GPRS

GPRS (General Packet Radio Service) is a non-voice service added to existing TDMA networks, one of the 2.5G technology upgrades. TDMA is the underlying transport mechanism used by GSM networks.

GPRS provides the transmission of IP packets over existing cellular networks.


How does it work?

GPRS is a network overlay to the existing cellular network. It uses the nature of IP transmissions to its advantage. Because IP traffic is made of “packets”, the network does not need to have continuous data transmission. Thus, IP traffic can easily share the channels. A user may be receiving or transmitting data while another one is reading information. The second user does not need to use the channel during this time, and it makes packet networks more efficient than circuit-switched networks (2G), where the channel would be in use, regardless of the user transmitting or not.

Each channel is divided into eight time slots, with a maximum data transmission of 13.4Kbps. One of these time slots is used for control, and normal allocation would reserve two slots for voice traffic as well. Asymmetric traffic (more download than uploads) dictates the distribution of the remaining time slots:

Type	Meaning
2+1	two slots for download + 1 for upload
3+1	three slots for download + 1 for upload
4+1	four slots for download + 1 for upload

Also, the GPRS devices have a classification related to their ability to handle GSM voice calls and GPRS connections:

Class	Meaning
A	devices are capable of simultaneous voice and data transmission
B	devices support one type at a time, and switch automatically between data and voice
C	devices support one type at a time, and require user intervention to switch between data and voice

In theory the GPRS data rate is 171.2Kbps per channel, but users will experience something lower than this, because of the nature of channel usage explained before.

The maximum rate a user can have using a Type 4+1 device is 53.6Kbps.

The connection to the GPRS network is always on, but the device needs to connect to the data terminal (Pocket PC, notebook, cellular phone). The connection times are of course much lower than dial up. My personal tests show GPRS connection to be ready to use in 7 seconds, while a dial up requires around 35 seconds to be in a ready state.

How to use GPRS

A user can connect to a GPRS enable network in three different ways:

Using a mobile with a microbrowser to access WAP pages	A user can connect to the GPRS and browse WAP pages from a GPRS enabled mobile. Of course the impact of reduced content on WML format and using the little phone keypad to type in URLs is tiresome. Examples are mobile phones such as Ericsson T39m and T68i.
Using a handheld with built in GSM/GPRS capabilities	Using handhelds with GPRS capabilities a user can connect to the GPRS network. All Internet services will be available, providing the needed ports are not blocked. This includes web, e-mail, newsgroups, VPN, ftp, etc. The connection between the terminal and the GPRS modem can be via infrared, serial cable or bluetooth. Examples are Treo, XDA Pocket PC Phone Edition, and Smartphones.
Using a mobile as a GPRS modem	A user can connect a notebook, desktop or handheld to connect to a GPRS enabled mobile, and then connect to the Internet. All Internet services will be available, providing the needed ports are not blocked. This includes web, e-mail, newsgroups, VPN, ftp, etc. The connection between the terminal and the GPRS modem can be via infrared, serial cable or bluetooth. Examples are using a H3970 to link via Bluetooth to a T39m and from there to the Internet.
Using a PCMCIA card as a GPRS modem	A PCMCIA card can be used as a GPRS enabled modem, and once inserted a dial up connection can be created. From there the user can connect to the Internet. All Internet services will be available, providing the needed ports are not blocked. This includes web, e-mail, newsgroups, VPN, ftp, etc.

What can I do with GPRS?

Anything available on the Internet, really. Web browsing, email, VPN connections, Instant Messenger (AOL, ICQ, MSN Messenger) are services compatible with GPRS.

The question is really “What performance can I expect from my connection?”

The answer is “It depends”. It really depends on strength of signal, number of users in the channel. But don't expect blazing performance. In my test, with my local GSM provider, I've got results sometimes lower than dial up. But these are non-scientific results.