GPRS, often called 2.5G, is a enhancement to the initial 2G network. The 2G network was a phone network and data was sent (slowly) over the phone network in a dial-up way. So either data or a phone call was possible. It was clear that a stand alone always on data network was needed. This is what GPRS supplied.
The BSC (Base Station Controller) would switch the connection between the circuit switched MSC (Mobile Switching Centre) that provided the phone services or to the SGSN (Serving GRPS Support Node) that was the packet switched data network. this, therefore, separated voice from data and would provide the first always on data service,albeit at a slow speed based on the original 2G network. Speeds of 56-171kbps were available.
There are 2 components to the GRPS network :
When the phone makes an HTTP request this is sent to the BSC. Within the BSC the new PCU (packet Control Unit) will route the data to the SGSN. Here authentication and registration takes place. This is needs to be carried out again as this is a separate service to the user. The data is sent to the GGSN (Gateway GPRS Support Node) and from there connection to the Internet. the response reverses the route.
In GPRS a phone can be in one of 3 states :
Whist 2.5G is nowhere near the speed of 3G, the network core used in 2.5G is similar to that used in 3G. The mobile networks used this development as a way to test out the technology prior to the full roll out of 3G.
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