Multiple Directional Antennas in Suburban Ad-Hoc Networks

Ronald PoseMuhammad Mahmudul Islam

Carlo Kopp

School of Computer Science & Software EngineeringMonash University

Melbourne, Australia

Outline

Focus of this paper Overview of SAHN Effects of omni-directional and

directional antennas on SAHN Conclusions

Focus of this Paper

Routing performance using three antenna schemes multiple fixed directional multiple omni-directional single omni-directional

Estimation of achievable performance in a SAHN Interference of non-SAHN nodes on a near by SAHN

SAHN (1/3)

How to connect to a corporate network from home or how to link a community of broadband users

Commercial Wired Services Direct Dial-up Services Internet Services

Dial-up Broadband (cable modems, xDSL, etc)

Ad-Hoc Wireless Networks Single Hop Solutions

802.11b Multi Hop Solutions

Nokia Roof-Top SAHN MIT Roofnet

SAHN (2/3) Provides services not offered by commercial service

providers Bypass expensive centrally owned broadband

infrastructure Provide symmetric bandwidth Independent of wired infrastructure Avoid ongoing service charges for Telco independent

traffic Features multi-hop QoS routing

Security throughout all layers Utilizing link states (e.g. available bandwidth, link stability,

latency, jitter and security) to select suitable routes Avoid selfish routing strategy to avoid congestion Proper resource access control and management

SAHN (3/3) Ideal for cooperative nodes. E.g. spread over a suburban area,

connecting houses, businesses, branch offices, etc Topology is quasi static Uses wireless technology Symmetric broadband, multi Mbps bandwidth No charges for SAHN traffic SAHN services

run alongside

TCP/IP Conceived in 1997 by

Ronald Pose

Carlo Kopp

Application

Presentation

Session

Transport

Network

Data Link

Physical

TCP/UDP

IP

Application

Presentation

Session

Transport

Network

Data Link

Physical

TCP/UDP

IP

SAHN Routing

e.g. IEEE 802.11 variants

e .g. IEEE 802.11 variants

AUDIO

VEDIO

OTHER

Appears to host like a cable modem Functionally more like a

RF LAN repeater Embedded

microprocessor &

protocol engine

that implements all

SAHN protocols, manages

and configures the system Each SAHN node has at least 2 wireless links Capable of achieveing link rate throughput

A Standard SAHN Node

Omni-directional Antennas

Advantages Directional orientation is not required May provide more connecting links Installation is easy and quick Ideal for ad-hoc networks with high mobility

Drawbacks Power radiates in all directions Increases hidden and exposed terminal problems Increases multiple access intereferences (MAI) Increases collisions and packet loss Degrades network performance Easy to eavesdrop

Directional Antennas

Drawbacks Requires antenna direction alignment May provide fewer links Installation may be complicated Network planning is more difficult

Advantages Power can be beam formed Reduces hidden and exposed terminal problems Reduces multiple access intereferences (MAI) Reduces collisions and packet loss Improves network performance Eavesdropping is limited to the direction of communication Ideal for ad-hoc networks with less mobility

Assumptions in this Work Only the interference related effects on the routing protocol are

presented Each of the antenna elements in multiple antenna schemes are

allocated distinct non-overlapping frequency channels Multiple omni-directional antennas represent an omni-

directional antenna scheme that can operate simultaneously in multiple non-overlapping frequency channels

GloMoSim (version 2.02) has been used for simulating various layers and wireless media

The radio layer has been modified to use multiple directional and omni-directional antennas

The effect of secondary lobes on the primary lobe has been ignored while using directional antennas

A two-ray path loss scheme calculates the propagation path loss DSR has been used as the routing protocol

Different Stages in Simulation

Simulations have been divided into the following stages:

1. Find maximum achievable throughput, delivery ratio and response time in single and multiple hops

2. Investigate the effect of different packet sizes on network performance

3. Study the average network performance4. Investigate the impact on network performance

of the presence of other networks operating nearby

Simulation Setup in Stage 1 & 2

A chain of nodes and only one pair of nodes were active at a time

Adjacent nodes have been separated by 350 metres UDP packets have been used to avoid additional delays for

hand shaking and end-to-end acknowledgements in TCP Loads at the sources have been changed from 10% to 85%

to get the critical point beyond which performance remains unchangedA node operating at 25% load means that it is generating traffic at 2.75Mbps (maximum bit rate is 11Mbps)

Simulation Results for Stages 1 & 2 (a)

Maximum delivery ratio, throughput and effects of different packet sizes in a single hop

Observations for Stages 1 & 2 (a) Due to single hop scenarios the impact of directional and

omni-directional antenna was all the same At around 55% load, the communicating link seems to

saturate Above 55% load, the minimum time required to serve each

data frame becomes more than the time slot needed to sustain the data rate

Smaller packets (e.g. 500 bytes) can reduce the peak performance of the network by almost 50%Since each data frame involves various delays (e.g. time for RTS, CTS, DIFS, SIFS etc), smaller packets increase the delay overhead per bit, hence reduce network efficiency

