Xirrus Application Note High-Density Wi-Fi

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Application Note

High-Density Wi-Fi


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Application Note High-Density Wi-FiTable of Contents

Background .............................................................................................................................. 3Description ................................................................................................................................ 3Theory of Operation ................................................................................................................. 3Application Examples ............................................................................................................ 11Tips and Recommendations .................................................................................................. 17

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Application Note High-Density Wi-Fi

Background

One of the biggest challenges faced by 802.11 networks is dealing with high-density userdeployments. Wi-Fi was initially intended to provide LAN access for a moderate number of users.The evolution and overwhelming success of this technology has brought 802.11 deployments to

environments that go well beyond a few users to a point where it can now be the primary accessto the LAN. It is not difficult to find Wi-Fi networks deployed across campuses, industrial areas,and even municipalities offering a wide variety of services. In these networks, it is often seen thatthe number of users connected to the network surpasses the initial design considerations and asa result, performance no longer meets expectations.

Description

This Application Note presents Xirrus solution for providing high quality network access for highuser densities over a Wi-Fi infrastructure. Traditional 1 or 2 radio APs have proven inadequate inhandling such environments. Truly successful high-density deployment requires Wi-Fi equipmentthat is designed with scalability and performance in mind.

There are a number of design elements in Xirrus Wi-Fi Arrays that make them uniquely powerfulfor high-density networks. These innovations include, but are not limited to:

Multi-radio System (IEEE 802.11a, IEEE802.11b, IEEE 802.11g and 802.11n): XirrusWi-Fi Arrays incorporate 4, 8, 12, or 16 radios into a single device. Each radio can beassigned to a unique channel providing dedicated bandwidth.

Antenna Sectorization: Directional, high gain antennas in a sectored Array designprovide a key capability for channel re-use in confined environments.

Auto Cell Sizing: Automatic control of power and sensitivity per radio allow control of thesize and performance of the coverage area.

Station Load Balancing and Association Limits: Appropriate distribution of users

among radios is key for high-density without requiring modifications to the Wi-Fi client andavoid radio overloading.

Traffic Shaping: Controlling user traffic prevents any one station from clogging thenetwork.

Broadcast /Multicast Control and Station Privacy: Broadcast/multicast traffic canextract a large toll on any network, so minimizing its effect improves network performance.

Radio Monitoring: Spectrum Analysis is an important troubleshooting aid.

Theory of Operation

Among the many challenges found in high-density Wi-Fi environments, the one that can be mostdifficult is channel reutilization. The best way to provide bandwidth to a high number of

simultaneous users is to leverage as much of the RF spectrum available to Wi-Fi as possible andas many times as possible. This means a multi-radio with intelligent antenna design to use asmany separate channels as possible while avoiding co-channel interference.

The following sections explain how the Xirrus Wi-Fi Arrays can create high-density user networks.

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Application Note High-Density Wi-FiMulti-Radio SystemPerhaps the most intuitive assumption is that in order to provide the best throughput to Wi-Ficlients, the connection between the client and the access point (AP) must be established at themaximum possible data rate (also known as link rate). In the case of 802.11a and 802.11g, thedata rates will be 54Mbps, and with 802.11b, it is 11Mbps. The new 802.11n standard takes datarates up to 600Mbps.

The table below summarizes the current 802.11 technologies.

802.11b/g 802.11a 802.11nBand 2.4GHz 5 GHz 2.4 GHz, 5GHzChannels 1,2,3,4,5,6,7,8,9,

10,11,12,13,1436,40,44,48,52,5660,64,100,104,108,112,116,120,124128,132,136,140149,153,157,161,165

Same channels as802.11g and802.11a.

Number of non-overlapping

channels

3 24 27 non bonding13 with bonding

Channel Width 20MHz 20MHz 20MHz and40MHz

Max Data Rate 11Mbps / 54Mbps 54Mbps 288/600Mbps

Table 1

The data rate is a function of the signal quality that is affected by distance and the noise levelsgenerated by nearby Wi-Fi or other interference sources. A high error rate will force clients andAPs to negotiate lower data rate connections even if the signal level is strong enough to supporthigher data rates.

