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Medium Access Control Protocols Using Directional
Antennas in Ad Hoc Networks
CIS 888Prof. Anish Arora
The Ohio State University
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A paper by Young-Bae Ko,
Vinaychandra Shankarkumar and Nitin.H.Vaidya
Department of Computer ScienceTexas A&M university
http://www.cs.tamu.edu/faculty/vaidya/papers/mobile-computing/infocom00.ps
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Background and Motivation
• Wireless mobile networks
• Omnidirectional antennas• Wastage of network capacity
• Directional antennas• Improve network capacity
• Improve routing performance
• Physical size limitations
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Related work:
• Packet Radio Networks• Slotted ALOHA using RTDMA
• SDMA for performance improvement
• Mobile Broadband Systems• Dynamic slot assignment protocol
RTDMA – Random Time Division Multiple Access
SDMA – Space Division Multiple Access (simultaneous multiple receptions)
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Network Model:
• Shared wireless channel
• Multiple Directional antennas
• Interference assumption• Hidden terminal problem
• Fixed transmission range
• Unidirectional transmission
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IEEE 802.11 MAC Protocol:
• RTS/CTS mechanism• Sender Broadcasts RTS packet
• Intended Receiver replies with CTS packet
• Sender transmits data packet
• Receiver sends an ACK
RTS – Request To Send
CTS – Clear To Send
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RTS/CTS Mechanism in 802.11
A B C D E
RTS RTS
CTS CTS
DATA DATA
ACK ACK
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RTS/CTS (Contd…)
• RTS and CTS contain proposed duration of
data transmission
• All in-range nodes MUST wait for this duration before transmitting
• Adv – Elimination of Hidden terminals
• Disadv – Wastage of network capacity
(D cannot send anything to E)
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D-MAC Schemes (Overview)
• Similar to IEEE 802.11 – only on a per antenna basis.
• 802.11• if node N is aware of an on-going
transmission, N cannot send or receive itself
• D-MAC• if antenna A at node N is aware, N cannot send
or receive using antenna T.
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Contd…
• Even if one antenna is blocked, the node may transmit using unblocked antennas.
• Leads to performance enhancement
• Can be used omnidirectionally as well
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Scheme 1 – Using DRTS packets
• Directional RTS packets
• Omnidirectional CTS packets
• DRTS – sender’s location
• OCTS – sender’s and receiver’s locations
• Data packet and ACK sent using Directional antennas
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Scheme 1 (contd…)
• Other nodes CAN transmit AA B B C C xxD D E E
A A BB C C D D xxEE• All DRTS may not get an OCTS reply
(D & E in the above scenarios cannot send OCTS if anyone sends
them a DRTS because one of their antennas is blocked) • Control packets may collide
A B- -> C D E
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Scheme 2 – Using DRTS/ORTS
• Send either DRTS or ORTS based on rule • “if all D-antennas are unblocked, send ORTS
but if any D-antenna is blocked, send DRTS”
A B C D E
• C sends ORTS but B can only send DRTS
• Reduces, not eliminates control packet collisions
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Performance evaluation
• Modified ns-2 simulator to include D-antennas (90°) and location information
• Simulation model• 5 x 5 mesh – 200m apart
• 250m range for each node
• 2Mbps wireless link bandwidth
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5
4
10
1
3
2
9
8
7
15
14
2520
13
12
11
19
18
16
17 22
23
24
6 21
Network Topology
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Case 1:
Connections IEEE 802.11 Scheme 1 No.1(6-11) 1130.42 771.27
No.2(16-21) 214.57 1040.21
Total Throughput
1344.99 1811.48
Use of DRTS allows simultaneous transmissions hence throughput for Scheme 1 is better than 802.11
Fairness is also much better in Scheme 1
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Case 2:
Connections IEEE 802.11 Scheme 1 No.3(6-1) 653.64 1250.14
No.4(11-16) 634.58 1251.64
Total Throughput
1288.22 2501.79
Directions of data transfers differ, hence fewer collisions
Best case scenario for D-antennas
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Case 3:
Connections IEEE 802.11 Scheme 1 Scheme 2 No.1(1-21) 179.66 207.41 210.20
No.2(1-5) 179.46 209.53 216.53
Total Throughput
359.12 416.94 426.73
Scheme 2 performs better than Scheme 1 because probability of control packet collisions decreases.
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Case 4:Connections IEEE 802.11 Scheme 1 Scheme 2
No.7(1-21) 157.50 146.73 165.89
No.8(2-22) 89.90 85.31 81.30
No.9(3-23) 22.00 91.39 105.03
No.10(4-24) 89.29 82.30 82.83
No.11(5-25) 157.94 153.30 163.37
Total Throughput
516.63 559.03 598.42
Border connections have much higher throughput
Percentage performance enhancement of DMAC not so high because of increase in number of connections
Reasonable fairness (esp. to conn. 9) in DMAC schemes
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Case 5:
Connections IEEE 802.11 Scheme 1 Scheme 2 No.12(1-21) 76.38 112.57 87.00
No.13(2-22) 23.93 40.26 25.27
No.14(3-23) 7.08 36.03 23.66
No.15(4-24) 36.91 32.80 37.50
No.16(5-25) 128.75 98.10 120.23
No.17(1-5) 74.67 117.08 85.96
No.18(6-10) 21.60 42.17 28.98
No.19(11-15) 6.80 40.46 26.73
No.20(16-20) 36.48 36.87 35.76
No.21(21-25) 125.36 101.27 122.11
Total Throughput
537.96 657.61 593.20
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Case 5 (contd…)
• DMAC 1 outperforms DMAC 2• The use of ORTS in scheme 2 reduces the
possibility of simultaneous transmissions by neighbouring nodes
• Trade-off between probability of collisions and loss of simultaneous transmissions
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Case 5(contd…)
Scheme 1 Scheme 2
E
C
A
F
D
B
E
C
A
F
D
BDRTS
ORTS
ORTS
DRTS
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Optimization: Using DWTS
• DWTS – Directional Wait-To-Send
• Aim: to avoid DRTS retransmissionsA B C D E
DRTS(B)
OCTS(B,C)
OCTS(B,C)
DATA
ACK
DRTS(E)
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DWTS (contd…)
• DWTS – short control packet, includes time-to-wait before RTS retransmission
A B C D E
DRTS(B)
OCTS(B,C)
OCTS(B,C)
DATA
ACK
DRTS(E)
DRTS(E)
DWTS (D)
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Conflict-Free ACK
• 802.11 – Immediate ACK for reliability• Minimal ACK collisions due to reserved
transmission range• D-MAC – No guarantee on ACK collisions• Possible solutions –
• Use separate channels for DATA/ACK & RTS/CTS
• Use RTS/CTS for ACK packets
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Location Information
• DRTS – accurate node locations required
• Hard to achieve in mobile nodes
• Solution:• If location unknown - send ORTS (no loss of
correctness)
• If location known – send DRTS
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(Contd…)
• Stale location data:• Include location information in RTS/CTS
• Set a threshold for DRTS transmissions
• If no response, switch to ORTS
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Conclusions:
• Current MAC protocols – wasted bandwidth
• D-MAC – utilizes directional transmissions
• Scheme 1 – DRTS/OCTS
• Scheme 2 – DRTS, ORTS / OCTS
• Optimization using DWTS
• D-MAC outperforms 802.11 by allowing simultaneous transmissions