Planning and Analyzing
Wireless LAN
Hidden Node Scenario and RTS/CTS
Solution
Lab 10
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
Distributed Coordinated Function
(DCF)
Sense the
medium
If the medium is busy,
defer
When the medium
becomes idle again,
transmit after a
random backoff
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
Wireless LAN Topologies
• Basic building block:
Basic Service Set (BSS)
• Independent BSS
• Infrastructure BSS
• Infrastructure Extended Service
Set (ESS)
BSS 1
Internet
BSS 2
BSS 3
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
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
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
C
B
– 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.
Lab Objective
• Set up independent BSS networks and
evaluate their performance under different
traffic and configurations.
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 802.11 protocol to solve Hidden Node
problem
Project and Scenario
• Create new project
• Create Scenario “WLAN”
– Office, 100m x 100m range
– Select wireless_lan node model
• Drag and Drop
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–
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Application Config
Profile Config
1 Wlan_wkstn_adv(fix)
1 Wlan_wkstn_adv(mob)
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
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=192.168.1.1,
Subnet=Class C
– Static Routing Table: Destination Address=192.168.1.2,
Subnet=255.255.255.0, Next Hop=192.168.1.2
• 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=192.168.1.2,
Subnet=Class C
– Static Routing Table: Destination Address=192.168.1.1,
Subnet=255.255.255.0, Next Hop=192.168.1.1
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
Statistics
• Collect Individual Statistics: WLAN
– Delay(sec)
– Throughput(bits/sec)
– Data Dropped(Buffer Overflow)
• Global Statistics
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–
–
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Delay(Sec)
Throughput(bits/sec)
Retransmission Attempt(pkt)
Load(bits/sec)
• Run Simulation for 5 min
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
Duplicate Scenario: Scenario3
• Add another mobile nodes wlan_wkstn_adv(mob)
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–
–
–
–
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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=192.168.1.3, Subnet=Class C
– Static Routing Table: Destination Address=192.168.1.1, Subnet=255.255.255.0,
Next Hop=192.168.1.1
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–
–
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WLAN Parameter
Set Physical Characteristics: Direct Sequence
Data rate: 11Mbps
Packet Reception Power Th: 7.33 E -11 (Tr Range= 36m)
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.
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=192.168.1.3, Subnet=Class C
– Static Routing Table: Destination Address=192.168.1.1, Subnet=255.255.255.0, Next
Hop=192.168.1.1
• 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.