Analog Vs. Digital Modems

You probably know that modems are analog devices. You may not know, however, the telephone system in the US is a mix of analog and digital. Almost all calls in US are converted from analog to digital at your local phone company office, and stay digital until they reach the local loop of the place you call. So what is going on is we start with digital data at our computer, convert it to analog in our modem, and send it out to the phone system. The phone company switches it back to digital and transmits it, then converts it back to analog for the local loop at the other end. There, the modem converts it from analog back to digital for the remote computer. Eight conversions per byte for full-duplex (ie, round trip) V.34 modems -- when you think about it, it's amazing this stuff works at all! By the way, the trick used to allow 56k modems is to eliminate some of the analog/digital conversions -- more below.

Analog V.34 modems utilize nearly the full bandwidth of the present day phone system (actually, more than the "rated" bandwidth) -- that is, 33.6 kbps is pushing the speed limit of the current analog phone system. Note that MANY people who have 28.8 or 33.6 kbps modems will never achieve connections at those rates due to phone line conditions.

The next generation phone system will be all digital. In the later 1990s it appeared that the standard would be ISDN, which stands for "Integrated Services Digital Networks" -- ISDN is presently in common use in many businesses. ISDN is basically the existing telephone network turned all digital, using existing switches and wiring wherever possible. The type of ISDN most commonly used in residential service supports two 64K voice/data channels, and one signaling/packet networking channel. While ISDN is available in many parts of the US, its cost varies from reasonable to very expensive. One of my friends got ISDN at his home, and through a misunderstanding of how the service is billed, ended up with a $475 phone bill for the first month (yikes!). Now, however, a newer technology called DSL has become the phoneline high-speed option for the consumer market (more DSL information later). DSL is competitive speed-wise and cost-wise with broadband internet access through your TV cable company.

The switch from analog to digital will take the phone companies years to accomplish. DSL Internet access (and TV Cable Modem access -- the main competition to DSL) are not available in many parts of the country, won't be for years to come. What this means is that many of us are going to be using our analog 28.8/33.6/56k modems for quite a while -- so choose wisely when you buy!