Foundations For Carrier-Class Internet Telephony
BY LAURENCE J. FROMM
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?
BUILDING THE INFRASTRUCTURE
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.
MEETING THE NEEDS OF PUBLIC NETWORKS
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
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.