The actual throughput a user can achieve is a function of Free Air Time, which is the time themedia is available for the client to transmit or receive. The access to the media is controlled bythe CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) algorithm. When the clientdetects energy in the media due to other transmissions, certain levels of noise and interference oradjacent channel emissions, the client must wait until the media becomes free.

If the media is only available half the time for a particular client, the maximum throughput thatclient can transmit or receive will be half as well. This brings the concept of Available Capacity,which is the product of the Free Air Time and the data rate between the AP and client.

The best method to increase the Available Capacity is to provide the highest data rate andmaximum Free Air Time.

The use of more spectrum and more radio channels increases airtime availability. When clientsare able to associate to multiple APs operating on separate non-overlapping channels, thensimultaneous transmission can occur, thereby increasing overall throughput and system capacity.The Free Air Time increases proportionally to the number of channels that are being used. Thehigher the number of non-overlapping channels used in a particular area, the more AvailableCapacity for that area.

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Application Note High-Density Wi-FiFree Air Time is also related to the data rate of the links between APs and clients. The higher thedata rate, the less time the media will be used to transmit a given amount of information. A usertransmitting a 500byte packet at 1Mbps will be using the media (airtime) 54 times longer than auser transmitting the same 500byte packet at 54Mbps.

The Free Air Time also depends on the number of clients in a channel. For a particular traffic

pattern and data rate per client, Free Air Time linearly decreases with the number of clientspresent in a particular channel.

In summary, in order to increase the highest data rate and maximum Free Air Time, one shouldsimultaneously use multiple channels (radios) per system and reduce the number of users perchannel. The Xirrus Wi-Fi Array does this and goes even further with antenna sectorizationallowing for smaller coverage areas (small cells), guaranteeing higher data rates and channel re-use to provide the best combination for better overall Wi-Fi Available Capacity.

Antenna SectorizationAppropriate planning of coverage areas is another critical task when deploying high-densitywireless networks. The coverage area correlates to the maximum distance clients can be from a

given AP with enough signal to associate and operate at a usable data rate. Data rates within thecell will vary depending on the actual distance clients are from the AP.

In the previous section, it was mentioned how the use of multiple channels and radios allows agreater number of stations in a given area and thus provides greater capacity. In high-densityenvironments, the number of stations per channel might be too large for the AP to provideadequate throughput. In these cases, the Wi-Fi network must re-use the same channels severaltimes within a given area by creating smaller or sectorized coverage areas. This approach issimilar to the one cellular carriers use for mobile phones. By creating a larger number of smallercells, it is possible to achieve greater density, resulting in increased capacity. Because these cellsare smaller, the number of users per channel can be limited and provide additional re-use ofchannels at much closer distance.

The choice of omni-directional or directional antenna design will also have a significant impact inthe re-use of channels. The use of sectorized directional antennas presents several advantagesover omni-directional antennas:

Allows the use of several channels in the same AP system, minimizing co-channelinterference.

Allows better-defined cells by concentrating energy in a sector.

Limits the amount of interference received from other directions thus reducing packeterrors.

Improves receive sensitivity in the direction of the cell since antenna gains work in bothdirections.

Reduces multipath issues because RF is not blindly transmitted in all directions.

Helps in hidden node problems.

In Figure 1, there is an area where three non-overlapping channels are used with omni-directionalantennas. The black dots represent 30 wireless stations inside a cell. The closest availablechannel for those stations will be channel 6. The distance required to reuse channel 6 againwithin the coverage area is shown.

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Figure 1 Figure 2

Figure 2 shows the coverage provided using sectorized antennas using additional radios persystem. For the same station, there are now three channels covering the same area resulting ingreater distribution of stations only 10 stations share a channel. By tripling the density of radiosper system and using sectorized antennas, the available capacity of the area has been increasedby a factor of three.

