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Asynchronous means “without respect to time.” In terms of data transmission, asynchronous means that no clock or timing source is needed to keep both the sender and the receiver synchronized. Without the benefit of a clock, the sender must signal the start and stop of each character so that the receiver knows when to expect data.
Asynchronous transmission is often described as “character-framed” or “start/stop” communication because this method frames each character with a start and stop bit. Each character is typically a 7- or 8-bit value that can represent a number, a letter, a punctuation mark, etc. Each character is preceded by a start bit and followed by a stop bit, or in some cases, two stop bits (see Figure ). An additional bit may be added for parity error checking prior to the first stop bit.
Synchronous means “with time.” In terms of data transmission, “synchronous” means that a common timing signal is used between. A clock signal is either embedded in the data stream or is sent separately to the interfaces.
If two hosts use a timing signal to “synch up,” start and stop bits for every 8-bit character value are not necessary. Instead, a large amount of data (e.g., hundreds or even thousands of bytes) can be preceded by synchronization bits. For example, in Ethernet a field of synchronization bits precedes the data payload. This field of synchronization bits, called a preamble, forms a pattern of alternating ones and zeros. The receiver uses this pattern to synchronize with the sender.
Service providers offer a variety of synchronous and asynchronous WAN services. These services can be grouped into three categories depending on their connection type: