Transcript Document

Part 3
MAC and Routing with Directional
Antennas
Nitin H. Vaidya
University of Illinois at Urbana-Champaign
[email protected]
© 2003 Nitin Vaidya
Impact of Antennas on MAC
• Wireless hosts traditionally use
single-mode antennas
• Typically, the
single-mode = omni-directional
• Our interest here in antennas with multiple
(directional) modes
IEEE 802.11
RTS = Request-to-Send
RTS
A
B
C
D
E
F
Pretending a circular range
IEEE 802.11
RTS = Request-to-Send
RTS
A
B
C
D
E
F
NAV = 10
NAV = remaining duration to keep quiet
IEEE 802.11
CTS = Clear-to-Send
CTS
A
B
C
D
E
F
IEEE 802.11
CTS = Clear-to-Send
CTS
A
B
C
D
E
NAV = 8
F
IEEE 802.11
•DATA packet follows CTS. Successful data reception
acknowledged using ACK.
DATA
A
B
C
D
E
F
IEEE 802.11
ACK
A
B
C
D
E
F
IEEE 802.11
Reserved area
ACK
A
B
C
D
E
F
Omni-Directional Antennas
Red nodes
Cannot
Communicate
presently
X
D
C
Y
Directional Antennas
Not possible
using Omni
X
D
C
Y
Question
• How to exploit directional antennas in
ad hoc networks ?
– Medium access control
– Routing
MAC Protocols
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:
• Capable of beamforming in specified direction
• Directional Gain Gd (Gd > Go)
Symmetry: Transmit gain = Receive gain
Antenna Model
• More recent work models sidelobes
approximately
Directional Communication
Received Power

(Transmit power) *(Tx Gain) * (Rx Gain)
Directional gain is higher
Potential Benefits of
Directional Antennas
• Increase “range”, keeping transmit power
constant
• Reduce transmit power, keeping range
comparable with omni mode
– Reduces interference, potentially increasing
spatial reuse
Neighbors
• Notion of a “neighbor” needs to be
reconsidered
– Similarly, the notion of a “broadcast” must also be
reconsidered
Directional Neighborhood
Receive Beam
B
Transmit Beam
A
C
• When C transmits directionally
•Node A sufficiently close to receive in omni mode
•Node C and A are Directional-Omni (DO) neighbors
•Nodes C and B are not DO neighbors
Directional Neighborhood
Transmit Beam
Receive Beam
B
A
C
•When C transmits directionally
• Node B receives packets from C only in directional mode
•C and B are Directional-Directional (DD) neighbors
Potential Benefits of
Directional Antennas
• Increase “range”, keeping transmit power
constant
• Reduce transmit power, keeping range
comparable with omni mode
– Several proposal focus on this benefit
– Assume that range of omni-directional and
directional transmission is equal
Directional transmissions at lower power
Caveats
• Only most important features of the protocols
discussed here
• Antenna characteristics assumed are often
different in different papers
Simple Tone Sense (STS) Protocol
[Yum1992IEEE Trans. Comm.]
STS Protocol
Based on busy tone signaling:
• Each host is assigned a tone (sinusoidal wave at a
certain frequency)
• Tone frequency unique in each host’s neighborhood
• When a host detects a packet destined to itself, it
transmit a tone
• If a host receive a tone on directional antenna A,it
assumes that some host in that direction is receiving
a packet
– Cannot transmit using antenna A presently
– OK to transmit using other antennas
STS Protocol
• Tone duration used to encode information
– Duration t1 implies transmitting node is busy
– Duration t2 implies the transmitting node
successfully received a transmission from another
node
Example
Node A cannot
Initiate a
transmission.
A
Tone t1
But B can send
to C
S
DATA
R
Because B does
not receive t1
B
C
STS Protocol
Issues:
• Assigning tones to hosts
• Assigning hosts to antennas: It is assumed
that the directions/angles can be chosen
– distribute neighbor hosts evenly among the
antennas
– choose antenna angles such that adjacent
antennas have some minimum separation
D-MAC Protocol
[Ko2000Infocom]
IEEE 802.11
F
A
B
C
RTS
RTS
CTS
DATA
D
CTS
DATA
ACK
ACK
Reserved area
E
Directional MAC (D-MAC)
• Directional antenna can limit transmission to
a smaller region (e.g., 90 degrees).
• Basic philosophy: MAC protocol similar to
IEEE 802.11, but on a per-antenna basis
D-MAC
• IEEE802.11: Node X is blocked if node X has
received an RTS or CTS for on-going transfer
between two other nodes
• D-MAC: Antenna T at node X is blocked if antenna T
received an RTS or CTS for an on-going
transmission
• Transfer allowed using unblocked antennas
• If multiple transmissions are received on different
antennas, they are assumed to interfere
D-MAC Protocols
• Based on location information of the receiver,
sender selects an appropriate directional
antenna
• Several variations are possible
D-MAC Scheme 1
• Uses directional antenna for sending RTS,
DATA and ACK in a particular direction,
whereas CTS sent omni-directionally
• Directional RTS (DRTS) and
Omni-directional CTS (OCTS)
D-MAC Scheme 1: DRTS/OCTS
A
B
C
E
D
DRTS(B)
DRTS(B) - Directional RTS including
location information of node B
OCTS(B,C)
OCTS(B,C)
DRTS(D)
OCTS(D,E)
DATA
OCTS(B,C) – Omni-directional CTS
including location information
of nodes B and C
DATA
ACK
ACK
Drawback of Scheme 1
• Collision-free ACK transmission not
guaranteed
A
?
B
C
D
DRTS(B)
OCTS(B,C)
DRTS(A)
DATA
DRTS(A)
ACK
OCTS(B,C)
D-MAC Scheme 2
• Scheme 2 is similar to Scheme 1, except for
using two types of RTS
• Directional RTS (DRTS) / Omni-directional
RTS (ORTS) both used
– If none of the sender’s directional antennas are
blocked, send ORTS
– Otherwise, send DRTS when the desired antenna
is not blocked
D-MAC Scheme 2
• Probability of ACK collision lower than
scheme 1
• Possibilities for simultaneous transmission by
neighboring nodes reduced compared to
scheme 1
Variations
• Paper discusses further
variations on the theme
– Reducing ACK collisions
– Reducing wasteful transmission of RTS to busy
nodes
Performance Comparison
• Which scheme will perform better depends on
– location of various hosts
– traffic patterns
– antenna characteristics
Performance Evaluation
•
•
•
•
Mesh topology
No mobility
Bulk TCP traffic
2 Mbps channel
5
10
15
20
25
4
9
14
19
24
3
8
13
18
23
2
7
12
17
22
1
6
11
16
21
Performance Measurement
• Reference throughput of single TCP
connection using IEEE 802.11
–
–
–
–
1 hop (1383 Kbps)
2 hops (687 Kbps)
3 hops (412 Kbps)
4 hops (274 Kbps)
Performance Measurement
• Scenario 1
Connections
IEEE802.11 Scheme1 Scheme2
No.1
1130.42
No.2
214.57
1040.21 1303.64
1344.99
1811.48 1354.67
Total Throughput
771.27
5
10
15
20
25
4
9
14
19
24
3
8
13
18
23
2
7
12
17
22
1
6
11
16
21
51.03
1
2
Performance Measurement
• Scenario 2: Best case for scheme 1
Connections
IEEE802.11 Scheme1 Scheme2
No.3
653.64
1250.14
884.82
No.4
634.58
1251.64
867.69
Total Throughput
1288.22
5
10
15
20
25
4
9
14
19
24
3
8
13
18
23
2
7
12
17
22
1
6
11
16
21
2501.78 1752.51
3
4
Performance Measurement
IEEE802.11 Scheme1 Scheme2
No.5
179.66
207.41
210.20
No.6
179.46
209.53
216.53
Total Throughput
359.12
10
15
20
25
4
9
14
19
24
3
8
13
18
23
2
7
12
17
22
1
6
11
16
21
6
• Scenario 3
Connections
5
416.94
426.73
5
Performance Measurement
• Scenario 4
11 5
10
15
20
25
10 4
9
14
19
24
9
3
8
13
18
23
8
2
7
12
17
22
7
1
6
11
16
21
Connections IEEE802.11 Scheme1 Scheme2
No.7
No.8
No.9
No.10
No.11
Total
157.50
89.90
22.00
89.29
157.94
516.63
146.73
85.31
91.39
82.30
153.30
559.03
165.89
81.30
105.03
82.83
163.37
598.42
Limitations of D-MAC
• No guarantee of collision-free ACK
– Some improvements suggested in paper
• Inaccurate/outdated location information can
degrade performance
Conclusion
• Benefit: Can allow more simultaneous
transmissions by improving spatial reuse
• Disadvantage: Can increase Ack collisions
• Alternatives for determining location
information should be considered
• Location information does not always
correlate well with direction
Busy Tone Directional MAC
[Huang2002MILCOM]
• Extends the busy tone (DBTMA) protocol
originally proposed by omni-directional
antennas [Deng98ICUPC]
• Three channels
– Data channel
– Two Busy Tone channels
• Receive tone (BTr)
• Transmit tone (BTt)
DBTMA
• Sender:
– Sense BTr. If sensed busy, defer transmission.
