17. Transmission synchronization types

a. Asynchronous Systems

In asynchronous systems, a separate timing channel is not used. The transmitter and receiver must be preset in advance to an agreed-upon baud rate. A very accurate local oscillator within the receiver will then generate an internal clock signal that is equal to the transmitters within a fraction of a percent. For the most common serial protocol, data is sent in small packets of 10 or 11 bits, eight of which constitute message information. When the channel is idle, the signal voltage corresponds to a continuous logic '1'. A data packet always begins with a logic '0' (the start bit) to signal the receiver that a transmission is starting. The start bit triggers an internal timer in the receiver that generates the needed clock pulses. Following the start bit, eight bits of message data are sent bit by bit at the agreed upon baud rate. The packet is concluded with a parity bit and stop bit.

Parity Bit

Noise and momentary electrical disturbances may cause data to be changed as it passes through a communications channel. If the receiver fails to detect this, the received message will be incorrect, resulting in possibly serious consequences. As a first line of defense against data errors, they must be detected. If an error can be flagged, it might be possible to request that the faulty packet be resent, or to at least prevent the flawed data from being taken as correct. If sufficient redundant information is sent, one- or two-bit errors may be corrected by hardware within the receiver before the corrupted data ever reaches its destination.

A parity bit is added to a data packet for the purpose of error detection. In the even-parity convention, the value of the parity bit is chosen so that the total number of '1' digits in the combined data plus parity packet is an even number. Upon receipt of the packet, the parity needed for the data is recomputed by local hardware and compared to the parity bit received with the data. If any bit has changed state, the parity will not match, and an error will have been detected. In fact, if an odd number of bits (not just one) have been altered, the parity will not match. If an even number of bits has been reversed, the parity will match even though an error has occurred. However, a statistical analysis of data communication errors has shown that a single-bit error is much more probable than a multibit error in the presence of random noise. Thus, parity is a reliable method of error detection.

b. Synchronous Systems

Although synchronous transmission is relatively simple to implement, it is efficient because of the start and stop bits required. For example, eleven or more bits is needed to transmit an 8 bit character.

In synchronous transmission, the start and stop bits are eliminated. The data is usually transmitted in one continuous block, rather than one character at a time. There are no pauses between characters. The blocks of data are separated by special characters called SYN character. The SYNC character usually consists of a 8 bit data code, although more can be used.

It should also be noted that transmission efficiency is higher for systems that sends out large packets. As proof, we shall consider the example below:

Amount of data bits to be sent: 1024 bits

With asynchronous transmission, 8 bits will be sent at a time, and therefore the efficiency is fixed at 8/11 ~ 72 % (with 1 start bit, 1 stop bit and 1 parity bit). However, with synchronous transmission, the efficiency is calculated to be 1024/(1024+8) assuming a 8-bit synchronization character which gives 99.4 % efficiency.

For small data packets however, it is entirely possible asynchronous transmission will be preferred.

Next topic will be on Multiplexing methods