A Temporary Block Flow (TBF) is a logical connection between the BSS and the MS. A TBF is set up when there is data to send at the BSS or MS side.
Let us take an example to understand the concept of throughput.
Total LLC data volume of GMM/SM signaling transferred
100 kbit with 20 kbit/s + 200 kbit with 30 kbit/s
Num = 100*20+200*30 = 2000 + 6000 = 8000 kbit2/s
Denom = 100 + 200 = 300 kbit