High-Density Wireless Networks for Auditoriums

High-Density Wireless
Networks for Auditoriums
Validated Reference Design
SolutionGuide

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High-Density Wireless Networks for Auditoriums Validated Reference Design | Solution Guide October 2010
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High-Density Wireless Networks for Auditoriums VRD | Solution Guide Contents | 3
Contents
Chapter 1 Introduction 7
About Aruba Networks 7
Aruba Validated Reference Designs 7
Solution Guide Assumptions and Scope 8
Design Validation and Testing 9
Reference Documents 9
Chapter 2 Design Requirements for Auditorium HD WLANs 11
Functional Requirements 12
Technical Requirements—Client Devices 13
Technical Requirements—Wired Infrastructure 13
Technical Requirements – Wireless Infrastructure 14
Chapter 3 Capacity Planning for HD-WLANs 17
HD WLAN Capacity Planning Methodology 17
Step #1: Choose a High-Density WLAN Capacity Goal 18
Step #2: Determine the Usable Number of Channels 19
20-MHz vs. 40-MHz Channels 19
Available 5-GHz Channels 20
To DFS or Not to DFS? 22
Site-Specific Restrictions 22
5-GHz Channel Reuse 23
Available 2.4-GHz Channels 24
2.4-GHz Channel Reuse 24
Step #3: Choose a Concurrent User Target 25
Mixed Auditoriums with Both 802.11n and Legacy Clients 25
Choosing a Concurrent User Target 27
Step #4: Predict Total Capacity 27
5-GHz Capacity 27
2.4-GHz Capacity 29
Step #5: Validate the Capacity Goal 29
Chapter 4 RF Design for HD WLANs 31
Coverage Strategies for Auditoriums 31
Overhead Coverage 32
Side Coverage (Walls or Pillars) 35
Floor Coverage (Picocells) 38
Choosing Access Points and Antennas 40
Recommended Products 41
Choosing an Access Point 44
External Antenna Selection 44
Minimum Spacing Between Adjacent Channel APs 45
AP and Antenna Spacing – Overhead and Underfloor Strategies 45
AP and Antenna Spacing – Side Coverage Strategy 46
Aesthetic Considerations 47

4 | Contents High-Density Wireless Networks for Auditoriums VRD | Solution Guide
General Installation Best Practices 48
Managing Adjacent HD WLANs 48
Managing Clients 48
Overhead or Floor Coverage 49
Side Coverage with Directional Antennas in Series 49
Side Coverage with Back-to-Back APs and Directional Antennas 50
Chapter 5 Infrastructure Optimizations for HD WLANs 51
Essential ArubaOS Features for HD WLANs 51
Achieving Optimal Channel Distribution 51
ARM Channel Selection 52
Mode-Aware ARM 52
Achieving Optimal Client Distribution 53
Band Steering 53
Spectrum Load Balancing 54
Optimal Power Control 54
How ACI and CCI Reduce WLAN Performance 54
How the 802.11 Carrier Sense Works 55
How Adjacent Channel Interference Reduces WLAN Performance 55
How Co-Channel Interference Reduces WLAN Performance 58
Limiting AP Transmitter Power 60
Limiting Client Transmitter Power 60
Enabling the Aruba RX Sensitivity Tuning-Based Channel Reuse Feature 60
Optimal Airtime Management 61
Ensuring Equal Access with Airtime Fairness 61
Limiting “Chatty” Protocols 63
Maximizing Data Rate of Multicast traffic 64
Enabling Dynamic Multicast Optimization for Video 64
Limiting Supported Legacy Data Rates 65
Other Required Infrastructure Settings 65
VLAN Pooling 65
Chapter 6 Configuring ArubaOS for HD-WLANs 67
Achieving Optimal Channel Distribution 68
Enabling ARM Channel/Power Selection 68
Enabling Mode-Aware ARM 69
Enabling DFS Channels 70
Achieving Optimal Client Distribution 71
Enabling Band Steering 71
Enabling ARM Spectrum Load Balancing 72
Achieving Optimal Power Control 73
Reducing AP Transmitter Power 73
Limiting Client Transmitter Power 74
Minimizing CCI with RX Sensitivity Tuning-Based Channel Reuse 74
Achieving Optimal Airtime Management 76
Enabling Airtime Fairness 76
Limiting “Chatty” Protocols 77
Implementing Multicast Enhancements 78
Enabling Multicast Rate Optimization 78
Enabling IGMP Snooping 80
Enabling Dynamic Multicast Optimization for Video 80
Video Scalability 81
Reducing Rate Adaptation by Eliminating Low Legacy Data Rates 82
Other Required Infrastructure Settings 83
VLAN Pooling 83

High-Density Wireless Networks for Auditoriums VRD | Solution Guide Contents | 5
Chapter 7 Troubleshooting for HD WLANs 85
Scoping the Problem 85
End-to-End Solution Framework 86
HD WLAN Troubleshooting 86
Troubleshooting Flow Chart 87
Symptom #1: Device cannot see any SSIDs 88
Symptom #2: Device can see SSIDs but not the one it needs 88
Symptom #3: Device successfully authenticates but cannot communicate 90
Symptom #4: Device has Connection Loss and/or Poor Performance 91
Before You Contact Aruba Support 92
Appendix A HD WLAN Testbed 95
Testbed Design 95
What is a Client Scaling Test? 95
Testbed Design 95
Test Plan Summary 96
20-MHz Channel Tests 96
40-MHz Channel Tests 97
Adjacent Channel Interference Tests 98
Co-Channel Interference Tests 98
Test Results: 20-MHz Channel 99
How does total channel capacity change as clients are added? 99
How does per-client throughput change as clients are added? 101
How much does throughput decrease as legacy stations are added? 102
How many stations can contend before channel capacity declines? 102
Is there a limit to the number of concurrent users an AP can serve? 103
Test Results: 40-MHz Channel 103
How does total HT40 channel capacity change as clients are added? 103
How does per-client HT40 throughput change as clients are added? 104
Appendix B Advanced Capacity Planning Theory for HD WLANs 107
Predicting Total Capacity 107
Predicting Device Counts Using a Radio Budget 107
Predicting Performance Using a Throughput Budget 109
Capacity Planning Methodology for HD WLANs 111
Appendix C Basic Picocell Design 113
RF Design for Picocell 113
Understanding Structure of a Picocell 114
Link Budget Analysis 115
Minimum Channel Reuse Distance 116
Capacity Planning for Picocell 117
Reconciling the RF and Capacity Plans 117
Appendix D Dynamic Frequency Selection Operation 119
Behavior of 5-GHz Client Devices in Presence of Radar 119
Behavior and Capabilities of 5 GHz Client Devices 120
DFS Summary 120
Appendix E Aruba Contact Information 121
Contacting Aruba Networks 121

6 | Contents High-Density Wireless Networks for Auditoriums VRD | Solution Guide

High-Density Wireless Networks for Auditoriums VRD | Solution Guide Introduction | 7
Chapter 1
Introduction
This guide explains how to implement an Aruba 802.11n wireless network that must provide high-speed
access to an auditorium-style room with 500 or more seats. Aruba Networks refers to such networks as
high-density wireless LANs (HD WLANs). Lecture halls, hotel ballrooms, and convention centers are
common examples of spaces with this requirement. Because the number of concurrent users on an AP
is limited, to serve such a large number of devices requires access point (AP) densities well in excess of
the usual AP per 2,500 – 5,000 ft2 (225 – 450 m2). Such coverage areas therefore have many special
technical design challenges. This validated reference design provides the design principles, capacity
planning methods, and physical installation knowledge needed to successfully deploy HD WLANs.
About Aruba Networks
Aruba delivers secure enterprise networks wherever users work or roam. Our mobility solutions bring
the network to you — reliably, securely, and cost-effectively — whether you're working in a corporate
office, teaching space, hospital, warehouse, or outdoors. Aruba 802.11n WLANs reduce the need for
wired ports, which lowers operating costs. Our remote access point technology brings the network to
branch offices, home offices, or temporary locations with plug-and-play simplicity, and all of the heavy
lifting stays at the data center. For customers with legacy wireless LANs, our AirWave multivendor
management tool supports WLAN devices from 16 manufacturers, which allows you to seamlessly
manage old and new networks from a single console.
Aruba Validated Reference Designs
An Aruba validated reference design (VRD) is a package of product selections, network decisions,
configuration procedures and deployment best practices that comprise a reference model for common
customer deployment scenarios. Each Aruba VRD has been constructed in a lab environment and
thoroughly tested by Aruba engineers. By using these proven designs, our customers are able to rapidly
deploy Aruba solutions in production with the assurance that they will perform and scale as expected.
Aruba publishes two types of validated reference designs, base designs and incremental designs.
Figure 1 illustrates the relationship between these two types of designs in the Aruba validated reference
design library.
Figure 1 Aruba Validated Reference Design Library
HD_190
Campus
Wireless
Networks
Retail
Wireless
Networks
Virtual
Branch
Networks
Optimizing
Aruba WLANs
for Roaming
Devices
Wired
Multiplexer
(MUX)
High-Density
Wireless
Networks
Incremental
Designs
Base
Designs

