usb - Baud rate vs. Bit rate

When we initialize a serial communication, we usually have to set the baud rate (e.g. 115200) I notice that many people assume that 115200 means 115200 bits per second. However, each baud(signal change) can encode more than 1 bit. baud 115200 means 115200 bits per second only if each baud = 1 bit. If each baud is 5 bits, then having baud 115200 means a bit rate of 115200 * 5 bits per seconds.

If what I said above is true, what is the baud to bit ratio for UART and USB3.0 assuming the baud rate is set to 115200?

It cannot be 1 to 1 because 115200 bits per seconds is 14.0625 KBps (kilo byte per second), which is too slow.

Answer

A minor point: by definition, a Baud (named after Emile Baudot) is one symbol per second. So your statement,

baud 115200 means 115200 bits per second only if each baud = 1 bit. If each baud is 5 bits, then having baud 115200 means a bit rate of 115200 * 5 bits per seconds.

is correct in spirit, but you should replace the word "baud" with "symbol". If you have an alphabet of 32 symbols, then each symbol encodes 5 bits, and a signalling rate of 115200 symbols per second, which is 115200 Baud (by definition), will give you 115200 * 5 = 576000 bits per second.

The commonly used phrase "Baud rate" is grammatically incorrect in essentially the same way that "ATM machine" is incorrect. The definition of Baud has the concept of rate built into it, so saying "Baud rate" is grammatically the same as saying "symbol rate rate" (which might make sense if you're talking about a changing symbol rate, but that's not how most people use it). Likewise, "ATM" has "machine" already incorporated into it, so "ATM machine" is equivalent to saying "automated teller machine machine" (which, again, could make sense if you were talking about a machine that produces ATMs, but that's usually not what people mean).

Grammar aside, most low-to-moderate speed short-range signaling over wire uses binary signalling, so the symbol rate equals the bit rate. Long range signalling over wire (e.g., cable modems) and via radio is more likely to use multi-bit symbols, and hence the bit rate will be higher than the symbol rate.