Answer:
If each user in a "cellular" system is assigned a particular FIXED slot, at which he can transmit, it is said to be FIXED TDMA. If the load is heavy only for a certain number of users, it becomes highly inefficient .
So , we go for dynamic TDMA .
One nice question is , what does the throughput for a fixed TDMA system look like for different sizes of packets. Try answering it, using the theory I just provided.