Structure of MAC frame.

The MAC frame is divided into down-stream and up-stream parts where down-stream transmissions are from base-station to mobile. The frame length varies between 1 and around 20 ATM cell times.

Each transmission unit has a preamble, synchronisation word and CRC. So each can be receivied and checked independently. This gives us better error immunity.

The MAC frame consists of:

• Turn-round time between up-stream and down-stream transmissions
• Acknowledgments for the previous frames up-stream cells
• Reservation grants for reservation requests in the previous frame
• A frame description sequence
• Down-stream cells
• Turn-round time
• Acknowledgments for the down-stream slots
• Up-stream cells
• Up-stream reservation request slots.