1. Planning and Analyzing Wireless LAN
Hidden Node Scenario and RTS/CTS Solution
Lab 10

2. WLAN Support in Opnet Based on IEEE 802.11 and IEEE 802.11b standards
Modeled data rates
1.0 Mbps
2.0 Mbps
5.5 Mbps
11.0 Mbps
Supported physical layers
Direct-sequence spread-spectrum (DSSS)
Frequency Hopping spread-spectrum (FHSS)
Infrared light (IR)
DCF MAC operation: Contention based (CSMA/CA)
PCF MAC operation: Poll based

3. Distributed Coordinated Function (DCF)
Sense the medium
If the medium is busy, defer
When the medium becomes idle again, transmit after a random backoff

4. Point Coordination Function PCF
Requires centralized coordination
Introduces contention free period (CFP)
Use for “near” real-time services
Forces a “fair” access to the medium during the CFP

5. Wireless LAN Topologies
Basic building block: Basic Service Set (BSS)
Independent BSS
Infrastructure BSS

6. Infrastructure Extended Service Set (ESS)
BSS 1
BSS 2
BSS 3
Internet

7. Opnet WLAN Node Models Wireless LAN Station (Non-IP based)
Wireless LAN Workstation
Wireless LAN Server
Bridge with WLAN Port (Access Point)
Router with WLAN interface (Access Point*)
* Unless the interface belongs to a WLAN backbone

8. WLAN Model Attributes RTS Threshold (bytes)
Set the packet size threshold for which the ready to send (RTS)/clear to send (CTS) WLAN mechanism will be used
Solution to hidden terminal problem
Prevent large packets to be dropped
Overhead due to the RTS/CTS frame exchange
Short Retry Limit
Maximum transmission attempts for data frames with a size shorter than or equal to RTS Threshold
High values for retry limit will produce a more reliable transmissions but will create overhead
Long Retry Limit
Maximum transmission attempts for data frames with a size greater than RTS Threshold
Set a lower value than Short Retry Limit will help to decrease the amount of buffer required

9. Hidden Node Problem Hidden terminals A and C cannot hear each other.
A sends to B, C cannot receive A.
C wants to send to B, C senses a “free” medium (CS fails)
Collision occurs at B.
A cannot receive the collision (CD fails).
A is “hidden” for C.
Solution?
Hidden terminal is peculiar to wireless (not found in wired)
Need to sense carrier at receiver, not sender!
“virtual carrier sensing”: Sender “asks” receiver whether it can hear something. If so, behave as if channel busy. A B C

10. Lab Objective
Set up independent BSS networks and evaluate their performance under different traffic and configurations.

11. Lab Overview
In this lab you will set up a Wireless LAN to study the impact of different datarates on throughput and delay.
Also analyze the use of RTS and CTS as part of IEEE protocol to solve Hidden Node problem

12. Project and Scenario Create new project Create Scenario “WLAN”
Office, 100m x 100m range
Select wireless_lan node model
Drag and Drop
Application Config
Profile Config
1 Wlan_wkstn_adv(fix)
1 Wlan_wkstn_adv(mob)

13. Application Configuration
Edit attributes of Application Config
Add application
Name: vdo_app
Description: Video conferencing low resolution
Edit attribute of Profile Config
Add profile
Name: vdo_pro
Application: vdo_app
Start time offset (sec): No Offset
Start Time: Constant(0)
Operation Mode: Simultaneous

14. WLAN Nodes attributes WLAN Fixed node Set name wlan_fixed
X_position:10
Y_position:50
Application Supported Services: vdo_app
IP Host parameters:
Interface Information: Address= , Subnet=Class C
Static Routing Table: Destination Address= , Subnet= , Next Hop=

15. WLAN Mobile node Set name wlan_mob X_position:40 Y_position:50
Trajectory: none (to make it stationary)
Application: supported profile= vdo_pro
IP Host parameters:
Interface Information: Address= , Subnet=Class C
Static Routing Table: Destination Address= , Subnet= , Next Hop=

16. WLAN Parameter
Expand WLAN in Edit attributes of Mobile_node and Fixed_node
Set Physical Characteristics: Direct Sequence
Data rate: 11Mbps
Packet Reception Power Th: 7.33 E -11 (Tr Range= 35m)
Save Project

17. Statistics Collect Individual Statistics: WLAN Global Statistics
Delay(sec)
Throughput(bits/sec)
Data Dropped(Buffer Overflow)
Global Statistics
Delay(Sec)
Retransmission Attempt(pkt)
Load(bits/sec)
Run Simulation for 5 min

18. Duplicate Scenario:Scenario2
Duplicate Scenario: Basic_Datarate
Edit WLAN parameters of both nodes
Change datarate to 2Mbps
Run and collect statistics
What Difference have you observed in delay and Throughput?
Check data drop rate due to buffer overflow. Explain the graph

19. Duplicate Scenario: Scenario3
Add another mobile nodes wlan_wkstn_adv(mob)
Edit Attributes
X_position:10
Y_position:80
Trajectory: none (to make it stationary)
Application: supported profile= vdo_pro
IP Host parameters:
Interface Information: Address= , Subnet=Class C
Static Routing Table: Destination Address= , Subnet= , Next Hop=
WLAN Parameter
Set Physical Characteristics: Direct Sequence
Data rate: 11Mbps
Packet Reception Power Th: 7.33 E -11 (Tr Range= 36m)

20. Duplicate Scenario3 Duplicate Scenario 3
Set WLAN Datarate=2Mbps
Compare statistics of all scenarios
Observe and Explain the difference of Throughput, Delay, and Load for all four scenarios.

21. Lab Task
Duplicate Scenario 1, add another mobile node to a distance such that the network represents Hidden Node problem (as explained in lab) i.e the difference between there x-position is equal to 36m, if y-position is fixed
IP Host parameters of new Mobile node:
Interface Information: Address= , Subnet=Class C
Static Routing Table: Destination Address= , Subnet= , Next Hop=
Edit Application Config:
Select Print Application, Description: Print Inter-arrival time= Constant(0.001), File Size=Constant(1024)
Run and Record WLAN throughput, Data Dropped, Load and Media access delay for all stations
Duplicate scenario and Enable RTS Threshold from WLAN parameters of all nodes. Set RTS Threshold=256
Observe the difference in Global attributes: Data Dropped, Throughput, Load and Delay
Explain Hidden Node Problem and the effect caused by enabling RTS on network performance.