1. Dual Busy Tone Multiple Access : A New Medium Access Control for Packet Radio Networks
ICUPC’98
Jing Deng and Zygmunt J. Hass
School of Electrical Engineering
Cornell University

2. Introduction Fundamental problems in wireless network
hidden terminal problem
exposed terminal problem
Goal:
A MAC protocol with enhancement of busy tones
Busy tone: a function similar to carrier sense
IEEE , MACA, MACAW
RTS/CTS to solve the hidden terminal problem
However, due to mobility, propagation delay, and transmission delay, the probability of packet collision may be high (to be analyzed later).

3. Effect of Mobility
Nm = number of nodes which will move into the transmission range of A during a propagation delay and thus interfere with the reception of A
To find Nm, we can
Find out the angle .
Integrate over the dotted circle.
Through calculus, it can be shown that Nm = 0.01
Insignificant!!

4. Effect of Packet Collision
A sends an RTS to B.
B replies a CTS to A.
At the same time C initiates an RTS to D. (This is OK with A.)
Thus, B’s CTS is destroyed at D.
During the transmission of data packet from A to B, D can potentially destroy this data packet by sending anything (such as an RTS, CTS, …).

5. Analysis of the Probability
The vulnerability period of B’s CTS packet is 2Tc.
Recall the ALOHA definition.
The number of nodes C that start a new RTS during the vulnerability period is:
b = percentage of ready nodes
Sshaded = that covered by B’s but not A’s transmission range
d = nodal density
Tw = the optimal backoff time (assuming an ideal situation)

6. Now that D does not know B’s RTS.
possible location of D = in both B’s and C’s ranges
During B’s reception of data packet, the number of D’s that will make damage is:
TD = transmission time of data packet
Note: TD (data packet) is much greater than TC (control packet)
It can be calculated that under heavy load, the probability that data packet being destroyed is about 0.6.

7. Observation of DBTMA DBTMA = dual busy tone multiple access
Frequency Assignment:
data channel
control channel :
bandwidth for control packets
BTt (transmit busy tone)
BTr (receive busy tone)
BTr
BTt
frequency
control
channel
data
channel

8. DBTMA Operations Still based on RTS/CTS. Busy Tone Usage Rules:
When one is sending, turn on its BTt.
When one is receiving, turn on its BTr.
Packet Sending/Receiving Rules:
When sending RTS, make sure that there is no BTr (receive busy tone).
When sending CTS, make sure that there is no BTt (transmit busy tone).

9. DBTMA State Diagram

10. Performance Evaluation
Simulation parameters
Coverage area (S) : 6 * 6 [km]
Nodal transmission radius (R) : 0.5, 2 [km]
Number of nodes (N) : 400
The length of control packets (LC) : 48 [bits]
The length of data packets (LD) : 1024 [bits]
Link data rate (Rd) : 2.048, 20.48, 2048 [kbps]

11. Performance Evaluation
Traffic Load :
aggregated traffic load in the whole coverage area
Network Utilization :
total number of packets being transmitted and received successfully per packet transmission time
Percentage of packet collisions
percentage of collided data packets among data packets transmitted

12. Network Utilization
R = 0.5
R = 2

13. Collision Ratio
R = 0.5
R = 2

14. Effect of Mobility on Network Utilization
negligible
difference
V = 0 and 100 km/hour r = 0.1 radian/sec Tm = sec

15. Conclusions Two busy tones
decouple the two communication directions
similar to the carrier sense in wired networks
doubling the network capacity
Effect of mobility at the MAC layer is less significant than packet collision.