After 300 baud modems, people started saying things like "300 baud is as fast as modems will ever go, so you might as well stop asking us for faster modems". People should have learned a long time ago not to say that sort of thing, because now they look really silly. It wasn’t easy getting from 1200 baud to the 33.6 and 56K modems we have today, and it’s not easy living with the 33.6 and 56K technologies we have today, because they didn’t just get faster, they got more complex. At the same time, computers got more and more powerful, and then suddenly everyone and their dog wants to dial into the internet, and download multi-megabyte files from their home PCs. A lot of people take modems for granted, or treat them like magic, but they aren't magic, and they don’t always work as designed.

The 1200 baud (or bps) modem replaced the 300 baud modem by the early 80s. It was four times faster than the 300 baud modem, but it used some new tricks to do this. Unlike 300 baud modems which had only two transmission frequencies, the 1200 baud modem used four separate signal frequencies for transmission. Using these four different frequencies, it could transmit two bits at a time, instead of one. It used a physical baud rate of 600 bps, and a four frequency transmission scheme. This means that technically 1200 baud modems are really 600 baud modems that transmit 1200 bits per second. Unfortunately, the meaning of baud (audible bit) gets distorted a lot. The most accurate way to refer to modem speed is in bits per second, abbreviated as "bps". Since people use both interchangeably even though they really aren’t the same, you just have to bear that in mind as I will freely intersperse them through the rest of this document to confuse you.

The other thing that 1200 bps modems introduced was fallback capabilities. If you went out and spent $1000 on a new 1200 bps modem, you would still want to revert to 300 bps operation. If a connection at 1200 bps failed due to noise, or if the modem on the other side was a 300 bps, the modem had to detect this. Changes were made to the carrier detection logic in 1200 bps modems to allow this to occur. This process is known as link negotiation. Generally, every modem from 1200 bps and faster has link negotiation capabilities, that allow it to attempt to connect with the remote modem at the fastest possible mutually supported speed. Sometimes this negotiation process fails, and modems need to be specifically instructed what speeds to allow connections at, and what speeds not to allow. This is part of the setup of your modem, and it is one of the reasons you need to understand your modem’s command set.

Domestically the Bell 212A spec became the North-American standard for 1200 bps modems.

Internationally the ITU V.22 1200 spec was endorsed by the of the ITU / CCITT (International Telecommunication Union, formerly the/ Comité Consultatif International Téléphonique et Télégraphique). It was a half-duplex protocol. It did not catch on in North America. Later standards are fixed in Europe and North America, so that you are guaranteed worldwide connectivity.

2400 Bps ITU V-22bis Modems

The 2400 bps modem was a logical evolution from the 1200 bps modem. It used 8 audio frequencies to transmit up to 4 bits at a time. It had more sophisticated fallback capabilities, allowing it to connect to 1200 bps modems at 1200 bps, and to 300 bps modems at 300 bps.

This modem standard applied in North America and Europe. Since then no further Bell standards have applied to modems, and further standards have all been ratified by the CCITT.

Several new features were introduced in modems while 2400 bps was the default speed. With the increase in speed from 1200 to 2400 bps, the unavoidable side effect was that modems were more affected by line noise and cross-talk on phone lines. This meant that a lot of garbage would be inadvertently read in by the modem. If you were using a terminal program you’d see this as an unintelligible mess of letters, numbers and symbols on your screen. Modem manufacturers started to address this by adding Error Detection and Error Correction to their modems. These standards were actually complex protocols that were implemented inside the modem. The most common of these are MNP, LAP-M, and V.42. These protocols sent data, along with additional checksum information, and could detect transmission errors, and automatically retransmit the corrupted data, all without the two computers even noticing that something had gone wrong. MNP, the Microcom Networking Protocol, was implemented in levels. Error Detection and Correction is done by MNP Level 4. The most popular general purpose error detection and error correction scheme, which is still used now in an improved form is V.42. The V.42 standard was approved by the CCITT, an international telephony standards organization.

