USS Clueless CDMA FAQ -- Can I use a normal modem with my CDMA cell phone?

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Can I use a normal modem with my CDMA cell phone?

Short answer: No.

Long answer: Sorry, but the long answer is really long.

This isn't a plot by the cell phone manufacturers to force you to discard your perfectly good modem. To understand why, you have to understand how normal modems work and how they relate to the regular land-line phone system.

Originally the phone system was a circuit switch, and modems were analog. This is in the days of the cross-bar switch, and after a circuit had been established a single wire carried traffic both ways (possibly with an analog amplifier in between). Modems were devices which moved digital information from one place to another using what was known as "frequency shift keying".

That meant that the modem alternated between two frequencies when it transmitted, with one frequency meaning "1" and the other "0".

Time marches on, and the modems got faster. The new approach was called "phase shift keying", and it worked by encoding four bits into a single wave.

There things stalled. Meanwhile, the phone companies had been switching to digital systems which were not cross-bars. Instead, they worked by digitizing the analog waveform being fed to them, packetizing the results, and interleaving it at very high speed into a TDMA bit stream inside the switch. So to connect two lines to each other, instead of a physical switch making an electrical connection between them, the switch tells each which TDMA time slot to use. This massively simplifies the switch and improves reliability by removing all the mechanical moving parts which were associated with the older cross-bar approach.

But it also meant that the connection from one end of the phone line to the other was no longer electrically isolated. Rather, it was being digitized and thus converted to a step function. Any approach which depended on pure analog waveforms was doomed to fail. But it opened the way for a crafty approach to modem design.

The fastest modems in existence today for standard phone lines (and the fastest which will ever exist, because they use the full bandwidth which is available) are designed specifically for how the phone system A/D converters work. They happen to be non-linear, but the real point is that the threshold for each A/D step is published. When two of these modems establish contact with each other, they "negotiate" and test the line to see just how high an amplitude the line will permit. (They also test various steps in between to make sure they understand how the line is being amplified.) If they can get up to step 237, for instance, then thereafter they will communicate with each other in modulo 237. On each A/D digitization time (which is also published) the transmitting modem sends to the phone system a flat voltage representing one of the 237 voltages which the line permits, and thus this is what will pop out at the receiving modem. Converting binary streams into arbitrary modulus transmissions and back out again is left as an exercise to the student.

So the modems are designed precisely around the exact characteristics of the land-line, in published standards.

Unfortunately, CDMA cell phones work entirely differently. For one thing, while the transmissions in the land-line system are lossless (as long as you stick to the amplitude and step-function levels which the line will support) that is not the case for CDMA.

CDMA begins with a far more restricted bandwidth per phone call than a landline does. When you speak into your phone, it is digitized, but then it is passed through a lossy compression device called a codec. The codec algorithm is specifically designed to take advantage of the fact that human spoken language is enormously redundant and that the human ear can compensate for certain kinds of distortion. In fact, humans are extremely good at this, as you'll realize if you've ever had a conversation at a busy party, or next to high surf, or by a revving motorcycle. Needless to say, CDMA's codecs don't induce distortion like that.

The CDMA codec deliberately discards useless detail, and by doing so is capable of achieving a tenfold reduction in the data stream -- or even more in some cases. Now the emphasis here is on the word useless. Human ears will barely notice that anything has changed, but test equipment (and modems) can pinpoint the differences very clearly.

This works beautifully for a human voice, and most people find that CDMA with a 13K codec actually sounds as good as or even better than a landline does. (Landlines suffer from the fact that the voice traffic covers several miles from the last stage switch to the home, in analog, on copper wires with little shielding. Distortion and noise are inevitable.) But what the CDMA codec is doing is completely wrong for how a standard modem wants to use the link.

For one thing, the traffic that a standard modem tries to feed to the phone looks nothing like a human voice, and the codec is lost at sea. If a standard modem were connected to such a phone, what would come out at the far end would bear only a passing resemblance to what went in. The negotiation between the two modems would fail completely and no connection would take place.

The highest transmission rate available as this is written (May 1999) in most CDMA systems is 14.4 kilobits. There simply is no way to cram 56 kilobits through such a channel; Claude Shannon's Information Theory doesn't permit it. And even at lower rates, what the modem is feeding the phone is not what the phone is designed to carry. [Soon the cell systems will deploy a new form of data services which will support much higher data rates. SCDB 2/2000]

The right solution is for digital communications through CDMA to take advantage of the characteristics of the medium, just as it does through a landline. In this case, it takes advantage of the fact that the actual link between the cell system and the phone is digital, unlike landlines. When the phone carries digital data, it bypasses the codec entirely and transmits the digital information given it by some external digital device (typically a notebook computer or PIM). At the cell system the resulting digital stream either is passed through a standard modem before interfacing into the standard landline phone system, or increasingly it will be gatewayed directly onto the Internet. By this means, the full digital bandwidth of the phone channel is made available to the user.

Captured by MemoWeb from on 9/16/2004