Why AT commands?

AT commands are designed to control modems. In fact, these are character strings that begin with the letters AT, and end with carriage return signs. However, in the initialization line, the closing elements are automatically generated. The AT command symbols can be either uppercase or lowercase. There may be spaces between individual teams.

Additional Features

In addition to performing the basic operations associated with switching the modem modes, the AT commands can control the S registers of the communication device. They contain a variety of service information, which is necessary to ensure the efficiency of the modem. For example, in the S-register, constants can be stored that determine the timeout, the types of protocols used, analog signals, the levels of the input and output signals, and so on. The number of registers is determined by the modem developer, sometimes their total number is several hundred. However, it is accepted that the first 13 such nodes retain the designation determined by the manufacturer. Despite the fact that each user can modify the contents of S-registers and read at their own discretion, we should not forget that there is a very complex relationship between the data in different nodes of the system. So an arbitrary change of any registers can lead to the fact that in the future it will be impossible to use standard programs for working on the Internet.

The basic AT commands that are most often used in modems

1. L0-L3 - means the speaker volume. The figures correspond to the minimum, low, medium and maximum levels.

2. In - means a request for data from the permanent memory of the modem, where n corresponds to the number of the first command.

3. M0-M3 - the state of the dynamics, where:

- 0 - the device is off;

- 1 - the speaker is switched on from "off-hook" and until the connection is established;

- 2 - the device is always off;

- 3 - the speaker turns on only after pressing the last digit in the number, and turns off after establishing a communication session.

4. P and T - corresponds to the use of pulse (PULSE) and tonal (TONE) dialing.

5. S - work with registers of the modem. SO? Matches the request for the content of the SO node. SO = 1 means assigning a state to a given register.

6. Z - implies that the contents of all nodes are set to the initial level.

7. & F - corresponds to the recovery of information in the S-registers of the modem from the permanent memory. These data are recorded by the modem manufacturer, they can not be changed by the user.

8. & W - means writing the contents of all registers to a special nonvolatile memory in order to save the user's settings.

AT commands of GSM-modems

According to the GSM 07.07 standards, the command line to define the "context" must have the following form: + CGDCONT = [[, [, [, [, [, [, [, ... [ , PdN]]]]]]]]]. Let's look at what this record means (AT command):

1. cid is the context identifier. Always an integer with a "+" sign. The maximum value is determined by the device model.

2. PDP_type - means the protocol type.

3. APN - the name of the access point, set by the cellular operator.

4. d_comp - means data compression PDP, where 0 - off, 1 - enabled.

5. h_comp - corresponds to the compression of the PDP header, where 0 is off, 1 is turned on.

6. pd1 ... pdN - implies string parameters specific for this type of protocol.