This scenario can be further improved by adding more radios per system. Since the number ofnon-overlapping channels in the 2.4GHz band is only three, the recommendation is to add radiosin the 5GHz band. Figure 3 shows an example of a Xirrus Array sectorizing 6 channels in onecell and Figure 4 shows 12 channels in one cell.

Figure 3 Figure 4

Auto Cell SizingThe size of the cell or coverage areas is determined by the transmit power and receive sensitivityof both the AP and the stations. By tuning those values, the cell size can be adjusted to

accommodate the dimensions and client density requirements. Adequate power control is alsoimportant to mitigate the interference between radios operating in the same channel. XirrusArrays have the option to either set the cell sizes manually or automatically via the Auto Cellfeature.

Auto Cell is an automatic, self-tuning mechanism that balances cell size between Arrays toguarantee coverage while limiting the RF energy that could extend beyond the organizationalboundary. This is accomplished by setting radio power dynamically so that complete coverage is

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Application Note High-Density Wi-Fiprovided to all areas, yet at the minimum power level required to diminish potential interferencewith neighboring networks. Additionally, Arrays running Auto Cell can detect and compensate forcoverage gaps caused by system interruptions.

Several Auto Cell parameters can be defined including minimum cell size, scheduled RFassessment/adjustment, and the ability to define a coverage overlap percentage for roaming. An

overlap of 15% to 20% between cells is recommended for seamless roaming.

Without Auto Cell

Figure 5. Without Auto Cell

Figure 5 shows that when two radios are operating, stations B and C could potentially experienceinterference from neighboring Arrays 1 and 2 resulting in corrupted packets. Strong co-channelinterference could cause stations in adjacent cells (cells 1 and 3) to defer communication whilestations in cell 2 are transmitting, resulting in the reduction of overall throughput. In the event ofArray 2 failure,stations A and D could lose all connectivity.

With Auto Cell

Array 1

Array 2

Array 3Array 1

Array 2

Array 3

Client DClient D

Client BClient B

Client AClient A

Client CClient C

Figure 6. Without Auto Cell

In Figure 6 above, Array radios balance power levels between themselves to guarantee clientcoverage, but without the potential of interfering with other cells. In the event of Array 2 failure,Arrays 1 and 3 automatically detect the loss of energy from Array 2 and raise their own radiopower levels to compensate for the loss. Stations who were on Array 2 then re-associate to theother Arrays.

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Application Note High-Density Wi-FiIn the case of manually configuring the cell sizes, the Xirrus Arrays have predetermined cell sizesfor different applications. In the case of high-density environments where channel re-use isdefinitely required, mediumor smallcell sizes should be used.

Table 2 below shows the values of TX power and RX sensitivity for different cell sizes.

Cell Size Transmit Power(dBm)

Receive Sensitivity(dBm)

Small 5 -75Medium 12 -81Large 19 -87Max (Default) 20 -90Auto AutoCell AutoCellManual 1 to 20 -95 dBM to -65 dBm

Table 2.Power and Receive Sensitivity settings on Array

Other combinations of values with more granularity can be manually configured to accommodate

other deployment requirements.

Station Load BalancingLoad Balancing allows the Array to distribute stations among all available radios in an area withthe goal of providing maximum bandwidth to all stations. By moving a station from onecongested radio to a less congested radio, load balancing allows that station to have moreavailable bandwidth. Additionally, it also decongests the radio where the station was initiallyconnected allowing the remaining stations on that radio greater overall bandwidth. This is veryimportant as described in the first point of this section where the available capacity linearlydecreases with the number of stations on a channel.

The Xirrus Wi-Fi Array supports automatic Load Balancing to distribute Wi-Fi stations across

multiple radios. In 802.11, it is the station that decides to which radio it will associate. The Arraycannot actually force station association to a specific radio, however the Array can encouragestations to associate in a more uniform fashion across all of the radios of the Array.

In high-density environments where the stations are uniformly distributed within the coveragearea, it would be expected that the load distribution among radios would be also uniform. This isnot always the case. Typically the station will connect to the AP that has the strongest signalstrength, but when the station moves to another location, it may remain connected to the previousradio until the signal drops below a certain threshold. Stations that are constantly moving cancreate inconsistent load distribution conditions.

