Xirrus Sectored WiFi Benefits

8/9/2019 Xirrus Sectored WiFi Benefits

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November, 2005

B E N E F I T S

Sectored Wi-Fi

Architecture

TM


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1 Sectored Wi-F i Architecture

Introduction

IT Administrators are facing an ever-increasing demand for wireless capacity (bandwidth). With theincreased number and types of devices and new applications such as voice, current enterprise Wi-Finetworks will not scale to meet these demands.

This White Paper details the issues surrounding the use of legacy Omni-directional access pointsthat populate todays Wi-Fi networks and presents how Xirrus sectorized Wireless LAN (WLAN)architecture addresses these issues; creating a high-performance, scalable Wi-Fi network.

Omni-Directional and Sectored Antennas

Most legacy access points today make use of Omni-directional antennas. Omni-directionalantennas transmit and receive RF energy in all directions much like a light bulb. Sectorizedantennas (also known as high-gain antennas) focus RF energy into a single direction, therebyintensifying the strength of the signal that is transmitted much like a flashlight directing a strongbeam of light in the direction that it is pointed. Another physical property of high-gain sectoredantennas is the ability to transmit further and listen better to the signals of wireless stations(clients). Some sectored antenna systems have multiple radios and multi-sectored or sectorized

antennas that allow for 360 degrees of coverage.

Executive Summary

There are many problems that legacy Omni-directional access points have in creating highperformance Wi-FiTM networks. Xirrus has delivered a sectorized approach to Wi-Fi networkingthrough the Wireless LAN Array. The Xirrus Wireless LAN Array provides the bandwidth andcapacity benefits of channel re-use, elimination of hidden nodes, reduced multi-path, andenhanced RF performance inherent to a sectored architecture.

Sectored Antenna Pattern

Shown from the top down

Omni-directional Antenna Pattern

Shown from the top down


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Omni-directional Access Point ProblemsThe vast majority of access point deploymentstoday consist of products that use omni-directional antennas. For the most part, thistype of deployment has served the market wellfor home use and light use in the enterprise.But as the number of wireless users hasdramatically increased and the number ofaccess points deployed to meet the needs ofusers increases, the omni-directional antennabecomes its own worst enemy in the battle toaddress improved performance.

No matter what type of access point is used(fat or thin) blasting RF energy in alldirections becomes a barrier to the performanceneeded for todays wireless networks. Thisproblem consists of a number of issues that alllimit high-performance deployments: cell size,channel reuse, hidden nodes and multi-path.These problems become even more acute withOmni-directional access points, and aredescribed in the following sections.

Cell Size and Co-channel Interference

In early wireless LAN deployments, coveragewas king and little attention was paid tocapacity. What was soon realized is that thelarger the cell, the more users you hadconnected to the same access point. Acceptableperformance became quickly problematic.Providing acceptable levels of performance(capacity) is important and with data ratesof 54Mbps per access point (and real-worldthroughputs of about 22Mbps), only 10-12active users can be handled per access point.

One common approach to increasing capacity is

to simply add more access points on differentchannels and to make sure that any two accesspoints that are on the same channel cannot heareach other. Adjacent cells must be on differentchannels (or clients from one cell will wait forclients on another cell before transmitting),therefore careful channel planning needs tobe done with the limited number of non-overlapping channels available (3 802.11b/gchannels, and 12 802.11a channels).

The next most widely used approach toincrease capacity is to try to use smallerand smaller cell sizes to increase the numberof cells (access points) where one re-uses thelimited set of non-overlapping channels moreoften. Smaller cells also allow higher sustaineddata rates to be used. While these approachesare commonly used, they create a number ofmajor issues.

First, shrinking the cell size of an access pointby lowering the transmit power does not lowerthe transmit power of the wireless station

(client). Second, the clients transmit powerand receiver settings are not under the controlof the access point and do not change. Bydecreasing the transmit power of the accesspoint, the overall cell size shrinks only slightlyand anyone deploying a wireless network mustrealize that the real size of a wireless cell is notjust the transmission range of the access point.In fact, the real size of a wireless cell is thetransmission range of the access point and thetransmission range of all the wireless clientsin that cell.

Sectored Wi-F i Architecture

Figure 1: The true size of a Wi-Fi cell is

determined by the access point and the

wireless clients in the cell

Extended Cell Size


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IT administrators need to have at least one cell

on another channel between cells on the samechannel (in all directions) and make the cellslarge enough so that wireless stations withinthose cells do not hear each other. This requireddistance between cells sets a hard limit to theamount of capacity that can be provided.

This concept of the extended cell is veryimportant because it sets a limit on how smallcells can really be. This is why some recentsolutions prescribe the placement of an accesspoint in every cubicle and turning its transmitpower down. However, this solution wontwork as the wireless stations (clients) willtransmit right over the top of any adjacentaccess points. In fact, cell sizes should only(at most) be half as small as the transmissionrange of the wireless clients; any smaller andthe wireless stations will bleed over to at leasttwo cells over.

