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 BSS 2 BSS 3 Internet

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

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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    Prevent large packets to be dropped Overhead due to the RTS/CTS frame exchange  Short Retry Limit  Solution to hidden terminal problem 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 Collision occurs at B.

A cannot receive the collision A is “hidden” for C.

(CD fails).

C

(CS fails)

B A

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 – 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.