Post on 18-Jan-2016
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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.