session abstract

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

Understanding WLAN Model Internals, Interfaces, and Performance Discrete Event Simulation for R&D

Session 1529

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

Session 1529: Understanding WLAN Model Internals, Interfaces, and Performance

Session Abstract



Agenda

IntroductionNetwork Configurations, Node Models

Interfacing WLAN with higher and lower layersMAC Process Model

States, interrupt types, state transitionsLab: Customizing the Back-off AlgorithmPhysical Layer Model

Pipeline Stage Model Modifications Improving Model PerformanceFAQsRelated SessionsTakeaway pointsAppendix



Agenda




Pipeline Stage Model Modifications Improving Model PerformanceFAQsRelated SessionsTakeaway PointsAppendix



Implemented Standards

OPNET WLAN suite implemented based on the following standards: Base standard: IEEE 802.11

Standard for Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications

Defines a MAC sub-layer and three physical layers FHSS, DSSS, and Infrared Data rates: 1 and 2 Mbps

Amendments: 802.11a, 802.11b and 802.11g Higher Data Rates

DSSS: 5.5 and 11 Mbps in the 2.4 GHz band (11b) OFDM: 6, 9, 12, 18, 24, 36, 48 and 54 Mbps

In the 5.0 GHz band (11a) In the 2.4 GHz band (11g)



OPNET’s WLAN Model FeaturesDistributed Coordination Function (DCF)Point Coordination Function (PCF)Reliable data transmission via RTS-CTS exchange (threshold

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

CTS-to-self or regular RTS/CTS exchangePhysical Layer Technologies

FHSS, IR, DSSS, OFDM, Extended Rate PHY-OFDM Auto-assignment of channels to BSSs (optional)

Data Rates (Mbps): 1, 2, 5.5, 6, 9, 11, 12, 18, 24, 36, 48, 54Modulations: DPSK, BPSK, QPSK, CCK, QAM-16, QAM-64 Roaming (can be turned on/off)



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



WLAN Node Models: Workstation, Server, and Station

Medium Access Layer

Network Layer

Transport Layer

Application Layer

Physical Layer

Use this model for studies that focus only on MAC and

physical layers



WLAN Node Model: Bridge/Switch

You can create custom node models with desired number/type of ports and

interfaces using device creator utility



WLAN Node Model: Router

Routing domain

Wired interface

Wireless interface



Wireless LAN Module

Packet stream characteristics Statwire characteristics1. Packet-based communication 1. Used for physical carrier sensing2. Information exchange between 2. Communicate values between the attached subsystems source and destination ports3. Interrupt-based data sensing 3. Scheduled interrupt-based carrier sensing

4. Indicates changes in transmitter/receiver status

Packet Streams

Statwires

Radio Receiver Radio Transmitter

Packet Streamsto/from Higher Layer



Antenna Module

Isotropic antenna pattern Since no antenna module is attached to the node model, the default

isotropic antenna pattern is associated by the simulation kernel 0 dB gain in all directions



WLAN MAC and Higher Layers

Higher LayerHigher Layer

WLAN MAC WLAN MAC

Data Packets to Tx +

an ICI with “dest_addr” field

(e.g.: wlan_mac_request)

Data Packets Rx +

ICI (wlan_mac_ind)



WLAN Packet Format (Data)

Data Packet (wlan_mac)



WLAN Packet Format (Control)

Control Packet

(wlan_control)



Agenda







WLAN Process Model (wlan_mac)



WLAN Interrupts

Self Interrupts Deference (SIFS, PIFS, DIFS and EIFS time intervals) Back-off elapse Contention window elapse Frame timeout Beacon transmission time (APs only) Contention free period end (APs only) NAV Reset AP Evaluate and channel scan end (roaming stations only)

Stream Interrupts Higher layer data arrival Lower (physical) layer

data arrival

Statwire Interrupts Receiver busy Receiver idle Transmitter idle



WLAN INIT

WLAN process registration Initialize all state variables Initiate MAC auto-addressing