Another case of inconsistent distribution occurs when many stations move into a concentrated

space such as meeting rooms, conferences or auditoriums. In this case, the strongest signallevel for those stations may come from one particular radio and all those users will associate tothat radio leaving other radios in the vicinity lightly loaded.

The Array decides if a particular radio is over utilized and should not allow any more associations.This decision process is based on a load-balancing algorithm that takes 3 key factors into affect:

Fewer stations on a radio is preferable

The strength of the signal

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5GHz channels preferred over 2.4GHz channels

Station Limits

To Load Balancing stations, the Arrays have the option to limit the number of associations perradio. When the number of associations reaches the configured limit, the radio sends the stationa message indicating that the radio has reached its limit, encouraging the stations to associate to

another radio.

Traffic Shaping802.11 is a shared medium. In a high-density Wi-Fi environment, traffic shaping is recommendedto optimize or guarantee performance levels. It is commonly applied at the network edges tocontrol traffic entering the network. Traffic shaping causes additional delays by serving queuesmore slowly than if traffic shaping was not applied. Xirrus has incorporated different trafficshaping methods such as:

Traffic Classification: Support for WMM /IEE802.11e (Quality of Service)

Rate Limiting: To control the maximum rate at which traffic is sent

Traffic classification can be configured on Xirrus Wi-Fi Arrays to provide traffic prioritization for

delay sensitive applications such as voice and video.

Rate limiting can be configured on the Array per SSID or User Group in the following ways:

Traffic overall

Traffic per station

Broadcast and Multicast ControlUnnecessary broadcast and multicast packets are a type of traffic that is undesirable in Wi-Finetworks. The reason broadcast and multicast traffic packets are so detrimental is becausethose packets are sent at the lowest basic data rates. Because these packets are intended toreach multiple stations that might be located at different distances, only the lowest data rate canguarantee reception to all of them. As mentioned, low data rate transmissions require more time

to send the same amount of information.

To optimize performance in high-density environments, it is important to minimize the amount ofunnecessary broadcast and multicast traffic. Both types of traffic are typically required andcannot be eliminated, but the amount of this type of traffic transmitted over the air needs to becontrolled.

Xirrus Wi-Fi Arrays incorporate several features to control broadcast/multicast traffic in the air:

ARP Filtering: Address Resolution Protocol finds the MAC address of a device with agiven IP address by sending out a broadcast message requesting this information. ARPfiltering allows the proliferation of ARP messages to be reduced by restricting how theyare forwarded across the network.

The following are options for handling ARP requests:o Off: ARP filtering is disabled and requests are broadcasted to stations.

This is the default value.o Pass-thru: The Array forwards the ARP request. It passes along only ARP

messages that are associated to the target the stations.o Proxy: The Array replies on behalf of the stations to which it is associated.

The ARP request is not broadcasted to the stations.

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Broadcast/Multicast Optimization:o Broadcast/Multicast Optimization: Restricts all broadcast/multicast

packets to only those radios that need to forward them. For instance, if abroadcast comes in from VLAN 10, and there are no VLAN 10 users on aradio, then that radio will not send out that broadcast. This increasesavailable airtime for other traffic.

o Rate Optimization: Changes the rates of broadcast traffic sent by theArray (including beacons). When set to Optimized, each broadcast ormulticast packet that is transmitted on each radio is sent at the lowesttransmit rate used by any station associated to that radio at that time. Thisresults in each IAP broadcasting at the highest Array TX data rate that canbe heard by all associated stations, thus improving system performance.The rate is determined dynamically to ensure the best broadcast/multicastperformance possible. The benefit is dramatic; consider a properlydesigned network (one that has -70db or better everywhere), where virtuallyevery station should have a 54Mbps connection. In this case, broadcastsand multicasts will all go out at 54Mbps vs. the standard rate. This meansthat with the broadcast rate optimization on, broadcasts and multicasts use

between 2% and 10% of the bandwidth that they would in Standard mode.