8 | Introduction High-Density Wireless Networks for Auditoriums VRD | Solution Guide
A base design is a complete, end-to-end reference design for common customer scenarios. Aruba
publishes the following base designs:
 Campus Wireless Networks VRD: This guide describes the best practices for implementing a
large campus wireless LAN (WLAN) that serve thousands of users spread across many different
buildings joined by SONET, MPLS, or any other high-speed, high-availability backbone.
 Retail Wireless Networks VRD: This guide describes the best practices for implementing retail
networks for merchants who want to deploy centrally managed and secure WLANs with wireless
intrusion detection capability across distribution centers, warehouses, and hundreds or thousands
of stores.
 Virtual Branch Networks VRD: This guide describes the best practices for implementing small
remote networks that serve fewer than 100 wired and wireless devices that are centrally managed
and secured in a manner that replicates the simplicity and ease of use of a software VPN solution.
An incremental design provides an optimization or enhancement that can be applied to any base design.
Aruba publishes the following incremental designs:
 High-Density Wireless Networks VRD (this guide): This guide describes the best practices for
implementing coverage zones with high numbers of wireless clients and APs in a single room such
as lecture halls and auditoriums.
 Optimizing Aruba WLANs for Roaming Devices VRD: This guide describes best the practices
for implementing an Aruba 802.11 wireless network that supports thousands of highly mobile
devices such as Wi-Fi® phones, handheld scanning terminals, voice badges, and computers mounted
to vehicles.
 Wired Multiplexer (MUX) VRD: This guide describes the best practices for implementing a wired
network access control system that enables specific wired Ethernet ports on a customer network to
benefit from Aruba role-based security features.
Solution Guide Assumptions and Scope
This guide is an incremental design. It addresses advanced radio frequency (RF) design topics, and it is
intended for experienced WLAN engineers. This design builds on the base VRDs that Aruba has
published (Campus Wireless Networks, Retail Wireless Networks, and Virtual Branch Networks). A
properly implemented master/local design is a prerequisite to proceed with this High-Density VRD.
This guide is based on ArubaOS version 3.4.2.3. This guide makes assumptions about the knowledge
level of the engineer, the existing architecture and configuration of the Aruba WLAN, and the AP type
and wireless frequency band that will be used. Table 1 lists these assumptions.
Table 1 Solution Guide Assumptions
Category Assumption
Engineer Knowledge Level  Thorough understanding of and experience with RF design principles,
link budgets, RF behaviors, antenna selection, regulatory bodies, and
allowable channel/power combinations, with Certified Wireless
Network Administrator (CWNA) level or equivalent.
 Thorough understanding of 802.11 MAC layer operation, beacons,
probes, rate adaption, retries, CSMA/CA.
 Experience with spectrum analysis and troubleshooting RF problems.
 Comfort with controller-based WLAN architectures that employ thin
APs.
 Thorough understanding of Aruba controller design, master/local
architectures, and controller and AP redundancy.

High-Density Wireless Networks for Auditoriums VRD | Solution Guide Introduction | 9
Design Validation and Testing
Test cases for this VRD were executed against the RF design and physical architecture recommended in
this guide using a heterogenous mix of up to 50 late-model laptops with varying operating systems,
CPUs, and wireless network adapters. This mix approximates actual conditions in a typical auditorium.
Aruba 3000 Series controllers were tested with AP-120 Series and AP-105 Series access points. ArubaOS
release 3.4.2.3 was used to conduct these tests. Ixia Chariot 7.1 was used to produce repeatable
controlled test loads that were used to characterize relative performance of various design choices.
More information on test methodology can be found in Chapter 3, “Capacity Planning for HD-WLANs”
on page 17 and Appendix A, “HD WLAN Testbed” on page 95.
Reference Documents
The following technical documents provide additional detail on the technical issues found in
HD WLANs:
 ARM Yourself to Increase Enterprise WLAN Data Capacity, Gokul Rajagopalan and Peter
Thornycroft, Aruba Networks, 2009
 Adaptive CSMA for Scalable Network Capacity in High-Density WLAN: a Hardware Prototyping
Approach, Jing Zhu, Benjamin Metzler, Xingang Guo and York Liu, Intel Corporation, 2006
 Next Generation Wireless LANs: Throughput, Robustness, and Reliability in 802.11n,
Eldad Perahia and Robert Stacey, Cambridge University Press, 2008
 Own the Air: Testing Aruba Networks’ Adaptive Radio Management (ARM) in a High-Density
Client Environment, Network Test Inc., July 2010
 Data sheets for Aruba AP-105, AP-124, and AP-125 access points
 Data sheets for Aruba AP-ANT-13B, AP-ANT-16, AP-ANT-17, and AP-ANT-18 external antennas
Existing Aruba Configuration  Base design was architected using one or more master/local clusters
that conforms to the Campus, Retail (for example, distributed), or
Virtual Branch Networks VRDs.
 Complete control over the RF airspace; freedom to choose any
combination of channels and power levels that are legal within the
country/regulatory domain.
High-Density WLAN Design  5 GHz is the primary band for servicing clients and all 5-GHz-capable
clients will be steered to that band; 2.4 GHz will accommodate legacy
devices or provide overflow capacity for 5 GHz.
 802.11n is required, with Gigabit Ethernet connections between each
AP and the IDF to support peak AP throughputs.
 High-throughput 20-MHz (HT20) channels are used exclusively in HD
WLAN coverage zones to increase capacity. 40-MHz channels are not
used in HD WLAN coverage zones.
 Channels are not reused inside any single auditorium. However, reuse
may occur for adjacent HD WLANs or adjacent conventional WLAN
deployments. (See Appendix C, “Basic Picocell Design” on page 113
for discussion of advanced designs requiring reuse in a single room.)
 Clients are stationary and evenly distributed within each auditorium.
The infrastructure may influence them to roam to balance the load.
Table 1 Solution Guide Assumptions (Continued)
Category Assumption

10 | Introduction High-Density Wireless Networks for Auditoriums VRD | Solution Guide

High-Density Wireless Networks for Auditoriums VRD | Solution Guide Design Requirements for Auditorium HD WLANs | 11
Chapter 2
Design Requirements
for Auditorium HD WLANs
HD WLANs are defined as RF coverage zones with a large number of wireless clients and APs in a single
room. With the proliferation of wireless-enabled personal and enterprise mobile devices, a surprisingly
diverse range of facilities need this type of connectivity:
 Large meeting rooms
 Lecture halls and auditoriums
 Convention center meeting halls
 Hotel ballrooms
 Stadiums, arenas, and ballparks
 Press areas at public events
 Concert halls and ampitheaters
 Airport concourses
 Financial trading floors
 Casinos
This VRD addresses auditorium-style areas. When you understand the auditorium scenario, it is quite
straightforward to apply the design principles to almost any type of high-density coverage zone.
The high concentration of users in any high-density environment presents challenges for designing and
deploying a wireless network. The explosion of Wi-Fi-enabled smartphones means that each person
could have two or more 802.11 NICs vying for service, some of which may be capable of only 2.4-GHz
communication. At the same time, maximum HD WLAN capacity varies from country to country based
on the number of available radio channels. Balancing demand, capacity, and performance in this type of
wireless network requires careful planning.
This chapter defines the functional and technical requirements of the auditorium scenario, including
those for client devices, wired infrastructure, and wireless infrastructure. Understanding these
requirements sets the stage for the design, configuration, and troubleshooting chapters to follow.

12 | Design Requirements for Auditorium HD WLANs High-Density Wireless Networks for Auditoriums VRD | Solution Guide
Functional Requirements
The typical auditorium addressed by this VRD has a total target capacity of 500 seats. If each user is
carrying a laptop and a Wi-Fi-enabled PDA or smartphone, the total WLAN client count could be as high
as 1,000 devices. The average real-world, per-client bandwidth need is usually no more than 1 Mbps
even for many video streaming deployments. In Chapter 3, “Capacity Planning for HD-WLANs” on
page 17, we discuss how higher or lower throughput targets alter the total capacity of an HD WLAN.
Figure 2 500 Seat University Lecture Hall
The users in an auditorium are evenly distributed across the space because they are usually sitting in
rows of stadium-type seating. The user density in the seating areas is an average of 1 user per 15 ft2
(5 m2), including aisles and other common areas. As many as 20 APs could be deployed in a single
auditorium, depending on the total number of allowed channels in the regulatory domain. Available
mounting locations are often less than ideal, and aesthetic and cable routing considerations limit
installation choices.
Figure 3 shows the user density in a typical auditorium or lecture hall environment.
Figure 3 Auditorium of 320 Seats with Typical Dimensions