WLAN BSS_INIT

Complete MAC auto-addressingNetwork configuration validationFor PCF enabled networks, form a list of CF-Pollable

stations



WLAN IDLE

Idling MACs wait in this state Empty transmission buffer, no ongoing transmissions, no response to send, no CFP After a successful completion of congestion window and no further data to send After completion of contention free period and no data to send Note: higher layer packet arrivals are processed in any unforced state



WLAN DEFER

A frame or response to transmit, or in contention free period Check for receiver status and network allocation vector (NAV)

If busy, wait until it gets idle If idle, wait for inter-frame spacing (SIFS, PIFS, DIFS or EIFS) before

advancing to the next state



WLAN BKOFF_NEEDED

Decide whether back-off is needed If needed, check whether starting a new back-off or resuming; if a new one:

Increase the contention window in case of a retransmission Compute total back-off duration



WLAN BACKOFF

Wait for the completion of back-off period If the back-off is suspended, compute the remaining back-

off duration



WLAN TRANSMIT

Data/Control packet transmissionsDetect collisions if any packet is received during

transmission



WLAN FRM_END

Decide the state transition following the completion of packet transmission by the transmitter



WLAN WAIT_FOR_RESPONSE

Wait for the response message until: The expected ACK or any type of message is received, or ACK-waiting timer expires



WLAN SCANVisited only by roaming-enabled non-AP MACs when the

connection with the current AP is lost or weakenedLeft only when a new, reliable AP (WLAN channel) is found



Life Cycle of Packet : RTS/CTS

RTS Transmission, RTS lost due to collision

RTS collides

Frame Timeout

RTS to send, medium is idle for longer than

DIFS



Life Cycle of Packet : RTS/CTS (cont.)

Suspending back-off

Wait for DIFS

Received interrupt, suspend

back-off




Resuming back-off, RTS retransmission, CTS reception

CTS received

Wait for DIFS

Complete the pending back-off




Data transmission, ACK reception

ACK received

Data transmission

Wait for SIFS




Back to idle if no more data to send

Wait for DIFS



Life Cycle of Packet : PCF POLL

AP transmitting Poll frame

Wait for PIFS

Transmit Poll

Polled station responds with either a Data, Data-Null, Data-ACK or ACK frame



Life Cycle of Packet : CTS-to-self

Transmitting CTS-to-self with op_pk_send() AND op_pk_deliver_delayed()

Transmit CTS-to-self

Own CTS-to-self

Received



Key WLAN Functions

wlan_mac_sv_init() State variable initialization Read the “Wireless LAN Parameters” configuration

wlan_higher_layer_data_arrival() Packet queuing

In the DCF mode all packets inserted into the hld_list_ptr Packets received by the AP during contention free period inserted into the cfpd_list_ptr

wlan_interrupts_process() Handles the appropriate processing needed for each interrupt. This function is called in the “exit exec” of unforced state



Key WLAN Functions (cont.) wlan_frame_transmit()

Determines the frame type for the next transmission If an AP polling stations, selects the next station to poll

wlan_prepare_frame_to_send() Called by wlan_frame_transmit() Creates a packet of requested type and populates its MAC header fields Saves a copy of the packet for possible retransmissions if required Fragments the higher layer data packets if required

wlan_physical_layer_data_arrival() Processes the frames received by the station from the lower layer Course of action taken depends on the packet received

wlan_data_process() Called by wlan_physical_layer_data_arrival() Handles the de-fragmentation process Data sent to higher layer if the receiver is the destination of the packet If the received packet is a broadcast, action taken depends on the station type



Key WLAN Roaming Related Functions

wlan_begin_new_scan() Switches the MAC’s transceivers to the next WLAN channel Initiates the evaluation of the new channel

wlan_find_new_ap_virtual() Evaluates the currently scanned channel Used by distance based approach

wlan_ap_switch() Performs the de-association from the current AP and re-association

with the new AP De-association and re-association are implicitly modeled



Agenda







Lab 1: Network Model

Application configuration FTP Traffic

50% GET, 50% PUT One request every 1 second Size of each file 5000 bytes (40K

bits) Total application traffic in the

network (40000 bits / 1 sec) * 15 clients = 0.6

Mbps Sufficient to saturate a wireless LAN

operating at 1 Mbps and therefore to cause retransmissions

Configuring the type and frequency of FTP requests

Wireless FTP Servers

Wireless FTP Clients



How Much Throughput Can I have in a Wireless LAN?

