| Telephone network modem standards |  |
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ITU V-Series | V.92 | K56flex | X2 | MNP | Hayes command set |
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Most dialup modems follow to a large extent the specifications of the Hayes command set, a specific command-language originally developed for the Hayes Smartmodem 300 baud modem. The command set consists of a series of short strings which combine together to produce complete commands for operations such as dialing, hanging up, and changing the parameters of the connection.
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History
Prior to the introduction of the Bulletin Board System (BBS), modems typically operated on direct-dial telephone lines that always began and ended with a single modem at each end. The user would usually dial the phone manually before connecting, or pick it up if it rang. In a few cases the computers themselves had to call a selection of numbers, and for this task they used a separate peripheral device, a "dialer" plugged into a different input/output port on the computer (typically an RS-232 port). This method of operation worked satisfactorily in the 1960s and early 1970s when modems generally connected to only large mainframe configurations. But 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 a separate port, which a microcomputer might not have available. One solution could have used 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. But both of these suffered from the similar problem that these pins might not be available, or connected, in many microcomputers. Hayes Communications introduced a solution in its 1977 product, the Smartmodem, by re-using the existing data pins with no modification. Instead, the modem itself could switch itself 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 that the local modem should execute).
To switch from data mode to command mode sessions sent an escape sequence string of three plus signs ("+++") followed by a pause of about a second (to go to the command mode without losing the connection); and to switch back they sent the online command, O. 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. This use of in-band signalling leads to a potential serious problem: what happens if the data sent in data-mode contains three consecutive plus signs? This could happen randomly quite easily, and it would happen any time someone attempted to send data describing the system – this document for instance. In order to reduce the chance of this problem the pause at the end of the escape sequence was required, 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-is. The command set includes commands for various phone-line manipulations, dialing and hanging-up for instance. It also includes various controls to set up the modem, including a set of 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 command set itself had no intellectual-property protection, but Hayes Communications patented the concept of a "guard time" after the escape sequence. In the late 1980s Hayes started enforcing the patent, charging $1 per modem that used it. Many later modems thus did not implement the guard time; this eventually (when modems began to be used to dial up to the Internet) led to a nasty denial-of-service attack involving an ICMP echo request ("ping") packet containing three plusses and ATH, the hangup command. The expansion to 1200 and 2400 baud required the addition of a small set of new commands, some of them prefixed with a & to denote those dedicated to new functionality. However Hayes Communications moved only slowly to higher speeds or the use of compression, and three other companies led the way here – Microcom, U.S. Robotics and Telebit. Each of these three used its own additional command-sets instead of waiting for Hayes to lead the way. Soon a plethora of new models appeared, including new ones from Hayes, following a variety of "standards". Although these shared the same commands for simple instructions such as dialing, the higher-speed options differed dramatically. Nevertheless, all of them referred to themselves as using the "Hayes command set". Things became simpler again during the widespread introduction of 14.4 and 28.8 kbaud modems in the early 1990s. Slowly a set of commands based heavily on the original Hayes extended set using "&" commands became popular, and then universal. Only one other command set has remained popular, the US Robotics set from their popular line of modems.
Example session
The following represents two computers, computer A and computer B, both with modems attached, and the user controlling the modems with terminal-emulator software. Terminal-emulator software typically allows the user to send Hayes commands directly to the modem, and to see the responses. In this example, the user of computer A makes the modem dial the phone number of modem B at phone number 555-1234. Note that after every command and response, there is a carriage return sent to complete the command.
| Modem A | Modem B | Comment |
|---|---|---|
| ATDT5551234 | User at modem A issues a dial command. | |
| RING | Modem A begins dialing. Modem B's phone-line rings, and the modem reports the fact. | |
| ATA | Computer at modem B issues answer command. | |
| CONNECT | CONNECT | The modems connect, and both modems report "connect". (In practice, most modems report more information after the word CONNECT — specifying the speed of the connection.) Also, at this time, both modems will raise the DCD, or Data Carrier Detect signal, on the serial port. |
| abcdef | abcdef | When the modems are connected, any characters typed at either side will appear on the other side. The person at computer A starts typing. The characters pass through the modem and appear on computer B's screen. (User A may not see his own typed characters — depending on the duplex setting). |
| +++ | The person at computer B issues the modem escape command. (Alternately, and more commonly, the computer B could drop the DTR, or Data Terminal Ready signal, to achieve a hangup, without needing to use +++ or ATH.) | |
| OK | The modem acknowledges it. | |
| ATH | The person at computer B issues a hang up command. | |
| NO CARRIER | OK | Both modems report that the connection has ended. Modem B responds "OK" as the expected result of the command; modem A says NO CARRIER to report that the remote side interrupted the connection. The modems on both sides drop their DCD signals as well. |
Description
The following text lists part of the Hayes command set (also called the AT commands: "AT" meaning attention). The Hayes command set can subdivide into four groups:
- basic command set - A capital character followed by a digit. For example, M1.
