Busy Elimination Multiple Access
Dramatically
reduces collisions in data
broadcasting due to the hidden terminal
problem
Geared to support prioritization of data
transmissions
Geared for use in mobile ad-hoc
networks
Has little overall control overhead and
provides impressive good-put
1
Reliable broadcast problem
•Transmissions collide at the common neighbor
•SOLUTION: Common neighbor should arbitrate between its
neighbors
2
Channel Reservation to overcome
hidden terminal problem
• Common neighbor warns other neighbors when it is receiving transmission
from any neighbor
C
D
A
B
E
busy signal
•Busy Tone Multiple Access
(BTMA) –
Channel reservation
with distinct frequency busy
signal (FDMA)
data transmission
A
B
C
•Busy Elimination Multiple
Access (BEMA)–
Channel
reservation with a busy
timeslot (TDMA)
busy signal (Double power
transmission)
3
BEMA protocol
•Rounds consist of BUSY/CONTROL phase and DATA phase
•Each potential sender transmits for random/priority-based
period of time bounded by Δ - β
•Contender listens for a busy signal or collision AFTER it
completes its busy signal transmission
•Transmitter of longest duration signal wins
4
BEMA in action
1
i
2
3
DATA
4
DATA
j
k
1. i and k compete
2. i wins and transmits data for 2 rounds
3. j and k transmit busy signal for entire busy timeslot
in the meantime
4. k competes again
5
BEMA: Protocol Actions
{idle, candidate, waiting, leader, locked}
DATA PHASE ACTIONS – 3,4,5
CONTROL PHASE ACTIONS – 1,2,6
6
Logical Proof
Lemma 1: If no leader in the beginning of a round  at most
one leader in one-hop neighborhood of any node k
Lemma 2: If j is a neighbor of k and j is a leader  k must be
‘locked’ to j
Lemma 3: If j is a neighbor of k and j is a leader in the
beginning of a round  no other node can transmit in the
DATA phase of the round
Lemma 4: Starting from the initial state  at most one leader
in one-hop neighborhood of any node k
Hence hidden terminal problem does not arise
7
Number of Collisions
• Collisions in BEMA and BMMM` remain largely
constant with increase in traffic load
8
Good-put (True Data throughput)
• BMMM` suffers heavily due to high control overhead
• BSMA’s good-put decrease almost linearly as the number of
collisions increase.
• CSMA’s good-put is high and constant because the data loss due to
collisions is made up with the increase in transmitters transmitting
with NO overhead.
9