Performance analysis of 802.11 DCF in presence of hidden nodes and collision prevention mechanism.

- Ruchir Bhanushali. - Sagar. Shah.

Outline (What ?)

• RTS/CTS vs. Basic access mechanism using OPNET Modeler:Case 1:Netwok nodes are hidden (0 – 5).Case 2: Additions of hidden nodes ( 0 – 4).

• Performance parameters:– Global Statistics: Throughput, MAC delay,

Retransmission Attempts.– Node Statistics: Control traffic sent/received.

Assumptions (What ? contd.)

• Physical Characteristics: – 802.11b @ 11Mbps.– Channel 1: center Frequency 2.412Ghz ; BW: 22Mhz.

• PT: 5mW, Receiver Sensitivity: -95dbm, Coverage: 1200m.– Path loss: Lp = L0 + 10α log10d ; L0: 40.04dB for 802.11b.

Assumptions (What ? contd.)

• Data & Control traffic:– Generation: • Throughout the simulation with an inter-arrival time of

0.05sec.• Min Outcome :256 bytes & Max. Outcome: 2000bytes.

– RTS threshold : 512 Bytes.

• Simulation time: 20 minutes.

Typical node settings

Case 1

Network Nodes become hidden.

Case 1: Assumptions

• Configuration: Adhoc, Star Topology.• Number of periphery nodes: 16.• Destination for every periphery node: Center node.

Configuration

Hidden node creation (HOW ?)

Scenarios (How ?)Scenario No. Basic

mechanismRTS/CTS

mechanismMaximum

Distance (R) m.No. of hidden

nodes

1 500 0

2 500 0

3 610 1

4 610 1

5 650 3

6 650 3

7 700 5

8 700 5

9 800 7

10 800 7

11 900 9

12 900 9

Observations:Traffic Sent(bits/sec)

Throughput(bits/sec)• Basic RTS/CTS

Retransmission Attempts(packets)

MAC delay (sec)

Data Dropped(bits/sec)

Control traffic Sent by Central Node (bits/sec)

Throughput Vs. No. of hidden nodes

Case 1: Conclusions (So What ?)

• Robustness of RTS/CTS access mechanism. • Weakness of basic access mechanism.• Overhead of RTS/CTS frames degrades the

performance.

Case 2

• Introduction of hidden nodes.

Assumptions

• Existing WLAN: Basic Service Set.– Star topology: periphery nodes – 16.

• Random destinations for Center node.• Periphery nodes do not transmit.

Assumptions (contd.)

• Data & Control traffic:– Generation: • Uniform distribution: Min Outcome :256 bytes & Max.

Outcome: 2000bytes.

– RTS threshold : 512 Bytes.

• Simulation time: 20 minutes.

Scenario 2

Scenario 3 & 4

Scenario 5

Configuration

Hidden node Configuration

Graphs

• Throughput: – Basic Mechanism:

Graphs

• Throughput: – RTS/CTS

Mechanism:

Retransmission Attempts (packets)

End to End Delay (sec)

Data dropped (bits/sec)

Conclusion

• Whether its basic or RTS/CTS mechanism, hidden node effect is more prominent when network nodes are hidden.

• Overall performance of the basic access method strongly depends on the number of stations in the WLAN and gets degraded with increasing number of nodes in both the cases.

• On the other hand, the RTS/CTS access method is very robust to hidden station effect in a WLAN environment.

• Accounting the capability of the RTS/CTS scheme to cope with hidden terminals, we conclude that this access method should be used in the majority of the practical cases.

References• Performance Modeling and Analysis of the IEEE 802.11

Distribution Coordination Function in Presence of Hidden Stations; Fu-Yi Hung; Pai, S.; Marsic, I.;Oct. 2006.

• Analyzing the Throughput of IEEE 802.11 DCF Scheme with Hidden Nodes; Ting-Chao Hou, Ling-Fan Tsao, and Hsin-Chiao Liu

• Performance analysis of the IEEE 802.11 distributed coordination function; Bianchi, G.; Volume 18, Issue 3, March 2000 .

• Performance evaluation of distributed co-ordination function for IEEE 802.11 wireless LAN protocol in presence of mobile and hidden terminals; Khurana, S.; Kahol, A.; Gupta, S.K.S.; Srimani, P.K.;24-28 Oct. 1999.

References (contd.)

• Evaluation Analysis of the Performance of IEEE 802.11b and IEEE 802.11g Standards; Athanasopoulos, A.; Topalis, E.; Antonopoulos, C.; Koubias, S.;23-29 April 2006.

• Wireless Information networks; Kaveh Pahlavan, Allen h. Levesque; Wiley publication; second edition.

• IEEE Std 802.11, 1999 edition.• OPNET Modeler v 12.0 model documentation.