What Is a Symbol Rate?

Symbol rate is a term used in digital communications to describe the transmission rate of signals along a particular connection. Measuring the symbol rate allows the end-user of the system to know how quickly the computers or other electronic devices are exchanging data. The most typically method for writing symbol rate is by listing it in terms of baud (Bd) — also known as symbols — per second. Another commonly used method to discuss line code is tracking the number of pulses transmitted along the wire per second.

Understand that a "symbol" is really nothing more than a collection of bits, specifically defined as X number of bits, where X is a whole number. Information traveling along digital networks is not sent in giant chucks of data, but rather in smaller, "bite-sized" packets of information. When the receiving computer collects the transmitted data, it reassembles the big-picture from the smaller packets of data.

The primary advantage to knowing the symbol rate of a transmission is as a raw measurement of speed. This is akin to measuring the number of miles per hour an automobile is traveling, allowing the driver and passengers to reach a rough estimation of the length of time it will take to reach the final destination. Without knowing the symbol rate for a particular transmission, both the sending and receiving computers would exist in a vacuum: neither would have any real idea of how long their networking hardware would be tied up by the transmission.

Another significant advantage to measuring symbol rate is for error-checking and troubleshooting purposes. If the symbol rate of a line drops dramatically in a relatively short period of time, chances are good that something has gone wrong with the network connection or the connecting cables along the way. Like water moving through a hose, its flow will not be impeded unless the hose becomes blocked or kinked in some way. Symbol rate provides a useful way to estimate the fluctuating impedance for a particular line.

The size of the bits has a rough approximation to the likelihood of bit errors along the line. When sending larger symbols--symbols with a greater number of bits per item--it is far more likely that something could go wrong along the way. Conversely, sending smaller symbols will decrease the probability of error, but increase the overall length of time for the transmission. Thus, the rate of symbol exchange becomes a balance between accuracy and expedience.