Just one more thought about this.

To make the routines really useable You should also include the reading of LAC. Although most GSM system vendors make the CI unique it is not restricted by the GSM standard. Therefore Ericsson system allows duplicate CI:s considered that they are on a different MSC.

The BSC (Base station) is fully and globally qualified by four digits: MCC, MNC, LAC and CI.

MCC - Country code
MNC - Network (operator) code
LAC - Local area code
CI - Cell ID

As cell densities grow to meet capacity demands the MSC areas tend to become smaller. The possibility to cross a MSC border (LAC changes) is greater and it becomes more "popular" all the time. If You are only using the CI to determine which cell is this You are actually resting on the goodwill of the operator. It is their decision whether the CI:s are really unique or not. LAC+CI is uniques unless You change the operator on the fly.

And one more thing. The CI is not a permanent property of a cell. It may change and some systems (Nokia among others) do this automatically whenever a cell is broke or othervice disturbed (power loss).

There is also this new concept of "Slave Cell" which shares the CI of the master cell.

All in all - location based on GSM requires very tight cooperation with the network operator to be even close to a reliable system.