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A couple of questions about modem:

  • I have been reading up on AT commands, and I see they're used to control the modem. They are sent over a serial connection so does that mean they are sent over UART?

  • The modems used in smartphone and other embedded modules like GPS/GSM serve a similar purpose as a modem connected to the ISP does, which is to modulate and demodulate the incoming/outgoing signals, yeah?

  • What does it take to implement AT commands? say you want to add some for your custom project. Is it as easy as parsing strings from the console?

  • Is there any point in having antennas on the modem particularly for one that's connected to the ISP via a cable? Having antennas on routers make sense since the signals are being "routed" to multiple devices

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  • \$\begingroup\$ 1: only if you are using a UART for serial communication .... 2: maybe .... 3: depends on the project .... 4: unclear what you are asking, I do not think that the antenna is on the modem \$\endgroup\$
    – jsotola
    Commented Oct 19, 2020 at 19:19
  • \$\begingroup\$ say you open a putty or minicom session. doesn't that use UART? \$\endgroup\$
    – xyf
    Commented Oct 19, 2020 at 19:21
  • \$\begingroup\$ not if you telnet into the destination device \$\endgroup\$
    – jsotola
    Commented Oct 19, 2020 at 19:24
  • \$\begingroup\$ Antennas are only used if there is some sort of radio involved, llike WiFi or bluetooth. \$\endgroup\$ Commented Oct 19, 2020 at 19:30
  • \$\begingroup\$ @jsotola cause you're talking over TCP? \$\endgroup\$
    – xyf
    Commented Oct 19, 2020 at 19:34

1 Answer 1

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AT commands are an in-band signalling method originally developed for controlling voice modems. They were introduced by Hayes, and were quickly adopted by other modem makers as an industry standard. The basic AT command function is to place the telephone call or terminate it.

Early wireless WAN data modems like CDPD that used serial interfaces emulated AT commands for backwards compatibility, even though they're IP devices. GSM, CDPD's immediate successor, supported them too and they even persist to this day with LTE modems. AT commands can be used to interrogate the device link status like signal strength, IMEI, and other operating parameters.

WLAN (802.11) devices do not support AT commands. Unlike dialup modems, link establishment and management is set up by lower levels of the protocol stack rather than the application, and any control is sent out-of-band. LAN and WLAN controllers provide separate channels for this, such MII that the controller uses to manage the PHY. The MII programmer's model is supported for integrated LAN and WAN as well.

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