Planning and Analyzing Wireless LAN

Hidden Node Scenario and RTS/CTS SolutionLab 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 BSS 2 BSS 3

Internet

Opnet WLAN Node Models

Wireless LAN Workstation

Wireless LAN Server

Router with WLAN interface (Access Point*)

Wireless LAN Station (Non-IP based)

Bridge with WLAN Port (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 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

BC

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– 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– 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)

– 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– 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.