Increasingly embedded programmers are faced with situations where they need to interface their applications with a commercial wireless communication protocol stack.
Typical application scenarios include a web client running on a handheld device like a mobile phone, a security application sending images from a remote site over the GPRS network or a more specialized tracking application marking the progress of a physical process using short message service (SMS).
In all of these scenarios, the application programmer is mostly relies on simple send/receive type functions to complete the job. However, at the middleware level some configuration/setup changes in software are required before one can use the typical networking APIs.
The procedure, involving wireless modem commands is a simple one and does not merit implementation as a full scale middleware product. Therefore the onus is on the networking programmer to figure out this interfacing mechanism. This paper provides useful information for the embedded developer who wants her embedded application to communicate with the outside world using the widely deployed 2/2.5G cellular network
This article contrasts two cellular networking architectures used in practice, i) the Telephony architecture and ii) the Machine-to-Machine (M2M) architecture [1]. The steps required to setup a data connection are common among these two cases; mainly it requires sending appropriate AT commands [2] to the modem.
However, there are important differences in how the programmer interacts with the wireless protocol stack in these architectures as explained later in this paper.
Second generation cellular systems especially the GSM standard has been a huge commercial success for voice communications around the globe. Although the wireless standard offers limited bandwidth for high speed data applications like live streaming and video conferencing etc. still its wide deployment ensures a competitive market for emerging data centric standards like UMTS and WiMax.
In the meantime, enhancements have been made to the original GSM standard with GPRS and EDGE technologies deployed and tagged as 2.5G communication systems. There are three major components of GSM protocol software stack, i.e. the physical layer, data link layer and signaling layer [3].
In typical GSM/GPRS modems, the physical layer is implemented in a DSP while the other layers are implemented in a RISC architecture processor. There is also a separate hardware RF interface. (See for instance the solution from BroadCom, BCM2124 [4])
