What is the difference between baud rate and bit rate in digital communication?

[Newtons Apple]

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?

 

[jedishrfu]

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

 

[Bandit127]

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

Typical RS232 is 10 bits / character Transmission at 9600 baud is 9600 characters / sec which equals 9600 ch/sec * 10 bits/ch = 96,000 bits/sec

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.

 

[rbelli1]

Typical RS232 is 10 bits / character Transmission at 9600 baud is 9600 characters / sec which equals 9600 ch/sec * 10 bits/ch = 96,000 bits/sec

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

[/PLAIN]
The term “baud” originates from the French engineer Emile Baudot, who invented the 5-bit teletype code. Baud rate refers to the number of signal or symbol changes that occur per second. A symbol is one of several voltage, frequency, or phase changes.

NRZ binary has two symbols, one for each bit 0 or 1, that represent voltage levels. In this case, the baud or symbol rate is the same as the bit rate.

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.

Given the RS232 connection is synchronous though,

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

BoB

 

[nsaspook]

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

BoB

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

 

[rbelli1]

RS232 is a physical/electrical interface specification,

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

BoB

 

[nsaspook]

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

BoB

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

http://www.comportco.com/~w5alt/cw/cwindex.php?pg=4

So that means that if you send CW at a speed of W words per minute, you are sending (50 W) symbols per minute or (50 W/60) = (0.8333 W) symbols per second. It also means that the time required for a dit - 1 unit - is (60/50 W) = 1.2/W seconds. And since the definition of baud is the frequency of the smallest meaningful unit of data, the CW baud rate is 0.8333 W baud. So when you send CW at 5 WPM, you are manually generating a 4.17 baud signal, while at 20 WPM you're up to 16.67 baud.

 

[Svein]

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.

 

[lastfsdfsd23]

i agree with Svein. it's really refers this two thing. and if you want you can check it.

 

[tech99]

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.

This is why it is nonsense that people talk about "Bandwidth" when discussing Internet connections. Marketing hype.