- extended command set - An “&” (ampersand) and a capital character followed by a digit. This extends the basic command set. For example, &M1. Note that M1 is different from &M1.
- proprietary command set - Usually starting either with a backslash (“\”) or with a percent sign (“%”); these commands vary widely among modem-manufacturers.
- register commands - Sr=n where r is the number of the register to be changed, and n is the new value that is assigned.
A register represents a specific physical location in memory. Modems have small amounts of memory onboard. The fourth set of commands serves for entering values into a particular register (memory location). The register will store a particular variable (alpha-numeric information) which the modem and the communications software can utilize. For example, S7=60 instructs the computer to "Set register #7 to the value 60". Although the command-set syntax defines most commands by a letter-number combination (L0, L1 etc.), the use of a zero is optional. In this example, "L0" equates to a plain "L". Keep this in mind when reading the table below. Some of the most important characters that may appear in the modem initialization string follow. Normally one should not change these characters.
- AT - "Attention": tells the modem that modem-commands follow. This must begin each line of commands.
- Z - Resets the modem to its default state.
- , (a comma) - Makes the software pause for a second. More than one comma can appear in succession: for example, ",,,," tells the software to pause four seconds. (The setting of register S8 governs the duration of the pause.)
- ^M - Sends the terminating Carriage Return character to the modem. This is a control code that most communication software translates as a carriage return. (Note, when this is sent to the modem, it is sent as a single byte, ASCII CR (0x0D), or "Control-M" not the two characters ^ and M.)
- ; (a semi-colon) - Return to command mode immediately after dialing. This makes it possible, for example, to dial more than 45 digits numbers, or to walk through interactive menus.
- W - wait for dialtone. The modem will wait for a dialtone before dialing numbers following the W. For this to work, waiting must not exceed a timeout, generally configured in the S7 register.
- ! - flash hook. put quickly the modem on/off hook.
When in data-mode an escape sequence can return the modem to command mode. The normal escape sequence is three plus signs ("+++"), and to disambiguate it from possible real data, a guard timer is used: it must be preceded by a pause, not have any pauses between the plus signs, and be followed by a pause; by default a "pause" is one second and "no pause" is anything less.
Compatibility
While the original Hayes command set represented a huge leap forward in modem-based communications, with time many problems set in, almost none of them due to Hayes per se:
- Due to the lack of a written standard, other modem manufacturers just copied the external visible commands and (roughly) the basic actions. This led to a wide variety of subtle differences in how modems changed from state to state, and how they handled error conditions, hangups, and timeouts.
- Each manufacturer tended to add new commands to handle emerging needs, often incompatible with other modems.
For example, setting up hardware or software handshaking often required many different commands for different modems. This undermined the handy universality of the basic "AT" command-set.
- Many "Hayes-compatible" modems had serious quirks that made them effectively incompatible. For example, many modems required a pause of several seconds after receiving the "AT Z" reset command. Some modems required spaces between commands, while others did not. Some would unhelpfully change baud-rate of their own "volition", which would leave the computer with no clue how to handle the incoming bits.
As a result of all this, eventually many communiucations programs had to give up any sense of being able to talk to all "Hayes-compatible" modems, and instead the programs had to try to determine the modem type from its responses, or provide the user with some option whereby they could enter whatever special commands it took to coerce their particular modem into acting properly.