When set to standard(default), broadcasts are sent out at the lowest basicrate only6 Mbps for 5GHz stations, or 1 Mbps for 2.4GHz stations.

In addition to those features, traffic filters can be created to limit or block any other type of trafficthat is not necessary or is slowing down the air portion of the network.

Station-to-Station BlockingStation-to-station blocking prevents stations connected to the same Array from sending trafficdirectly to each other. As a result, only traffic to/from the wired network is allowed.

This feature provides multiple benefits. First, it improves security since it prevents users fromaccessing other devices on the network without the need to implement firewalling or filtering.Second, it helps optimize broadcast and multicast traffic in the air. Broadcast traffic sent by astation will not be sent back to the air by the other radios in that Array. Since broadcasts are sentat the lowest data rates, this helps increase the overall available air time.

Station-to-station blocking may also be enabled in the network switches where the Arrays areconnected; this will prevent stations from communicating with each other when associated todifferent Arrays.

Radio MonitoringSpectrum Analyzers are used for troubleshooting Wi-Fi networks as well as for spectrumanalysis. There are numerous devices including microwave ovens, cordless phones, andBluetooth devices that can cause RF interference and degrade the performance of an 802.11 Wi-Fi network. Often the sources of interference are signals from nearby Wi-Fi networks and can belocated with the use of a spectrum analyzer.

There is an integrated Spectrum Analyzer function (Figure 8) in every Xirrus Wi-Fi Array thatallows network administrators to monitor and troubleshoot their wireless networks in both 2.4GHzand 5GHz spectrum in a distributed manner. They are mostly used to detect existing sources ofinterference.

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Figure 8. Radio Monitoring

Using the Spectrum Analyzer on the Array provides users with the following real-time statistics ineach and every channel in the 2.4GHz and 5 GHz band.

Packets/ sec: Total number of packet per second

Bytes/sec: Total number of Bytes per second

802.11 Busy: % of time that 802.11 traffic is seen on that channel

Other Busy: % of time that non-802.11 traffic is seen on that channel

Signal to Noise: Average Signal to Noise Ratio

Noise Floor: Average noise floor seen on that channel

Error Rate: % of 802.11 packet with CRC errors

Average RSSI: Average RSSI level seen on 802.11 packets Average Data Rate: Average Data Rate over time

Application Examples

This section shows two real-world examples of high-density environments that have beendeployed using Xirrus Wi-Fi Arrays. In these examples the configuration guidelines are outlinedfor the important Xirrus features discussed previously. For additional details and how to configurethese features, please refer to the Xirrus Array User Guide as well as documents that areavailable on the Xirrus website.

College Classroom/Auditorium

A large college classroom or auditorium is a typical case of a high-density environment. In suchapplications, laptop-to-student ratio can reach 1:1 when all students are required to have a laptopto access online or school resources during class.

In this example, the design goal was to provide wireless access to 140 simultaneous students ina lecture hall. Only one Xirrus Wi-Fi Array, model XS8, was deployed. On this 8 radio Array, 3were deployed at 2.4GHz, 4 at 5GHz, and one as a dual band monitor radio. It is important tonote that the monitor radio can also be used as a regular access radio in case additional capacityis required.

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A ratio of 20 wireless stations per radio is used in this deployment, which represents a goodbalance for bandwidth and coverage. Within the coverage area, the expected signal level issufficient to guarantee the highest possible data rates. The most important Array features for thisscenario are multi-radio, sectorization and load balancing.

Network TopologyThe topology in this example is very simple. Just one Array was deployed and powered throughPoE and installed hanging from the ceiling in the center of the facility as shown in Figure 9.

Figure 9. Lecture hall

Configuration

Channel Plan: The Array is configured for Auto Channel selection. After scanning thearea, the Array will select the most optimal channels in which to operate. Since three ofthe 2.4GHz radios are enabled, the Array will pick channels 1, 6, and 11. For the 5GHz,the Array will select four channels taking into consideration outside RF interferenceconditions. The eighth radio is typically used in a threat sensor mode.

