1. Multihop Wireless Networks: What’s Wrong With Min Hopcount?
Douglas S. J. De Couto
Daniel Aguayo, Benjamin A. Chambers, and Robert Morris
1. NOT shortest path is bad, but we need to be more discerning

2. Wired Nets: Why Min Hopcount?
Prefer route with fewest links (“hops”)
Many more sophisticated alternatives
End-to-end latency, lowest congestion, etc.
Min hopcount works in practice
Can engineer a decent network
Alternatives complex, less tested

3. Min Hopcount Assumption
All important route state in hopcount
Link quality is bimodal
Links are either “good”, or “bad”
All “good” links are equivalent
Sufficient condition for success
What about wireless?

4. Indoor Wireless Network
18 nodes
6 on 6th, 12 on 5th
802.11b radios (fixed tx power), PCs, DSDV
5th floor
6th floor

5. Min Hopcount Underperforms
Packets/sec
Worse Better
Why DSDV, doesn’t it suck? No, not on static nets.
Max small packet throughput is ~400—750 pps, depending on bit rate
DSDV worse on 152/185 pairs
18 * 17 = 296 total possible pairs; no data for some of the pairs (either missing DSDV or SR)
Best half of SR routes are mostly 1-hop with a few scattered 2-hop. The middle third and bottom are mostly 2-hop, with a few 3-hop among the bottom half.
124 bytes * 500 pkts/sec ~= kilobits/sec (kbps)
Why best sometimes not better: experiments at different times, didn’t try all possible routes
Node Pair
DSDV
124-byte packets for 30 seconds
“Best” static route
“Best” is maximum pkts/sec over multiple static source routes

6. Not All Min Hopcount Paths Are Equal
3-hop paths between
a single node pair
14 paths, all shortest paths between same 2 nodes: 3 hops
Different experiment from other slides, same packet size: 124 bytes
Different throughput for different paths!
Pair: 10 to 18

7. Not All Links Are Equal 171 non-zero links Links are not bimodal
Broadcast delivery rate
Link
171 non-zero out of 342 links measured. But why not 18*17=306 links? Oops! In data collection…
171 non-zero links
Links are not bimodal
Min hopcount assumptions are false

8. Intermediate Quality Wireless Links Always Exist
In a random ad hoc network, there will be nodes at all distances from each other
Since link quality depends on signal strength, and signal strength is a function of distance, there will nodes with all different link qualities from any given node.
Min hopcount -> Max distance per hop -> min hopcount more likely to pick marginal links!
There are nodes at every distance
Link quality depends on signal strength (distance)
Min hopcount maximizes link distance
Marginal links are more likely!

9. Should We Give Up Hopcount?
Intuition from wired networks is wrong
Links share spectrum
Capacity penalty for more hops
What should we optimize?
Per-route throughput, network capacity, power?
Capacity = 1/2
Capacity = 1

10. Low Quality 802.11 Links 802.11 has link-level retransmissions
Lossy links become narrow links
0% effective loss rate
Good pings
But, low packet-per-second throughput
Retransmissions waste capacity
Other nodes could have transmitted
Will retransmit until ACK, up until a fixed number of maximum retransmits, e.g. 16

11. A Reasonable (Wrong) Idea
Maximize bottleneck throughput: A-D-C
50%
100%
51% D A B C
A-B-C: ABBABBABB = 1/3
A-D-C: AADDAADD = 1/4
Actual throughput:

12. A Better Idea Insight: spectrum use is important!
Idea: minimize transmissions per packet
Num tx 1 2 2

13. Transmissions Per PacketB
100%1 tx
50% 2 tx A C
51% 2 tx
51% 2 tx D
Transmissions per packet (A-B-C) = 3
Transmissions per packet (A-D-C) = 4

14. Research Agenda Explore performance of other protocols, e.g. DSR, AODV
Explain route performance by underlying link performance
Confirm usefulness of transmissions per packet as metric
Handle link variation over time

15. Summary
Minimum hopcount protocols are unlikely to achieve best performance
Quality varies between links
Transmissions per packet nicely quantifies link and path quality for routing