Because of the collision avoidance schemethat governs data transmissions in an 802.11network, clients and access points will wait

until the medium is free before attempting totransmit their own packets. Additional accesspoints that are deployed within range of otheraccess points on the same channel will not addany additional capacity. Access points or clientsthat can hear each other will result in themwaiting for the other transmitter to cease beforeattempting to transmit their own packets. Ineffect, adding more access points on the samechannel simply causes the cells to merge intoa larger cell which actually degrades performance.


Shrinking a Cell

Figure 2: Decreasing the access points cell

size only slightly decreases the transmit range

of the extended cell

Interfering Stations

Figure 3: When two access points are placed

near each other on the same channel, they may

not hear each other but the stations (clients)

that associate to those access points will

interfere with stations on the adjacent cell and

vice versa. Interfering stations are shown in

orange [top]

Minimum Cell Spacing

Figure 4: Access points on the same channel must

have at least one cell between them so wireless

stations cannot hear each other


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Hidden Node Problems

Omni-directional access points have anotherinherent problem. A wireless station on oneedge of a cell may not hear a station on theother side of the cell. Because of this, wirelessstations will not be able to hear when the otheris transmitting, incorrectly assuming the air isidle and begin to transmit its own packets.This will cause the two transmissions to colliderequiring both stations to re-transmit greatlyreducing the effective bandwidth within thecell. A protection mechanism exists called CTS-RTS that can help address this issue requiringeach client to ask for permission from theaccess point before transmitting. But the useof this protocol will reduce overall performanceby 30%.

Omni-directional Access Points and Multi-path

Omni-directional access points are not only badneighbors for adjacent cells but also generate avast amount of performance-reducing multi-path. Multi-path occurs when signals bounceoff multiple objects in the environment and arereflected back to the receiver. The effect is thatweaker copies of the original signal arriveslightly later than the primary signal. Thiscauses inter-symbol interference and gets worseas the delay spread, or time between thereception of the primary signal and secondarysignal increases. The end result is corruptpackets that must be re-transmitted, loweringnetwork performance.


Hidden Node Problem

Station 1 Station 2

Multi-path Interference

Secondary Reflected Signal

Time

Primary Desired Signal

Multi-path Time Delay

Figure 5: Station 1 and Station 2 cannot hear

each others transmissions


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The Benefits of a Sectorized Architecture

Xirrus developed the Wireless LAN Array forenterprise Wi-Fi networks using a sectoredantenna system to focus RF energy into a singledirection. This next generation architectureprovides multiple sectors and multiple radiostogether in a single platform providing a 360degree coverage pattern with extremely highcapacity. Xirrus architecture delivers a numberof important advantages over legacy Omni-directional access points including highcapacity channel re-use, elimination of hiddennodes and enhanced wireless performance.

Sectored Antenna System and Channel Re-use

A sectored approach is vastly superior tothat of omni-directional access point. As thefollowing diagrams indicate, a sectoredapproach creates sectors or slices of cellcoverage. Each sector uses a unique channeland by its very design, clusters clients togetheron the same channel under the area of thesectorized antenna pattern. If a wireless stationmoves too far to the left or right of any one

sector, it will automatically re-associate to thenext sector.

By grouping or clustering wireless stationstogether, the sectored architecture createsenough separation between sectors and stationson the same channel that the entire pattern ofsectors can be repeated by an adjacent XirrusWireless LAN Array in such a way that wirelessstations do not interfere with each other.


Single and Multiple Sectors Covering 3600

Two Sectored Systems Side-by-Side

Figure 6: Stations on the same channel are

isolated so that they do not interfere

Cellular network operators have long understoodthis benefit and make use of sectored basestations. The result of a sectorized approach isthat the indoor wireless network can operate atfull wireless capacity and allows an adjacentWireless LAN Array to use the same set ofchannels. In fact, each Wireless LAN Array canprovide up to 810Mbps per cell if all twelve802.11a channels and three 802.11bg channels are

used. This pattern can be used over and over.Contrast this with 108Mbps in an Omni-directional deployment, the Wireless LAN Arrayprovides eight times the capacity.

Multiple Sectored Cells

Figure 7: Multiple Sectored Cells can be deployed

where each Wireless LAN Array re-uses all channels


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Sectored Systems; Multi-path and Performance

As previously discussed, Omni-directional

access points inherently create large amountsof performance-robbing multi-path. With asectorized approach, this problem is greatlyreduced because RF energy is not blindlytransmitted in all directions. RF signals aretransmitted in the direction of the wirelessclient within a given sector and not in thedirection opposite the wireless client thatwould otherwise come back as distortingmulti-path.


