1) Data rate: A GSM modem is limited to 9600bps. With GPRS, speeds of up to 50kbps can be achieved.
2)Always on connection: GPRS is an always on connection allowing data to be sent and received at will. GSM connections are ‘dial-up’ which causes delays.
3) Cost: GSM connections are billed just like a mobile phone call, so an idle connection will cost the user by the second regardless of how much data is transferred. GPRS is charged per MB of data only. Recent price reductions now make it a very attractive prospect with tariffs as low as £1 per MB now available.
4) IP based: Unlike GSM, GPRS is IP-based and can transfer data direct from mobile devices to the internet or to a private network.
With GPRS, freight can be tracked in real-time across an entire continent. Substations in isolated areas can be monitored for power outages. Traffic levels across the whole of the UK’s motorway network can be viewed from a single PC. These large-scale applications already exist but integrators wanting to use GPRS for small to medium sized projects have found that there’s more to this technology than first meets the eye.
The hardware that is designed to provide access to GPRS networks to send serial, Ethernet, digital or analogue I/O signals is relatively straightforward. Most of it ‘can’ do exactly what it says on the tin. This encourages the less-experienced user to dream up expansive and exciting applications based on the capabilities of the hardware with little or no thought given to the SIM card or the required network topology. This is where the problems tend to begin.
A Subscriber Identity Module or SIM Card is essential for anyone wanting to access GSM or GPRS services. We are most familiar with them through the use of mobile phones but the same principles apply regardless of the hardware being used. When a device that is GPRS enabled is first powered on, it contacts the network to request an IP address. A private IP address is allocated allowing the phone to communicate on the service provider’s own network or with the public internet via the service provider’s APN.
The APN can allow connection requests from the private network to the internet, (eg a GPRS enabled laptop connecting to bbc.co.uk for example), but connections cannot be initiated in the other direction. The mobile device is not visible (on the network) from the internet. This is a key stumbling block for all those that want to use GPRS to connect to dumb devices. If it is a PC or a laptop initiating a GPRS connection it is only a configuration issue within the operating system to make the connection happen. If a dumb device is in the field, it will have no way to initiate the connection and there is no way to connect to it through the APN from the internet.
This scenario is a classic problem for GPRS users and the workarounds or fixes are rarely documented. Those that have hit this brick-wall will find the following solutions of great interest:
1) It is possible to buy a direct connection into the service provider’s network. This will provide you with your own APN (gateway) but can be very expensive. This is only cost-effective for large applications but well worth considering if you plan to have lots of devices in the field.
2) There are a handful of specialist devices with built-in intelligence that can initiate a connection from inside the GPRS network to an internet server designed to receive and maintain a connection. Once the connection is in place, bi-directional TCP/IP traffic can be sent carrying data from almost any application.
3) MVNOs – Mobile Virtual Network Operators act as resellers for GSM & GPRS air-time. Some specialist MVNOs can offer SIM cards with a public (internet) IP address. This allows a connection to a GPRS device from any PC connected to the internet.
4) Another service offered by MVNOs is a Virtual Private Network (VPN) connection into their infrastructure. This makes the GPRS device appear as though it is on a network local to you and provides a more secure connection than across the open internet.
5) Some GPRS modems can be configured to dial a GPRS connection upon receipt of an incoming call from a remote modem. Because the call is never answered, there is no charge for the incoming call but the phone number can be logged, verified and used to determine which remote server the GPRS device should connect to.
It has been said that there are “no plug and play solutions for GPRS.” In essence this is true because the requirements of each application are different and the constraints not always immediately clear. It is because of this that Amplicon have pulled together hardware and service providers from around the globe to offer proven and reliable solutions in the sometimes daunting arena of GPRS.