The user density of the typical auditorium is approximately 20 times greater than an office environment.
In a typical office environment with a mix of cubicles and offices, a typical client density is 250 – 350 ft2
(23 to 33 m2) per person, including common areas, with a per-client bandwidth need of 500 Kbps or less.
It is common to deploy one AP every 2,500 to 5,000 ft2 (225 to 450 m2), which provides for average
received signal strengths of -65 to -75 dBm depending on the walls and other structures in the area.
Also, the office environment provides much more flexibility in AP mounting and placement choices.
In universities and convention centers, it is common for several auditoriums of varying capacities to
exist side-by-side or above-and-below. This situation makes the design aspect even more challenging
because the rooms are almost always adjacent and close enough to require careful management of co-
channel interference (CCI) and adjacent channel interference (ACI) between auditoriums. This
situation can include intended and unintended RF interaction between APs, clients, and between
clients in different rooms. As a result, such facilities require special RF design consideration, which is
covered in Chapter 4, “RF Design for HD WLANs” on page 31.
Technical Requirements—Client Devices
Understanding and controlling the output power and roaming behavior of the client devices is an
essential requirement for any HD WLAN. Client radios greatly outnumber AP radios in any high-density
coverage zone and therefore they dominate the CCI/ACI problem. 802.11h and Transmit Power Control
(TPC) are critical, but they are totally dependent on the client WLAN hardware driver. Encouraging or
requiring users to implement these features will greatly improve overall client satisfaction.
The usage profile of most dense auditorium environments is a heterogeneous, uncontrolled mix of
client types. The devices are not owned and controlled by the facility operator, so they cannot be
optimized or guaranteed to have the latest drivers, wireless adapters, or even application versions. Any
operating system of any vintage or device form factor could be in use. Network adapters could be any
combination of 802.11a, 802.11b, 802.11g, and 802.11n.
Users of the wireless network in an auditorium expect moderate throughput, high reliability, and low
latency. Concurrent usage and initial connection is of primary concern in the design and configuration.
Some common small handheld devices, such as the iPhone, go into a low power state frequently and
cause a reconnection to the WLAN periodically. This demand puts more control path load on the WLAN
infrastructure and it must be considered in the design.
The user traffic in an auditorium WLAN is a variety of application types. Some of the most common
applications in the auditorium WLAN are HTTP/HTTPS traffic, email, and collaboration and custom
classroom applications. Custom applications in an auditorium include classroom presentation and
exam applications, as well as multicast streaming video applications. With the exception of video, these
applications are bursty in nature and require concurrent usage by many or all of the wireless clients.
Therefore, this VRD assumes that fair access to the medium is a fundamental requirement.
Technical Requirements—Wired Infrastructure
The user density and heterogeneous client mix inherent in the auditorium HD WLAN scenario also
places a number of unique requirements on the wired network infrastructure. Some key requirements
are:
 Gigabit Ethernet (GbE) Edge Ports with 802.3af or 802.3at: This guide assumes 802.11n APs,
which provide up to 300 Mbps per radio. This speed in turn requires gigabit connections at the edge.
 10-Gigabit Ethernet Uplinks to Distribution Switches: Most, if not all, APs in each auditorium
will terminate on the same IDF, so edge switch backplanes and uplinks must be sized for the
expected peak aggregate throughput from the HD WLAN.

 Simultaneous Logins/Logoffs: The RADIUS or other authentication server must be able to handle
the inrush and outrush of users at fixed times (such as a class start and stop bell). Ensure that the
AAA server can accommodate the expected peak number of authentications per second. You can
use the Aruba command “show aaa authentication-server radius statistics“ to monitor
average response time.
 IP Address Space: Sufficient addresses must be available to support not only laptops but also
smartphones and other future Wi-Fi-compatible devices that may expect connectivity. Some surplus
space will be necessary to support inrush and outrush of users in a transparent fashion and in
concert with the DHCP service lease times in order to prevent address exhaustion.
 DHCP Service: The DHCP server for the HD WLAN must also be able to accommodate an
appropriate inrush peak load of leases per second. Lease times must be optimized to the length of
sessions in the room so that the address space can be turned over smoothly between classes or
meetings.
Technical Requirements – Wireless Infrastructure
HD WLANs also require specific capabilities in the wireless infrastructure, including:
 Adaptive Radio Management (ARM) Dynamic RF Management: To minimize the IT
administration burden and enable HD WLANs to adapt to changing RF conditions, dynamic channel
and power selection features are a requirement. So are dynamic client distribution features
including the ability to steer 5-GHz-capable clients to that band and spectrum load balancing to
ensure even allocation of clients across available channels. Because there are many fewer 2.4-GHz
channels than 5-GHz channels, another requirement is that the minimum number of 2.4-GHz radios
are enabled inside each HD WLAN. This requires either an automatic coverage-management feature,
such as the Aruba Mode-Aware ARM to convert surplus 2.4-GHz radios into air monitors to prevent
unnecessary CCI. Alternatively, a static channel plan may be used in the 2.4-GHz band in parallel
with ARM in the 5-GHz band.
 ARM Airtime Fairness: Airtime fairness is basic requirement of any heterogenous client
environment with an unpredictable mix of legacy and new wireless adapters. Older 802.11a/b/g
clients that require more airtime to transmit frames must not be allowed to starve newer high-
throughput clients. The ARM Airtime Fairness algorithm uses infrastructure control to dynamically
manage the per-client airtime allocation. This algorithm takes into account the traffic type, client
activity, and traffic volume before allocating airtime on a per-client basis for all its downstream
transmissions. This ensures that with multiple clients associated to the same radio, no client is
starved of airtime and all clients have acceptable performance.
 VLAN Pooling: There must be adequate address space to accommodate all of the expected devices,
including a reserve capacity for leases that straddle different meetings in the same room. At the
same time, limiting the broadcast domain size is crucial to limiting over-the-air management traffic.
Aruba’s VLAN Pooling feature provides a simple way to allocate multiple /24 subnets to
accommodate any size auditorium.
 Disabling Low Rates: By definition, any high-density coverage area has APs and clients in a single
room or space. To minimize unnecessary rate adaptation due to higher collision activity, it is a
requirement to reduce the number of supported rates. This may be accomplished by just enabling
24-54-Mbps legacy OFDM rates. However, all 802.11n MCS rates must be enabled for compatibility
with client device drivers.
 “Chatty” Protocols: A “chatty” protocol is one that sends small frames at frequent intervals,
usually as part of its control plane. Small frames are the least efficient use of scarce airtime, and
they should be reduced whenever possible unless part of actual data transmissions. Wherever chatty
protocols are not needed, they should be blocked or firewalled. These protocols include IPv6 if it is
not in production use, netbios-ns, netbios-dgm, Bonjour, mDNS, UPnP, and SSDP.

 Dynamic Multicast Optimization (DMO): DMO makes reliable, high-quality multicast
transmissions over WLAN possible. To ensure that video data is transmitted reliably, multicast video
data is transmitted as unicast, which can be transmitted at much higher speeds and has an
acknowledgement mechanism to ensure reliability. Transmission automatically switches back to
multicast when the client count increases high enough that the efficiency of unicast is lost.
 IGMP Snooping: Ensures that the wired infrastructure sends video traffic to only those APs that
have subscribers.
 Multicast-Rate-Optimization (MRO): Multicast over WLAN, by provision of the 802.11 standard,
needs to be transmitted at the lowest supported rate so that all clients can decode it. MRO keeps
track of the transmit rates sustainable for each associated client and uses the highest possible
common rate for multicast transmissions.
 Quality of Service (QoS): If voice or video clients are expected in the HD WLAN, it is essential
that QoS be implemented both in the air as well as on the wire, end-to-end between the APs and the
media distribution infrastructure.
 Receive Sensitivity Tuning: Receive sensitivity tuning can be used to fine tune the APs to “ignore”
clients that attempt to associate at a signal level below what is determined to be the minimum
acceptable for a client in the intended coverage zone. This tuning helps to reduce network
degradation to outside interference and/or client associations that may be attempted below the
minimum acceptable signal level based on the desired performance criteria.

High-Density Wireless Networks for Auditoriums VRD | Solution Guide Capacity Planning for HD-WLANs | 17
Chapter 3
Capacity Planning
for HD-WLANs
Over the next four chapters you will learn capacity planning, RF design, configuration, and validation
for HD WLANs. In this chapter, you will learn the basic approach to planning an HD WLAN and making
a first-order assessment of whether the desired level of performance is possible for an area of a given
size.
This chapter uses charts and lookup tables to provide the wireless architect with the necessary sizing
parameters. These tables are based on extensive validation testing conducted in the Aruba labs. For
those interested in the mathematics and theory of HD WLAN design behind the charts, Appendix B,
“Advanced Capacity Planning Theory for HD WLANs” on page 107 provides a technical explanation of
the process.
HD WLAN Capacity Planning Methodology
The process of sizing an HD WLAN is straightforward if you have the benefit of certain test data and an
accurate database of allowable channels in each country. You will follow the same five steps for each
coverage zone you plan:
1. Choose a capacity goal: The first step is to pick an application-layer throughput target linked to
the seating capacity of the auditorium.
2. Determine the usable number of channels: For each band, decide how many nonoverlapping
channels are usable for the HD WLAN. Use a database of regulatory information included here,
augmented by site-specific decisions such as whether or not Dynamic Frequency Selection (DFS)
channels are available.
3. Choose a concurrent user target: Determine the maximum number of simultaneously
transmitting clients that each AP will handle. Use a lookup table based on test data supplied by
Aruba. You must do this for each radio on the AP.
4. Predict total capacity: Use the channel and concurrent user count limits to estimate the maximum
capacity of the auditorium using lookup tables supplied by Aruba.
5. Validate against capacity goal: Compare the capacity prediction with the capacity goal from
step 1. If the prediction falls short, you must start over and adjust the goal, concurrent user limit, or
HD_277
Choose capacity
goal
Determine usable
channel count
Choose concurrent
user target
Predict total
capacity
Validate
against goal

