session abstract

Copyright © 2004 OPNET Technologies, Inc. Confidential, not for distribution to third parties.

Planning and Analyzing Wireless LANs

Network Analysis, Planning, and Troubleshooting

Session 1332

Copyright © 2004 OPNET Technologies, Inc. Confidential, not for distribution to third parties. 2

1332 Planning and Analyzing Wireless LANs

Session Abstract



Agenda

Wireless LAN (WLAN) overviewOPNET WLAN models use casesWLAN model support

Network configurations Node models Statistics Node attributes Global attributes Mobility modeling

Lab 1: Hidden node scenarioBreakLab 2: Infrastructure Extended Service Set (ESS)Lab 3: PCF access mode Lab 4: Mixed 11b/11g WLAN Performance



Agenda

Wireless LAN overviewOPNET WLAN models use casesWLAN model support





Why Wireless LAN?

Availability Open specifications

Mobility Users do not have to be plugged in

Ease of installation No need for cabling through/around walls Can go where wires cannot

Reduced cost-of-ownership Easier to move, add, and change Uses license-free radio spectrum



Wireless LAN Support in OPNET

Based on IEEE standardsModeled data rates

802.11: 1 and 2 Mbps 802.11b: 5.5 and 11 Mbps 802.11a and 802.11g: 6, 9, 12, 18, 24, 36, 48, and 54 Mbps

Supported physical layers Direct-Sequence Spread-Spectrum (DSSS) Frequency Hopping Spread-Spectrum (FHSS) Infrared light (IR) Orthogonal Frequency Division Multiplexing (OFDM) Extended Rate PHY-OFDM (ERP-OFDM)

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) OperationRequires centralized coordination Introduces contention free period

(CFP)Use for “near” real-time servicesForces a “fair” access to the

medium during the CFP



Wireless LAN Support in OPNET (cont.)

Reliable data transmission via RTS-CTS exchange (threshold based) Request To Send – Clear To Send

Fragmentation (threshold based)Exponential back-off – reduced collision probabilityProtection for mixed 11b/11g wireless LANs

CTS-to-self or regular RTS/CTS exchange

Roaming (can be turned on/off)



Agenda






WLAN Models: Typical Use Cases

Study wireless LANs as an alternate/supplemental local area network technology

Analyze network performance by varying network load (e.g., number of nodes, application traffic) for independent and infrastructure BSS network configurations

Evaluate optional protocol-specific features like fragmentation and reassembly or RTS/CTS frame exchange against various network conditions

Tune PCF parameters to achieve maximum performance for different applications



WLAN Models: Typical Use Cases (cont.)

Investigate the impact of mobility on applications running on mobile nodes and the efficiency of the wireless LANs being visited

Find out what to expect when upgrading your 11b WLAN to an 11g WLAN

Study the effects of different operational channel assignment choices on overall performance in networks with large number of wireless LANs

(R&D) Modify the logic of standard WLAN algorithms to conduct experiments with new ideas and prospective improvements



Agenda






Supported Network Configurations

Infrastructure BSS

Ad Hoc Network

Extended Service Set

BSS 1

Internet

BSS 2 BSS 3



Supported Network Configurations (Cont.)

Wireless Backbone



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



Statistics

Global Statistics

Node Statistics



Statistics (cont.)



Attributes



Model Attribute Definitions

BSS Identifier Identifies the BSS to which a WLAN MAC belongs

Also needed for roaming enabled nodes for initial association If set to “Auto Assigned,” the entire OPNET subnet will be considered as a

single BSS If configured for one WLAN node, then it needs to be configured for all

WLAN nodes in the network

Access Point Functionality Enable or disable access-point operation in the node Used to configure BSS and ESS topologies Required to be Enabled

For PCF operation To support roaming



Configuring PHY and Data Rate

First select the physical layer technology

Then select the data rate for data transmissions among the available data rates



Configuring Operation Channel

Channel assignments must be consistent within BSS



Auto-Allocation of WLAN channels to BSSs

Example 5 BSSs: from “BSS A” to BSS “E” where A < B < C < D < E

Ch 1 Ch 2 Ch 3 Ch 4 Ch 5 Ch 6 Ch 7 Ch 8 Ch 9 Ch 10 Ch 11

2,401 MHz 2,451 MHz 2,473 MHz

Reserved Frequency Band for WLAN Channels in U.S. at 2.4 GHz (11/11b/11g)

BSS A Ch 1 BSS B Ch 6 BSS C Ch 11

BSS D Ch 2 BSS E Ch 7

Ch 36 Ch 40 Ch 44 Ch 48

5,170 MHz 5,210 MHz 5,230 MHz

Reserved Frequency Band for WLAN Channels in U.S. at 5 GHz (11a)

BSS A Ch 36 BSS B Ch 40 BSS C Ch 44 BSS D Ch 48

5,190 MHz



Transmitting and Receiving

Transmit Power Nodes fixed transmission power in Watts

Packet Reception Power Threshold Defines the receiver sensitivity in dBm

Vendor specific Packets whose reception power is less than threshold will not be

sensed by the MAC Such packets may still cause interference noise at the receiver

* Two key attributes that determine the sensing and communication distance between WLAN nodes



Model Attribute Definitions (cont.) RTS Threshold (bytes)

Sets the packet size threshold for which the request-to-send (RTS)/clear-to-send (CTS) mechanism will be used

Solution to hidden terminal problem Prevent large packets to be dropped Overhead due to the RTS/CTS frame exchange

