1. How 802.11 MAC interacts with Capacity of Ad-hoc Networks – Interference problem
Capacity of Wireless Networks – Part Page 1

2. MAC Background
Use of DCF (Distributed Coordination Funktion)
access method used in ad-hoc mode
four-way (RTS/CTS/Data/Ack) exchange
nearly CSMA/CA
binary exponential backoff scheme
Capacity of Wireless Networks – Part Page 2

3. MAC Interactions described by simulations with ns
tuned to model 2 Mbps data rate
transmission range 250 m
interfering range 550 m
only stationary nodes separated by 200 m
5 runs lasting 300 sec.
Capacity of Wireless Networks – Part Page 3

4. Single Cell Capacity pattern: min. contention on 2-node-cell
200 m² cell
nodes sends as fast as allowed
random destination
min. contention on 2-node-cell
overhead reduces data throughput-limit to 1,7 Mbps
Capacity of Wireless Networks – Part Page 4

5. Capacity of a Chain of Nodes (1)
Assumption: no interferences caused by non-neighbor nodes (beyond 250 m)
channel utilization of ⅓
However: with 550 m interfering range
expected channel utilization of ¼
Capacity of Wireless Networks – Part Page 5

6. Capacity of a Chain of Nodes (2)
Data flow form node 1 to last node
max. throughput of 1,7 Mbps at 2-node-chain
longer chains approach a utilization of 0,25 Mbps ≈ 1/7*1,7 Mbps
Capacity of Wireless Networks – Part Page 6

7. Capacity of a Chain of Nodes (3)
How is the discrepancy between ¼ and 1/7 caused?
achieving the max. through- put at 0,41 Mbps delivered by controlled send rates
very close to 1,7 Mbps* ¼ = 0,425
peak rate isn't maintained by , scheduling greedy ad-hoc-forwarding senders
Capacity of Wireless Networks – Part Page 7

8. Capacity of a Chain of Nodes (4)
Why fails to achieve the optimum chain schedule?
node's ability to send is affected by its experienced competitions
a chain source injects more packets than subsequent nodes can forward
eventually dropped at forwarding nodes causes resends
decreasing throughput since it prevents transmissions of subsequent nodes
backoff window can dramatically increase
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9. Real Hardware Verification
6 radios configured to mimic simulation parameters
matches fairly
no major errors
average difference only 6% (1500-byte packet)
Capacity of Wireless Networks – Part Page 9

10. Capacity of a Regular Lattice Nettwork (1)
200 m from its radio neighbors
every third chain of left scenario can operate without inter-chain interference
expected flow-throughput of ¼*⅓
1/12 * 1,7 Mbps = 0,14 Mbps (1500-byte packet)
Capacity of Wireless Networks – Part Page 10

11. Capacity of a Regular Lattice Nettwork (2)
Per Flow Throughput settels at about 0,1 Mbps
inefficiencies found in chain scenarios are still present
Capacity of Wireless Networks – Part Page 11

12. Cross Traffic in a Lattice (1)
vertical and horizontal flows
theoretical schedule:
one time cycle operating all verticals
and the horizontal in the next
assumed: each flow see half of its normal throughput
since there are twice as many flows, the overall capacity is the same
However, 802,11 may not schedule this efficently
caused by head-of-queue blocking
Capacity of Wireless Networks – Part Page 12

13. One-hop Throughput under different Topologies
the sum total of data-bits send by all nodes per second
including forwarded data-bits
excluding non-successfully sink-arriving data
a constant factor decrease cross-traffic-one-hop capacity to uncrossing ones
~ node number
Capacity of Wireless Networks – Part Page 13

14. Random Traffic in Random Layout
Simulating realistic scenarios
non-uniformly placed nodes in a square
each node sends to randomly chosen sink
adjustable send rates to keep total drop rate < 20%
3*lattice-node-density to guarantee conectivity
similar one-hop capacity to that of cross-traffic-nets
However: irregular placement leads to free areas -> lowers capacity
random destination -> tendential routing through center
resulting in a capacity limitation by the throughput of the center
Capacity of Wireless Networks – Part Page 14