Data Rate: 11Mbps Traffic Load: 8Mbps Packet Size: 1024 bytes

(constant)

0

1

2

3

4

5

6

2 5 10 20 30 40 50

Number of Contending Nodes

Th

rou

gh

pu

t(M

bp

s)

Physical Layer: DSSS RTS/CTS: Off Fragmentation: Off



Agenda







Module Attributes

Physical Layer Configuration

Node Attributes

*Module attribute values are overwritten



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



Receiver Sensitivity Configuration

Attribute “high threshold trigger” of the statwire will be overwritten by Wireless LAN Parameters Packet Reception-Power Threshold



WLAN Transmitter Pipeline Stages

wlan_rxgroup Exclude your own receiver Exclude the receivers in different OPNET subnets (an optional feature for faster simulation run)

wlan_txdel Retrieve the transmission data rate from the packet

wlan_chanmatch Match conditions are based only on the transmitting and receiving frequency and bandwidth

Tx Antenna Gain: None

dra_closure Three modes:

No occlusion Spherical Earth line-of-sight Terrain based using TMM



WLAN Receiver Pipeline Stages

wlan_power In addition to the default behavior, Mark a received packet as “Noise” if the

Receiver is busy Received power is lower than the threshold specified

Monitor the beacons for channel evaluation

wlan_propdel Warning message added to “Simulation Log” if the distance between the transmitter and receiver exceeds 300 meters

Rx Antenna Gain: None




wlan_ber Loads all WLAN modulations into an array using the new kernel procedure op_tbl_modulation_get()

Picks the corresponding modulation based on the transmission rate of the received packet instead of using receiver’s modulation

Computes the processing gain by itself using the data rate information conveyed in the packet




wlan_ecc Forces the simulation kernel to always accept packets even in the event of collision

Later bad are dropped packets at the MAC layer

Implements signal extension for 11g nodes

wlan_error Optimized approach:

If the packet already has enough bit errors for rejection, don’t find all the bit errors

Retrieves the data rate from the packet



Implementing 11g Signal Extension

ECC pipeline stage (wlan_ecc): Sets the delay attribute of the stream when an ERP-OFDM packet is successfully received at the receiver of an 11g nodes

Packet stream: Introduces a delay between the receiver and MAC module

MAC module: Resets the delay attribute after receiving the ERP-ODFM packet

Packet Stream



Agenda







Improving Simulation Performance

Disabling communication and interference across subnets Configured using simulation attribute: WLAN Transmission

Candidacy Values: Objects in Same Subnet / Objects Across Entire Network

Implemented in wlan_rxgroup()

Reducing the set of potential receivers for each transmitter Exclude the receivers after operational channel assignments if:

No overlap between channels (no communication or interference) No roaming (operational channel remains same)

Implemented by calling dynamic rxgroup kernel procedures in wlan_rxgroup_reduce() op_radio_txch_rxgroup_get() op_radio_txch_rxgroup_set()



Improving Simulation Performance (Cont.)

Support for parallel processing WLAN MAC process model and all radio pipeline

stage models are multi-thread safe Process models: Interfaces Support Parallel

Execution is checked Main function names in pipeline stages have

been renamed For instance: wlan_error() wlan_error_mt()

Initialization (t = 0) phase is sequentialAccess of global variables are managed by

use of mutexes For instance, at each invocation the MAC locks a

local mutex for accessing roaming related information

Same information is also read and/or overwritten by wlan_power pipeline stage



Improving Simulation Performance (Cont.)

