Signal-Strength-Aware Routing in Ad hoc Networks
Abhinav GuptaIan WormsbeckerCarey WilliamsonDept. of Computer ScienceUniversity of Calgary
October 5, 2004 MASCOTS 2004 2
Ad hoc Networks
An ad hoc network is dynamically formed when two or more mobile hosts with wireless capability come into transmission range of each other
Advantage of ad hoc networks: Can be set up ‘on-the-fly’ Requires no existing infrastructure
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Ad hoc Network
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Objectives
Implementation of Signal-strength-aware AODV
Study the effect of AODV routing, user mobility, and number of hops on TCP throughput.
Study the effectiveness of rate-based-pacing of TCP (TCP-RBP)
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AODV Operation
Source
Destination
RREQ
RREP
Data
RERR
Data
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Ad hoc On-Demand Distance Vector Routing Protocol (AODV)
Reactive in operation Route discovery and maintenance
using control packets (RREQ, RREP, RERR and HELLO)
The route freshness determined using sequence numbers associated with the control packets.
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AODV UU
Implementation carried out in user-space. Consists of 3 kernel modules and some
user space modules. Uses Netfilter hooks for packet mangling
from kernel space to user space. Packets analyzed in user-space to trigger
AODV events. Packets are queued on to user-space
using libipq which communicates with ip_queue, standard queue handler for IPv4.
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AODV Modules
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Signal-strength-aware AODV
Rationale: Don’t allow route freshness to be determined solely on the basis of sequence numbers
Checks signal strength of control packets coming from adjacent Mobile Host to before it creates, updates or deletes routes.
Signal strength of control packets determined by link_strength module and used by aodv_socket to choose whether or not to let the packet through.
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Variation of Signal Strength
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Operation
If the packet is a broadcast or is intended for the current host, then it is handled as usual by Linux.
If the packet is not intended for the current host and a route exists, it is forwarded to the next hop.
If no route exists, the packet is dropped. If the packet is generated by the local
host, it is buffered in user-space, and a route discovery initiated and routed to next hop when a route is found.
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Locally Generated Packet
NF_IP_LOCAL_OUT
Packet
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Experimental Environment
Redhat Linux 8.0 (kernel 2.4.18-14) 5 laptops
3 IBM Thinkpads (Processor P4) 2 Compaq Evos (Processor P3)
Cisco Aironet 350 PCMCIA Wireless Cards Netperf and Netserver used as source and
sink for TCP traffic. Kernel Probes added to TCP code to monitor
the TCP statistics Airopeek Sniffer used to count the control
packets
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Testbed TopologyNode1
Runningnetserver
Node2
Node3
Node4Node5
RunningnetperfAiropeek Sniffer
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Routing Discovery Time (ms)
Hops Min Median Max Mean StdDev
2 3 7 965 50.8 173.6
3 6 10 3,212 292.8 633.9
4 9 331 5,183 613.3 999.6
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Round Trip Time
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TCP Throughput (Mbps)
Hops Stationary Slow Medium Fast
2 2.23 2.40 2.37 1.96
3 1.45 2.17 1.95 1.66
4 1.24 1.59 1.31 1.18
Slow Speed – 0.33 m/sFast Speed – 1.0m/s
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Routing Overhead
Hops Data Packets Control Packets Overhead Ratio
2 24,369 347 0.0142394
24,822 349 0.0140601
24,516 361 0.0147251
24,559 358 0.0145771
24,384 349 0.0143127
3 17,369 405 0.0233174
16,911 376 0.0222340
16,825 376 0.0223477
17,072 398 0.0233130
17,151 376 0.0219229
4 13,249 499 0.0376632
12,857 476 0.0370226
11,922 507 0.0425264
14,075 452 0.0321137
13,324 401 0.0300961
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TCP rate-based-pacing (TCP RBP)
Rationale: “Spread-out” the TCP Packets in time to improve TCP performance
InterPacketDelay=RTT/(CurrentWindow+V) Performed simulations and experiments with
Reno TCP and RBP TCP
Related Work:[1] Z.Fu et al, “The Impact of Multi-hop Wireless Channel on TCP Throughput and Loss”, Proceedings of IEEE INFOCOM’03, San Francisco, April 2003[2] J.Ke and C.Williamson, “Towards a Rate-Based TCP Protocol for the Web”, Proceedings of MASCOTS 2000, San Francisco, pp. 36-45, October 2000
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TCP rate-based-pacing
PacketPacketPacket
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Simulations Results (TCP-RBP)
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Experimental Results
Num Hops TCP Reno RBP TCP
Mean SDev Mean SDev
1 4.69 0.02 4.21 0.22
2 2.22 0.06 2.13 0.03
3 1.44 0.04 1.31 0.11
4 1.24 0.01 1.08 0.03
•TCP Throughput (Mbps)
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Conclusions
Signal-strength-aware AODV is a good choice for ad hoc networks because of low overhead and good performance
Design choices made for signal-strength-aware AODV were effective
Performance of RBP TCP is highly sensitive to channel contention and AODV routing dynamics
Simulation results should be interpreted with caution, unless validated against experimental measurements
Thank You!