These commands were used to instruct the modem to dial numbers, negotiate connection speeds, and all sorts of other things. In order for a computer to connect to these devices, they contained a small bit of firmware called an “AT Command Interpreter.” This firmware would listen on some kind of interface and respond to “AT Commands” sent to it. Many of you who’ve been brought up in the broadband era might never have seen a modem (at least not in the classical sense of one, anyway), but if you ever have you simply can’t forget the classic squawking sound they used to make while dialling the remote node and connecting. If you’re programming on a Windows Embedded CE device, there are APIs in the operating system to help you with this, but if you’re running on a standard Linux system or on a regular Windows build, you’ll very likely need to open a serial COM port, or a USB device to get access to the GSM Modem itself. Many small devices are equipped with a simple GSM modem and a SIM card, and the only means you have of communicating with the outside world is via the SMS (Smart Messaging Service) that’s exposed on many of these devices. Quite often, if you’re using a small device (such as something that may be used at the core of an IoT-enabled device) you may not always have a full standard TCP or UDP stack.
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