Taming Voice

Taming millions of voice streams to travel over a single packet-based backbone is a major challenge for VoIP.

Physicists dream that one day there will be a single equation to explain all physical forces. Although this strongly attracts many, other physicists believe this is the impossible dream. In telecommunications, the parallel is the all-IP network, where video, sound, text files, control information (signaling) and voice all travel over a single massive packet-based backbone. The most challenging aspect of this may be taming the millions of streams of voice.

To people who are used to computer communications, the requirements of voice seem minimal — only 32kb/s to 64kb/s of data per conversation in the fixed network and as low as 8kb/s on many wireless communications protocols. Meanwhile, even low-end communications is using 10,000kb/s wired Ethernet or its wireless equivalent, IEEE 802.11.

The major challenge with VoIP is obtaining a high QoS. Technically, QoS is a list of characteristics that must be met for a protocol stack to support a particular application. E-mail, for example, is flexible on some aspects, allowing delays in transmission; it can accept delivery of messages in any order, but it has no tolerance for the delivery of erroneous information to the end user.

Voice, by contrast, is an example of isochronous communications. There is some tolerance in the delivery of this information, but this delay must be relatively consistent, as voice has low tolerance for jitter. To deliver one packet with a latency of 400ms and the next one with only 10ms (and out of order) could totally destroy the voice quality. Making life somewhat easier, voice is relatively tolerant to errors or packet loss, more so than most forms of data communications.

Some QoS characteristics for important services are listed in Figure 1, but they vary significantly based on consumer expectations.

Voice in the traditional phone network is transmitted over facilities and routed through switches that are adapted perfectly to its isochronous nature. Voice is digitized to a continuous 32kb/s-to-64kb/s stream and transmitted over facilities that assign that amount of bandwidth for the duration of a call.

Switching equipment is simple, merely copying bits of digitized voice from an incoming timeslot to an outgoing timeslot. Apart from ensuring high quality of the facilities, there is no provision for error handling; systems rely on high-quality facilities to make errors extremely rare. Overload cannot occur once a call is established because the stream of voice traffic for each call never varies in size. Overload only can occur with signaling traffic, which only affects the ability to set up calls.

This seemingly perfect world has a number of drawbacks. Telecom equipment is, on a bit-per-second basis, very expensive, and it operates in a network that is largely separate from public and private Internets.

The dream is to put digitized packets of voice on the same network as everything else, meaning that only one network has to be built, managed and upgraded for a carrier to provide a full suite of services.

Many convergent services can integrate a number of types of information, such as a videoconference integrating streaming video with voice, text and graphics. Another advantage of IP telephony is the ability to avoid international tariffs, which further (although artificially) reduces the cost.

Yet voice is not handled well by an IP network unless considerable adjustments are made. One of the problems is that IP works on a best-effort basis to deliver packets, assuming that applications such as TCP can re-send packets if they disappear. Routing of IP packets is much more complex than copying bits across a timeslot interchange switch.

The robust routing used by IP allows different packets to take different routes, arriving at different times. Therefore, unless something is done to handle isochronous traffic such as voice in a different fashion, the quality will vary tremendously, declining as the amount of traffic increases.

QoS Characteristic	E-Mail	Voice	Fax	Web Surfing	Streaming Audio/Video
Mis-Sequencing of Packets	Tolerant	Intolerant	Intolerant	Tolerant	Intolerant
Delay	Tolerant	Intolerant	Intolerant	Varies	Intolerant
Jitter (Variation in Delay)	Tolerant	Intolerant	Intolerant	Tolerant	Tolerant
Errors	Intolerant	Tolerant	Tolerant	Intolerant	Tolerant
Figure 1. QoS characteristics for various services

Maintaining an adequate QoS for voice services is acknowledged as one of the major challenges for VoIP. ETSI (www.etsi.org) has set up its TIPHON group for this purpose, aiming to enhance existing protocols such as SIP and H.323.

One way to improve VoIP QoS is to run a managed network. Enough extra bandwidth can be provided that traffic peaks do not cause serious congestion. Another method is to work harder to optimize the path through which the voice traffic passes.

Avaya (www.avaya.com) has created techniques called “shuffling” and “hairpinning” that can remove servers from the voice-traffic stream, eliminating some of the delays, jitter and sequencing errors. This is similar to the redirection and path-minimization techniques used in ANSI-41 for optimizing the path of wireless calls.

Other techniques are differentiated services and resource-reservation protocol, which provide information that IP routers can use to give voice packets more desirable treatment.

Although major challenges remain for VoIP in fixed networks, even greater hurdles confront those trying to support wireless access. Although wireless protocols already digitize voice efficiently, the addition of IP overhead will destroy these gains.

According to an Ericsson white paper, compression and other techniques will help achieve VoIP over wireless with 3G systems but still will not achieve the quality or robustness of circuit-switched connections without further work.

VoIP systems likely will achieve the most success in areas where either cost or the benefits of convergence with data services are of greater importance than voice quality. Ignoring VoIP now, because of its imperfections, would be to make the same mistake as those who ignored wireless in the early 1980s.

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Crowe (crowed@cnp-wireless.com) is a wireless-standards consultant and editor of Cellular Networking Perspectives, a wireless-standards and -technology bulletin.