Cell Size: Although Auto Cell is not relevant when deploying only one Array, it should beconsidered in case other Arrays are deployed within the vicinity, such as in otherclassrooms.

The cell sizes of each radio in the Wi-Fi Array can be customized. In Figure 10 below,each radio is set to a different cell size for illustration purposes. However, in most

implementations, all the radios are set to similar cell size. When configuring cell sizemanually, the cell size can be set as small, medium, large, max, or manual.

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Figure 10. Auto Cell

Please refer to the AutoCell Application Note for additional configuration details.

Load Balancing

Having the stations homogenously distributed on the Array is very important to offer maximumbandwidth.

There are three possible settings for Load Balancing: Off: Load balancing is disabled.

On: The overloaded radio will ignore Probe Requests for a few seconds expecting that thestation will go to another radio. During that time, the radio will still respond to Associationand Authentication Requests (i.e. if the station already knows the radio is there). After afew seconds if the station keeps sending Probe Requests, the radio will start respondingadmit access.

Aggressive: In this case, if a radio is overloaded it will ignore all Probe Requests,Association Requests, and Authorization Requests from additional stations. It will do thisindefinitely until its load decreases. This way the Array makes sure the users haveassociated to other radios less congested.

Please refer to the Station Load Balancing Application Note for configuration details.

Station Limits

If Load Balancing is not used, another possibility is to set a station limit. When a radio reachesthe established limit it will not accept connection from additional stations.

Traffic Shaping

The type of applications that will be accessed during class will determine the amount of allowedtraffic per user. Traffic shaping should be enabled to prevent students from utilizing morebandwidth than necessary for the purpose of the class or to from transferring large amounts ofinformation between them.

The traffic limitation can be enabled on per SSID or per user group. Per SSID

o At SSIDs >SSID Management, select the SSID that rate limiting is to beconfigured. User can choose to configure either one or both of the following:

- Uncheck Unlimited and enter a value for the overall traffic for theselected SSID.

- Uncheck Unlimited and enter a value for maximum rate allow foreach station associated to the selected SSID.

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Figure 11. Traffic Shaping Per SSID

Per User Groupo At Groups >Group Management, select the Group that rate limiting is to be

configured. User can choose to configure either one or both of the following:- Uncheck Unlimited and enter a value for the overall traffic for the

selected Group- Uncheck Unlimited and enter a value for maximum rate allow for

each station in that Group.

Figure 12. Traffic Shaping Per User Group

Broadcast and Multicast Control

ARP filtering should be set to Pass-thru or Proxy.

To control of ARP traffic with Proxy and Pass-through mode

Figure 13. ARP Filtering

Station-to-Station Blocking

Go to IAPs > Global Settings to enable/disable intra-station traffic

Figure 14. Station-to-Station Blocking

This feature was not enabled at the above example.

Radio MonitoringAs mentioned before, the monitor radio can be also used as a regular access radio. In thisexample, the number of radios available in the Arrays is sufficient to accommodate the number ofstudents, so the monitor radio is used for spectrum analysis and intrusion detection.

In order for the spectrum analyzer to function, there are 2 things that needs to be enabledon the Array:

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The abg2 (XS series) or abgn2 (XN series) IAP must be configured as a monitorusing the omni-directional antenna.

Intrusion Detection Mode must be set to Standard.

Go to IAPs > IAP Settings, configure abg2/abgn2 as monitor

Figure 15. Radio Monitoring

At IAPs> Advanced RF Settings, select Standard mode for Intrusion Detection

Figure 16. Intrusion Detection

In RF Monitor >Spectrum Analyzer, an overview of the 802.11 and non-802.11 activity in both2.4GHz and 5GHz channels can be monitored.

Convention CenterA convention center is a perfect example of a high user density wireless environment. In thiscase, high-speed wireless Internet access is provided to 7000 attendees with approximately 3000

simultaneous users in a 150,000 square-foot hall. Various types of applications are supportedincluding web access, email, corporate VPN, text messaging, and VoIP stations connecting inboth the 2.4GHz and 5GHz bands.

