Comparison of Routing Metrics for Static MultiHop Wireless Networks

1
Comparison of Routing Metrics for Static
Multi-Hop Wireless Networks

• Richard Draves, Jitendra Padhye and Brian Zill
• Microsoft Research

Presented by Hoang Nguyen CS598JH - Spring 06
Some slides adopted from the authors and from
Professor Robin Kravets
2
Multi-hop Wireless Networks
3
Routing in Multi-hop Wireless Networks

• Mobile Networks
• Minimum-Hop
• DSR, AODV
• Static Networks
• Minimum-Hop vs. More Hops
• Link-quality Based Routing
• Signal-to-Noise ratio
• Packet loss rate
• Round-trip-time
• Bandwidth

Experimental Comparison
4
Contributions

• Design and Implementation of LQSR (Link Quality
  Source Routing) protocol
• LQSR DSR with link quality metrics
• Experimental Comparison of link quality metrics
• Minimum-hop (HOP)
• Per-Hop Round Trip Time (RTT)
• Per-hop Packet Pair (PktPair)
• Expected Transmission (ETX)

5
Outline

• DSR Revisited
• Link quality metrics revisited
• LQSR (Link Quality Source Routing)
• Experimental results
• Conclusion

6
Outline

• DSR Revisited
• Link quality metrics revisited
• LQSR (Link Quality Source Routing)
• Experimental results
• Conclusion

7
DSR Route Discovery
Adopted from Professor Robin Kravets lectures
8
DSR Route Discovery
X,Y Represents list of identifiers appended
to RREQ
Broadcast transmission
Adopted from Professor Robin Kravets lectures
9
DSR Route Discovery
Node H receives packet RREQ from two neighbors
potential for collision
Adopted from Professor Robin Kravets lectures
10
DSR Route Discovery
Node C receives RREQ from G and H, but does not
forward it again, because node C has already
forwarded RREQ once
Adopted from Professor Robin Kravets lectures
11
DSR Route Discovery
Nodes J and K both broadcast RREQ to node D Since
nodes J and K are hidden from each other, their
transmissions may collide
Adopted from Professor Robin Kravets lectures
12
DSR Route Discovery
Node D does not forward RREQ, because node D
is the intended target of the route discovery
Adopted from Professor Robin Kravets lectures
13
DSR Route Reply
Adopted from Professor Robin Kravets lectures
14
DSR Data Delivery
Packet header size grows with route length
Adopted from Professor Robin Kravets lectures
15
DSR Route Caching
P,Q,R Represents cached route at a node
(DSR maintains the cached routes in a
tree format)
Adopted from Professor Robin Kravets lectures
16
DSR Route Caching
Assume that there is no link between D and
Z. Route Reply (RREP) from node K limits flooding
of RREQ. In general, the reduction may be less
dramatic.
Adopted from Professor Robin Kravets lectures
17
DSR Route Error
J sends a route error to S along J-F-E-S when its
attempt to forward the data packet S (with route
SEFJD) on J-D fails Nodes hearing RERR update
their route cache to remove link J-D
Z
RERR J-D
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Adopted from Professor Robin Kravets lectures
18
Outline

• DSR Revisited
• Link quality metrics revisited
• LQSR (Link Quality Source Routing)
• Experimental results
• Conclusion

19
Per-hop Round Trip Time (RTT)

• Node periodically pings each of its neighbors
• RTT samples are averaged using exponentially
  weighted moving average
• Path with least sum of RTTs is selected

20
Per-hop Round Trip Time (RTT)

• Advantages
• Easy to implement
• Accounts for link load and bandwidth
• Also accounts for link loss rate
• 802.11 retransmits lost packets up to 7 times
• Lossy links will have higher RTT
• Disadvantages
• Expensive
• Self-interference due to queuing

21
Per-hop Packet Pair (PktPair)

• Node periodically sends two back-to-back probes
  to each neighbor
• First probe is small, second is large
• Neighbor measures delay between the arrival of
  the two probes reports back to the sender
• Sender averages delay samples using low-pass
  filter
• Path with least sum of delays is selected

22
Per-hop Packet Pair (PktPair)

• Advantages
• Self-interference due to queuing is not a problem
• Implicitly takes load, bandwidth and loss rate
  into account
• Disadvantages
• More expensive than RTT

23
Expected Transmissions (ETX)

• Estimate number of times a packet has to be
  retransmitted on each hop
• Each node periodically broadcasts a probe
• 802.11 does not retransmit broadcast packets
• Probe carries information about probes received
  from neighbors
• Node can calculate loss rate on forward (Pf) and
  reverse (Pr) link to each neighbor
• Select the path with least total ETX

24
Expected Transmissions (ETX)

• Advantages
• Low overhead
• Explicitly takes loss rate into account
• Disadvantages
• Loss rate of broadcast probe packets is not the
  same as loss rate of data packets
• Probe packets are smaller than data packets
• Broadcast packets are sent at lower data rate
• Does not take data rate or link load into account

25
Outline

• DSR Revisited
• Link quality metrics revisited
• LQSR (Link Quality Source Routing)
• Experimental results
• Conclusion

26
LQSR

• DSR-like
• Route Request, Route Reply and Route Error
• Link-quality metrics is appended
• Link-state-like
• Link caching (not Route caching)
• Reactive maintenance
• On active routes
• Proactive maintenance
• Periodically floods RouteRequest-like Link Info

27
Outline

• DSR Revisited
• Link quality metrics revisited
• LQSR (Link Quality Source Routing)
• Experimental results
• Conclusion

28
Mesh Testbed
23 Laptops running Windows XP. 802.11a cards
mix of Proxim and Netgear. Diameter 6-7 hops.
29
Link bandwidths in the testbed

• Cards use Autorate
• Total node pairs
• 23x22/2 253
• 90 pairs have non-zero bandwidth in both
  directions.

Bandwidths vary significantly lot of asymmetry.
30
Experiments

• Bulk-transfer TCP Flows
• Impact of mobility

31
Experiment 1

• 3-Minute TCP transfer between each node pair
• 23 x 22 506 pairs
• 1 transfer at a time
• Long transfers essential for consistent results
• For each transfer, record
• Throughput
• Number of paths
• Path may change during transfer
• Average path length
• Weighted by fraction of packets along each path

32
Median Throughput
ETX performs best. RTT performs worst.
33
Why does ETX perform well?
ETX performs better by avoiding low-throughput
paths.
34
Impact on Path Lengths
Path length is generally higher under ETX.
35
Why does RTT perform so poorly?
RTT suffers heavily from self-interference
36
Why PktPair gets worse?
PktPair suffers from self-interference only on
multi-hop paths.
37
Summary of Experiment 1

• ETX performs well despite ignoring link bandwidth
• Self-interference is the main reason behind poor
  performance of RTT and PktPair.

38
Experiment 2
39
Median
40
Outline

• DSR Revisited
• Link quality metrics revisited
• LQSR (Link Quality Source Routing)
• Experimental results
• Conclusion

41
Conclusions

• ETX metric performs best in static scenarios
• RTT performs worst
• PacketPair suffers from self-interference on
  multi-hop paths
• Shortest path routing seems to perform best in
  mobile scenarios