Simulation Results for Stages 1 & 2 (b)

Maximum delivery ratio (1500 bytes/packet) with multiple hops

Traffic load 25% Traffic load 55%

Maximum throughput (1500 bytes/packet) with multiple hops

Simulation Results for Stages 1 & 2 (c)

Traffic load 25% Traffic load 55%

Observations for Stages 1 & 2 (b, c)

Performance with single and multiple omni-directional antennas reduced almost by 60% in most cases whereas the performance achieved with multiple directional antennas remained almost unchanged

Due the mechanism of DCF, some of the nodes along a route have to wait while others are transmitting if omni-directional antennas are used

As a result data transmission via multiple hops suffers more back-off delays and collisions than single hop communication

Directional antennas solved this problem with the sacrifice of a range of directions

Simulation Results for Stages 1 & 2 (d)Minimum response time (1500 bytes/packet)

Observations for Stages 1 & 2 (d) Response time can be as small as 2.6 milliseconds for single

hop communication With multiple directional antennas, a response time of 6.4

millisecond is possible at the 11th hop At this distance, response times for a single and multiple

omni-directional antennas are 13.6 milliseconds and 8.4 milliseconds respectively

With the increase of number of hops, distance traveled by a packet increases, hence more time is needed to get a reply for a request

Moreover, time required for resolving interference can make a response more delayed

The later problem is more common for omni-directional antennas than directional ones

TDMA based schemes may exhibit better results

Simulation Setup in Stage 3 77 nodes were placed on a 3000x3000 sq km flat terrain where

each node had at most 6 neighbors Separation between neighboring nodes was 350 metres All antenna configurations had the same transmission range Channel allocation to multiple antenna elements was random On average each antenna channel connected 2 neighbors Multiple directional antennas were allowed to communicate to

at most 3 neighbors at 3 different frequency channels which effectively reduced the degree of connectivity per node

CBR and interactive applications generated random traffic The number of nodes for interactive traffic was 6 To vary traffic, the number of nodes for CBR terminals were

increased in 5 steps (4, 8, 12, 16 and 20). For each configuration, loads at CBR sources were varied at 4 different levels (10%, 25%, 40% and 55%)

Each data packet was 1500 bytes long

Simulation Results for Stage 3 (a)

Average delivery ratio (1500 bytes/packet) with multiple hops

Traffic load 5.19% Traffic load 20.78%

Simulation Results for Stage 3 (b)

Average throughput (1500 bytes/packet) with multiple hops

Traffic load 5.19% Traffic load 20.78%

Simulation Results for Stage 3 (c)

Average response time (250 bytes/packet) with multiple hops

Traffic load 5.19% Traffic load 20.78%

Observations for Stage 3

At low load and for the same number of hops, multiple omni-directional and multiple directional antennas perform similarly

As the loads at the CBR sources increase, their performance start to differ significantly up to a certain limit (i.e. for moderate traffic)

With the increase of the number of nodes and the rate of traffic generation, fewer routes remain unsaturated to balance the aggregated network load

As a result, a small performance gain can be achieved with multiple directional antennas over multiple or single omni-directional antennas

Simulation Setup in Stage 4

In omni-directional mode, both networks use the same frequency channel

In the multiple omni-directional antenna scheme, SAHN uses a frequency channel different from the neighboring network

Simulation Results for Stage 4

Effect on throughput (1500 bytes/packet) due to interference from other networks

Observations for Stage 4 If the nearby node belongs to the same network, they are

supposed to co-operate, e.g. route others' packets A node can decide not to allow packets coming from other

nodes belonging to a different network A node cannot stop nodes of a different network from

transmitting. Instead, it can stop listening from that direction to avoid interference

Two ways to do this operating in different frequency channel or using directional antennas

Directional and multiple omni-directional antenna achieved similar performance (i.e. throughput was constant) despite the increasing load at the nearby non SAHN node whereas the omni-directional antenna suffered from interference

Conclusions

If no route exists in configured directions antennas may

need to be redirected and it may be difficult with multiple

fixed directional antennas Multiple fixed directional antennas may be expensive to

buy and install A smart directional antenna can be an alternative

solution at low cost We plan to optimize a routing protocol and the MAC

layer to efficiently handle the real life problems with

smart antennas in the context of SAHN

References

R. Pose and C. Kopp. Bypassing the Home Computing

Bottleneck: The Suburban Area Network. 3rd Australasian

Comp. Architecture Conf. (ACAC). February, 1998. pp.87-

100. A. Bickerstaffe, E. Makalic and S. Garic. CS honours theses.

Monash University. www.csse.monash.edu.au/~rdp/SAN/

2001 MIT Roofnet. http://www.pdos.lcs.mit.edu/roofnet/

Thank You

?