Editing Asynchronous serial communication (section)

== Operation ==
Before signaling will work, the sender and receiver must agree on the signaling parameters:

* Full or half-[[duplex (telecommunications)|duplex]] operation
* The number of bits per character -- currently almost always [[8-bit character]]s, but historically some transmitters have used a [[five-bit character code]], [[six-bit character code]], or a [[7-bit ASCII]].
* [[Endianness]]: the order in which the bits are sent
* The speed or bits per second of the line (equal to the [[Baud]] rate when each symbol represents one bit). Some systems use automatic speed detection, also called [[automatic baud rate detection]].
* Whether to use or not use [[parity_bit|parity]]
** Odd or even parity, if used
* The number of stop bits sent must be chosen (the number sent must be at least what the receiver needs)
* Mark and space symbols (current directions in early telegraphy, later voltage polarities in [[Electronic Industries Alliance|EIA]] [[RS-232]] and so on, frequency-shift polarities in [[frequency-shift keying]] and so on)

Asynchronous start-stop signaling was widely used for dial-up [[modem]] access to [[time-sharing]] computers and [[Bulletin board system|BBS]] systems. These systems used either seven or eight data bits, transmitted [[least-significant bit]] first, in accordance with the [[ASCII]] standard.

Between computers, the most common configuration used was "8N1": eight-bit characters, with one start bit, one stop bit, and no parity bit. Thus 10 Baud times are used to send a single character, and so dividing the signaling bit-rate by ten results in the overall transmission speed in characters per second.

Asynchronous start-stop is the lower [[data-link layer]] used to connect computers to modems for many dial-up Internet access applications, using a second (encapsulating) data link [[frame (networking) | framing]] protocol such as [[Point-to-Point Protocol|PPP]] to create [[network packet | packets]] made up out of asynchronous serial characters. The most common physical layer interface used is RS-232D. The performance loss relative to synchronous access is negligible, as most modern modems will use a private synchronous protocol to send the data between themselves, and the asynchronous links at each end are operated faster than this data link, with [[flow control (data)|flow control]] being used to throttle the data rate to prevent overrun.