Transcript Impact of Directional Antennas on Ad Hoc Routing

Impact of Directional Antennas
on Ad Hoc Routing
Romit Roy Choudhury
Nitin H. Vaidya
Ad Hoc Networks
Typically assume Omnidirectional antennas
A silenced
node
C
B
A
D
Using Directional Antennas …
 Spatial reuse increases
C
 Wireless interference reduces
 Range extension possible
B
D
A
MAC layer performance
shown to improve.
[Zander, Ramanathan, Takai,
RoyChoudhury, Kalyanaraman]
C
B
A
D
Are directional antennas also beneficial to ad hoc routing ?
Do routing protocols need to be adapted to suit directional
antenna systems ?
This Paper
 Proposes a simple DiMAC protocol
 Evaluates impact of DSR over DiMAC
 Identifies key tradeoffs
 Proposes optimizations to suit directional antennas
– Directional DSR (DDSR)
 Discusses issues where directional antennas may
or may not be suitable
Antenna Model
2 Operation Modes: Omni and Directional
A node may operate in any one mode at any given
time
Antenna Model
In Omni Mode:
• Nodes receive signals with Gain Go
• While idle a node stays in Omni mode
In Directional Mode:
• Beamforms in any one of N static beams (switched)
• Directional Gain Gd (Gd > Go)
Directional MAC – DiMAC
 A node listens omni-directionally when idle
 Sender transmits Directional-RTS (DRTS)
– Receiver receives RTS in the omni mode (DO links)
 Receiver sends Directional-CTS (DCTS)
 DATA,ACK transmitted and received directionally
B
RTS
CTS
C
Directional MAC – DiMAC
 A node listens omni-directionally when idle
 Sender transmits Directional-RTS (DRTS)
– Receiver receives RTS in the omni mode (DO links)
 Receiver sends Directional-CTS (DCTS)
 DATA,ACK transmitted and received directionally
B
Data
ACK
C
Directional MAC – DiMAC
 Directional Network Allocation Vector (DNAV)
 Defer only in the direction of ongoing communication
 Broadcast implemented through sweeping
 Beam Handoffs (due to node mobility) handled
through scanning
 Send probe packets on recently used beams
 Update neighbor cache based on replies to probes
Routing Protocols
• Many routing protocols for ad hoc networks rely
on broadcast messages
– For instance, flood of route requests (RREQ)
• Using omni broadcast will not discover far-away
neighbors
• Need to implement broadcast using directional
transmissions
– A directional transmission, Omni reception = DO link
Dynamic Source Routing
[Johnson]
• Sender floods RREQ through the network
• Nodes forward RREQs after appending their
names
• Destination node receives RREQ and unicasts a
RREP back to sender node, using the route in
which RREQ traveled
Route Discovery in DSR
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
I
D
N
Represents a node that has received RREQ for D from S
Route Discovery in DSR
Y
Broadcast transmission
[S]
S
Z
E
F
B
C
M
J
A
L
G
H
K
I
D
N
Represents transmission of RREQ
[X,Y]
Represents list of identifiers appended to RREQ
Route Discovery in DSR
Y
Z
S
E
[S,E]
F
B
C
A
M
J
[S,C]
H
G
K
I
L
D
N
Route Discovery in DSR
Y
Z
S
E
F
B
[S,E,F]
C
M
J
A
L
G
H
I
[S,C,G] K
D
N
• Node C receives RREQ from G and H, but does not forward
it again, because node C has already forwarded RREQ once
Route Discovery in DSR
Y
Z
S
E
[S,E,F,J]
F
B
C
M
J
A
L
G
H
D
K
I
[S,C,G,K]
• Nodes J and K both broadcast RREQ to node D
N
Route Reply in DSR
Y
Z
S
E
RREP [S,E,F,J,D]
F
B
C
M
J
A
L
G
H
K
I
Represents RREP control message
D
N
DSR over DiMAC
 DiMAC broadcast – RREQ transmitted
sequentially on all N beams – sweeping
 Sweeping allows DO links
 Higher delay
 Higher Overhead
Tradeoffs
Higher tx range
 Fewer hop routes
 Lower end to end delay
 Fewer link failures
 Narrow beamwidth
Narrow beamwidth
 High sweeping delay
 High sweeping overhead
 Frequent handoffs
Motivation to evaluate impact of directional antennas on routing
Evaluation
 Simulation
–
–
–
–
–
Qualnet simulator 3.1
Constant Bit Rate (CBR) traffic
Packet Size – 512 Bytes
802.11 transmission range = 250meters
Channel bandwidth 2 Mbps
– DSR  DSR + 802.11 + Omni Antenna
– DDSRx  DSR + DiMAC + x-Beam Antenna
• E.g., DDSR6  DSR over DiMAC, with
beamwidth = 60 degrees
Route discovery latency …
Single flow, grid topology (200 m distance)
DDSR4
DDSR6
DSR
Throughput
DDSR18
DDSR9
DSR
Observations
• Advantage of higher transmit range significant only
at higher separation between source-destination
• Grid distance = 200 m -- thus no gain with higher tx
range of DDSR4 (350 m) over 802.11 (250 m).