The basic Hayes command set
| Command | Description | Comments |
|---|---|---|
| A0 or A | Answer incoming call | |
| A/ | Repeat last command | Don't preface with AT, don't follow with carriage return. Enter usually aborts. |
| B0 or B | Call negotiation | V32 Mode/CCITT Answer Seq. Today, most modems ignore this command, though they answer "OK" for compatibility. |
| B1 | Call negotiation | Bell 212A Answer Seq. Ignored by most modern modems. |
| B2 | Call negotiation | Verbose/Quiet On Answer. Ignored by most modern modems. |
| D | Dial |
Dial the following number and then handshake in originate mode. P - Pulse Dial |
| E0 or E | No Echo | Will not echo commands to the computer |
| E1 | Echo | Will echo commands to the computer (so one can see what one types) |
| H0 | Hook Status | On hook - Hang up |
| H1 | Hook status | Off hook - phone picked up |
| I0 or I | Inquiry, Information, or Interrogation | This command is very model specific. I0 usually returns a number or code,
while higher numbers often provide much more useful information. Today, Windows uses this for Plug and Play detection of specific modem types. |
| I3 or I | Information (3) | This command, on newer modems, usually returns the make and model number of the modem. For example: ATI3 returns LT V.90 Data+Fax Modem Version 6.00. |
| L0 or L | Speaker Loudness. Modems with volume control knobs will not always have these options. | Off or low volume |
| L1 | Low Volume | |
| L2 | Medium Volume | |
| L3 | Loud or High Volume | |
| M0 or M | Speaker off | M3 is also common, but different on many brands |
| M1 | Speaker on until remote carrier detected (i.e. until the other modem is heard) | |
| M2 | Speaker always on (data sounds are heard after CONNECT) | |
| N0 or N | Handshake Speed | Handshake only at speed in S37 |
| N1 | Handshake at highest speed larger than S37 | |
| O0 or O | Return Online | Returns the modem back to the normal connected state after being interrupted by the "+++" escape code. |
| O1 | Return Online after an equalizer retrain sequence | |
| Q0 or Q1 | Quiet Mode | Off - Displays result codes, user sees command responses (e.g. OK) |
| Q1 | Quiet Mode | On - Result codes are suppressed, user does not see responses. |
| Sn | Select current register
Note that Sn, ? and =r are actually three separate commands, and can be given in separate AT commands. |
Select register n as the current register |
| Sn? | Select register n as the current register, and query its value. Using ? on its own will query whichever register was most recently selected. | |
| Sn=r | Select register n as the current register, and store r in it. Using =r on its own will store into whichever register was most recently selected. | |
| V0 or V | Verbose | Numeric result codes |
| V1 | English result codes (e.g. CONNECT, BUSY, NO CARRIER etc.) | |
| X0 or X | Smartmodem | Hayes Smartmodem 300 compatible result codes |
| X1 | Usually adds connection speed to basic result codes (e.g. CONNECT 1200) | |
| X2 | Usually adds dial tone detection (preventing blind dial, and sometimes preventing ATO) | |
| X3 | Usually adds busy signal detection. | |
| X4 | Usually adds both busy signal and dial tone detection | |
| Z0 or Z | Reset | Reset modem to stored configuration. Use Z0, Z1etc. for multiple profiles. This is the same as &F for factory default on modems without NVRAM (non volatile memory) |
Note: a command string is terminated with a CR (\r) character
V.250
The ITU-T established a standard in its V-Series Recommendations, V.25 ter, in 1995 in an attempt to establish a standard for the command set again. It was renamed V.250 in 1998 with an annex that was not concerning the Hayes command set renamed as V.251. A V.250 compliant modem implements the A, D, E, H, I, L, M, N, O, P, Q, T, V, X, Z, &C, &D, and &F commands in the way specified by the standard. It must also implement S registers and must use registers S0, S3, S4, S5, S6, S7, S8, and S10 for the purposes given in the standard. Lastly it also must implement any command beginning with the plus sign, "+" followed by any letter A to Z, only in accordance with ITU recommendations. Modem manufacturers are free to implement other commands and S-registers as they see fit, and may add options to standard commands.
| Leading characters |
Includes commands related to |
|---|---|
| +A | Call control (network Addressing) issues, common, PSTN, ISDN, ITU-T Rec. X.25, switched digital |
| +C | Digital Cellular extensions |
| +D | Data Compression, ITU-T Rec. V.42 bis |
| +E | Error Control, ITU-T Rec. V.42 |
| +F | Facsimile, ITU-T Rec. T.30, etc. |
| +G | Generic issues such as identity and capabilities |
| +I | DTE-DCE Interface issues, ITU-T Rec. V.24, etc. |
| +M | Modulation, ITU-T Rec. V.32 bis, etc. |
| +P | PCM DCE commands, ITU-T Rec. V.92 |
| +S | Switched or Simultaneous Data Types |
| +T | Test issues |
| +V | Voice extensions |
| +W | Wireless extensions |
Voice command set
- Main article: Voice modem command set
Modems with voice or answering-machine capabilities support a superset of these commands to enable digital audio playback and recording.
See also
