Tech Notes

New Digital Telephone Technologies

There’s been a lot of talk about new digital telephony technologies in the past few years. Lots of work has been done to deliver higher speed data to the home or desktop than is available via ISDN or 33.6 kb/s modems. Some of these technologies hold promise for real-time audio connections, and some don’t. Here’s a sampling of some of the systems you may have heard about, and what they really mean to you.

Key:

Good chance of real-time audio applications

Possibilities for real-time audio applications

Not suitable for real-time audio applications

 

56K modems

If you’re still surfing the internet on a 28.8K modem, these new modems (V.90, X2, and K56 Flex) can really make a difference. They utilize the fact that the telephone company digitizes voice audio into 64 kb/s streams. So by "tickling" the telco analog/digital converters the right way, a similar digital stream can be created and utilized. They do have some pretty serious limitations, primarily in that the connection must be purely digital on one end (like an ISDN line), and that the really high throughput link is only available from the digital to the analog side. This makes them great for web surfing, where the user’s connected to an analog phone line and downloads lots more than he uploads. It works the wrong way for remote audio, unfortunately, since the high speed would be available only in the direction to the remote site (assuming ISDN in the studio).

xDSL

Ma Bell’s foray into internet access. They take an existing analog telephone line and put very high-speed data on it, often leaving the line usable for telephone service as well. It can’t be deployed on long telephone loops, and can’t be served from the line concentrators on your street corner. Some varieties (ADSL) don’t have quite as huge an upload rate as a download rate, and your telephone company won’t be pleased about "dropping in a line for an afternoon remote." The important thing to understand about these new internet connection technologies is that one end terminates on premises, and the other end lands on a TCP/IP server. In the case of xDSL, the server may be at the telco central office, or the line may "pass through" the telco to an Internet Service Provider. Either way, any point to point connections must be internet based, with all the delays, packet loss, and congestion issues associated with the internet.

Cable modems

Like the telephone company, the cable company would like to provide your internet service. Like xDSL, the modem at your premises is tied directly to an internet server at your ISP who, in this instance, is your cable company. We’ve heard from cable modem users who experience very high speed connections, but they can be subject to throttling not only due to internet congestion, but cable network congestion as well (many users share one pipe). For real-time audio, cable modems have the same limitations as other internet access devices, namely, the internet.

Frame Relay

This service is being offered instead of dedicated digital circuits in some areas. It’s similar to a packet based protocol which runs over a private network (e.g. not the internet) and provides guaranteed data throughput, specified as bits/second average over a certain period of time. While designed primarily for bursty, lan-type data traffic, some work has been done to simulate dedicated, synchronous networks for applications like video conferencing. It’s of interest because the throughputs it offers are typical audio codec data rates (56-256 kb/s) and the cost is much lower than dedicated point to point links. The fact that "frames" are "relayed" between network routers implies some time delay, so its duplex applications may be limited, but keep watching for audio applications using frame relay.

3G and GPRS

Lots of work is being done to turn cellular and PCS networks into high speed data networks. GPRS is an interim solution, while 3G is the working name for the yet-to-be standardized "third generation wireless." There isn’t any deployment yet, and it remains to be seen which technology will find its market, but wireless networks hold out the hope for wideband digital audio with mobility. The ultimate solution will be network based, although hopefully not inexorably tied to the internet. It may be that a broadcaster will need a high speed data link dedicated to his wireless provider, or perhaps receive digital audio transmissions on a wireless phone in his studio!

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