The unit of frequency, number of cycles per second, in a periodic signal. Another important aspect of transmission is how much data can be sent in any one signal time. We commonly represent computer data in binary digits, or bits, which are usually grouped together in bytes for convenience. Bytes may be transmitted by providing one physical path for each bit so that an entire character is sent at once, all bits in parallel , or they may be transferred as a sequence of bits on a single, serial connection. The more bits that can be sent at once, the faster the data can be transferred.
Transmission speed is characterized in two different ways.
The baud rate is the speed, or number of signals per second, successive, independent signals can be sent so that they still remain distinguishable. Mode Asynchronous devices transmit characters individually with arbitrary time intervals in between. To ensure that the receiver is paying attention and ready to handle the bits in the data character, a start bit is sent first, followed by the data bits, and finally one or more stop bits. At low speeds, these ten or eleven bits are relatively easy for the receiver to handle, because the slight variations in clocking between the transmitter and receiver will be small compared to the length of each bit signal, and should not add up to a problem even over the total length of the character. When higher data rates are desirable, synchronous transmission is preferred to transfer a large number of characters consecutively.