– If BTr idle, transmit RTS to receiver
• Receiver
– On receiving RTS, sense BTt.
– If BTt idle, reply with a CTS, and transmit BTr until DATA is
completely received
• Sender
– On receiving CTS, transmit DATA and BTt both
DBTMA + Directional Antennas
• DBTMA reduces reduction in throughput
caused by collisions by hidden terminals
• Directional antennas can be used to transmit
the busy tones directionally
– RTS/CTS, DATA, busy tones all may be sent
directionally
– Trade-offs similar to directional versus omnidirectional transmission of RTS/CTS
Another Directional MAC protocol
[Roychoudhury02mobicom]
• Derived from IEEE 802.11 (similar to
[Takai02mobihoc])
• A node listens omni-directionally when idle
• Sender transmits Directional-RTS (DRTS) towards
receiver
• RTS received in Omni mode (idle receiver in when
idle)
• Receiver sends Directional-CTS (DCTS)
• DATA, ACK transmitted and received directionally
Directional MAC
RTS = Request-to-Send
X
RTS
A
B
C
D
E
F
Pretending a circular range for omni
Directional MAC
CTS = Clear-to-Send
X
CTS
A
B
C
D
E
F
Directional MAC
•DATA packet follows CTS. Successful data reception
acknowledged using ACK.
X
DATA
A
B
C
D
E
F
Directional MAC
X
ACK
A
B
C
D
E
F
Directional NAV (DNAV)
• Nodes overhearing RTS or CTS set up directional NAV
(DNAV) for that Direction of Arrival (DoA)
D
CTS
C
X
Y
Directional NAV (DNAV)
• Nodes overhearing RTS or CTS set up directional NAV
(DNAV) for that Direction of Arrival (DoA)
D
C
X
DNAV
Y
Directional NAV (DNAV)
• New transmission initiated only if direction of
transmission does not overlap with DNAV,
i.e., if (θ > 0)
B
D
A
DNAV
θ
RTS
C
DMAC Example
C
E
D
B
B and C communicate
D and E cannot: D blocked with DNAV from C
D and A communicate
A
Issues with DMAC
• Two types of Hidden Terminal Problems
– Due to asymmetry in gain
Data
RTS
A
B
C
A is unaware of communication between B and C
A’s RTS may interfere with C’s reception of DATA
Issues with DMAC
• Two types of Hidden Terminal Problems
– Due to unheard RTS/CTS
D
B
A
C
• Node A beamformed in direction of D
• Node A does not hear RTS/CTS from B & C
Issues with DMAC
• Two types of Hidden Terminal Problems
– Due to unheard RTS/CTS
D
B
A
C
Node A may now interfere at node C by transmitting
in C’s direction
Issues with DMAC
• Deafness
Z
RTS
A
B
DATA
RTS
Y
RTS
X
X does not know node A is busy.
X keeps transmitting RTSs to node A
Using omni antennas, X would be aware that A is
busy, and defer its own transmission
Issues with DMAC
• Uses DO links, but not DD links
DMAC Tradeoffs
• Benefits
• Disadvantages
– Better Network
Connectivity
– Hidden terminals
– Spatial Reuse
– Deafness
– No DD Links
Using Training Sequences
[Bellofiore2002IEEETrans.Ant.Prop]
• Training packets used for DoA determination,
after RTS/CTS exchange omni-directionally
Sender
Receiver
RTS
RXTRN
CTS
DATA
TXTRN
ACK
• Performance depends on the TXTRN and
RXTRN delays
• If direction is known a priori, then these
delays can potentially be avoided
– But mobility can change direction over time
Another Variation
[Nasipuri2000WCNC]
• Similar to 802.11, but adapted for directional
antennas
• Assumptions:
– Antenna model: Several directional antennas which can all
be used simultaneously
– Omni-directional reception is possible (by using all
directional antennas together)
– Direction of arrival (DoA) can be determined when receiving
omni-directionally
– Range of directional and omni transmissions are identical
Protocol Description
• Sender sends omni-directional RTS
• Receiver sends omni-directional CTS
– Receiver also records direction of sender by determining the
antenna on which the RTS signal was received with highest
power level
– Similarly, the sender, on receiving CTS, records the direction
of the receiver
• All nodes overhearing RTS/CTS defer
transmissions
• Sender then sends DATA directionally to the
receiver
• Receiver sends directional ACK
Discussion
• Protocol takes advantage of reduction in
interference due to directional
transmission/reception of DATA
• All neighbors of sender/receiver defer
transmission on receiving omni-directional
RTS/CTS
 spatial reuse benefit not realized
Enhancing DMAC
• Are improvements possible to make DMAC
more effective ?