18 | Capacity Planning for HD-WLANs High-Density Wireless Networks for Auditoriums VRD | Solution Guide
channel count until you have a plan that you can live with. For large auditoriums over 500 seats, you
should be prepared to accept a per-client throughput of 500 Kbps or less, assuming a 50/50 mix of
.11n and .11a stations and nine usable channels.
If Channel reuse is required to achieve the capacity goal, see Appendix C, “Basic Picocell Design” on
page 113 for an advanced discussion of the theoretical issues involved in managing AP-to-AP and client-
to-client interference. In practice, reuse is extremely difficult to achieve in most auditoriums due to
their relatively small size and the signal propagation characteristics of multiple-in multiple-out (MIMO)
radios. Reuse requires more complex calculations and testing as well as the potential for modifying
physical structures in the user environment.
Step #1: Choose a High-Density WLAN Capacity Goal
Every HD WLAN design begins by defining a capacity goal. This goal has two parts, which are the key
factors are necessary for the designer to properly scale and produce a HD WLAN project design.
 Total number of devices: Often, this is just equal to the seating capacity of the area. Sometimes,
each seat may contain more than one client (that is, one laptop and one Wi-Fi-capable smartphone).
This is important because every MAC address consumes airtime, an IP address, and other network
resources.
 Minimum bandwidth per device: This is primarily driven by the mix of data, voice, and video
applications that will be used in the room. Aruba recommends using LAN traffic studies to precisely
quantify this value.
Here are some common examples of a complete capacity goal:
 “Each classroom has 30 students who each need 2 Mbps of symmetrical throughput.”
 “The auditorium holds 500 people. Each one has a laptop that must have at least 350 Kbps for data
and a voice handset that requires at least 128 Kbps.”
 “The trading floor must serve 800 people with at least 512 Kbps each.”
Each of these scenarios provides the wireless architect with a clear, concise, and measurable end state.
It’s a good idea to build in future capacity needs. While the number of seats in the auditorium is not
likely to change, it is nearly certain that the number of 802.11 radios per seat will increase in the future.
Be sure to consider the actual duty cycle of each device type when setting the capacity goal. In many
cases, it is unlikely that every device will need access to the maximum capacity simultaneously (unless
there are specific applications that require it such as interactive learning systems). It's a good idea to
use a wireless packet capture utility to study the actual bandwidth requirements of a typical user. Many
customers initially overestimate their bandwidth requirements.
N O T E
This guide assumes that channels will not be reused within a single auditorium.

Step #2: Determine the Usable Number of Channels
In any HD WLAN, we need to use as many nonoverlapping RF channels as possible, because data
capacity increases linearly with the number of channels. Figure 4 shows two colocated APs on different
nonoverlapping channels provide roughly twice the capacity of a single AP. With three APs on different
channels in the same room, capacity is roughly tripled.
Figure 4 Using Additional Channels to Increase WLAN Capacity
Wi-Fi operates in the 2.4-GHz band and in different segments of the 5-GHz band. The available RF
channels are subject to national regulations, but generally there is 83 MHz available at 2.4 GHz and
around 460 MHz at 5 GHz. The 802.11 standard uses 20-MHz or 40-MHz (for 802.11n) channels, so
standard Wi-Fi equipment is also constrained by these parameters. The number of allowed
nonoverlapping channels is the primary capacity constraint on an HD WLAN. For this reason, HD
WLANs should always use the 5-GHz band for primary client service because most regulatory domains
have many more channels in this band.
20-MHz vs. 40-MHz Channels
Most HD WLANs including auditoriums should only use 20-MHz channel widths, also known as HT20.
Using high-throughput 40-MHz (HT40) channels reduces the number of radio channels by bonding them
together. This forces each AP to serve more users. It is better to have 50 users each on two different
HT20 channels than 100 users on one HT40 channel. Also, most handheld devices are not capable of
taking full advantage of 40-MHz channels due to their limited processing power single spatial stream
radios. HT40 channels are never expected to be used on the 2.4-GHz band for reasons that are beyond
the scope of this guide.
The main benefit to using HT40 channels is the ability for individual stations to burst at the maximum
PHY rate when only a portion of the users are trying to use the WLAN. However, in the auditorium
scenario, we must support so many users in a single room that we need every possible channel. In this
case, we accept a reduction in the maximum per-station burst rate during light loads in exchange for a
greater total user capacity at all times.
HD_246
x
w y
If one channel provides x Mbps capacity… Two APs covering the same area on
non-overlapping channels provide 2x Mbps capacity.
Channel
A
Channel
A
Channel
C
CA
z
v
C
z
v
x
w
A
y

Available 5-GHz Channels
The 5-GHz band(s) allow many more nonoverlapping channels than 2.4 GHz. In the United States before
2007, the UNII-I, -II, and –III bands allowed the use of a total of thirteen 20-MHz channels (or six 40-MHz
channels). The number of available 5-GHz channels varies significantly from country to country.
Figure 5 shows the number of 20-MHz channels and 40 MHz channel pairs available for use in the 5-GHz
band.
Figure 5 5-GHz Nonoverlapping Channels
In 2007 the radio regulatory bodies in many countries allowed the use of the “UNII-II extended” band
from 5470 MHz to 5725 MHz as long as UNII-II equipment was capable of Dynamic Frequency Selection
(DFS). DFS requires that the AP monitor all RF channels for the presence of radar pulses and switch to
a different channel if a radar system is located. Wi-Fi equipment that is DFS-certified can use the
extended band, which adds up to another eleven 20-MHz channels or five 40-MHz channels (depending
on the radio regulatory rules in each country).
Channels definedfor 5 GHz band(US regulations), showing common 20 MHz channelplan and 40 MHz o ptions
Channel
Frequency (MHz)
US UNII I and UNII II bands
UNII I: 5150-5250 MHz
UNII II: 5250-5350 MHz
8x 20 MHz channels
4x 40 MHz channels
UNII II requires DFS
149 161157153 Band
Edge
Channel
Frequency (MHz) 5745 5765 5785 5805 5850
Band
Edge
5725
US UNII III / ISM band
5725-5850 MHz
4x 20 MHz channels
2x 40 MHz channels
Channel
Frequency (MHz)
US intermediate band
(UNII II extended)
5450-5725 MHz
11x 20 MHz channels
5x 40 MHz channels
Requires DFS
36 4844 5240 56 6460 Band
Edge
5180 5200 5220 5240 5260 5280 5300 5320 5350
Band
Edge
5150
100 112108 116104 120 128124
5500 5520 5540 5560 5580 5600 5620 5640
Band
Edge
5450
136 140 Band
Edge
5680 5700 5725
132
5660
US Intermediate Band
(UNII-II Extended)
5470-5725 MHz
11x20 MHz channels
5x40 MHz channels
Requires DFS
US UNII-I and UNII-II Bands
UNII-I: 5150-5250 MHz
UNII-II: 5250-5350 MHz
8x20 MHz channels
4x40 MHz channels
UNII-II requires DFS
US UNI-III / ISM Band
5725-5850 MHz
4x20 MHz channels
2x40 MHz channels
Channels defined for 5-GHz Band (US Regulations), Showing Common 20-Mhz Channel Plan and 40-Mhz Options
165

Table 2 lists the typical channels available for some example regulatory domains at the time of
publication.
Actual channel availability for any given installation depends on the specific AP model selected, the
present status of regulations, and any local country specific deviations or changes from this table since
the time of publication. Aruba recommends that you contact our Technical Assistance Center or a
professional installer to obtain a specific list for your deployment. An Aruba controller will also report
Table 2 Typical 5GHz Channels Available for Use in Selected Regulatory Domains
Channel #
Frequency
(MHz)
USA Europe Japan Singapore China Israel Korea Brazil
36 5180 Yes Yes Yes Yes No Yes Yes Yes
DFSChannels
100 5500 Yes Yes Yes No No No Yes Yes
120 5600 No No Yes No No No Yes No
132 5660 No No Yes No No No No No
136 5680 Yes Yes Yes No No No No Yes
140 5700 Yes Yes Yes No No No No Yes
149 5745 Yes No No Yes Yes No Yes Yes
Total without DFS 9 4 4 9 5 4 9 9
Total with DFS 20 15 19 13 5 8 21 20

