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How were standard serial port baud rates chosen? 1200, 9600, 115200, aren't the numbers we'd expect (powers of 2 or 10), but don't seem random either.

Wikipedia states

Many of these standard modem baud rates are multiples of either 0.9 kbps (e.g., 19200, 38400, 76800) or 1.2 kbps (e.g., 57600, 115200).[23] Crystal oscillators with a frequency of 1.843200 MHz are sold specifically for this purpose. This is 16 times the fastest bit rate, and the serial port circuit can easily divide this down to lower frequencies as required.

but that doesn't seem to fully explain it either.

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    \$\begingroup\$ There are no standard that defines what serial port baud rates must be. There are only baud rates that are commonly used so that is what devices commonly offer. Some specific standard intefaces like DMX512 or MIDI do define the baud rate, but you really don't think it as serial port but a specific interface. \$\endgroup\$
    – Justme
    Sep 22, 2023 at 16:50

4 Answers 4

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The first modems I ever worked with were 110 baud, but the Bell 103 modem set the mainstream speed ball rolling with a 300 baud FSK setup. This was the best which could reliably be achieved at the time over the phone lines of the day, given the state of the art analog tone generators and detectors.

From there things increased in powers of 2, because that allowed a single crystal to maintain back compatibility with simple dividers.

300->600->1200->2400->4800->9600->19200->38400

Serial ports were matched to these speeds.

The digital phone network samples at 8ksps and 8bits, which in theory limits to 64kbp. However, the use of "robbed bit signaling" in the US limited the maximum speed for analog PSTN modems to 56Kbps.

There was therefore a secondary series of speeds related to 56Kbps, which were not power of 2 multiples of 300, but were still whole multiples (14400, 57600, etc).

115200 is the most common limit at 3*38400, by which point most really high-speed links moved to different physical protocols such as USB and Ethernet (which have entirely different bit rates).

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    \$\begingroup\$ Nostalgia: First modem I used operated in asymmetric speed 1200/75 \$\endgroup\$
    – MiNiMe
    Sep 22, 2023 at 17:44
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    \$\begingroup\$ colintd, I remember 50 baud/bps (they aren't the same thing, baud and bps, but at the time they were the same) in the 1960's. To be specific, the date I recall was 1961 for that. Then it was upgraded to 110 (circa 1968/69 from my recollection.) Those are at least two data points, if not the transition points where they first appeared. \$\endgroup\$ Sep 22, 2023 at 17:54
  • \$\begingroup\$ You win! However I've still got a type 33 teletype that uses 5 bit bauddot instead of ASCII! \$\endgroup\$
    – colintd
    Sep 22, 2023 at 18:55
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    \$\begingroup\$ @colintd: you misremember. Model 32 was Baudot generated by 3-row keyboard sent at 75 baud = 10x(1+5+1.5); 33 was mechanically similar (same case, basically same printer, similar tape reader and punch if ASR) but ASCII with 4-row keyboard sent at 110 baud = 10x(1+7+1+2). \$\endgroup\$ Sep 24, 2023 at 2:08
  • \$\begingroup\$ You could well be right, as it has been over 40 years since I ran it. .'ll have to dig it out from storage and check. \$\endgroup\$
    – colintd
    Sep 24, 2023 at 7:20
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The short version

300 baud is the maximum rate that the telephone line could support using modulation and demodulation technology that was easily available at the time. The specific number is, ultimately, related to the ordinary frequency range of sounds that can be heard by the human ear. The phone line was designed and optimized so people could speak normally and be understood on the other end.

How does this work?

Telephone lines carry electrically encoded audio. That is to say, there's a circuit that turns sound into electrical impulses, which are sent over the phone line. What frequencies need to be carried is determined by how humans speak. Whatever language you speak, the maximum frequency is determined by what frequencies the human ear can perceive.

A modem (modulator/demodulator) converts digital data into audio, which is then sent over the same phone line.

Early modems used old-style telephones with rubber cups that had speakers in them. One cup had a speaker and the other a microphone. You'd set your phone in them and it would physically make audio sounds that were converted by the phone into the electrical signaling required for the phone line.

Later on, the extra step was eliminated and the modem would plug directly into the phone line. It had the circuitry in it to emulate the same procedure, including the part that the telephone receiver did.

If you think about sending digital data over an audio channel, you may think something like Morse code. ITU modems (the ones that most people used that ran at 300 baud) didn't use Morse code, but there was a different encoding scheme called frequency shift keying that worked the same. FSK is much more difficult (generally impossible) for humans to encode and decode by hand, unlike Morse code, but faster. For reference, 100 words per minute (WPM) of Morse code is, "pretty good," and correlates to 75 baud.

Right, but still, why 300?

The speed you can use is based on what you can modulate or demodulate (often one is easier than the other), what your line can handle and how clean (error-free) your line is. The bandwidth on original analog lines was fairly limited. The rubber cups made a pretty good connection to your phone but it wasn't super fantastic. Further, the world of digital to analog conversion was relatively new (compared to now) so less was known about how to do it efficiently and cost-effectively.

Like any system, the maximum rate is determined by the maximum rate achievable by the slowest part of the system.

After weighing all the factors, the ITU published a specification for inter-device communication. For a long period of time, the quality of the existing phone lines and other factors put tolerances in the range of 300 baud FSK range.

Adaptive rate protocols were newer at the time these first standards were rolled out. Fixed-rate systems or user-configured rates (you agreed with your friend that you were both using 300 baud) were more common, although by the end of the phone modem, adaptive rates became the standards. If you knew your line and your modem could handle at least 300 baud, you used 300 baud.

Recap

The frequency of speech (people talking to each other) is determined by the frequency response of the human ear, figured out over thousands of years of human society. The bandwidth of the phone line was determined by the frequency of speech. The bandwidth of the modem was determined by what could be sent over the phone line. The symbol rate was determined by the available bandwidth.

It is my personal belief that the actual math (FSK symbol rate calculations, etc.) is fairly pointless unless you're trying to re-design this system, which has already been designed pretty well. Given the audio response of the ear and the mechanics of FSK, 300 was the closest round number.

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How were standard serial port baud rates chosen? 1200, 9600, 115200, aren't the numbers we'd expect (powers of 2 or 10), but don't seem random either.

If you take 1200 as the base then

  • 9600 is 16 × 0.5 times higher
  • 19200 is 16 × 1 times higher
  • 38400 is 16 × 2 times higher
  • 57600 is 16 × 3 times higher
  • 76800 is 16 × 4 times higher
  • 115200 is 16 × 6 times higher

So, there is some logic but why the original 1200 bps? I expect it had something to do with basic telephony channels that only had a usable bandwidth that could support up to 1200 bps. Of course, this doesn't rule out lower rates.

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    \$\begingroup\$ It might be 300 baud is the highest usable in a telephony bandwidth for the earliest technology modems \$\endgroup\$
    – Neil_UK
    Sep 22, 2023 at 16:41
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    \$\begingroup\$ The old style acoustic coupler modems were 300 baud. \$\endgroup\$
    – KristoferA
    Sep 22, 2023 at 16:43
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I'm uncomfortable assuming that the speed was determined by electronic considerations: I believe that 75 bits per second was used for at least some mechanical teleprinters (e.g. Creed). There were also non-standard speeds: 110 bps, 134.5 bps and so on.

If we do however have to view this from an electronic POV, I'd suggest that the important thing is to consider Bell's selection of two frequencies which passed reliably through their voice-oriented telephone system (without triggering early switching electronics), and then look at what data rate those frequencies could support.

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