Network Topology

The area is covered using twelve Xirrus Wi-Fi XS16 Arrays. The XS16 Array includes twelve802.11a, three 802.11bg and one monitor spectrum/analyzer radio.

The physical distribution of Arrays within the area is mostly uniform with half of the Arraysstanding on tripods along the perimeter of the hall and the other half hanging from the ceiling.

In this configuration (Figure 17), a total of 180 radios are available 36 operating in 2.4GHz

proving 802.11bg access and 144 operating in 5GHz providing 802.11a access.

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Figure 17. Array Deployment with Channel Allocation

All Arrays are wired to GigE switch ports and powered via PoE.

Configuration

Channel Plan: In such an environment with a high density of radios, close proximitybetween Arrays, and no considerable obstructions between them, the channels can be

easily assigned manually. Twelve 5GHz channels and three 2.4GHz channels are usedfor a total of 15 per Array. For the 2.4GHz band, only three non-overlapping channels inthe US regulatory domain are used: 1, 6 and 11. In the 5 GHz band, if the UN-II bandchannels are not considered, there are a total of 13 non-overlapping channels to allocateto 12 radios. The channels are set manually, replicating the same channel plan on all theArrays and placing each Array in the same orientation (rotation).

Cell Size: Considering the distance between Arrays and a static and controlledenvironment, the cells size are set to mediumbecause the average distance from stationto Array is relatively short. Channel interference between Arrays will determine the cellsize rather than the size of the coverage area or required data rate.

Load Balancing: This feature is enabled in all the Arrays and set as normal. Thebehavior of the users should be monitored at peak time and if the distribution does notlook uniform, Load Balancing may be set to aggressive. Consider that in thisenvironment, users are free to move and they could be creating areas of higher densitythan others.

Traffic Shaping: In this particular deployment, traffic shaping was set to 2000 packetsper second per user. Other traffic limitations can be set depending on the networkrequirements, for example applying filters to limit application support.

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Broadcast Control: ARP filtering is set to proxy and station-to-station blocking isenabled. In this example, these two features are more critical since the amount ofstations in the same broadcast domain is much higher than in the previous example.

Tips and RecommendationsHigh user density environments can push Wi-Fi networks to their limits, largely based on thecontention created by many stations accessing a shared communication medium. The mostimportant capabilities of the Xirrus Wi-Fi solution to enable successful deployment in suchenvironments are its multiradio/multi-channel design, antenna sectorization, load balancing, andbroadcast control. Beyond these facilities, the following recommendations may be used tooptimize these types of deployments:

Monitor the number of users per radio and radio utilization during peak times using theXMS to understand the user distribution. Enable aggressive load balancing if necessary.

Monitor that the radios in all Arrays are functional and reporting information for future data

collection and statistics generation.

In cases where access for legacy 802.11b stations is not required, configure the Arrays for802.11g only. This will eliminate the lowest data rates from the air and improve the overallperformance on those radios for 802.11g stations.

Analyze the type of traffic that is being sent by the stations using a packet sniffer. Ifcertain traffic types are seen that are not desired and can be eliminated (e.g. videostreaming, P2P, etc.), create filters in the Arrays to block them.

Implement station-to-station blocking on the switches to which the Arrays are connectedto prevent users on different Arrays from communicating with each other.

Monitor the backhaul and WAN utilization of the network to understand the traffic flow andmagnitude to ensure these components of the network are not becoming a bottleneck.

Use the XMS to monitor the traffic utilization of each Array to see if some are carryingmore traffic than others or have many more users associated to them. Use the results tobetter distribute usage accordingly. For example, a lower user limit can be set in theArrays that carry more traffic.

Look at the traffic statistics and check the number of retransmissions. If a highpercentage of retransmissions is detected, there may be too much inter-Arrayinterference. In this case, lower the TX power of the Array radios

Check the Array IAP and station statistics to understand the number of users connected atlow data rates. A lot of traffic at low data rates may indicate users are roaming and notselecting the closest radio. In this case, aggressive load balancing may be enabled andadditionally increase the sensitivity threshold.

Documents

Xirrus Application Note High-Density Wi-Fi