Also the older technology of MODE A and C (Identity and Height) was supposed to be switched off at the end of this year but this has been put back till the 31st March 2012 (for now!) Think of this as a big digital switch over. Why is the switchover happening and how will it affect self loading freight, sorry , passengers? It won't but will allow more aircraft to fly in a given area (continent) as the number of MODE A addresses was limited to 4096 individual codes (7777 in octal) whereas the new technology will allow 16,777,214 individual addresses, currently there are around 600,000 various type of aircraft able to fly so it's reasonably future proof.
Need to know more? Come to Bournemouth Airport and I'll bore you even more!