Definition of: AT command set

１） A series of machine instructions used to activate features on an intelligent modem. Developed by Hayes Microcomputer Products and officially known as the Hayes Standard AT Command Set, it is used entirely or partially by most every modem manufacturer. AT is a mnemonic code for ATtention, which is the prefix that initiates each command to the modem.

２）Also known as the Hayes Standard AT Command Set. A language that enables PC communications software to get an asynchronous and "Hayes-compatible modem" to do what you want it to do. So called "AT" because all the commands begin with "AT," which is short for ATtention. The most common commands include ATDT (touchtone a number), ATA (manually answer the phone), ATZ (reset modem — it will answer OK), ATSO=O (disable auto-answer), and ATH (hang up the phone).

See more reference:

http://en.wikipedia.org/wiki/AT_command

http://en.wikipedia.org/wiki/Motorola_Phone_AT_Commands

See :http://www.pcmag.com/encyclopedia_term/0,2542,t=Hayes+Smartmodem&i=44139,00.asp

Definition of: Hayes Smartmodem
A family of modems developed by Hayes, which was the industry leader for many years (see Hayes). Hayes developed the "intelligent modem" for the first personal computers in 1978, and its command language became the de facto standard for modem control (Hayes Standard AT Command Set).
Two States of Operation
An intelligent modem has a command state and an online state. In the command state, it accepts instructions. In the online state, it dials, answers, transmits and receives.
Handshaking
Once connected, the modem performs the handshaking with the remote modem, which are the whistles and tones you hear from the speaker. This is similar to the opening exchange of a human telephone call: called party says "hello;" calling party says "hello, this is..."; after that, the conversation begins. After the handshake is completed, you are online with the other computer, and data can be transmitted back and forth.
Escape Sequence
The escape sequence tells the modem to switch from online to command state. It consists of three plus signs in sequence (+++) with a Hayes-patented, one-second guard time interval before and after, which prevents the modem from mistaking a random occurrence of plus signs for an escape sequence. The escape sequence and guard time interval can be programmed in the modem's status registers.
To issue an escape sequence, hold down the shift key and press + three times with a pause of at least one second before and after the sequence. The modem will return the OK result code, indicating it is ready to accept commands.  

See: http://en.wikibooks.org/wiki/Serial_Programming:Modems_and_AT_Commands

What is Hayes?

Hayes Microcomputer Products, Inc. was a modem manufacturer from the beginning of the 1980s until the end of the 1990s, with its heyday in the early '90s. The name Hayes still exists as a brand name, owned by Zoom Telephonics, Inc. (as of Fall 2004).

In 1981, Hayes developed the Hayes Smartmodem. This was a unique product at the time, because this modem was no longer simply a "dumb" device blindly converting serial data to and from audio tones, but contained some "intelligence". It was possible to send commands to the modem to configure it, to execute certain operations (such as dialing a number, quieting the speaker, hanging up, etc.), and to read the current status of the connection. Hayes developed and published a command set to control the modem over a serial line. This command set became popular among consumer modem manufacturers, and was cloned a thousand times. Known as both the "Hayes command set" and the "AT command set", it has long been the de-facto standard for controlling consumer modems and also many professional modems. Modems which support this command set are called Hayes-compatible.

The commands were standardised at some point in time, however, as it is typical with standards, there are several standards. Plus, of course, there are still vendor-specific extensions and implementations in different modems vary slightly. Some of these enhancements were required to support at that time emerging features, such as data compression and FAX support. As a result, the command sets of modern modems are not fully compatible with each other. The original Hayes commands, however, should still work, and still form the core of almost all consumer modem command sets.

The basic set of commands was at some point in time standardised as TIA/EIA-602 and the syntax as EIA/TIA-615. But as already mentioned, modem manufacturers added their extensions. A larger extended set, particular under the presure from cell phone manufacturers, was standardised as ITU V.250 (old name V.25ter). That one usually forms the base for professional Hayes-compatible modems, and cell phones with build in data modems. ITU V.250 further referes to a bunch of other standards (e.g. V.251, V.252, V.253) for particular applications and extensions, and also has some suplements. Plus, of course there are the many standards defining other aspects of a modem, like compression and transmission.

What are AT Commands?

Almost all of the Hayes modem commands start with the two letter sequence AT - for getting the modem's attention. Because of this, modem commands are often called AT Commands. This still holds for many of the manufacturer specific command set extensions. Most of them also start with AT, and are called AT Commands, too. Please note, that just because an AT command contains a & does not make it an extensions. & commands were already part of the original Hayes command set.

The exact usage of the term AT command set slightly varies from manufacturer to manufacturer, often subject to marketing blurbs. In general, it can be assumed that a modem with an AT command set

• uses commands mostly starting with AT,
• uses the original Hayes way of separating data and commands, and
• supports the original Hayes commands and register settings as a subset.

What is a Modem?

A modem in the classic sense is a modulator/demodulator for transmitting digital information over analog wires, such as the analog telephone system's two-wire or four-wire lines. The term has come to be used as acceptable slang for many communication devices used to link a computer to either another computer, or a wide-area network (Wikipedia:WAN). For example, the Ricochet radio data transceivers were commonly known as "Ricochet modems".

This module deals with the classic type of smart modems, designed to convert data from/to a serial interface to/from an analog line. The module also applies to modems which provide the classic serial interface but connect over a different physical layer, such as a digital line, as well as devices providing a serial modem-like interface for other purposes. For our purpose, the modem is a classic DCE (data communications equipment) device, controlled via serial line by a classic DTE device (such as a computer).

Depending on the type of modem, the modem can use a number of different technologies and speeds to transmit the data over the analog line. The details of these technologies are of no particular interest here, other than to note that it is possible with most modems to specify these communication parameters (for example, to disable compression, or to change modulation techniques). The data this module deals with is not the data on the analog line, but the data as it appears on the serial interface between the DTE and DCE.

(Smart) Modems also provide auxiliary services, such as dialing a particular number to set up a connection. As a consequence, a modem can be in a number of different states and modes, which are not always orthogonal. It is possible, for example, for a modem to be in the command mode while still keeping a connection (see the +++ sequence for details).