Synchronous and Asynchronous communication refers to the manner in which data is transmitted over a transmission medium.

Synchronous communication devices or systems must use an external clock to synch the devices to the same time. This is commonly done via the use of GPS satellites which provide a timing reference if the systems are geographically far apart.

The reason this timing signal is needed is to provide an agreed upon start and stop condition to the data stream. The receiving device must know when the transmitting device has started to send data and when the transmitting device has stopped sending data. The timing signal is agreed upon before data is exchanged and during periods of idle activity, the devices actually exchange special characters or messages to remain in synch with one another.

Example, if the transmitting device sent the following message in frames: “The”, “Information”, “Systems”, “Security”, “Association” and the receiving device was in disagreement as to when each separate frame began and ended, it may actually interpret this to be “TheInfor”, “matiom”, “Systems”, “Securi”, “tyAsso”, “ciation”. After these frames are processed and presented in the manner they should have originally been, they will be unintelligible.

The timing signal, which is an electrical signal varying in voltage, is usually sent on a separate wire in order to avoid confusion on the data wire, however, this is not always the case. Due to this timing source, synchronous communication devices perform minimal error checking.

Asynchronous communication, which many modems use, has no additional timing source. The transmitting device inserts start bits, stop bits, and optional parity bits into the data stream which inform the receiving device when the data frame began, when it ended, and if it was transmitted with or without errors.

A message is broken up into numerous frames which are limited in their size (MTU) via bytes or ASCII characters. Each frame then receives a start and stop bit and are then transmitted.

Modems employ asynchronous communication due to their unreliable nature. Modems using an RJ-11 connector were often victim to line spikes and induced noise and would unsynch the timing signal needed for synchronous communication.

Examples of asynchronous communication include the RS-232 serial interface (commonly used in communication equipment) and V.2 modems.

An asynchronous communication service or application does not require a constant bit rate (it is variable). Examples are file transfer, e-mail and the World Wide Web. A synchronous communication service can be realtime streaming media, for example IP telephony, IP-TV and video conferencing.