• Possible improvements:
– Make Use of DD Links
– Overcome deafness [Roychoudhury03 – UIUC Tech
report under preparation]
Using DD Links
Exploit larger range of Directional antennas
Receive Beam
A
Transmit Beam
C
A and C are DD neighbors, but cannot communicate
using DMAC
Multi Hop RTS (MMAC) – Basic Idea
D
C
A
B
DO neighbors
E
DD neighbors
F
G
A source-routes RTS to D through adjacent
DO neighbors (i.e., A-B-C-D)
When D receives RTS, it beamforms towards
A, forming a DD link
Impact of Topology
D
A
A
E
B
B
F
C
C
Aggregate throughput
802.11 – 1.19 Mbps
DMAC – 2.7 Mbps
Nodes arranged in
“linear” configuration
reduce spatial reuse
Aggregate throughput
802.11 – 1.19 Mbps
DMAC – 1.42 Mbps
Power control may
improve performance
Aggregate Throughput (Kbps)
Aligned Routes in Grid
1200
802.11
DMAC
MMAC
1000
800
600
400
200
0
0
500
1000
1500
Sending Rate (Kbps)
2000
2500
Aggregate Throughput (Kbps)
Unaligned Routes in Grid
1200
1000
802.11
DMAC
MMAC
800
600
400
200
0
0
500
1000
1500
Sending Rate (Kbps)
2000
2500
“Random” Topology
Aggregate Throughput
1200
1000
802.11
DMAC
MMAC
800
600
400
200
0
0
500
1000
1500
Sending Rate (Kbps)
2000
2500
Avg. End to End Delay (s)
“Random” Topology: delay
2
1.5
1
DMAC
MMAC
0.5
0
0
500
1000
1500
Sending Rate (Kbps)
2000
2500
MMAC - Concerns
• Lower probability of RTS delivery
• Multi-hop RTS may not reach DD neighbor due to
deafness or collision
• Neighbor discovery overheads may offset the
advantages of MMAC
TDMA with Directional Antennas
[Bao2002MobiCom]
• Each node uses multiple beams, and can
participate in multiple transmissions
simultaneously
• Link activation schedule determined for each
slot, by a priori coordination among the nodes
• Protocol needs neighborhood information
(obtained using periodic broadcasts on a
common control channel)
Directional MAC: Summary
• Directional MAC protocols show improvement
in aggregate throughput and delay
– But not always
• Performance dependent on topology
Routing
Routing Protocols
• Many routing protocols for ad hoc networks
rely on broadcast messages
– For instance, flood of route requests (RREQ)
• Using omni antennas for broadcast will not
discover DD links
• Need to implement broadcast using
directional transmissions
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 Directional Antennas
[Roychoudhury03PWC,
Roychoudhury02UIUC Techrep]
• RREQ broadcast by sweeping
– To use DD links
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
Directional Routing
Broadcast by sweeping
Tradeoffs
Larger Tx Range
Few Hop Routes
Fewer Hop Routes
Low Data Latency
Small Beamwidth
More Sweeping
High Sweep Delay
High Overhead
Issues
• Sub-optimal routes may be chosen if destination node
misses shortest request, while beamformed
F
D misses request from K
J
RREP
J
D
K RREQ
N
Optimize by having
destination wait before
replying
• Broadcast storm: Using broadcasts, nodes receive
multiple copies of same packet
Use K antenna elements to
forward broadcast packet
Performance
• Preliminary results indicate that routing
performance can be improved using
directional antennas
Route discovery latency … Single
flow, grid topology (200 m distance)
DDSR4
DDSR6
DSR
Observations
• Advantage of higher transmit range
significant only at higher distance of
separation.