the valid channels for a given regulatory domain with the “show ap allowed-channels country-
code <country code>” command.
Enabling or disabling specific channels is done through the Regulatory Domain Profile of the AP Group
to which the auditorium APs belong. Configuration of channel availability is covered in Chapter 6,
“Configuring ArubaOS for HD-WLANs” on page 67.
To DFS or Not to DFS?
With as many as twenty 20-MHz channels (different vendors support slightly different numbers), the
5-GHz band with DFS now has sufficient channels to achieve high performance in a 500-seat auditorium
without channel reuse in dozens of countries. Without DFS channels, the goal can still be achieved, but
the radios will be oversubscribed and the per-client average throughput will be much lower. So why
wouldn’t everyone use DFS?
Three significant exceptions could adversely affect HD WLAN performance with DFS enabled. The
wireless architect must assess whether either of these exceptions applies to their organization:
 Proximity to radar sources in the 5250-MHz to 5725-MHz band.
 Lack of DFS support on critical client devices.
 The Receive Sensitivity Tuning-Based Channel Reuse feature of ArubaOS is needed.
First, actual or false positive radar events can be extremely disruptive to a WLAN that attempts to use
DFS channels. Users on DFS channels can potentially experience lengthy service interruptions from
radar events. Because radar frequencies do not align with 802.11 channelization, such events can
impact multiple Wi-Fi channels simultaneously. See Appendix D, “Dynamic Frequency
Selection Operation” on page 119 for a more detailed discussion of radar operation and DFS
compatibility.
Second, as of this writing, many 802.11 client Network Interface Cards (NICs) do not support DFS
channels, especially outside the United States. Client devices in an auditorium are not generally under
the control of the facility operator, so always be sure to include non-DFS channels in your HD WLAN
channel plan for these devices.
Third, ArubaOS will not allow the Aruba Receive Sensitivity Tuning-Based Channel Reuse feature to be
used with DFS channels, because it could result in the AP missing radar events. This feature is only
available on the non-DFS channels in any regulatory domain.
Site-Specific Restrictions
Because high-density coverage zones are just one part of a larger facility, the channel plan for the rest
of the site may also impose constraints on channel availability. Be sure to consider any reserved
channels that are required for indoor or outdoor mesh operations, or for dedicated applications such as
N O T E
As of October 5, 2009, the United States FCC and European Technical Standardization Institute
have disallowed 5600 to 5650MHz (approximately channels 120-132) for use with WLANs. This is to
avoid interference with airport terminal doppler radar systems. Aruba APs with approvals as of that
date, including AP-120 series and AP-105, are allowed to continue using those channels, but future
AP models may not support them.
N O T E
The question of usability is also a function of the client and what channels its chipset/driver
combination supports for that regulatory profile. For example, with driver version 13.1.1.1, both the
Intel 5100agn and 5300agn WLAN NICs support all DFS channels in the US (both 52-64 and 100-
140). However, with the same driver, the Intel 4965agn does not support channels 100-140. Another
example is the Cisco 7925g voice handset, which does not support channel 165.

IP surveillance video. It is prudent to conduct a spectrum clearing survey to ensure that no fixed
frequency interference sources would further reduce channel selection.
5-GHz Channel Reuse
Since wireless signal strength decays over distance, a given RF channel can be re-used at intervals. This
concept has long been used by mobile telephone networks, and it is central to most WLAN
architectures. All enterprise WLANs reuse channels in clusters to serve large areas, where the radios
are separated from one another by free space, walls, or other structures. In this case, the purpose of
reuse is to provide a consistent signal level everywhere in a facility, regardless of the actual number of
client devices. Figure 6 shows two channel reuse clusters and the relative position of reused channels.
Figure 6 Channel Plan with 13 Channels in 5GHz with Minimum Separation of Two Cells
However, in an auditorium, channel reuse is driven by the number of devices to be served. Because
each radio can serve a finite number of devices, there is a limit to the total number of clients that can be
in an area without either oversubscribing the APs or reusing the allowed radio channels.
Achieving channel reuse in a single room of less than 10,000 ft2 (930 m2) is technically challenging,
requires expensive directional antennas and costly physical installation. The antennas and cables can
negatively impact the room aesthetics, which is a concern in most buildings. However, no channel reuse
is needed for auditoriums of up to nearly 1,000 devices in the United States, Europe, Japan and Korea
with DFS enabled (assuming 50 simultaneously transmitting clients per radio). Without DFS, up to 650
devices can be accommodated in the US and 400 devices in Europe.
As this covers most common auditorium sizes, the main body of this VRD uses a simple lookup table
approach for capacity planning assuming that no channel reuse occurs. Appendix C, “Basic Picocell
Design” on page 113 presents the mathematics behind channel reuse distances. If your high-density
coverage zone does require reuse, picocells with under-floor mounting will likely be required. This is
described in Chapter 4, “RF Design for HD WLANs” on page 31.
161
36
149
48
44
40
60
52
157153
153
64
161
36
149
48
2 cell isolation
44
64
HD_247

Available 2.4-GHz Channels
This solution guide assumes that the 5-GHz band is the primary service band for all auditoriums.
However, many of today’s personal smartphones and enterprise single-mode voice handsets are not 5-
GHz-capable. Therefore, many high-density coverage zones must be dual-band to provide some reduced
level of service to those devices. The IEEE 802.11b/g standard allows only three nonoverlapping
channels in 2.4 GHz, installed facing downward, as shown in Figure 7.
Figure 7 2.4-GHz Nonoverlapping Channels
These channels are available in most countries today. With a small amount of overlap, four channels
have sometimes been employed to increase overall system capacity. However, four-channel plans are
not advisable in HD WLANs due to the very high levels of ACI already present in the environment.
Because of the very limited number of nonoverlapping channels in the 2.4-GHz band, it is vital to
anticipate how many of those radios will be on that band and to conduct a basic traffic study for the
applications expected in your high-density coverage area. Aruba has found that most smartphones that
provide basic push email service have low duty cycles and consume 256 Kbps or less. Voice-over-Wi-Fi
handsets using higher quality G.711 codecs generate 128 Kbps of bidirectional traffic.
2.4-GHz Channel Reuse
Because 2.4-GHz radio signals travel nearly twice as far in free space as 5-GHz signals and experience
less attenuation when penetrating objects or people, channel reuse is even harder to achieve in 2.4 GHz.
Overhead and wall-mount coverage strategies will not succeed in most auditoriums.Figure 8 shows the
maximum number of simultaneous transmitters is just 150 in the 2.4-GHz band, assuming 50 users per
radio.
Figure 8 2.4-GHz Channel Reuse vs. Users
HD_248
Users
Number of 2.4 GHz APs
No
Reuse
1
Reuse
2
Reuses
3
Reuses
NOT
PRACTICAL
NOT
PRACTICAL
600
450
300
150
0
3 6 9 12

In planning mixed 2.4-GHz and 5-GHz deployments with dual-band APs, only one 2.4-GHz radio should
be enabled on each of the three channels. Don't forget that these channels are very likely already being
reused outside the auditorium, which will further reduce overall capacity of each 2.4-GHz channel.
Step #3: Choose a Concurrent User Target
The next step is to figure out the practical limit for the number of client devices that can transmit
simultaneously on a radio in your environment while still achieving your capacity goal. This is one of
two main constraints on HD WLAN performance (the other being available channel count). The
concurrent user limit is determined by looking up the per-client throughput value that best matches the
capacity goal you picked in Step #1, adjusted for the expected mix of legacy and high-throughput
stations.
Some vendors attempt to simplify this with blanket rules, such as recommending no more than 10
active voice calls or 25 active data clients. This works well enough for standard WLAN deployments,
but is nowhere near precise enough for HD WLANs that need to serve large numbers of heterogeneous
users with relatively few radios. The wireless architect trying to serve 500 auditorium users with just 10
available channels needs to know for sure how far each AP can scale and whether channel reuse can be
avoided. If it cannot, then many more radios and a much more expensive and complex physical
installation will be required.
Aruba’s research has shown that per-client limits are primarily determined by the mix of legacy 802.11a/
b/g and 802.11n devices expected in the auditorium. The more legacy devices that are present, the lower
the limit will be. For further information on the testbed Aruba constructed for this VRD, including
detailed test results for both 20-MHz and 40-MHz channels, see Appendix A, “HD WLAN Testbed” on
page 95.
Mixed Auditoriums with Both 802.11n and Legacy Clients
In most auditoriums, it is probable that there will be a mix of 802.11a, 802.11g, and even 802.11b devices
coexisting with faster 802.11n clients. The important parameter here is time on the medium, because an
802.11a client with a top rate of 54 Mbps will tend to slow down a population of 802.11n HT20 clients at
150 Mbps if all have data to send. The same phenomenon exists in the 2.4-GHz band. Therefore, the
presence of even one older device can dramatically reduce the aggregate channel capacity, which in
turn reduces the maximum per-client limit per radio.
Mixed WLAN environments support the latest high throughput standards while still supporting the
legacy technologies 802.11g, 802.11b, and 802.11a through a protection mode mechanism that is part of
the 802.11n standard. This is an automatic response by the APs and high throughput clients in the
presence of legacy clients as detected in management frame capability fields. High throughput devices
support legacy clients by transmitting additional management frames that can be decoded by the legacy
clients. This support results in significantly reduced throughput for both HT and legacy station types. It
is important to note that the legacy client does NOT need to be associated to the HD WLAN to cause a
protection mode to be triggered. The mere presence of a legacy client will reduce throughput. It is very
difficult to create an environment where no legacy devices are present.
N O T E
With under-floor mounting, it may be possible to reuse each 2.4-GHz channel one time in a very
large auditorium over 10,000 ft2 (930 m2). If this is a requirement in your environment, see the
section on picocells using under-floor mounting in Chapter 4, “RF Design for HD WLANs” on
page 31.

