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Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.
When devices exchange data, there is a flow or stream of information between the two. In any data transmission, the sender and receiver must have a way to extract individual characters or blocks (frames) of information. Imagine standing at the end of a data pipe. Characters arrive in a continuous stream of bits, so you need a way to separate one block of bits from another. In asynchronous communications, each character is separated by the equivalent of a flag so you know exactly where characters are located. In synchronous communications, both the sender and receiver are synchronized with a clock or a signal encoded into the data stream.
In synchronous communications, the sender and receiver must synchronize with one another before data is sent. To maintain clock synchronization over long periods, a special bit-transition pattern is embedded in the digital signal that assists in maintaining the timing between sender and receiver. One method of embedding timing information is called bipolar encoding, as pictured in Figure S-14. In this method, the bit stream pictured at the top is meshed with the clock pulse pictured in the middle to produce the transmission signal shown at the bottom.
Synchronous communications are either character oriented or bit oriented. Character-oriented transmissions are used to send blocks of characters such as those found in ASCII (American Standard Code for Information Interchange) files. Each block must have a starting flag similar to asynchronous communications so the receiving system can initially synchronize with the bit stream and locate the beginning of the characters. Two or more control characters, known as SYN (synchronous idle) characters, are inserted at the beginning of the bit stream by the sender. These characters are used to synchronize a block of information. Once correct synchronization has been established between sender and receiver, the receiver places the block it receives as characters in a memory buffer.
Bit-oriented synchronous communication is used primarily for the transmission of binary data. It is not tied to any particular character set, and the frame contents don't need to include multiples of eight bits. A unique 8-bit pattern (01111110) is used as a flag to start the frame.
An entirely different form of synchronous communications can be seen in the form of chat and instant messaging. Like a voice telephone call, a chat or instant messaging session is live and each user responds to the other in real time. In contrast, discussion forums and electronic mail are asynchronous communications. Some amount of time may pass before a person responds to a message. In a discussion forum, a message sits in a message queue for other people to read and respond to at any time, or until the message falls out of the queue. These two forms of communication, which are accessible to any Internet user from just about any Web-attached system, may be the most profound aspect of the Internet. They promote a new form of instant global communication and collaboration. In the case of discussion forums and e-mail, delayed communication gives respondents time to think about their response and gather information from other sources before responding.
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.