[Networking] veletron:
Hi

For the networks, supporting GPRS meant a relatively simple firmware update to all but the oldest base stations. GPRS is pretty crude, if offers higher data rates at the expense of network capacity and makes poor use of the available spectrum (compared to W-CDMA G3 modulation schemes).

A basestation will have available to it a number of standard frequencies for GSM comms, say 50 channels, these channels will be different to the channels used on the neighboring base stations.

Each of these channels is split up into 8 time slots. In a GSM config, a mobile gets 1 timeslot, so 8 calls can take place 'at the same time' on the same channel. The base station and mobiles decide who is gonna transmit and when. This happens so fast that as far as the user is concerned they have a comms channel to themselves. This is TDMA Time Domain, Multiple Access.

Interestingly the time slots are so narrow that if you have phone A on located 20KM from the base station on timeslot 8, and phone B next to the base station on timeslot 2, then phone A will transmit its data before phone B, with the data arriving at the base station in correct time-slot order!

Mobile networks also support time-slot and channel hopping to get around network interference problems, and more importantly to counteract the issues of dopler shift when the mobile is moving at speed.

GPRS works simply by making more of these timeslots available to a single mobile. The fact that data is bursty means that the base station can hand out timeslots fairly efficiently. The 2 current systems for GPRS are class 8 and class 10

Class 8 offers 4 download timeslots and 1 upload timeslot

Class 10 offers 3 download timeslot and 2 upload timeslots

Each slot offers from 9.6K to 21.4K depending on network configuration (basically thats the width the RF channel, perhaps also the no of slots that the channel is split into - cant remember). In the UK I beleieve that each timeslot runs at 14.4K. You will however not see that since some of it is used for error correction.

O2 defaults to class 10. I dont know whether this is locked at the network level and I never tried changing it.

Class 10 will hence give max 43.2K down / 28800 up and

class 8 is 57.6K down, 14.4K up

You can knock 20% of those figures for error correction duties.

It would be possible for GPRS to run in 7/1 mode to give 7x14K4 = 100.8K down / 14.4K up. I dont think any networks or devices support such a configuration though!

Its been a while since I worked in this field so this info may be out of date by now. GPRS is not that much faster than HSCSD (high speed circuit switched data), but its much more efficient in its use of the bandwidth.

Ever been up a mountain miles from nowhere, had a screen full of signal bars but never managed to make a call?? Its a limitation of GSM (or rather the config of the network). A phone must be able to respond within the correct timeslot, if the phone is too far away then it can miss its slot altogether, hence full bars, but no calling! In the UK, the network setup means that the limit is around 25KM, in australia, they have extended this to around 100KM in rural locations.

Roll on EDGE - 2.5G++ its a new modulation scheme for GSM that makes *much much* better use of the spectrum. Should give us 256K on the move - i.e it will be pushing into 3G terratory bandwidth wise. Trouble is, GPRS was a firmware update for most base stations, EDGE is gonna require hardware. I would not be surprised though if we did not see networks giving up on taking 3G coverage beyond 70-80% of population, and rely on EDGE/GPRS to cover the dregs.

Nigel