As part of the validation testing for this VRD, Aruba completed open air client scaling test runs for five
different mixes of 802.11n and 802.11a clients:
 100% HT20 clients
 75% HT20 and 25% 802.11a clients
 50% HT20 and 50% 802.11a clients
 25% HT20 and 75% 802.11a clients
 100% 802.11a clients
The testbed included a heterogeneous mix of 50 different laptops and netbooks with a wide variety of
operating systems and wireless NICs, just as you would find in a real auditorium. Ixia Chariot was used
as the traffic generator.
Figure 9 shows the effect of these combinations on application-layer throughput. The left vertical axis is
the average per-client application-layer throughput in Mbps (shown by the lines). The right vertical axis
shows the total channel capacity relative to the total throughput for 10 clients transmitting at one time
(shown by the bars). When we change just 25% of the clients on a 5-GHz HT20 channel to be 802.11a
only, the average per-client throughput is reduced by between 20% and 25%, depending on the number
of stations in the test. Increasing the .11a client mix to 50/50 results in another 25% reduction in both
aggregate and per-client throughput. Interestingly, little difference was observed with less than 50%
HT20 clients.
Figure 9 5-GHz Per-Client Mixed-Mode TCP Client Scaling Performance
These results were obtained with airtime fairness enabled using “preferred” access mode which
provides somewhat more transmit slots to HT clients. Without airtime fairness, legacy clients starve
newer 802.11n clients by consuming a greater share of the airtime. Airtime fairness effectively reduces
the amount of time that is made available to legacy stations to transmit, which essentially penalizes
them to allow the fastest clients to obtain the bulk of the airtime. In Chapter 5, “Infrastructure
Optimizations for HD WLANs” on page 51, you will learn more about how to leverage this feature in
your HD WLAN.

Choosing a Concurrent User Target
Use Table 3 to choose the concurrent user limit for each 5-GHz HT20 AP. First, choose the row that
corresponds to your expected mix of legacy and 802.11n clients. Then find the column whose
throughput is closest to the capacity goal you chose in Step #1. Note the client count at the top of the
column and proceed to Step #4: Predict Total Capacity on page 27.
Step #4: Predict Total Capacity
By combining channel count with the concurrent user target, we can construct a simple chart that
allows the wireless designer to quickly determine the number of devices that are supportable for a
given number of nonoverlapping channels.
5-GHz Capacity
Use Figure 10 to quickly arrive at the total device capacity of your HD WLAN in 5 GHz. Choose your
country and whether DFS is available or not. Follow that upward to the line that matches the
concurrent user target you picked in Step #3: Choose a Concurrent User Target on page 25. The total
user/device count can be seen on the Y axis.
Figure 10 HD WLAN User Capacity Predictor
Table 3 TCP Bidirectional Mixed PHY Scaling Test (Per Client)
Clients
10 20 30 40 50
100% HT20 5.99 Mbps 2.99 Mbps 1.81 Mbps 1.30 Mbps 0.94 Mbps
75% HT20 / 25% 11a 4.69 Mbps 2.20 Mbps 1.46 Mbps 1.03 Mbps 0.77 Mbps
100% 11a 1.50 Mbps 0.75 Mbps 0.50 Mbps 0.36 Mbps 0.28 Mbps
HD_249no
1,200
200
400
600
Users
Available 20 MHz Channels
800
1,000
1,200
200
0 0
400
600
800
1,000
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
10 Users/AP
50 Users/AP
40 Users/AP
30 Users/AP
20 Users/AP
10 Users/Radio
20 Users/Radio
30 Users/Radio
40 Users/Radio
50 Users/Radio

The chart allows a wireless designer to rapidly assess the capacity limit of a given auditorium. Table 4
provides the same information in tabular form.
Table 4 HD WLAN User Capacity Matrix - 5 GHz
Radios 10/radio 20/radio 30/radio 40/radio 50/radio
1 10 20 30 40 50
2 20 40 60 80 100
3 30 60 90 120 150
4 40 80 120 160 200
5 50 100 150 200 250
6 60 120 180 240 300
7 70 140 210 280 350
8 80 160 240 320 400
9 90 180 270 360 450
10 100 200 300 400 500
11 110 220 330 440 550
12 120 240 360 480 600
13 130 260 390 520 650
14 140 280 420 560 700
15 150 300 450 600 750
16 160 320 480 640 800
17 170 340 510 680 850
18 180 360 540 720 900
19 190 380 570 760 950
20 200 400 600 800 1,000
21 210 420 630 840 1,050
22 220 440 660 880 1,100
23 230 460 690 920 1,150
24 240 480 720 960 1,200

2.4-GHz Capacity
We begin by determining how large the current population of 2.4-GHz-only devices is and what type of
growth to expect on that band. The following approaches can be used to answer these questions:
 Simply assume that each user has one 5-GHz and one 2.4-GHz client (such as a laptop and a
smartphone). This is the worst case.
 If dual-band coverage exists elsewhere in the facility, use historical WLAN client association data
from a network monitoring system, such as the AirWave Wireless Management Suite, to obtain a
ratio of 2.4-GHz to 5-GHz users as well as per-station bandwidth consumption.
In the second case, you would then multiply the base occupancy of the auditorium by the ratio of users
to get the 2.4-GHz population. To be conservative, increase the ratio by 5-10% to provide a safety margin
for near-term growth in the 2.4-GHz band.
Table 5 lists the maximum number of 2.4-GHz devices that are supportable for a given number of
nonoverlapping channels.
The obvious problem with this chart is how to support a 500-seat or larger auditorium where every user
has an iPhone, BlackBerry, or other 2.4-GHz-only-capable smartphone. If picocells are not feasible, then
the only solution is to oversubscribe each radio. Use Aruba's airtime fairness feature to help distribute
capacity evenly among the users associated to each AP.
Step #5: Validate the Capacity Goal
You now have the tools to validate whether the entire auditorium will meet the capacity goal you chose
in Step #1. It is common for the wireless architect to have to follow an iterative process, compromising
between channel count, radio loading, and minimum per-client throughput. If the capacity prediction in
Step #4 falls short of the capacity goal, repeat the first four steps until you achieve a balance you can
live with.
Table 5 HD WLAN User Capacity Matrix - 2.4 GHz
Radios 10/radio 20/radio 30/radio 40/radio 50/radio
1 10 20 30 40 50
2 20 40 60 80 100
3 30 60 90 120 150
4*
* CAUTION: 1 reuse is required, which requires picocell deployment. See Chapter 4, “RF Design for HD WLANs” on
page 31 and Appendix C, “Basic Picocell Design” on page 113 for more information.
40 80 120 160 200
5* 50 100 150 200 250
6* 60 120 180 240 300

High-Density Wireless Networks for Auditoriums VRD | Solution Guide RF Design for HD WLANs | 31
Chapter 4
RF Design for HD WLANs
Coverage in HD WLANs is achieved by carefully combining the number of APs as determined in the
previous chapter with the physical space for which the designer is providing wireless services.
Placing many APs in close proximity to one another and enabling them to operate with minimal
interference requires the use of a several specific wireless design principles. These principles must be
balanced against building limitations like mounting restrictions, cabling requirements, room shape, and
room size. This chapter will teach you how to achieve this balance successfully.
Coverage Strategies for Auditoriums
A coverage strategy is a specific method or approach for locating APs inside a wireless service area.
Generally, any given coverage strategy will also call for a specific antenna pattern providing required
directionality (even if it is just using integrated antennas in the AP).
Three basic coverage strategies for auditoriums are available to the wireless architect. Each strategy
has advantages and disadvantages that we will explore in this chapter. These methods should never be
combined to ensure that signal levels are as consistent as possible throughout the coverage area.
Overhead Coverage: This refers to placing APs on the ceiling above the seats in the
auditorium, usually with a special low-gain antenna with a radiation pattern directing the
signal at the floor.
Side Coverage: The AP is mounted to walls and/or pillars that exist in the auditorium,
generally no more than 12 ft (4 m) above the floor. Either directional or omnidirectional
antennas can be used, with the direction of maximum gain aimed sideways across the seats.
Floor Coverage: This design creates picocells using APs mounted in, under, or just above
the floor of the auditorium, with a low-gain downtilt antenna reversed to face straight up at
the ceiling. This strategy is the only one that can allow for multiple channel reuse inside a
room of 10,000 ft2 (930 m2) or less.
Within each of these approaches, a number of choices must be made, such as whether to use integrated
or external antennas, mounting method, minimum AP spacing, how APs will connect to the LAN, and so
forth.

32 | RF Design for HD WLANs High-Density Wireless Networks for Auditoriums VRD | Solution Guide
Overhead Coverage
Ceilings are a common AP mounting location because they generally allow an unobstructed view down
to the wireless clients. By distributing APs consistently and evenly across a ceiling, you are able to limit
AP-AP interference (also known as “coupling”) while providing very uniform signal levels for all client
devices at floor level. Figure 11 shows what an overhead coverage deployment would conceptually look
like.
Figure 11 Simplified Overhead Coverage Example
Overhead coverage is a good choice when uniform signal is desired everywhere in the auditorium.
Overhead APs are usually out of view above eye level. It is even possible to conceal the system
completely by flush mounting external antennas to the ceiling. Of course, it must be possible to access
the ceiling without too much difficulty or expense to pull cable and install equipment. No channel reuse
is possible with overhead coverage because the signal spreads. This applies to areas underneath
balconies of up to 10 rows, because APs in the front portion of the auditorium will generally have
favorable line-of-sight even if the AP immediately above is obstructed. Every AP will be available with
high signal strength everywhere in the auditorium.
HD_250
60
149
36
44
52
52
48
56
Overhead View
Side View
40
36 48

