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Baud or Symbols per second?

  1. Aug 10, 2016 #1
    Hi everyone, I was reading up on bit rate vs Baud speed, and sort of got myself straight with it, but I have a few questions.. First of all why isn't the term Baud still used? We see devices and speeds quoted at their bits per second speeds, but is this just by convention now, or is baud speed someone irrelevant?

    Second, just to check for myself, I went to Newegg.com, and looked up an arbitrary modem and looked for the specs. Below is a listing,

    Channel Bonding: Up to 4

    Channel Width: 200KHz / 400KHz / 800KHz / 1.6MHz / 3.2MHz / 6.4MHz

    Modulation: QPSK, 8/ 16 / 32/ 64 / 128 QAM

    Symbol Rate: 160 / 320 / 640 / 1280 / 2560 / 5120 Ksym/s

    Maximum Data Rate: Up to 143 Mbps

    Frequency Range: 5 to 42 MHz (edge to edge)​

    WIthout getting too weedy or into the nitty gritty, I'm not sure how the numbers here work out. Is Symbol Rate here, specified as Msym/s and Ksym/s the same as baud? wouldn't symbols per second be the same number as the baud? So for example, when bonding 4 channels as shown below... it can have a max of 5120 Ksym/s.....meaning that it's 5120 baud?

    So basically if the modulation is using say, 32-QAM, that's 32 different symbols, and each symbol can then be 160/320/640/1280,2560, or 5120 thousand? (because of the Ksym/s)?

    Again I know it's probably an overly simplified look, but do I have the idea right?
     
  2. jcsd
  3. Aug 11, 2016 #2

    jedishrfu

    Staff: Mentor

    Wikipedia talks about baud here:

    https://en.wikipedia.org/wiki/Baud

    It seems that it can be used as a measure of data transfer for analog systems where bits aren't appropriate.

    My superficial understanding of baud from long ago was that characters per second was somewhat misleading when comparing different systems of transmission because at the time there were 6-bit character vs 7-bit vs 8-bit. The 6-bit character sets didn't have lowercase lettering and so were inferior to the newer more modern 8-bit encodings and so two systems while having the same characters per second data transmission rate were in fact transmitting at different bit rates.

    I found this more detailed comparison of bit rate vs baud rate that may help:

    http://electronicdesign.com/communications/what-s-difference-between-bit-rate-and-baud-rate
     
  4. Aug 11, 2016 #3

    Bandit127

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    Gold Member

    There is a good comment at the bottom of the article which matches my understanding.

    Since RS232 can have a variable amount of bits to a character the bitrate could differ for a given baud rate. For a character the data was usually 7 bits (it could be 6, 7 or 8) and there were options for a parity bit and a stop bit. So potentially 6 to 10 bits per baud.

    Given the RS232 connection is synchronous though, I don't know if a nominal baud rate was varied around a constant bit rate or vice versa.
     
    Last edited: Aug 11, 2016
  5. Aug 11, 2016 #4

    rbelli1

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    Gold Member

    This is a comment on the article that directly contradicts the article.

    RS232 8N1 has 8 bits of payload data per 10 baud (1 start, 8 bits data, 1 stop) maximum. Many times there are additional framing bytes so actual throughput is lower.

    RS232 is asynchronous. The hardware often used is referred to as a UART. Universal Asynchronous Receiver Transmitter.

    BoB
     
    Last edited by a moderator: May 8, 2017
  6. Aug 12, 2016 #5

    nsaspook

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    Science Advisor

    RS232 is a physical/electrical interface specification, it does not define encoding or framing. The framing and encoding of data could be asynchronous or synchronous. Most of the 'High Level' TTY interfaces were 60mA current loops for 5 bit Baudot systems (with a start bit and one or two stop bits for the mechanical encode/decoder inside) but we had some 8 bit asynchronous link systems that still used 60ma for TTY in the early days so we had converters to send and receive 5 bit data on 8 bit links by stuffing 'marks' in the extra lsb bits after the normal data 'marks and spaces'.
    http://www.baudot.net/docs/harvey--mark-and-space.pdf
     
    Last edited: Aug 12, 2016
  7. Aug 12, 2016 #6

    rbelli1

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    I stand corrected. The specification may be used for either type of communication.

    BoB
     
  8. Aug 12, 2016 #7

    nsaspook

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    It can even be used for a Morse key interface.:biggrin:
    http://kob.sdf.org/morsekob/interface.htm

    http://www.comportco.com/~w5alt/cw/cwindex.php?pg=4
     
    Last edited: Aug 12, 2016
  9. Aug 19, 2016 #8

    Svein

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    Baud rate and bit rate refers to two very different things.
    • Bit rate: The number of bits transferred from one digital device to another digital device in one second. As such, it is an end-to-end specification, it does not care how you get the bits from one device to another.
    • Baud rate: Whenever you put a digital data stream onto a transmission medium, you convert the bits into symbols. You can have several bits per symbol, you can have several symbols per bit and so on. The baud rate is defined as the inverse of the duration of the shortest symbol.
    Thus, the maximum baud rate you can have is determined by the bandwidth of the communication channel. The maximum bit rate is determined by your choice of bit encoding. For an efficient encoding, check out https://en.wikipedia.org/wiki/Trellis_modulation.
     
  10. Aug 24, 2016 #9
    i agree with Svein. it's really refers this two thing. and if you want you can check it.
     
  11. Aug 24, 2016 #10
    This is why it is nonsense that people talk about "Bandwidth" when discussing Internet connections. Marketing hype.
     
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