In addition to what Bob said, the most basic definition, when considering communications systems, of course:

Bit rate : The actual data rate in bits per second that is to be transmitted

Baud rate : The maximum number of line signaling symbols per second when transmitting the above data in the transmission medium. (Sometimes we refer to line transitions, but that gets a little sticky for FSK, but the principle holds)

We encode data so there are multiple bits per line transition, usually for bandwidth reasons (there's a correlation between encoding and S/S+N degradation). The Baud rate is sometimes also known as **the Symbol Rate**, the imply the number of symbols, not the original data rate.

Just to aid with the homework, some encoding techniques that are common (not line coding, but multi-level codes) are:

QAM PAM PSK QPSK DQAM DQPSK DPSK

There's a lot more - I suggest some light reading :)

Baud has been misused a lot, as a fancier word for bps. Especially when talking about modems you see that Baud is often used incorrectly.

"Data speed used to be specified in terms of baud, which is a measure of the number of times a digital signal changes state in one second. Baud, sometimes called the "baud rate," is almost always a lower figure than bps for a given digital signal because some signal modulation techniques allow more than one data bit to be transmitted per change state."

The advertising departments of modem manufacturers wanted to tell the users about the higher number, the bps, but they also wanted to use the fancier word Baud, so they often used Baud in place of bps.

From

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"While taking about modems, the transmission speed is the source of a lot of confusion. The root of the problem is the fact that the terms "baud" and "bits per second" are used interchangeably. This is a result of the fact that it's easier to say "baud" than "bits per second," though misinformation has a hand in it, too. A baud is "A change in signal from positive to negative or vice-versa that is used as a measure of transmission speed" and bits per second is a measure of the number of data bits (digital 0's and 1's) transmitted each second in a communications channel. This is sometimes referred to as "bit rate." Individual characters (letters, numbers, spaces, etc.), also referred to as bytes, are composed of 8 bits. Technically, baud is the number of times per second that the carrier signal shifts value, for example a
1200 bit-per-second modem actually runs at 300 baud, but it moves 4 bits per baud (4 x 300 = 1200 bits per second)."

When digital information is transmitted it is sometimes more efficient to encode the bits in a way such as to reduce the total bandwidth necessary for that transmission.

If you use only two different values of a signal's characteristic (let say its amplitude) to encode a binary signal then the bandwidth required to send that signal is fairly high.

If, on the other hand, you use more than two different values of that signal to encode a binary signal then the same bitrate can be transmitted using a lower-bandwidth channel.

For example, a two-level voltage signal's spectral content is easily calculated using the Fourier transform of the time-domain signal. Use a

1-0-1-0-1-0... data pattern with a given rise/fall time to see the highest needed bandwidth of the channel (and get the signal through with minimal time-domain distortion).

Now, try using four levels to encode the bitstream. Each pair of bits can be encoded with a single voltage level. Do the same Fourier transform and see its reduced spectral width. Note that this type of encoding requires the bitstream to be scrambled such that if you have each pair of bits repeat themselves (over and over) you always get some voltage level changes (which is necessary to recover the level-change boundary information [aka, clock recovery]).

A baud is the symbol rate (per second). With encoding, you send more than one bit/second (bps) per baud.

If you send 1M bit per second, but encode 16 bits per symbol, then the baud rate is 62.5K baud. This could be done (for example) by using 4 voltage levels per symbol or by using 4 different phases per symbol.

If I recall correctly, fast dial-up modems (say 38.4Kbps and 56Kbps) are all

2400 baud. This high bps per baud encoding is required because dial-up lines only have about 3KHz of bandwidth (they're all digitally sampled at 8Ksps).

By the way, if this is a homework question then I hope (at least) you force yourself to really understand this. If my explanation doesn't click with you, then do some research. Please don't just copy this text and paste it. Please.

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