Why GPRS modem provides embedded TCP/IP stack

Question

My colleague and I are mining the GPRS MODEM market for a module suitable for use with embedded Linux. During the market scan, we see that several vendors highlight that their MODEMs include an embedded TCP/IP stack.

This makes me wonder: when we are using embedded Linux which already contains a TCP/IP stack and connects using PPP, will it make use of the stack included in the GPRS MODEM at all?

My current assumption is that the stack is included for use with tiny microcontroller OS that do not supply their own stack. Also some of the MODEMs allow for running small applications IN the MODEM baseband processor which could explain the embedded stack...

So: is the TCP/IP stack supplied by the GPRS MODEM superfluous when using it with an HL OS or did I overlook something?

Answer 1

It is almost certainly superfluous in your use case.

Most cellular modem products are cut-down versions of products designed for use in mobile phones. Obviously, in a phone application, the TCP/IP stack is required, along with a whole pile of other functionality.

A typical GPRS modem probably contains an ARM9 processor, and this is not greatly taxted running just the modem software. For many smaller applications it certainly provides sufficient performance to run the entire application (think of something like a vending machine indicating that it is nearly empty, for example), and a TCP/IP stack might be helpful here.

There is a slightly cynical possible explanation, too. Many mobile phone stacks have a bit more software coupling than their manufacturers would like to admit to, and it may turn out that it is simply not worth the effort to remove the TCP/IP stack.

In your application, it is almost certainly the best option to use the AT command interface (this is an extension of old-fashioned dial-up modem command set to allow you to fetch information such as signal strength, network status etc.

Answer 2

This is the EXACTLY same question I been asking myself these two days. ^^

After some study and asking around, I found this:

In the case of a smart phone such iPhone/android, TCP/IP are running on Application Processors(AP) as part of the OS. Baseband Processors(BP) are simply network modems (think of the 56k dial-up modem and PC setup in ancient time). Of course BP will be running mobile network stack(GSM, CDMA, LTE...) to hop on cellular network, but to AP, it's transparent and simply does modulation/demodulation work for the wireless network. Modems receives AT commands and can switch between command mode and data mode in operations. In data mode, the protocol between AP and BP is normally PPP over serial (! correct me if i am wrong here). So TCP/IP/PPP/serial.

Embedded TCP/IP stack found in some BP are meant to provide an entire abstraction for certain applications whereby network stack is not available due to system constraints or simply made to be simple. A TCP/IP stack is then very useful in BP. As you mentioned, some BP (e.g infenion) does have extra processing power for user application and/or network stack. AP, in this case, is even not needed. This is a typical setup of a function phone(such as Nokia brick). Extension of AT command sets are then provided by BP to create a socket or even a FTP connection.