Thomas Kunz
Systems and Computer Engineering
15
CDPD: MAC Protocol
–downlink/forward
channel: no contention, only one sender: the MDBS. All frames
are broadcasted, each M-ES picks out the ones destined for it
or for everyone
–uplink/reverse
channel: contention is a problem
naccess to channel follows a DSMA/CD protocol:
–uses
time slots of 60 bit times (see structure of forward channel)
–“digital
sense”: watch forward channel to determine whether reverse
channel is busy or idle (busy/idle flags every 60 bits)
–if
busy, skip a random number of slots and try again. If still busy, wait
for longer period (statistically twice as long) and retry
–if
idle, start transmitting
–“collision
detection”: decode flag in forward channel indicates with
delay whether there was a collision
–keep
sending until collision is detected or until maximum number
of slots is set or until MDBS tells
M-ES to shut down