GPRS Architecture


The General Packet Radio Service is a mobile data upgrade to a GSM mobile phone network. This provides users with packet data services (similar to the Internet) using the GSM digital radio network. Each voice circuit in GSM transmits the speech on a secure 14kbps digital radio link between the mobile phone and a nearby GSM transceiver station.

The GPRS service joins together multiple speech channels to provide higher bandwidth data connections for GPRS data users. The radio bandwidth remains the same, it is just shared between the voice users and the data users. The network operator has the choice of prioritizing one or the other.

GPRS users will also benefit from being able to use GPRS while traveling as the GSM system should transparently hand over the GPRS connection from one base station to another.

Radio Interface

Each GSM radio transceiver uses Time Division Multiplexing to deliver eight voice circuits on one radio channel Each radio site may have one or more transceivers to provide sufficient channels to end users (maximum numbers are limited by many factors including - operators radio license, interference with other nearby GSM cells, cost of equipment, capacity of radio site infrastructure etc.)

A GPRS user may theoretically use all voice channels on one transceiver - (8 * 14 kbps) but radios to support this are not available and the operators will probably reserve at least some channels for voice circuits.

Each 14kbps channel may be shared by multiple 'connected' GPRS users (many users will be connected to the network but transmitting very little data). As a user's data requirements grow, they will use more of the available capacity within that timeslot, and then more available timeslots up to the maximum available or the maximum supported by their device.

 

In general the higher the data rate, the more power the mobile device will use and the shorter the battery life and the higher the transmitted RF power. If you are using GPRS with a mobile phone, do not keep it near your ear for long periods while data transfers are taking place.

GPRS Mobile devices

The key use for GPRS is to send and receive data to a computer application such as Email, web browsing or even telemetry (telemetry refers to devices not being controlled by humans such as cash point machines or traffic monitoring cameras etc.). To use GPRS the service is 'dialed' in a similar manner to a standard data call (though there is no phone no.) at which point the user is 'attached' and an IP address is allocated. From then on data can flow to and from the Internet until either the network unattaches you (maybe because of a time-out, fault or congestion) or you manually unattach.

Mobile workers usually have a mobile phone, when this includes GPRS then it can also be used to transfer data to an connected computer.

Some of the key issues are:

Using GPRS will not stop you making or receiving voice calls.

Current phones will usually suspend the data session while a voice call takes place.

Battery life will be reduced when using GPRS.

The data needs to be connected with your computer.

The three standard methods to connect your computer to GPRS mobile phone are:

Infrared - available on most business mobile phones - just align the IR. port on the phone with the IR. port on the Laptop.
Data-cable - reliable and doesn't require the careful alignment of IR. which may be difficult when traveling

Bluetooth - My preferred solution - often difficult to set up but once its configured Bluetooth provides a very convenient connection. Bluetooth is available for connecting to Laptops via USB, PC-cards or CF-cards in addition to cards for PDAs such as those offered by PALM. Older Compaq IPAQs will require an expansion jacket but newer Pocket PC devices usually include a suitable expansion port (check at the time of purchase). One very important point is that Bluetooth devices are very low powered so do not drain your computer battery or phone battery too much. Many people will be tempted by the all-in-one phone/PDA, but consider will you be happy with the relatively short battery life, large size and weight and unreliability of many PocketPC devices.

GPRS data cards are also available, the issues here are:

Fully integrated solution
Best in Laptops with PC card expansion slots
GPRS will drain your battery so expect reduced life
You can subscribe to a different network than your GSM voice supplier
GPRS data cards will have their own SIM card and hence will need another subscription to your mobile network

 

GPRS Roaming

In the short term don't expect to be able to roam to many countries with GPRS, many networks are still negotiating to set up roaming agreements. Technically there are two type of GPRS Roaming

Home Network Roaming - Here all data is transmitted from wherever you connect to a GPRS network to your home GPRS network where it is connected to the Internet or your company LAN as if you were indeed in your home country.
Local Network Roaming - Data is just connected to a local Internet connection point and will be subject to local conditions for security and performance.

GPRS users would be advised to ensure they also are able to use either GSM or High Speed GSM data (HSCSD) to retrieve their data when traveling because of the changing state of GPRS roaming agreements. They can either phone their ISP or RAS server on their home network or subscribe to an ISP which provides local access points in each country visited.

 

GPRS Security

The radio interface is considered to be relatively secure being controlled by the GSM network's security - (SIM card + HLR). Security issues arise when data needs to leave the GPRS network to be delivered to either the Internet or a company LAN.

Internet connectivity is the cheapest and most common - and here you can take charge of security by encrypting sensitive data. If your GPRS network supplier allows it you can set up encrypted VPN connections to your company systems - though there could be a performance hit. Treat the connection as a standard dial-up Internet connection to an ISP and take similar security precautions.

 

Network Connectivity

As a business GPRS user you will have a choice of methods to connect to the GPRS network - by far the most common method will be via the Internet. For larger users you may connect your company LAN to the GPRS networks using leased lines or Frame Relay virtual circuits.

 

Internet

Your company probable already has an Internet connection (though you may need more capacity if you add many GPRS users) and this provides a quick and easy way of connection to GPRS.

The key problem is to deliver your data SECURELY to your users, using strong encryption such as with SSL (128 bit) or VPN (162 bit).

For secure company Email access you have a number of choices. These include:

VPN firewalls - this will provide secure access to everything on the company LAN from GPRS and other Internet users.
Microsoft Mobile Information Server
WAP interfaces to your Email system e.g. Peramon

POP server - set up a company POP server to provide Internet based Email, make sure to enable additional security if required.

Employees (often senior managers) often bypass a companies security systems by redirecting to personal Internet Email accounts which provides them with a quick fix to mobile connectivity.

 

Leased Lines

Leased lines provide the most secure method of connecting to GPRS but are traditionally expensive and have long contract periods. (Min 1 year)

The protocol over the leased line would normally be frame relay but it is possible you could use ATM with some networks. You do not really need any CPE (Customer Premises Equipment) supplied by your GPRS network supplier, just a spare Frame relay port on an existing router. There may be economies to be made if you also use the leased line to carry standard voice and data and bulk SMS in addition to the GPRS traffic - in which case your network supplier will provide a device to route these onto your network. They may also try to sell you consultancy to design this interface - shop around to get the best solution.

Keep costs down by connecting to a geographically close connection point to the chosen GPRS network. Not all networks have the same number and location of connection points (GGSNs in GPRS terms).

   
DMC Firewall is developed by Dean Marshall Consultancy Ltd