Medium Access Control Protocols Using Directional

Antennas in Ad Hoc Networks

CIS 888Prof. Anish Arora

The Ohio State University

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

Background and Motivation

• Wireless mobile networks

• Omnidirectional antennas• Wastage of network capacity

• Directional antennas• Improve network capacity

• Improve routing performance

• Physical size limitations

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)

Network Model:

• Shared wireless channel

• Multiple Directional antennas

• Interference assumption• Hidden terminal problem

• Fixed transmission range

• Unidirectional transmission

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

RTS/CTS Mechanism in 802.11

A B C D E

RTS RTS

CTS CTS

DATA DATA

ACK ACK

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)

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.

Contd…

• Even if one antenna is blocked, the node may transmit using unblocked antennas.

• Leads to performance enhancement

• Can be used omnidirectionally as well

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

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

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

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

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

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

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

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.

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

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

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

Case 5(contd…)

Scheme 1 Scheme 2

E

C

A

F

D

B

E

C

A

F

D

BDRTS

ORTS

ORTS

DRTS

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)

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)

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

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

(Contd…)

• Stale location data:• Include location information in RTS/CTS

• Set a threshold for DRTS transmissions

• If no response, switch to ORTS

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