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I am working on a GPRS communication design. What I want to do to connects two modems together via GPRS. But I found that i could not connect.

I was able to connect both modems to the internet using TCP connection AT Commands, and able to establish a connection to a server with a known/fixed IP address( e.g google ), but i could not connect both modem's together, knowing each IP addresses.

I know the IP for the modems are dynamic, which shows to be the reason why i cant connect them, but i want to know if there is another way to go about this.

My design intention is to use a smart Mobile Device (e.g my phone), which connect to a Modem (say at Home), enabling me communicate with certain appliances at the house remotely.

Are there any way around this?

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    \$\begingroup\$ Have the house end run something which connects to a dynamic dns service (there are many) and then you can connect to a host name from your remote device instead of an ip address. \$\endgroup\$ – brhans Apr 8 '15 at 13:11
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A possible solution would be to connect the "appliances" to the internet and use your mobile device, via the internet, to control the appliances.

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No, you cannot connect one cell phone directly with another. Cell phones contain cell modems, which are controlled using "AT" commands, patterned after (but greatly extending) the AT commands developed in the original Hayes modems thirty years ago.

There is a core set of AT commands that are common across all cell modems; these generally have a prefix of "AT+". Then there are a number of proprietary AT commands developed by each cell modem manufacturer, they have prefixes like "AT$" or "AT%" etc where the third character is unique to the manufacturer.

I have worked with cell modems from four different manufacturers. The documentation for these commands typically runs over 500 pages long.

These are all designed to work only with a cell tower. There is a command to get the signal strength (AT+CSQ), which is used to update the bars on the screen. There are commands to check whether the cell modem has registered on the network (connected to a cell tower). Another command to get the list of carriers supported by the cell tower.

For voice calls there ia a command to dial a call (ADT, just like the dial up modems from the past), and another command (or rather set up of commands) to set up a data call via GPRS and request a TCP/IP connection as you found out. When a call answers, or a connection is made to the Internet, the response is CONNECT. The modem then goes into a data wheere data can be sent or received. To get back to control model, the familiar +++ sequence (patented by Hayes) is used.

If these were to work for point-to-point communication with another cell phone, that other cell phone would have to have most of the capability of a cell tower. But there is no provision for that. There are no commands in any cell modem AT set which allow a cell phone to behave that way.

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