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Circuit Switching Gets Better Over Packet Networks
28 September 2000 Geoff Johnson ITU and IETF have agreed on a new standard called H.248 to facilitate circuit-switched traffic in packet networks. Enterprises can expect more-robust delivery of their planned Voice over Packet Networks traffic.
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# Circuit Switching Gets Better Over Packet Networks

ITU and IETF have agreed on a new standard called H.248 to facilitate circuit-switched traffic in packet networks. Enterprises can expect more-robust delivery of their planned Voice over Packet Networks traffic.

Bottom Line

Key Issue What WAN designs and technologies will network planners adopt to best handle disparate (and changing) traffic, connectivity and application requirements of their enterprises?

Strategic Planning Assumption By 2002, enterprises will be able to rely on carrier-grade switching, signaling and billing of IP traffic as H.248 becomes used effectively in compliant gateways throughout carrier and Internet service provider networks (0.7 probability).

In Geneva on 4 August 2000, ITU-T Study Group 16, responsible for multimedia services and systems, and the Megaco Working Group of the Internet Engineering Task Force (IETF), the international body responsible for the Internet protocol (IP), agreed on a single standard for the control of IP/PSTN gateway devices — H.248. This agreement for multimedia standards facilitates scalable and relatively seamless implementation of services and applications over circuit-switched and IP-based networks. The new standard will support gateway devices passing voice, video, fax and data traffic between conventional telephone and packet-based data networks, such as commercial IP networks or the Internet. H.248 now enables highly scalable implementations of H.323 gateways because it decomposes the H.323 gateway function into functional subcomponents and specifies the protocols used to communicate. H.248 enables these gateways to include components from multiple vendors distributed across multiple physical platforms. Gartner estimates the total international telephone traffic over IP gateways to be 40 percent of all traffic by 2005. Sixty percent of major public telecommunication operators believe that IP telephony is capable of becoming the main means of telecommunication by 2004. IP Telephony. Different types of IP telephony provide, to varying degrees, alternative means of originating, transmitting and terminating voice and data transmissions that would otherwise be carried by the PSTN. So-called "next-generation telcos" are building vast global networks, based around IP, on which voice service can be provided alongside data. Although IP-based networks are optimized for the carriage of data, they can carry voice very competently and cheaply. IP telephony can be subdivided into two major groups: Voice over the Internet and Voice over enhanced IP. IP telephony often does not involve the public Internet at all, but rather only its underlying technology, the Internet Protocol suite. All types of IP telephony vary according to three characteristics: the type of terminal devices used, where gateways between IP networks and the PSTN are located, and the principal underlying means of transmission (see Note 1).

Note 1 Definitions IP Telephony: the transmission of voice, fax and related analog services over packet-switched IP-based networks. Internet telephony and VoIP are specific sub-sets of IP telephony Internet Telephony: IP telephony in which the principal transmission network is the public Internet, also referred to as “Voice-over-the-Net” (VON), “Internet Phone” and “Net Telephony” VoIP: IP telephony in which the principal transmission network or networks are private, managed IP-based networks (e.g., "Voice over frame relay," "Voice over cable” and “Voice over DSL” or “VoDSL.” The type of IP telephony differs according to the type of terminal used, where gateways are located and the underlying transmission means — PC-to-PC, PC-to-phone or phone-to-phone.

Packet Switching. The primary goal of IP telephony has been to replicate the functionality, reliability and service quality of circuit-switched telecommunications networks, using a protocol suite that was not optimized to facilitate real-time communications (IP). By contrast, on circuit-switched networks operating under a protocol such as Signaling System 7 (SS7), a call is routed through a hierarchy of local, inter-urban and international switches to establish a circuit between caller and called party. In a digital network with data compression, the differences between circuit and packet switching are diminishing. Gateways. The gateway is a link between an IP network and a telephone network at a point at which voice signals become digitized and packetized, or at which digitized packets are converted back to voice. Gateways make long distance or international calls "look" like local calls to the billing systems of PSTN operators at either end. Gateways bridge PSTN and Internet architectures. Regulatory Cautions. ITU "even-handedness" in regulatory issues means that it will support its members and their regulators rather than be overtly competitive (see Figure 1 and Figure 2). IP telephony is no longer a fringe service lying outside the telecommunication world. H.248 players include: Telstra, ITXC, iBasis, Concert Communications, BT and AT&T.

Figure 1 Table of Regulatory Status of IP Telephony by Country
Source: ITU and Organisation for Economic Co-operation and Development

Figure 2 Standard-Setting Bodies
Source: ITU

Bottom Line The initial enthusiasm for “free long distance on the Internet” now appears to have been subdued by the reality of the immense complexity of duplicating a global telecommunications system to reliably signal, switch and bill voice traffic. Enterprises planning Voice over Packet Networks should be encouraged by service providers' H.248 adoption. Users must understand the detailed implications of its use by equipment and service vendors (particularly H.323 merging with Media Gateway Control Protocol and displacing Session Initiation Protocol) in order to optimally migrate their WAN and application environments. Acronym Key DSL Digital subscriber line ITU International Telecommunication Union PSTN Public switched telephone network VoIP Voice over IP

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