1. Channel Assignment and Channel Hopping in IEEE 802.11

2. Control Knobs in Wireless Networks
Motivations
Limited wireless spectrum
Applications demand high bandwidth
Goal: maximize wireless network capacity
Control knobs
Node placement
Channel
Transmission power
Data rate
Scheduling
Routing
Antenna design: directional/smart/cognitive antennas …

3. Operating Channels for 802.11b
Europe (ETSI)
channel 1
channel 7
channel 13
2400
2412
2442
2472
2483.5
22 MHz
[MHz]
US (FCC)/Canada (IC)
channel 1
channel 6
channel 11
2400
2412
2437
2462
2483.5
22 MHz
[MHz]

4. Operating channels for 802.11a / US U-NII36 40 44 48 52 56 60 64
channel
5150
5180
5200
5220
5240
5260
5280
5300
5320
5350
[MHz]
16.6 MHz
center frequency =
*channel number [MHz]
149
153
157
161
channel
5725
5745
5765
5785
5805
5825
[MHz]
16.6 MHz

5. Channel Assignment and Hopping
Goal: Using right channels at right nodes at right time optimizes network throughput
Approaches
Channel assignment
Channel hopping
Problem domains
Wireless LANs
APs determine the channel
Clients share the same channel as their associated APs
Multihop wireless networks
Additional constraints: intermediate relay nodes need to share the same channel as the upstream and downstream node
Interaction with route selection

6. A Client-Driven Approach for Channel Assignment
A. Mishra, V. Brik, S. Banerjee, A. Srinivasan, and W. Arbaugh

7. Types of Conflicts in WLAN
D <= R
APs within communication range of each other

8. Types of Conflicts in WLAN
R < D <= 2R
APs within interference range of each other

9. Types of Conflicts in WLAN
2R < D <= RBSS

10. Types of Conflicts in WLAN
RBSS < D <= RBSS+R

11. Channel Assignment Algorithm
Objective: maximize # conflict free clients
Client c is conflict-free if at least one AP in the range set is assigned a color j and no other AP in the range set is assigned color j
Random compaction
Initialize AP assignment
X’= random-permute(X)
While true do
ncf = num_conflict_free(T,Θ)
for each AP i in X’
Θ(xi) = compaction_step(T, Θ, xi, k)
end
if num_conflict_free(T, Θ) == ncf then
stop

12. Channel Assignment Algorithm
Objective: maximize # conflict free clients
Client c is conflict-free if at least one AP in the range set is assigned a color j and no other AP in the range set is assigned color j
Random compaction
Initialize AP assignment
X’= random-permute(X)
While true do
ncf = num_conflict_free(T,Θ)
for each AP i in X’
Θ(xi) = compaction_step(T, Θ, xi, k)
end
if num_conflict_free(T, Θ) == ncf then
stop
Any drawback?

13. 4 steps in doing Research
Choosing the problem
Solving the problem
Evaluating solutions
Selling your work

14. Choosing the Problem Finding the right problem is important but hard
Relevance, significance
Read widely
Read conference papers and journals
Keep track of technology trends (tech news, industrial applications) and theoretical advances
Talk to others
People in different areas, in different organizations
Keep a journal of the problems you think important
Willing to take a risk

15. Solving the Problem Survey the related work (google!)
Avoid reinventing the wheel
Be skeptical
Acquire necessary knowledge (on demand)
Work hard
Ask for feedback
Mid-point report: include a short survey and critique of related work

16. Evaluating Your Ideas Be objective
Use simple scenarios to gain insights
Systematically explore the parameter space
Create realistic evaluation scenarios

17. Selling Your Work Three ways of selling your work
Publish papers
Deliver presentation
Spontaneous situations
Be clear about contribution and motivation
Learn from good/bad examples
Practice, practice, and practice
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