Some omnidirectional antennas are designed with built-in electrical downtilt. Aruba recommends the
use of these downtilt or squint antennas for overhead coverage, either integrated directly into the AP or
externally connected. Although they are omnidirectional in the horizontal plane, they have
directionality in the vertical plane. They focus substantial energy in the downward direction or, if
mounted under the floor facing up, they focus and receive energy upward. See Table 7 for specifications
of the models that Aruba recommends.
Figure 12 AP-ANT-16 Downtilt Antenna Flush-Mounted to Ceiling Grid
These antennas look like “patch” antennas but they are installed facing downward. They are electrically
designed to provide a full 360 degrees of omnidirectional coverage with standard vertical polarization.
However, when viewing the E-plane from the side, we can see that the antenna provides approximately
120 degrees of vertical beamwidth with the direction of maximum gain centered around a 45-degree
down angle, as shown in Figure 13. This produces a coverage pattern shaped like a “cone” underneath
the antenna.
Figure 13 E-Plane Antenna Pattern of AP-ANT-16
HD_116
0
330 30
60
90
120
150
180
210
240
270
300
3 dBi - 5 dBi = -2 dBi
directed at other
APs on ceiling
3 dBi - 1 dBi = 2 dB
at -90 degrees down
Max gain is directed to clients!
Direction of maximum gain
at -45° to ceiling,
max gain = +3 dBi

These are commonly referred to as “downtilt” or “squint” antennas. From the plot, it is clear that the
antenna pattern helps with interference rejection in two important ways:
 External room interference: Because the direction of maximum gain is straight down, 802.11
signals outside the room on the same floor will not be aligned within the 3-dB beamwidth of the
antenna. In the case of two auditoriums on top of one another, the back lobe is up to 12 dB down
from the main lobe.
 Reduced AP-AP interference at ceiling level: In the plane of the ceiling, the pattern of a downtilt
antenna is about 8 dB down from the main lobe, which allows APs to be spaced somewhat more
closely for a given EIRP.
A ceiling deployment can occur at, below, or above the level of the ceiling surface. Care should be taken
with above-ceiling installations when external antennas are not being used to leverage building
obstructions such as pillars, ductwork, or floor joists that can benefit the RF design by further reducing
AP-AP coupling within the room. The closer the obstruction, the greater the blocking effect. APs should
never be placed more than 6 inches above the ceiling material to minimize obstructions in the direction
of the users.
Figure 14 Use Attenuating Building Materials to Reduce AP-AP Coupling
Here is a summary of the advantages and disadvantages of overhead coverage for auditoriums:
Pros Cons
 APs can be concealed inside ceiling with flush-
mounted antennas
 APs can be mounted above eye level
 More uniform signal in the room when APs are
evenly distributed
 Clear line-of-sight to user devices and minimal
human-body attenuation
 Better CCI/ACI control between adjacent HD
WLANs (when downtilt antennas used)
 Channel reuse is not possible
 Difficulty of pulling cable to high ceiling locations
HD_251
Ceiling
material
g g
to reduce AP-AP coupling
HVAC
ductPipes
I-beam

Side Coverage (Walls or Pillars)
Wall installations are most often seen where ceiling or under-floor access is not possible or too
expensive. Wall installations come in every variety you can think of, because no two auditoriums are
the same. Common examples include:
 Co-located APs in an A/V area in the back of an auditorium with directional antennas facing
forwards.
 Hotel ballrooms where APs with integrated antennas can only be placed along the sides of the room,
mounted to speaker stands or simply placed on tables.
 Where pillars or columns exist in very large auditoriums, it is often practical to mount on them
3-6 ft (1-2 m) above the users.
 Structures with no overhead or under-floor access, which could include temporary structures like
tents or open air fairs.
As with overhead coverage, channel reuse is not possible when mounting to walls or pillars. Care must
be taken to orient antenna patterns to cover the intended area and reduce AP-to-AP interference.
Figure 15 shows what a wall-based side-coverage solution that uses integrated omnidirectional
antennas looks like conceptually.
Figure 15 Simplified Side Coverage Example with Integrated Antenna
The illustration is meant to show AP position and antenna pattern, not the actual signal propagation. In
fact, even in the very largest auditoriums every AP will likely be able to hear every other AP. It is vital
that adjacent channels, such as 36 and 40, not be adjacent on the wall. Aruba ARM will automatically
manage this for you, but the level of CCI/ACI in a side coverage design is much less desirable than in the
overhead or under-floor cases. You may find that mounting closer to the floor is more successful. For
example, one university customer experienced an issue when they side mounted the APs at 15-20 ft (3-5
m) above floor height. The APs all saw each other with strong enough signal strength that they auto
tuned their power down to match the ARM coverage index causing AP to client signals to be weaker
than required. This resulted in seated clients getting very inconsistent connectivity. When the APs were
moved to floor level, locating them underneath the desks/seats in a few locations, much better
performance was achieved.
You will note that half of the wall-mounted AP signals are lost to the next room (and 75% of the signal in
the corners). With multiple adjacent HD WLANs this can be exploited by the wireless designer, but
otherwise it represents a waste of signal.
HD_252
60
4036
44
52
48
56
Overhead View
Front View
149
4036

You can overcome the signal leakage problem through the use of low-gain external directional antennas
aimed sideways. This can also be achieved very inexpensively by mounting the Aruba AP-105 with its
integrated downtilt pattern vertically on the wall, pointing back to the seats. In this case, no special
antenna is required. See Table 8 for specifications on the models that Aruba recommends.
Figure 16 Simplified Side Coverage Example with Directional Antennas
This strategy also allows APs to be spaced slightly closer together for the same reasons explained under
Overhead Coverage. For details on computing minimum AP-AP separation, see Appendix C, “Basic
Picocell Design” on page 113.
Aruba strongly advises against the use of high-gain directional antennas (8 dBi or more) in auditoriums
for several reasons:
 Questionable benefit: With MIMO technology, signal scattering in typical size auditoriums negates
any value of a narrower beamwidth. At distances typically required in an HD WLAN, higher gain
antennas are not necessary for good coverage and can increase the interfering signal levels within
the coverage space significantly.
 Poor near-field signal: Narrow vertical-beamwidth antennas mounted just 12-15 ft (4-5 m) above
the floor do not actually reach the ground for dozens of yards (meters). Close in to the antenna,
clients may experience weak signal as a result of being outside the 3-dB beamwidth
 Increased interference outside room: High-gain directional antennas can adversely affect
WLANs outside the auditorium in the direction of maximum gain.
 Multiple radomes: The maximum gain for a dual-band antenna in a single radome is about 8 dBi.
Higher gain requires separate antenna radomes for each band. This can be unsightly.
 Aesthetics: MIMO panel antennas are relatively large, have multiple RF cables, and generally
require an azimuth-elevation swivel mount. This looks great on a rooftop mast, but not so good in an
ornate auditorium.
HD_253
Front View
Overhead View

Sometimes pillars or columns exist in an auditorium, and they may even have existing cable pathways
to them. These can be used by the wireless designer to achieve more uniform coverage of a room than
is possible from just the walls alone. When using integrated omnidirectional antennas, be sure to take
into account the “shadow” that a pillar or column creates on the opposite side from the AP. This can be
used to the designer’s advantage to limit AP-AP coupling. The closer the AP is to the pillar, the greater
the blocking effect.
Figure 17 Simplified Column Mounting Coverage Example
As you can see, an infinite variety of side-coverage scenarios are possible. Here is a summary of the
advantages and disadvantages of side coverage for auditoriums:
Pros Cons
 Easy access for installing APs and pulling cable
 Columns can be used to deliberately create
RF shadows
 Channel reuse is not possible
 Inconsistent signal levels on each channel due to
AP location
 Increased human body attenuation
 Harder to control CCI/ACI between rooms
 Wasted signal bleed outside desired
coverage area
HD_254

Floor Coverage (Picocells)
By far the best coverage strategy for auditoriums is mounting under, in, or just above the floor. In this
design, we flip the overhead model upside down and use either integrated or external downtilted
antennas that point back at the ceiling and use very low transmit power.
This is the only coverage strategy that allows for channel reuse in auditoriums smaller than 10,000 ft2
(930 m2). Aruba calls this a picocell design. By using very low EIRP and taking advantage of the
attenuation provided by human bodies in the seats, Aruba has successfully achieved single channel
reuse distances of just 30 ft (9 m).
Figure 18 Simplified Picocell Coverage Example
Floor mounting is the best choice when there is convenient access underneath the auditorium either for
locating APs or simply pulling cable up into the auditorium from beneath. APs can be located in small
enclosures that are permanently mounted underneath or behind seats.
This strategy has all the advantages of overhead coverage, without the maintenance access headaches.
Because signal is directed upward, impact on adjacent HD WLANs on the same floor is negligible. In
multifloor buildings, inter-floor isolation is also generally good.
HD_256
Overhead View
Side View
161
36
149
48
44
40
60
52
157153
153
64
161
36
149
48
44
64 44 157153 4464

It may also be possible to install APs in the ceiling of the floor or basement underneath, shooting up
through the floor. This method can allow for even finer control of the cell size. However, it may be
necessary to use directional antennas with 6-8 dBi higher gain to compensate for interfloor absorption,
such as the AP-ANT-18. Many invisible construction details can influence RF penetration of floor slabs.
Validation testing in a variety of possible configurations should be completed before this method is
selected. The distance from the AP to the slab and floor construction have a direct impact on the size of
the cell in the user space.
Figure 19 Effect of AP Distance on Picocell Width
Aruba has studied signal propagation of underfloor mounting. Figure 20 shows an AirMagnet survey of
an AP-124 with AP-ANT-16 facing up on channel 44 at 3 dBm conducted power, or 6 dBm total EIRP. It
is mounted underneath a layer of ¾-in plywood.
Figure 20 AirMagnet 2D Survey of AP-124 with AP-ANT-16 Picocell at 6 dBm EIRP
HD_257
Floor
slab
Signal
Signal
Channel 44 AP-ANT-16
20 ft (6 m)
20ft(6m)
AP