Short and Long Retry Limits Specifies maximum number of transmission

attempts for a data frame Two independent counters

Long retry count incremented only if a data transmission fails despite a successful RTS/CTS exchange

High retry limits may perform better in noisy networks

Low retry limits can be suitable for network with high mobility



Lab #1: Hidden NodeObjective

Show the impact of the RTS/CTS mechanism as a measure to prevent the hidden node problem



Break



Lab #2: Infrastructure BSS

Objective Become familiar with WLAN

model attributes needed to configure BSSs

Use the model to select an appropriate WLAN topology according to the application traffic



Model Attribute Definitions (cont.) Fragmentation Threshold (bytes)

MSDU Threshold fragmentation occurs

Smaller packet size reduces packet loss but increase overhead

Large Packet Processing Action taken in the case: higher layer packet

size maximum allowed data size Based on this, a packet will be dropped or

fragmented Outside the scope of the standard

Max Receive Lifetime (seconds) Maximum time for a packet to wait to be

reassembled at receiver’s reassembly buffer

Buffer Size (bits) Maximum length of higher-layer data

arrival buffer



PCF Configuration PCF Parameters

PCF Functionality Enables / disables use of PCF

Beacon Interval Specifies how often the beacons will be transmitted

CFP Interval The length of each contention free period in seconds

CFP Beacon Multiple Specifies the number of beacons between two CFPs

Max Failed Polls Specifies the maximum

number of consecutive polls by the AP without a valid response from MAC that is being polled

CFP CFP

CFP Interval = 10 msec.

Beacon Interval = 20 msec.

CFP Beacon Multiple = 2

0 2010 30 40 50 msec.

CFP Offset = 1

= TBTT

CFP CFP

CFP Interval = 10 msec.

Beacon Interval = 20 msec.

CFP Beacon Multiple = 2

0 2010 30 40 50 msec.

CFP Offset = 1

= TBTT= TBTT



Lab #3: PCF Access Mode

Objective Use PCF mechanism to improve the performance of real-time

applications over WLAN



Model Attribute Definitions (cont.)

CTS-to-self Option 802.11g specific Used as a protection mechanism in mixed 11b/11g networks Alternative to RTS/CTS frame exchange

Roaming Capability Enables the MAC to perform scanning procedures to associate with

another AP when the communication is lost with the current one Requires configuration of regular WLAN operational channels Cannot be turned on for APs



Global Attributes

Closure Method (non-TMM) Not used during terrain modeling By default no closure computation

Faster simulation execution Alternatively closure computation based on Earth’s line-of-sight

Requires setting the altitude of the nodes

WLAN Transmission Candidacy Provides an option to block any communication and interference

between the WLAN nodes of different subnets for faster simulations



Global Attributes (cont.)

WLAN Beacon Efficiency Mode An option to turn off APs’ periodic beacon messages for faster

simulation execution PCF enabled APs continue transmitting beacons Does not prevent roaming of stations and AP evaluation

A distance based approximation approach is used for AP evaluation

WLAN AP Connectivity Check Interval Used only by roaming capable stations when beacon efficiency is on Specifies how frequently the distance with the current AP will be

evaluated



Modeling Node Mobility

Three methods to enable node mobility Trajectories Specifying a “motion vector” via attributes Modifying node position programmatically, e.g., “Random Waypoint”



Random Waypoint Mobility

Node moves randomly from one waypoint to another Location of each waypoint is randomly chosen within specified

rectangle Speed between waypoints, and pause time at a waypoint follow

specified random distributions

Configure using Random Waypoint “utility” object Specify rectangular region via

coordinates, or graphically using the “wireless domains” object

Define random waypoint profiles by specifying speed, start time, stop time, and pause time

GUI support for assigning profiles to a set of mobile nodes

“Random_Mobility” example project



Lab #4: Mixed 11b/11g WLAN Performance

Objective Compare the total achievable WLAN throughputs measured in a

mixed 11b/11g WLAN and in an all-11g WLAN to study the performance degradation in 11g WLANs that support legacy nodes



Additional Resources

Wireless LAN Model Usage Guide Click on “Help” menu and select “Product Documentation” “Model Descriptions Model Usage Guides Wireless LAN

(802.11)”

IEEE standards IEEE 802.11-1999 IEEE 802.11a-1999, IEEE 802.11b-1999 and 802.11g-2003

Wireless LAN FAQs Go to Support Center at OPNET’s WWW site

http://www.opnet.com/support Click on “FAQs” link under “Technical Resources” Search the FAQ database using the keywords “Wireless LAN” or

“WLAN”



Related Wireless Sessions

Session 1348: Planning and Analyzing Mobile IP NetworksSession 1345: Planning and Analyzing Mobile Ad-Hoc

NetworksSession 1529: Understanding WLAN Model Internals,

Interfaces, and PerformanceSession 1530: Modeling Custom Wireless EffectsSession 1815: Introduction to Wireless LAN Protocols



Take-Away Points

DES models provides extensive support for modeling wireless LAN networks, e.g.,: Analyzing network performance, with and without mobility Studying the effects of transient conditions and protocol overhead Deployment of explicit traffic sources (e.g., TCP/IP-based

applications or raw traffic generators) over WLAN technologySimulate large wireless LAN network topologies

Reduce simulation execution time by using simulation efficiency modes (global attributes)

Study the interaction between legacy and new wireless LAN technologies

Find the most optimal configuration (e.g., when using PCF) to achieve optimum performance for all wireless applications

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