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November 1998

Foundations For Carrier-Class Internet Telephony


Most IP telephony gateway deployments to date have been in the public network. Service providers -- both large and small -- are taking advantage of the arbitrage opportunities in international long-distance calling by avoiding telephone taxes. But the public network market is only a small fraction of what the total market for IP telephony gateways will become over the next few years.

Service providers, capital in hand, are ready to deploy large volumes of gateways. What will it take to make it happen?

First, the physical and commercial infrastructure must be in place. This is happening. New ventures like Level 3 and Qwest have raised billions of dollars on the public markets to build massive packet/cell networks for all forms of communications, including IP telephony gateways on the edge to accommodate telephony traffic. Worldwide, established carriers are building out their own packet/cell networks for multimedia use. The steady expansion of bandwidth available through fiber-optic and dense, wave-division multiplexing technology is bringing the required bandwidth online. Where IP falls short (so far) in providing the Quality of Service (QoS) necessary for full-duplex audio conversations, underlying protocols -- including some combinations of ATM and SONET (Synchronous Optical NETwork) -- can provide the necessary quality of service.

Next, equipment vendors must gear up to meet the needs of the public network market. A range of vendors have developed, or are developing, gateways that meet carriers' availability, density, and cost requirements. The open systems side of the market is rallying around the CompactPCI form factor for carrier-class gateways. CompactPCI is a breakthrough for open systems telco deployments, combining high availability features with standard components to cost-effectively deliver scalable, reliable solutions. Important CompactPCI capabilities include card hot swap, backplane TDM bus (ECTF H.110), and features for easy installation and administration. A variety of vendors are developing CompactPCI chassis tuned for telco deployment, including models that comply with standards such as the Bellcore Network Equipment Building Standard (NEBS). Depending on vendor and carrier preference, carriers can deploy a variety of operating systems, including Solaris, Digital Unix, UnixWare 7, and Windows NT. Component vendors are delivering increasingly powerful, dense, and cost-effective solutions. In short, the systems are becoming available.

The most critical issue may well be standards. (For a closer look at standards, see "So You Say You Want a Standard?" in the October issue of Internet Telephony™.) Large-scale carrier deployment requires fault-resilient separation of the call control logic from the media stream. This way, the numerous media gateways can be relatively uncomplicated and less expensive. Also, different companies can focus on different elements of the value chain, making carriers less dependent on any one vendor. While the H.323 standard does enable call control via an outboard gatekeeper, there are two flaws to this approach. First, the gatekeeper standards do not yet include provisions for fault resiliency necessary for carrier deployment. More fundamentally, H.323 does not seamlessly fit into the existing telecom network -- a requirement for an IP telephony gateway that bridges the telecom and data networks.

The telecom industry has developed the Intelligent Network architecture, protocols, and equipment to separate application logic from the media stream in a fault resilient manner. IP telephony gateway functionality must encompass both media conversion between circuit and packet networks, as well as the critical control function conversion between the two networks. Signaling System 7 (SS7) is the foundation protocol for the public switched telephone network (PSTN) control function. Hence, IP telephony gateway standards must enable seamless interoperation with SS7. Unfortunately, H.323 may not meet this requirement as well as some alternatives. There are a number of proposals in the works to address this gap, notably the Simple Gateway Control Protocol (SGCP), led by Bellcore and Cisco, and the Internet Protocol Device Control (IPDC), led by the Technical Advisory Council organized by Level 3. Lest this appear too simple, note that a variety of other protocols have been proposed, including Reliable Signaling Gateway Control Protocol (RSGP), Diameter, Etheric, and (I am not making this up) Nothing Other than a Simple Internet Phone (NOTASIP).

An "SS7 Internet" Birds of a Feather gathering at the August Internet Engineering Task Force (IETF) was filled to capacity, and did reach some general conclusions. The general architecture is decomposed into Signaling Gateways (SG), Media Gateway Controllers (MC), and Media Gateways (MG). Different working groups are chartered to address the interface between SG and MC, and between MC and MG.

The objective of these SS7-Internet protocols is to enable the IP network elements to appear to the telecom network as peer telecom switches. Application logic can be independent of underlying network technology, and administration modules, like billing and numbering plans, can likewise span switch topology. Stay tuned.

Laurence J. Fromm is vice president, new business development for Dialogic Corporation. Dialogic is a leading manufacturer of high-performance, standards-based computer telephony components. Dialogic products are used in voice, fax, data, voice recognition, speech synthesis, and call center management CT applications. The company is headquartered in Parsippany, New Jersey, with regional headquarters in Tokyo and Brussels, and sales offices worldwide. For more information, visit the Dialogic Web site at www.dialogic.com.


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