SRL: A Bidirectional Abstraction for Unidirectional Ad Hoc Networks.

Venugopalan Ramasubramanian

Ranveer Chandra

Daniel Mosse

Introduction

Links in an ad hoc network could be unidirectional.

Many Ad hoc network routing protocols are not designed to handle unidirectional links (TORA).

Some handle unidirectional links but are very inefficient (DSR).

Noise: source of one-way link.

Transient unidirectional links. Go away when noise subsides or nodes move.

A

B

C

D

E

Asymmetry in Transmit Power

Topology Control Schemes: Sensor Network Heterogeneity of hardware: Home Network

AB

C

AB

C

Problems due to one-way links.

Collision avoidance (RTS/CTS) scheme is impaired Even across bidirectional links!

A B C

RTS

CTS XCTS

MSG MSG MSG

Problems due to one-way links

Collision avoidance (RTS/CTS) scheme is impaired Even across bidirectional links.

Unreliable transmissions through one-way link. May need multi-hop Acks at Data Link Layer.

Link outage can be discovered only at downstream nodes.

Problems for Routing Protocols

Route discovery mechanism. Cannot reply using inverse path of route request. Need to identify unidirectional links. (AODV)

Route Maintenance. Need explicit neighbor discovery mechanism.

Connectivity of the network. Gets worse (partitions!) if only bidirectional

links are used.

Average Bidirectional Connectivity

40

50

60

70

80

90

100

110

P 5 P 10 P 15 P 20 P 25 P 30 P 35

Probablity of one-way link (%)

Bid

irec

tio

nal

Co

nn

ecti

vity

(%)

Distribution of Bidirectional Connectivity.

0

5

10

15

20

25

30

35

40

45

50

Connectivity (%)

Dis

trib

uti

on

200 random topologies. Probablity of one-way link = 0.25

Reverse route for one-way link

Let A C be a one-way link. C B A is a 2-hop reverse route.

AB

C

Connectivity with reverse routes.

60%

65%

70%

75%

80%

85%

90%

95%

100%

P 5 P 10 P 15 P 20 P 25 P 30 P 35

Rts 5

Rts 4

Rts 3

Rts 2

Rts 1

One-way links with reverse routes.

40%

50%

60%

70%

80%

90%

100%

P 5 P 10 P 15 P 20 P 25 P 30 P 35

Probablity of unidirectional links (%)

len 4 %

len 3 %

len 2 %

len 1 %

Average Reverse Route Length

1

1.1

1.2

1.3

1.4

1.5

1.6

P 5 P 10 P 15 P 20 P 25 P 30 P 35

Probablity of unidirectional link (%)

reve

rse

rou

te l

eng

th (

ho

ps)

Observations from analysis.

Topologies generated with asymmetric transmit power also produce similar graphs.

The connectivity follows a long tail distribution.

Reverse routes are short (2 or 3 hops) for most one-way links.

SRL: Sub Routing Layer

Short reverse routes for one-way links Improve connectivity substantially. Also decrease route lengths.

SRL discovers and maintains reverse routes for one-way links.

It provides a bidirectional abstraction to the routing protocols.

Provides services such as reliable transmission and link breakage detection.

Internals of SRL

Reverse Distributed Belmanford Algorithm Distance vector based technique.

Each node maintains: Shortest path from other nodes in its locality. Periodically neighbor-casts this information.

Locality of node A: Set of nodes that can reach A in r hops. r: is the radius of locality.

Reverse Distributed Belmanford Algorithm.

AB

C

A; 2; CC; 1; B

C; 2; BB; 1; A

A; 1; C

Update Message Format: Source; #hops; First Hop

Reverse Route: C B A

RDBA contd.

Periodic update messages are neighbor-cast:Source ID : Hop Count : First Hop

Sources restricted to locality of radius r. r: called SRL radius is small (2 – 3). Scalable to large networks.

No counting to infinity problem. Ignore distances bigger than r.

No Route-loops. Use first hop information to check for loops.

SRL: Periodic Updates

Incremental Updates Most recent changes in hop count or first hop. Sent periodically at same rate as hello messages. Replaces hello messages.

Complete Updates Contains entire data for locality. Sent with much lower frequency. Random distribution to avoid co-ordination.

Hello Packets Sent when no incremental updates need to be sent.

Optimization 1: Dynamic SRL

The SRL radius of each node could be different. Each node increases radius until it can find reverse

routes. Radius decreases if reverse routes are shorter than

the radius. Decreases the number of updates that is neighbor-

cast: lower overhead.

Optimization 2: On-demand DSRL

Routing protocol requests DSRL to find reverse routes for certain one-way links.

Reverse routes maintained only for the chosen one-way links.

Routing strategy that uses one-way links only when route discovery along bidirectional links fail.

Services provided by SRL

Identification of one-way links (radius = 1): Routing protocols can avoid them.

Reverse route forwarding: Routing protocol uses reverse routes to send route replies and route

errors. Not good for data packets.

Link breakage detection: Several protocols rely on lower layers to do this.

Reliable Transmission across unidirectional links: Multi-hop Acks can be used if required by the protocol.

Simulation: AODV over SRL

AODV is adapted on top of SRL. Use reverse routes for RREPs and RERRs. Uses SRL’s link break discovery service.

Compared with traditional AODV. Routes only along bidirectional links. Uses black-list to identify unidirectional links. Runs on top of IEEE 802.11

Simulation Setup

80 nodes in 1300m x 1300m area. 220m nominal radio range (WaveLan). 360s total simulation time.

300s of data origination. 20 random src-dest pairs for each run. 50 random topology for each experiment. Packet Size: random between 64B – 1024B. Average data rate: 1 packet per sec.

Static Experiments: Packet Delivery.

50000

100000

150000

200000

D 0 D 60 D 140 D 220 D 280 D 320

Average Diversity in Range (m)

# P

acke

ts D

eliv

ered

.

RADIUS 1

RADIUS 2

RADIUS 3

AODV

Static Experiments: Average Route Length.

3

3.5

4

4.5

5

5.5

6

D 0 D 60 D 140 D 220 D 280 D 320

Average Diversity in Range (m)

Ave

rag

e #h

op

s p

er p

acke

t.

RADIUS 1

RADIUS 2

RADIUS 3

AODV

Mobility Experiments: Packets Originated

transmission range between 80m and 360m

0

20

40

60

80

100

P 0 P 60 P 120 P 180 P 240 P 300 P 360pause time (sec)

#Pac

kets

Ori

gin

ated

RADIUS 1

RADIUS 2

RADIUS 3

AODV

Mobility Experiments: Packet Delivery.

transmission range between 80m and 360m.

0

20

40

60

80

100

P 0 P 60 P 120 P 180 P 240 P 300 P 360pause time (sec)

#pac

kets

del

iver

ed

RADIUS 1

RADIUS 2

RADIUS 3

AODV

SRL Overhead: Average Length of Update Packets.

Transmission Range between 80m and 360m

0

50

100

150

200

250

P 0 P 60 P 120 P 180 P 240 P 300 P 360Pause time (sec)

Pac

ket

Len

gth

(B

ytes

)

RADIUS 0

RADIUS 1

RADIUS 2

Conclusions

SRL increases the packet delivery of AODV by 30%.

The overhead generated by SRL is not very significant and can be further reduced.

The effect of optimizations need to be studied. RTS/CTS implementation with SRL would be

interesting!