GPRS was invented as part of the move to what is called 3G or 3rd generation cellular phone service. That's the idea that cell phones can also be computers, email and web browsers and even TV receivers. It's a tall order for a technology that was originally designed to simply make telephones mobile.
So, how does GPRS work? It uses the idle radio capacity in GSM telephone systems. All cell phone systems have capacity that isn't being used except when everybody is on the phone in a particular cell. Normally that idle capacity wouldn't be doing anything. It would simply be held in reserve for the next callers. What GPRS does is to use that extra capacity to create a data network. If the system gets really busy with telephone calls, then the data gets a lower priority and data rates slow down.
GSM is the most popular cellular technology in the world with over a billion subscribers in 85 countries. It's based on TDMA or Time Division Multiple Access. GSM channels are 200 KHz wide and divided into 8 time slots. Each slot can carry a digital telephone call coded at 13 Kbps or 14.4 Kbps of IP or Internet Protocol data. Busy cells may use multiple channels to support the call demand.
If you could use an entire GSM channel to carry data, you could have a bandwidth of over 100 Kbps. With compression, the maximum bandwidth for GPRS is 170 Kbps. Don't get your heart set on that rate. GPRS is set up on a class system, with class 8 being the default. Class 8 is known as 4+1. You get 4 time slots for download and 1 for upload. Another popular class is GPRS class 10, also known as 4+2. That's 4 time slots for download and 2 for upload.
Typical download rates might be on the order of 40 Kbps or so. That's not the blazing speed of broadband but is plenty for SMS messaging, email, order entry and web browsing. Laptop computers can also access the Internet through GPRS with an adaptor. T-Mobile offers the Sony Ericsson GC79 PC Cellular Card that has both GPRS and WiFi capability. Within a WiFi hotspot you get the speed of WiFi. Elsewhere, within the coverage of the T-Mobile GSM cellular network, you use GPRS. This can be the perfect combination for mobile business people who need access to their Internet based tools while visiting clients.
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