Measurement Window in GPRS – ALU

Hi dear experts!

Can you please explain me the concept of Measurement Window in GPRS?
I can’t understand what do they mean in ALcatel’s docs :

“Tra is the minimum time between the last Tx window and the first Rx window of the next TDMA in order to be able to open a measurement window”

Does the process of measuring radiolink in GPRS differ from the similar process in GSM?

And where will we use these time-intervals:
– Tta
– Ttb
– Tra
– Trb ?

Please, help me in this question. Many thanks in advance!

BR, Pawa