Sectored Antenna Systems and Hidden Nodes

A sectorized approach is vastly superior to that of an Omni-directional access point in eliminatingHidden Node issues because all wireless stations (clients) in a given RF sector are associated to thesame radio; so they are geometrically on the same side of the Wireless LAN Array. Since the clientsexist in the same sector, the hidden node problem is eliminated as all stations are able to hear eachother and correctly determine when the air is busy or idle. This eliminates the performance-robbingissues found with legacy Omni-directional access points and the need to use the CTS-RTS protocol.

Reduced Sectored Multi-path

Omni-directional Hidden Nodes Sectored Approach Eliminates Hidden Nodes

Lastly, because a sectorized antenna system

typically has higher gain antennas than anOmni-directional system, it can transmitstronger signals to wireless clients and receiveweaker signals from others. These increasedrate and range performance metrics throughoutthe sectorized wireless cell results in the abilityto use and sustain higher data rates at alldistances within the cell improving overallwireless bandwidth.


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Wireless LAN Array: A Sectorized Approach

Xirrus has delivered the most efficient use of Wi-Fi spectrum with its family of Wireless LAN Arrays.By using a sectored antenna system and solving the fundamental problem of allowing multipleIntegrated Access Points to function in a single platform, Xirrus has created a new paradigm forenterprise Wi-Fi deployments. No longer do Wi-Fi networks require a sea of inconsiderate Omni-directional access points that limit channel reuse, create hidden node and multi-path issues that yielda poor performing network.


Xirrus: Powerful Wi-Fi DeploymentsThe Xirrus approach of using a Multi-sectorAntenna System has important benefits forhigh performance Wi-Fi deployments.

The Wireless LAN Array can create a sectorizedwireless cell of up to 864Mbps (16 x 54Mbps).By integrating up to 16 separate IntegratedAccess Points in a single platform, Xirrus hascreated a solution for today and tomorrowsbandwidth intensive applications. Each XirrusWireless LAN Array can take the place of up to16 separately installed legacy access points andhandle hundreds of simultaneous users.Additionally, one integrated access point can beused as a dedicated full-time RF sensor for thedetection of rogue access points and othersecurity threats.

Embedded Array Controller

Integrated Access Points

Multi-Sector Antenna System

Wireless LAN Array: Sectorized Antenna System

The sectorized approach of the Wireless LANArray provides a superior level of automaticchannel planning, load balancing, and stationroaming. Because the Wireless LAN Arraymakes use of every available non-overlappingWi-Fi channel, the best possible channel plancan automatically be used and changed asconditions change in the environment.

Xirrus eliminates re-architecting and re-deploymentof the wireless network. Legacy architecturesforce the need to continuously re-architect, re-wire and re-deploy the wireless network andmanage even more devices when additionalcapacity is needed. With the Wireless LANArray, the maximum possible bandwidth isinstantly provided such that you can deploy itonce and forget it.


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The Wireless LAN Array allows for high

density radio service at a fraction of the cost ofdeployment, and a fraction of the managementpains of legacy products. Instead of installinglarge numbers of access points that eachrequires a power and Ethernet connection, asingle Xirrus WLAN Array can be installed.

The Wireless LAN Array architecture iscompletely Wi-Fi interoperable. No specialclient devices are needed and the productssupport all three Wi-Fi modes: 802.11a, 802.11b,and 802.11g.

Lastly, the Xirrus approach frees IT administrators

from the hassles of managing a sea of legacyomni-directional access points. The Xirrusarchitecture allows for a greatly reducednumber of devices to manage by a factor ofsixteen. The ongoing expenses of managementand maintenance are costly in a Wi-Fi network,and the fewer devices to manage, maintain,troubleshoot and upgrade the better. Xirrusleads the way with extreme capacity andsimplicity of deployment.

Summary

This White Paper has shown the problems with legacy Omni-directional access points and how asectored approach solves the channel re-use, hidden node, and multi-path problems of the past.Todays legacy equipment cannot keep pace with the trajectory of needed wireless capacity. Xirrushas developed the Wireless LAN Array that utilizes a sectorized approach that puts IT administratorsahead of capacity demands, makes deployments a snap, and creates a high-performance Wi-Finetwork that is simple to deploy, easy to manage and flexible enough to handle anything that getsthrown at it.



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Copyright 2005 Xirrus, Inc. All Rights Reserved. The Xirrus logo is a registered trademark ofXirrus, Inc. All other trademarks and registered trademarks are the property of their respectiveowners. Content subject to change without notice.November 2005

Xirrus, Inc.370 North Westlake Blvd.Suite 200

Westlake VillageCalifornia 91362805.497.0955 Corporate Office805.497.0955 x300 Sales805.449.1180 Faxwww.xirrus.com

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Xirrus Sectored WiFi Benefits