– However, DDSR4 has sweeping delay. Thus route
discovery delay higher
• Sub-optimal routes chosen by DDSR because
destination misses shortest RREQ, when beamformed
Sub-optimal Routes in DDSR
F
J
RREP
J
D
K
RREQ
N
D receives RREQ from J, and replies with RREP
Meanwhile, D misses RREQ from K – called Deafness
Delayed RREP Optimization
• Due to sweeping – earliest RREQ need not have
traversed shortest hop path.
– RREQ packets “sweep-ed” to different neighbors at
different points of time
• If destination replies to first arriving RREP, it can
miss shorter-path RREQ
• Optimize by having DSR destination wait before
replying with RREP
– Waiting allows destination to gather all early RREQs
Bridging “Voids” using DDSR
For randomly located nodes
Using DDSR can be beneficial in sparse networks. Higher
transmission range of directional antennas can communicate
across “voids” in the topology.
Throughput and Beamwidth
For randomly located nodes
Routing Overhead
 Using omni broadcast, nodes receive
multiple copies of same packet – Redundant
 Broadcast Storm Problem
 Using directional Antennas – can do better ?
– Forward packets radially outward
Selective-Forward Optimization
Use K antenna elements to forward broadcast packet.
K = N/2 in simulations
Footprint
of Tx
Ctrl Overhead  =
 (No. Ctrl Tx)  (Footprint of Tx)
 No. Data Packets
Selective-Forward Optimization
Control overhead reduces
Beamwidth of antenna element (degrees)
Mobility
• Link lifetime increases using directional antennas.
– Higher transmission range - link failures are less
frequent
• Handoff: Nodes moving out of beam coverage in
order of packet-transmission-time
– Low probability
Mobility
• Antenna handoff
– If no response to RTS, MAC layer uses n adjacent antenna
elements to transmit same packet
– Route error avoided if communication re-established
[RoyChoudhury02UIUC Techrep]
Aggregate throughput
Aggregate Throughput (Kbps)
Over random mobile scenarios
DSR
DDSR4
DDSR6
DDSR9
DDSR18
2000
1500
1000
500
0
0
5
10
15
20
mobility (m/s)
25
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Observations
• Randomness in topology aids DDSR.
• Voids in network topology bridged by higher
transmission range (prevents partition)
• Higher transmission range increases link lifetime –
reduces frequency of link failure under mobility
• Antenna handoff due to nodes crossing antenna
elements – not too serious
Future work
• Directional route repair possible in DDSR
• Incorporate Anycasting in DDSR
• Reducing route alignment
– Choosing zig-zag routes increase spatial reuse
• Power control based on the knowledge of
neighborhood
Conclusion
• Directional antennas can be beneficial to routing
– Fewer hop-count
– Bridges network “voids” in sparse scenarios
– Higher link lifetime
• However tradeoffs exist
– Broadcast overhead higher
– Handoffs possible when node moves beam beams
– Deafness can cause sub-optimality
Conclusion
• Evaluation shows DDSR better than DSR when
– Sparse networks
– Large src-dest separation
– Moderately narrow beamwidth
Thank you
www.crhc.uiuc.edu/~nhv
Issues
 Broadcast storm: Using broadcasts, nodes receive
multiple copies of same packet
Optimize by using K out of N beams to
forward broadcast packets
Performance
• Results indicate that routing performance
can be improved using directional antennas
Issues: Sub-optimal Routes
 Due to sweeping, shortest path RREQ may reach
destination late
 Sub-optimal routes may be chosen if destination node
misses shortest request, while beamformed
D receives RREQ from J
D beamforms to send RREP
F
J
RREP
J
D misses RREQ from K
D
K
RREQ
N
Using Omni, D gets all RREQs
Performance
Throughput Vs Mobility
Aggregate Throughput (Kbps)
Control overhead
DSR
DDSR4
DDSR6
DDSR9
DDSR18
2000
1500
1000
500
0
0
5
10
15
20
25
mobility (m/s)
• Control overhead higher using DDSR
• Throughput of DDSR higher, even under mobility
• Latency in packet delivery lower using DDSR
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