At about the same time, engineers decided that data compression would be a good way to increase the throughput (total bytes per second) of modems, for compressible data. Data which repeats certain patterns over and over is highly compressible, and can be transmitted more efficiently through data compression. MNP Level 4 was extended to MNP Level 5, and V.42 was extended to V.42bis. This did not increase the raw bit transmission rate of the modem (the baud rate), or the raw transmission rate of the data bits (the bps rate), but it did increase the apparent transmission rate, by transparently compressing and decompressing the data as it went through the modem. Highly compressed data would actually transfer more slowly through MNP Level 5 than it did through an MNP Level 4 connection, because there was some time and additional data transmitted to allow MNP Level 5 to work. MNP 5 is not considered a current technology anymore, and it has fallen into general disuse. LAP-M is still supported, but is not highly used, either. V.42bis compression however, compressed not only the data sent through the link, but the V.42 error correction information as well, so even highly compressed data transferred through the modem slightly faster than it did using V.42 alone. In case you were wondering, "bis" is a French idiom for "plus", so V.42bis actually means "V.42 plus".

The 2400 bps Fax Modem was the beginning of the telephony modem. If a computer can talk to another computer, and if two fax machines can use a similar technology to send scanned images over a phone, why can’t your computer send and receive faxes. The first 2400 baud Modems with fax were known as 2400/9600 modems, because they usually sent 2400 bps as data modems, and 9600 as fax modems. Fax machines and fax modems use a very simplified signaling scheme that is not suitable for general data modem use. If you see a 2400/9600 modem, it’s really a 2400 bps modem with faxing, and it is not capable of 9600 bps for data use. Fax modems require additional fax software for them to be used for Faxing. One additional feature of these modems is that with the right software they can distinguish between incoming fax calls and incoming modem calls.

While 2400 bps modems were the mainstream, early attempts were made at 9600 bps modems. Most of these were a success in one industry or another, but were not generally standard. One of these was the Telepath 9600, which used a proprietary negotiation scheme requiring two identical Telepath modems in order to connect at 9600 bps. Another was the very popular US Robotics HST modem. The HST used a proprietary "Ping-Pong" half-duplex system, taking advantage of the fact that for some applications data is mostly transmitted in one direction and acknowledged in the other direction at a much slower data rate. The HST had a front channel and a back channel, which allowed transmission in one direction faster than the other direction. If the volume of data switched so that most of the data was now coming from the other side, the modem would do a "Ping-Pong", and the fast channel and slow channel would reverse directions. This was very effective, and was very popular in the days of hobbyist and home user Bulletin Board Systems, before the hobbyists and home users began frequenting the Internet instead.

Many people again thought that the phone line was being fully utilized, and that no further speed improvements could be made. Of course they were wrong.

The CCITT V.32 standard introduced the first standard 9600 bps modems that were full duplex. Both send and receive channels could operate at 9600 with full speed at once. These modems added advanced fallback and re-negotiation features so that the modem could temporarily decrease the modem speed and then increase it as line noise or other line conditions improved or worsened.

Modem acronyms were getting to be quite a mouthful by this time. A good modem would be a 14400 V.32/V.32bis/V.42/V.42.bis modem. A modem with "all that V stuff" was soon referred to as "V.Everything".

This was a significant jump. A doubling from 1440 to 28800 was a feat not easily achieved. At this point there was some serious arguments that the capacity of the phone lines were all used up. The next increment above this pretty much proved the point that we really were approaching saturation of the Plain Old Telephone System, which is known simply as POTS.

The move from 28.8K to 33.6K modems happened quietly and without much fanfare. The improvements to V.34 to support 33600 were incremental, and in many cases line quality and headroom were not high enough to allow 33.6K connections. The differences between a 28.8 and 33.6 modem are pretty minimal.

Although this sounds like a dramatic move, a V.90 modem is really a V.34 modem with a new twist: It lives halfway between the current analog phone system and the future all digital phone system. Since most central phone company equipment and internet service provider equipment no longer includes analog modems, the "modem" ceases to be a true modulator/demodulator, and becomes instead half modem, and have a digital decoder. The digital signal is encoded and transmitted digitally from the digital side, and when received at the analog side the data can be decoded.

That means that you can’t place a call between two analog lines and connect any faster than 33.6. In a sense we have finally hit the limits of analog phone technology, at least with the phone company infrastructure being in the half-analog/half-digital state that it is in. I probably shouldn’t say that, but it’s ingrained.