab’s PacketCable, ETSI’s (News - Alert) TISPAN and 3GPP2’s MMD) have decided to align. But it’s complex and expensive, and thus delayed.

An IMS network can guarantee bandwidth, latency and other Quality of Service (QoS) parameters on a per-session basis. This not only guarantees service quality but allows operators to charge different prices for different services. However, all communication between endpoints must be established by centralized network servers, so an IMS network has the complexity of the traditional telephone network and then some. As a result, it’s taken years to develop the standards, they are still evolving, and mobile operators are still searching for the killer application that justifies the expense.
Meanwhile, fixed-line VoIP networks continue to spread as they save money today without the complexity of IMS. There are a few dozen IMS networks in existence, but they are partial implementations and most are trials. In each case, IMS is being deployed to support a new service with new revenues, e.g., Push-to-talk over Cellular (PoC), Fixed-Mobile Convergence (News - Alert) (FMC) or video sharing, not traditional mobile voice services.
Circuits-over-Packets for Mobile Cost Savings
Mobile operators save money when all services share a single packet-based core network, so existing operators are migrating to IP. More to the point, new networks in rapidly growing markets like China, India and Africa are beginning to use packet-based backbones to interconnect conventional mobile switches and even radio base stations. But these networks are not using SIP or IMS for voice services — they are using circuits-over-IP.
Circuits-over-IP preserves conventional SS7-based mobile signaling but sends it using SIGTRAN protocols over an IP-MPLS backbone while a softswitch controls voice circuits over the same IP-MPLS backbone. Traditional signaling protocols remain, so roaming and handoffs work as always and subscriber management is untouched. Most important, there is no need to install IMS/SIP software on consumer handsets, so billions of existing handsets continue to work.
Service platforms look a little different. SS7 messages go over SCTP/IP rather than MTP & TDM and voice goes over RTP/UDP (News - Alert)/IP rather than TDM, but nothing fundamental changes. It’s the PSTN-over-IP or more properly, a PLMN (Public Land Mobile Network) over IP.
Long-Term Prospects for IMS
Because IMS is complex and equipment volumes are still small, IMS lacks the cost savings for mobile voice telephony that the circuits-over-packets approach provides. Operators continue to see IMS as their long-term solution, but they only invest to save money or to support new applications with new revenues. Thus we have a classic chicken-and-egg problem that will only be solved in increments over many years.

Brough Turner (News - Alert), co-founder and CTO of NMS Communications, writes the Next Wave Redux column for TMCnet. To read more of Brough’s articles, please visit his columnist page.

Edited by Greg Galitzine

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