• 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
Throughput
DDSR18
DDSR9
DSR
Sub-optimal routes chosen by DSR because destination
node misses the shortest RREQ, while beamformed.
Route Discovery in DSR
F
J
RREP
J
D
K
RREQ
D receives RREQ from J, and replies with RREP
D misses RREQ from K
N
Delayed RREP Optimization
• Due to sweeping – earliest RREQ need not
have traversed shortest hop path.
– RREQ packets sent to different neighbors at
different points of time
• If destination replies to first arriving RREP, it
might miss shorter-path RREQ
• Optimize by having DSR destination wait
before replying with RREP
Routing Overhead
• Using omni broadcast, nodes receive multiple
copies of same packet - Redundant !!!
• Broadcast Storm Problem
• Using directional Antennas – can do better ?
Routing Overhead
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
Routing Overhead
Control overhead reduces
Beamwidth of antenna element (degrees)
Mobility
• Link lifetime increases using directional
antennas.
– Higher transmission range - link failures are less
frequent
• 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 (Kbps)
Aggregate throughput over random
mobile scenarios
DSR
DDSR4
DDSR6
DDSR9
DDSR18
2000
1500
1000
500
0
0
5
10
15
20
mobility (m/s)
25
30
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
30
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
Other Approaches to Routing
with Directional Antennas
[Nasipuri2000ICCCN]
• Modified version of DSR
• Transmit Route Request in the last known
direction of the receiver
• If the source S perceives receiver R to have
been in direction d, then all nodes forward the
route request from S in direction d.
Example 1
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
I
D
N
Example 1
Y
Z
S
E
F
B
C
J
A
G
H
K
I
M
L
Route Reply
D
N
Example 2
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
I
D
N
D does
not
receive
RREQ
Limited Forwarding
• Benefit: Limits the forwarding of the Route
Request
• Disadvantage: Effectively assumes that each
node has a sense of orientation
Routing: Conclusion
• Directional antennas can improve routing
performance
• But suitable protocol adaptations necessary
Conclusion
• Directional antennas can potentially benefit
• But also create difficulties in MAC and routing
protocol design
End of Part 3
Slides to be made available at
http://www.crhc.uiuc.edu/~nhv
Chicken and Egg Problem !!
• DMAC/MMAC part of UDAAN project
– UDAAN performs 3 kinds of beam-forming for
neighbor discovery
– NBF, T-BF, TR-BF
– Send neighborhood information to K hops
– Using K hop-neighborhood information, probe using
each type of beam-form
– Multiple successful links may be established with the
same neighbor
Mobility
• Nodes moving out of beam coverage in order of
packet-transmission-time
– Low probability
• Antenna handoff required
–
–
–
–
MAC layer can cache active antenna beam
On disconnection, scan over adjacent beams
Cache updates possible using promiscuous mode
Evaluated in [RoyChoudhury02_TechReport]
Side Lobes
• Side lobes may affect performance
– Higher hidden terminal problems
B
A
C
Node B may interfere at A when A is receiving from C
Deafness in 802.11
• Deafness 2 hops away in 802.11
RTS
A
B
C
D
• C cannot reply to D’s RTS
– D assumes congestion, increases backoff
MMAC Hop Count
• Max MMAC hop count = 3
– Too many DO hops increases probability of failure of RTS
delivery
– Too many DO hops typically not necessary to establish DD link
C
B
A
D
DO neighbors
E
DD neighbors
F
G
Broadcast
• Several definitions of “broadcast”
– Broadcast region may be a sector, multiple sectors
Broadcast Region
A
– Omni broadcast may be performed through
sweeping antenna over all directions
[RoyChoudhury02_TechReport]
DoA Detection
• Signals received at each element combined with
different weights at the receiver
Why DO ?
• Antenna training required to beamform in
appropriate direction
– Training may take longer time than duration of pilot
signal [Balanis00_TechReport]
– We assume long training delay
• Also, quick DoA detection does not make
MMAC unnecessary
Queuing in MMAC
D
E
F
C
A
B
G
Impact of Topology
D
A
A
E
B
B
F
C
C
Aggregate throughput
802.11 – 1.19 Mbps
DMAC – 2.7 Mbps
Nodes arranged in
linear configurations
reduce spatial reuse
for D-antennas
Aggregate throughput
802.11 – 1.19 Mbps
DMAC – 1.42 Mbps
Organization
•
•
•
•
•
•
802.11 Basics
Related Work
Antenna Model
MAC
Routing
Conclusion