Disabling periodic access point beacons Configured using simulation attribute: WLAN Beacon Efficiency Mode Not allowed when PCF is enabled Allowed even if roaming capable stations are present

APs are evaluated using a distance-based approximation approach Faster, but less accurate The frequency of AP evaluations are configured using the

simulation attribute: WLAN AP Connectivity Check Interval



Improving Simulation Performance (Cont.): Pipeline StagesSkipped pipeline stages: tagain, ragainMinimum error allocation

Terminated when error threshold is reached Use wlan_error_all_stats() as alternative

Receiver “state” instead of “extended attributes”By default no closure computation

Assumes no occlusion Approach chosen based on simulation attribute: Closure Method (non-

TMM) Values: No Occlusion / Earth Line-of-Sight Not used when TMM enabled



Agenda







Assigning BSS IDs and IP Addresses to WLAN nodesBSS IDs auto-assigned: subnet boundaries define the BSS

boundaries If you configure one, then you need to configure all

When manually assigned, BSSs can expand over subnet boundaries

Same BSS ID Same IP network



Assigning BSS IDs and IP Addresses to WLAN nodes (Cont.)When an IP networks includes multiple BSSs

IP addresses have to be assigned manually



Some Typical Customizations

Changing the data transmission rate dynamically or introducing additional data rates Overwrite the state variable operational_speed at the beginning of the

function wlan_prepare_frame_to_send()Varying transmission power

Overwrite the “power” attribute of the channel of the transmitterChanging the receiver sensitivity dynamically

Overwrite the “high threshold trigger” attribute of the statwire from the receiver module to the MAC module

Introducing priority queueing Queues are implemented as lists (List*) Search for the state variable hld_list_ptr to find the locations where

the DCF queue is initialized, packets are added, removed, etc.



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 WLAN Standards IEEE 802.11-1999 IEEE 802.11a-1999, IEEE 802.11b-1999 and IEEE 802.11g-2003

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

http://www.opnet.com/support/home1.html Click on “FAQs” link under “Technical Resources” Search the FAQ database using the keywords “Wireless LAN” or

“WLAN”



Related Sessions

1332: Planning and Analyzing Wireless LANs1530: Modeling Custom Wireless Effects1815: Introduction to Wireless LAN Protocols1544: Case Studies: Wireless LANs – 802.11 Modeling1564: Case Studies: Wireless LANs – QoS1565: Case Studies: Wireless LANs – Mobility and Handoffs



Take-Away Points

OPNET WLAN suite supports a rich set of features of IEEE 802.11, 11a, 11b and 11g standards

In order to transmit packets using the WLAN MAC model, an ICI containing the destination MAC address must accompany each packet from the higher layer module

The WLAN MAC process model causes radio transceiver module attributes such as the bandwidth, data rate, and transmission power to be overridden by the corresponding node model attributes

The physical layer sensing mechanism required for CSMA/CA is accomplished via statistic wires that convey the state (busy/idle) of the wireless channel to the MAC module

Signal extension feature specified in IEEE 802.11g standard is implemented using the delay attribute of the packet stream connecting the radio receiver module to the MAC module



Appendix

DCF and PCF Overview

Acronyms



Overview: Protocol Basics

Medium access modes DCF

CSMA/CA Virtual Sensing (NAV) Physical Sensing (Receiver Status)

RTS - CTS exchange

Data – ACK exchange

CTS Data Ack

Nav (RTS)

NAV (CTS)

Nav (Data)

DIFS

SIFS

SIFS

SIFS

CWRTS

DataACK



Overview: Protocol Basics (Cont.)

PCF Polling

D1+Poll

PIFS

SIFS

SIFS

SIFS

B U1+ACK D2+ACK+Poll

….

CF-End

Contention Period DCF

Contention Free Period (PCF)

CFP Repetition Interval



Some WLAN Acronyms Referred to in the Presentation

BSS: Basic Service Set

FHSS: Frequency Hopping Spread SpectrumDSSS: Direct Sequence Spread Spectrum OFDM: Orthogonal Frequency Division Multiplexing

DPSK: Differential Phase Shift Keying BPSK: Binary Phase Shift Keying QPSK: Quadrature Phase Shift Keying CCK: Complimentary Code Keying QAM: Quadrature Amplitude Modulation

session abstract

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