Bit rate and Baud rate

In telecommunication Bit rate and baud rate are two important terms. Bit rate and baud rate play important role when we talk about Serial protocol  UART or modulation schemes like BPSK or QPSK  etc.

Bit rate:

Bit rate referes to Number of data bits transferred per second. Unit of Bit rate is bits per second(bps). For example bit rate is 10kbps means 10,000 bits are transferred within one second.

Baud rate:

Baud rate refers to number of signal or voltage level changes per second. Unit of baud rate is bauds per second. For example baud rate is 9600 means 9600 signal level changes are happening within a second.

Usually we say bit rate and baud rate are same but bit rate and baud rate are only same in Binary signaling.
Binary Signaling:

In binary signaling, we can represent two voltage or signal levels by using a single bit. Single bit has value  either logic 0 or logic  1 where  logic 0  represent ‘0’ volt and logic 1 represent ‘5’ volt. In binary signaling,  number  of signal level changes are equal to number of bits transferred within a second because one bit at time represents either of two signal levels. That’s why bit rate and baud rate are same in binary signaling. 

Multi level Signaling:

In multi level signaling, there are more than two voltage or signal levels. To represent those signal levels, we require more than one bit. Number of bits required to represent voltage levels is obtained by using following formula.

N=log2(m)
where N=Number of bits required to represent signal or voltage levels
m=number of signal or voltage levels.
Example: To represent 4 voltage levels you required at least 2 bits. Put m=4 in above formula , you will get N as 2.

Assume in the above figure, Number of bits transferred per second are 9600 and there are 4 voltage or signal levels[0V, 2V, 4V, 5V]. To represent those 4 voltage or signal levels you required at least 2 bits (using N=log2(m)). Assume 00 represents 0V, 01 represents 2V, 10 represents 4V and 11 represents 5V.

If you observed carefully in the above figure, the number of bits transferred per second are 9600 but the number of signal level  changes per second are 4800 because 2 bits are representing one voltage level. That’s why in multi level signaling  baud rate is not the same as  bit rate. In multi level signaling  baud rate always depends on the signal or voltage levels. In the above case, the Baud rate is half of bit rate  as two bits are required to represent one signal or voltage level.

COMMON DOUBTS

1)When are bit rate and baud rate equal

Bit rate and baud rate are the same only when we represent two voltage levels or signal levels by using a single bit i.e nothing but Binary signaling.

2)Does bit rate and baud rate are always same

No, they are only same in binary signaling. If we use multilevel signaling bit rate and baud rate different.

3)Difference between bit rate and baud rate

Important diffrence between bit rate and baud rate is that bit rate refers to the number of bits per second where baud rate refers to the number of signal level changes per second. Unit of bit rate is bits per second where the unit of baud rate is bauds per second. Bit rate and baud rate are same in the binary signaling. Bit rate and baud rate are not same in the multi level  signaling.

4)Bit rate and baud rate example

Assume 9600 bits are transferred per second and 1 bit used to represent voltage level or signal level i.e either logic 0 or logic 1 then bit rate is 9600 because number of bits transferred per second are 9600 and baud rate is 9600 because number of changes happening per seconds are also the same because one bit representing one voltage or signal level at time which is shown below.

5)Bit rate and baud rate example 2

Assume 9600 bits are transferred per second and 2 bits used to represent the voltage level or signal level then bit rate is 9600 because the number of bits transferred per second are 9600 and baud rate is 4800 because two bits representing one voltage or signal level which is shown below.

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