Multiple Directional Antennas in Suburban AdHoc Networks

1
Multiple Directional Antennas in Suburban Ad-Hoc
Networks

• Ronald Pose
• Muhammad Mahmudul Islam
• Carlo Kopp
• School of Computer Science Software Engineering
• Monash University
• Melbourne, Australia

2
Outline

• Focus of this paper
• Overview of SAHN
• Effects of omni-directional and directional
  antennas on SAHN
• Conclusions

3
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

4
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

5
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

6
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

7
A Standard SAHN Node

• 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

8
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

9
Directional Antennas

• 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

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

10
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

11
Different Stages in Simulation

• Simulations have been divided into the following
  stages
• Find maximum achievable throughput, delivery
  ratio and response time in single and multiple
  hops
• Investigate the effect of different packet sizes
  on network performance
• Study the average network performance
• Investigate the impact on network performance of
  the presence of other networks operating nearby

12
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 unchanged
• A node operating at 25 load means that it is
  generating traffic at 2.75Mbps (maximum bit rate
  is 11Mbps)

13
Simulation Results for Stages 1 2 (a)
Maximum delivery ratio, throughput and effects of
different packet sizes in a single hop
14
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

15
Simulation Results for Stages 1 2 (b)
Maximum delivery ratio (1500 bytes/packet) with
multiple hops
Traffic load 25
Traffic load 55
16
Simulation Results for Stages 1 2 (c)
Maximum throughput (1500 bytes/packet) with
multiple hops
Traffic load 25
Traffic load 55
17
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

18
Simulation Results for Stages 1 2 (d)
Minimum response time (1500 bytes/packet)
19
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

20
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

21
Simulation Results for Stage 3 (a)
Average delivery ratio (1500 bytes/packet) with
multiple hops
Traffic load 5.19
Traffic load 20.78
22
Simulation Results for Stage 3 (b)
Average throughput (1500 bytes/packet) with
multiple hops
Traffic load 5.19
Traffic load 20.78
23
Simulation Results for Stage 3 (c)
Average response time (250 bytes/packet) with
multiple hops
Traffic load 5.19
Traffic load 20.78
24
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

25
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

26
Simulation Results for Stage 4
Effect on throughput (1500 bytes/packet) due to
interference from other networks
27
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

28
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

29
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/

30
Thank You

• ?