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Over the past couple of years, there has been much discussion in the industry about the convergence of voice and data and the pros and cons of transmitting voice over a data network. The purpose of this white paper is to address the question of what is voice over IP and what does it mean to anyone grappling with the question of whether to buy a new telephone system.
In telecommunications (beyond the level of two cans and a piece of string) there are 2 bi-directional streams (possibly over the same channel. In fact, It was common for a number of years to have the control stream riding on the same medium as the voice stream. This is known as "in-band" control or signaling. However with the advent of digital transport systems, most in-band signaling has been replaced by a separate control channel which is known, appropriately enough, as "out-of-band" signaling.), a "voice" stream (which includes voice, music, fax squeals, modem shrieks, etc.) and a control stream. "Voice over IP" (VoIP) refers to the transport of a telecommunications voice stream over a data network using the data transport mechanisms associated with the Internet, called the Transmission Control Protocol/Internet Protocol (TCP/IP) suite.( You might think that the restriction to the voice stream was self evident. However, one vendor presents the following ingenious and useful application as part of their VoIP strategy. It is a solution primarily for remote workers. The worker needs an IP connected computer and a Plain Old Telephone System (POTS) line. The worker uses a browser to connect to a web-site which, in turn, communicates with the PBX. The user then identifies and authenticates, giving the phone number of the POTS phone. The PBX connects to the telephone, and provides all the sophisticated features of the PBX like hold, conference calling, etc. by means of the phone line plus feature access through the computer interface.)
Discussing Voice over IP is often difficult since voice over IP means different things to different people. To some it means voice over the Internet; to others it means voice over a point-to-point wide area network (WAN); to still others it means voice over the local area network (LAN). Complicating matters, various authors attempt to differentiate between the meanings by coining terms like "IP Telephony," "Internet Telephony," "Telephony over IP" or "Telecommunications over IP." For our purposes all these terms are synonymous. For the purposes of this white paper, we will be concentrating on VoIP in the LAN and WAN environments only, largely ignoring the issue of voice over the public Internet.
Traditional telephony, is based on circuit switching and time-division multiplexing and is so reliable that the industry "standard" calls for "five nines reliability" or availability 99.999% of the time. So then why is anyone considering voice over IP? In his article, IP Telephony, the End of the World as We Know It? ( Cause/Effect, Vol 22 No 2, 1999), Scott Street of Compass Consulting International, Inc. discusses the change drivers for IP telephony. They fall into 4 categories: economic, functional/technical, management, and hype. Scott's conclusion in 1999 was that none of the factors (except hype) truly favored IP telephony at that time. In an analysis for the August, 2000 ACUTA National Conference, Scott Street and Robert Kuhn (also of Compass) showed that, while there has been movement in all four areas, the earlier analysis remains fundamentally true. However, under exceptional circumstances, it is already possible to make the economic and technical case for VoIP. These exceptional cases require economically a short amortization period, lack of adequate building-to-building copper, a strong IT-Telecomm synergy, and a robust in-place data infrastructure. Typically early VoIP adoptions are administrative implementations without stringent e-911 or disabled-access requirements and a minimal need for advanced features.
One way to arrive at the core arguments in favor of VoIP is to compare voice and data technologies on a "bit-by-bit" basis. Take a typical large university as an example. The typical data network in a university today delivers 100 Megabits per second (10 8 bps) to the desktop, with the core switching devices approaching Terabit-per-second (10 12 bps) speeds. Compare this to a typical telecommunications network for a similar institution. The connections to the desktop are 64 kilobits-per-second (6.4x10 4 bps) and the telephone switch capacity would not reach 15,000 simultaneous calls (10 9 bps). This gives the data switches a thousand-fold edge in switching capacity.
So, while telephones are much cheaper than computers, and telephone networks cheaper than data networks, if you divide by the capacity, the data network equipment is much, much cheaper bit-for-bit. To put it another way, given the explosion in bandwidth requirements for data and video, voice can "ride for free" on the data networks in terms of both economics and management. In the long run this is leading vendors to favor VoIP strategies for the future of telephony, but the steps taken by different vendors vary widely in technical approach. To understand the current state of VoIP it is necessary to look more closely at what is actually going on in any particular implementation.
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