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!