Editing Hayes AT command set (section)

== History ==
=== Background ===
Before the introduction of the [[bulletin board system]] (BBS), modems typically operated on direct-dial [[telephone line]]s that began and ended with a known modem at each end. The modems operated in either "originate" or "answer" modes, manually switching between two sets of frequencies for data transfer. Generally, the user placing the call would switch their modem to "originate" and then dial the number by hand. When the remote modem answered, already set to "answer" mode, the telephone handset was switched off and communications continued until the caller manually disconnected.

When automation was required, it was commonly only needed on the answer side; for instance, a bank might need to take calls from a number of branch offices for end-of-day processing. To fill this role, some modems included the ability to pick up the phone automatically when it was in answer mode, and to clear the line when the other user manually disconnected. The need for automated outbound dialling was considerably less common, and was handled through a separate peripheral device: a "dialler". This was normally plugged into a separate [[computer port (hardware)|input/output port]] on the computer (typically an [[RS-232]] port) and programmed separately from the modem itself.

This method of operation worked satisfactorily in the 1960s and early 1970s, when modems were generally used to connect dumb devices like [[computer terminal]]s (dialling out) with smart [[mainframe computer]]s (answering). However, the [[microcomputer revolution]] of the 1970s led to the introduction of low-cost modems and the idea of a semi-dedicated point-to-point link was no longer appropriate. There were potentially thousands of users who might want to dial any of the other thousands of users, and the only solution at the time was to make the user dial manually.

The [[computer industry]] needed a way to tell the modem what number to dial through software. The earlier separate dialers had this capability, but only at the cost of [[out-of-band signaling|a separate port]], which a [[microcomputer]] might not have available. Another solution would have been to use a separate set of "command pins" dedicated to sending and receiving commands; another could have used a signal pin indicating that the modem should interpret incoming data as a command. Both of these had hardware support in the [[RS-232]] standard. However, many implementations of the RS-232 port on microcomputers were extremely basic, and some eliminated many of these pins to reduce cost.

=== Hayes' solution<span class="anchor" id="+++"></span> ===
{{further|Command mode and Data mode}}

[[Hayes Communications]] introduced a solution in its 1981 [[Smartmodem]] by using the existing data pins with no modification. Instead, the modem itself could be switched between one of two modes:
# ''Data mode'' in which the modem sends the data to the remote modem. (A modem in data mode treats everything it receives from the computer as data and sends it across the phone line).
# ''Command mode'' in which data is interpreted as commands to the local modem (commands the local modem should execute).

To switch from data mode to command mode, sessions sent an [[escape sequence]] string of three plus signs ({{precode|+++}}) followed by a pause of about a second. The pause at the end of the escape sequence was required to reduce the problem caused by [[in-band signaling]]: if any other data was received within one second of the three plus signs, it was not the escape sequence and would be sent as data. To switch back they sent the ''online'' command, {{precode|ATO}}. In actual use many of the commands automatically switched to the online mode after completion, and it is rare for a user to use the online command explicitly.

In order to avoid licensing Hayes's patent, some manufacturers implemented the escape sequence without the time guard interval ([[Time Independent Escape Sequence]] (TIES)). This had a major [[denial-of-service attack|denial of service]] security implication in that it would lead to the modem hanging up the connection should the computer ever try to transmit the byte sequence {{precode|+++ATH0}} in data mode. For any computer connected to the Internet through such a modem, this could be easily exploited by sending it a [[ping of death]] request containing the sequence {{precode|+++ATH0}} in the payload. The computer operating system would automatically try to reply the sender with the same payload, immediately disconnecting itself from the Internet, as the modem would interpret the [[Internet Control Message Protocol|ICMP]] data payload as a Hayes command.<ref>{{cite web|url=http://marc.info/?l=bugtraq&m=90695973308453&w=2|title=1+2=3, +++ATH0=Old school DoS|last=Max|first=Schau|date=27 September 1998|work=[[Bugtraq]] mailing list|access-date=8 December 2012}}</ref> The same error would also trigger if, for example, the user of the computer ever tried to send an e-mail containing the aforementioned string.

=== Commands ===
The Hayes command set includes commands for various phone-line operations such as dialing and hanging-up. It also includes various controls to set up the modem, including a set of ''[[Register (computing)|register]] commands'' which allowed the user to directly set the various memory locations in the original Hayes modem. The command set was copied largely verbatim, including the meaning of the registers, by almost all early 300 baud modem manufacturers, of which there were quite a few.

The expansion to 1200 and 2400 baud required the addition of new commands, some of them prefixed with an [[ampersand]] ({{code|&}}) to denote those dedicated to new functionality. Hayes itself was forced to quickly introduce a 2400 baud model shortly after their 1200, and the command sets were identical as a time-saving method.<ref>Frank Durda IV, [http://nemesis.lonestar.org/reference/telecom/modems/at/history.html "The AT Command Set Reference – History"] {{Webarchive|url=https://web.archive.org/web/20080415210519/http://nemesis.lonestar.org/reference/telecom/modems/at/history.html |date=2008-04-15 }}, 1993</ref> Essentially by accident, this allowed users of existing 1200 baud modems to use the new Hayes 2400 models without changing their software. This re-inforced the use of the Hayes versions of these commands. Years later, the [[Telecommunications Industry Association]] (TIA)/[[Electronic Industries Alliance]] (EIA) formally standardized the 2400-baud command set as '''Data Transmission Systems and Equipment – Serial Asynchronous Automatic Dialing and Control''', '''[[TIA/EIA-602]]'''.

However, Hayes Communications was slow to release modems supporting higher speeds or [[Modem compression|compression]], and three other companies led: [[Microcom]], [[U.S. Robotics]], and [[Telebit]]. Each of these three used its own additional command-sets. By the early-1990s, there were four major command sets in use, and a number of versions based on one of these. Things became simpler again during the widespread introduction of [[kbits/s|14.4 and {{Val|28.8|u=kbit|up=s}}]] modems in the early 1990s. Slowly, a set of commands based heavily on the original Hayes extended set using {{code|&}} commands became popular, and [[Informal standard|then universal]]. Only one other command set has remained popular, the U.S. Robotics set from their popular line of modems.