Download Asynchronous and Synchronous Protocols - Telecommunications - Lecture Notes and more Study notes Telecommunication electronics in PDF only on Docsity!
Part 8: Asynchronous and Synchronous Protocols
Introduction
This section briefly discusses the differences between two different methods of Serial transmission, namely, asynchronous and synchronous. A protocol establishes a means of communicating between two systems. As long as the sender and receiver each use the same protocol, information can be reliably exchanged between them. We shall look at two common protocols used in serial data communications, the first is knows as Asynchronous, the second as Binary Synchronous.
Protocols
A protocol is a set of rules which govern how data is sent from one point to another. In data communications, there are widely accepted protocols for sending data both the sender and receiver must use the same protocol when communicating. One such rule is
BY CONVENTION, THE LEAST SIGNIFICANT BIT IS TRANSMITTED FIRST
A SYNCHRONOUS TRANSMISSION
The asynchronous protocol evolved early in the history of telecommunications. It became popular with the invention of the early tele-typewriters that were used to send telegrams around the world.
Asynchronous systems ascend data bytes between the sender and receiver by packaging the data in an envelope. This envelope helps transport the character across the transmission link that separates the sender and receiver. The transmitter creates the envelope, and the receiver uses the envelope to extract the data. Each character (data byte) the sender transmits is preceded with a start bit, and suffixed with a stop bit. These extra bits server to synchronize the receiver with the sender.
In asynchronous serial transmission, each character is packaged in an envelope, and sent across a single wire, bit by bit, to a receiver. Because no signals lines are used to convey clock (timing) information, this method groups data together into a sequence of bits (five—eight), then prefixes them with a start bit and appends the data with a stop bit.
The purpose of the start and stop bits was introduced for the old electromechanical Tele- typewriter. These used motors driving cams that actuated solenoids that sampled the signal at specific time intervals. The motors took a while to get up to speed, thus by prefixing the first data bit with a start bit, this gave time for the motors to get up to speed. The cams generate a reference point for the start of the first data bit.
At the end of the character sequence, a stop bit was used to allow the motors/cams etc to get back to normal. In addition, it was needed to fill in time in case the character was an end of the line, when the Tele-typewriter would need to got to the beginning of a new-line. Without the stop character, the machine could not complete this before the next character arrived.
Its important to realize that the receiver and sender are sender are re-synchronized each time a character arrives. What that means is that the motors/cams are restarted each time a start bit arrives at the receiver. Nowadays, electronic clocks that provide the timing sequences necessary to decode the incoming signal have replaced the electromechanical motors.
This method of transmission is suitable for slow speeds less than about 32000 bits per second. In addition, notice that the signal that is sent does not contain any information that can be used to validate If it was received without modification. This means that this method does not contain error detection information, and is susceptible to errors.
In addition, for every character that is sent, additional two bits are also sent. Consider the sending of a text document which contains 1000 characters. Each character is eight bits, thus the total number of bits sent are 1000 (8 bits per character plus a start and stop bit for each character). These 10000 bits is actually 1250 character, meaning that an additional 21450 equivalent characters are sent due to the start and stop bits. This represents a large overhead in sending data, clearly making this method An inefficient means for sending large amounts of data
Summary Transmission of these extra bits (2 per byte) reduces data throughput. Synchronization is achieved for each character only. When the sender has no data to transmit, the line is idle and the sender and receiver are NOT in synchronization. Asynchronous protocols are suited for low speed data communications, and there is no method of error checking inherent in this protocol.
Synchronous Transmission
One of the problems associated wi9th asynchronous transmissions the high overhead associated with transmitting data. For instance, for every character of 8 bits transmitted, at least an additional overhead of 2 bits is required. For large amounts of data, this quickly adds up. For example, to transmit 1000 characters, this requires 12000 bits, an extra 2000 bits for the start and stops. This is equivalent to an overhead of 250 characters. Another problem is the complete lack of any form of error detection. This means the sender has no method of knowing whether receiver is correctly recognizing the data being transmitted.
In synchronous transmission, greater efficiency is achieved by grouping characters together, and doing away with the start and stop bits for each character. We still envelop the information in