The radius of the -70 dBm signal was approximately 10 ft (3 m) in this test. Aruba subsequently set up
two APs 40 ft (12 m) apart and measured signal roll off between them. Figure 21 shows that roughly 20
dB of isolation was achieved between these cells.
Figure 21 AirMagnet 3D Survey of Side-by-Side Picocells at 6 dBm EIRP
Here is a summary of the advantages and disadvantages of floor coverage for auditoriums. For more
detailed information on picocell design, see Appendix C, “Basic Picocell Design” on page 113 or contact
your local Aruba representative.
Choosing Access Points and Antennas
The process for deciding which AP and optional external antenna to use for an auditorium deployment
requires that you have chosen a preferred coverage strategy and are familiar with the physical
installation constraints in the coverage area.
Pros Cons
 Channel reuse is possible
 Higher AP densities can be achieved
 APs can be easily concealed
 More uniform signal in the room when APs are
evenly distributed
 Better CCI/ACI control between adjacent
HD WLANs
 Access underneath the auditorium
 Availability of cable pathways beneath the floor
 Higher signal attenuation requiring higher gain
antennas
 Validation testing is required to characterize floor
attenuation

Recommended Products
Aruba offers both integrated-antenna and external-antenna capable 802.11n APs to enable you to
implement the plan of your choice. Table 6 compares features of the Aruba 802.11n APs, particularly
antennas and RF performance.
Table 6 Aruba 802.11n APs
Integrated Antennas External Antenna
Model AP-105 AP-125 AP-124
Radios MIMO 2x2:2 3x3:2 3x3:2
Number Dual Radio Dual Radio Dual Radio
Antenna Integrated downtilt
antenna
Integrated dipole antenna 3 dual-band RPSMA
connectors
Transmit Power (5GHz) MCS15 = +15 dBm
MCS0 = +20 dBm
54Mbps = +17 dBm
6Mbps = +20 dBm
MCS15 = +12 dBm
MCS0 = +17 dBm
54 Mbps = +13 dBm
6 Mbps = +17 dBm
Same as AP-125
Receive Sensitivity
(5GHz)
MCS15 = -77 dBm
MCS0 = -96 dBm
54 Mbps = -83 dBm
6 Mbps = -96 dBm
MCS15 = -65 dBm
MCS0 = -91 dBm
54 Mbps = -77 dBm
6 Mbps = -91 dBm
Same as AP-125
Maximum Antenna
Gain
2.4 GHz = 2.5 dBi
5.150 GHz - 5.875 GHz =
4.0 dBi
2.4-2.5 GHz = 3.2 dBi
5.150- 5.875 GHz =
5.2 dBi
n/a
E-Plane (Vertical)
Antenna Pattern
Depends on selected
external antenna
Advantages  Best TX power
 Best RX sensitivity
 Lowest cost
 Integrated downtilt
antenna
 Smallest footprint
 Wall or ceiling mount
 3x3 MIMO
 High performance
CPU
 Integrated dipole
antenna
 Wall or ceiling mount
 3x3 MIMO
 High performance
CPU
 Supports external
antennas
 AP can be concealed
behind walls or
ceilings

Table 7 and Table 8 list the antennas that are recommended for use with the AP-124 in external antenna
deployments.
Table 7 Downtilt Antennas
Model AP-ANT-13B-KIT AP-ANT-16
Antenna
Elements
3 radomes / 1 element each 1 radome / 3 elements inside
Maximum
Antenna Gain
 2.4-2.5 GHz (4.4 dBi)
 4.9-5.9 GHz (3.3 dBi)
 2.4-2.5 GHz (3.9 dBi)
 4.9-5.9 GHz (4.7 dBi)
E-Plane (Vertical)
Antenna Pattern
> 60degrees
(centered at +/-45 degrees down angle)
> 60degrees
(centered at +/-45 degrees down angle)
H-Plane
(Horizontal)
Antenna Pattern
Omnidirectional Omnidirectional
Dimensions 2.0" x 2.0" x 0.7"
5.1 x 5.1 x 1.8 cm
12.1" x 3.6" x 0.9"
30.8 x 9.2 x 2.2 cm

Table 8 Low-Gain Directional Antenna
Model AP-ANT-17 AP-ANT-18
Antenna
Elements
3
(Linear vertical & dual slant +/- 45 degrees)
3
(Linear vertical & dual slant +/- 45 degrees)
Maximum
Antenna Gain
 2.4-2.5 GHz (6.0 dBi)
 4.9-5.875 GHz (5.0 dBi)
 2.4 - 2.5 GHz (7.5 dBi)
 5.15 - 5.875 GHz (7.5 dBi)
E-Plane (Vertical)
Antenna Pattern
60 degrees
(with 15 degree electrical downtilt)
60 degrees
(with 15 degree electrical downtilt)
H-Plane
(Horizontal)
Antenna Pattern
120 degrees 60 degrees
Dimensions 7.9" x 7.9" x 1.3"
20.1 x 20.1 x 3.2 cm
7.9" x 7.9" x 1.3"
20.1 x 20.1 x 3.2 cm

Choosing an Access Point
In general, the AP-105 is the most economical, flexible and aesthetically pleasing solution for
auditoriums. It can be directly mounted in the user space. The integrated downtilt antenna can be
oriented up, down, or sideways so it can be used with all three coverage strategies.
The AP-125 is the best choice for wall-mounted installs where the wireless designer wants an
omnidirectional pattern to serve both sides of a wall. It contains a higher performance CPU than the
AP-105. Otherwise, the AP-105 is more cost effective.
Where external antennas are needed or desired, the AP-124 is required. This could be to conceal the AP
outside the user space using flush-mounted antennas. Or it could be driven by the need to a specific
type of directional antennas.
Use the decision tree Figure 22 to simplify the decision of which AP model and corresponding antenna
is appropriate for your specific environment.
Figure 22 AP and Antenna Selection Tree
External Antenna Selection
Several of the recommended options above include a particular external antenna model. External
antennas can provide the designer with additional options when designing HD WLANs:
 If a wide horizontal beamwidth (120 degrees), low-gain directional is needed, the AP-ANT-17 should
be used.
 If a narrow horizontal beamwidth (60 degrees) is needed, the AP-ANT-18 should be used.
 If an external downtilt antenna is needed, and a very small antenna is desired, choose the AP-ANT-
13B-KIT. This includes three small units, each less than 2 in (5 cm) square. However, they must be
individually mounted with 4-6 in (10-15 cm) separation between them.
 Alternatively, if you prefer a single radome, choose the AP-ANT-16. While larger than all the AP-
ANT-13B antennas put together, it requires only a single installation.
 For underfloor picocell deployments with the AP on the ceiling below, the AP-ANT-18 is
recommended facing straight up. If the AP will be in the auditorium (in the floor itself or a floor-
mounted enclosure) then use the AP-105 with no external antenna facing up.
However, before you choose an external downtilt antenna, be aware that the RF performance of the
AP-105 with its integrated antenna is equal to or better than an AP-124 with either the AP-ANT-13B or
AP-ANT-16. In general, you will find that the AP-105 is the more economical and higher-performing
solution. Unless you have a need to conceal the AP outside the user space, the AP-105 is the better
choice.
HD_274
Choose
coverage
strategy
Conceal
AP above
ceiling?
Overhead Side coverage
Picocell
Above or
below
floor?
External
or integrated
antenna?
No Yes
Ceiling
over
20 ft?
No Yes
Either
AP-105
or AP-125
AP-105
Above Below
AP-105
facing up
AP-124 +
ANT-18
facing up
Single
Radome?
No Yes
AP-124
plus
ANT-13B-Kit
AP-124
plus
ANT-16
Integrated External
Omni or
directional
Omni
Direct-
ional
AP-125
wall
mounted
AP-105
wall
mounted
120° or
60° beam
120° 60°
AP-124
plus
ANT-17
AP-124
plus
ANT-18

Minimum Spacing Between Adjacent Channel APs
As mentioned previously, this solution guide assumes no channel reuse due to the relatively small size
of auditoriums. So you need not compute a single channel reuse distance. However, in HD WLAN
designs it is also important to isolate APs from each other to reduce ACI. This can be done by ensuring
a minimum separation distance between APs. A wireless designer may also deliberately interpose
building structures, including existing floors and walls or newly-installed shielded boxes, to control AP-
AP coupling.
The impact of ACI is especially important to consider in an HD WLAN because the overall effect of ACI
is to reduce the total channel capacity. These two considerations are critical for determining the
minimum recommended AP spacing:
 Spacing between the integrated or external installed antennas
 Spacing between the APs themselves
Typically, if the APs are co-located with their antennas, the second distance can be ignored because the
characteristics of antennas used will solely determine the recommended distance. This is typically the
case with an integrated antenna AP or an external antenna that is at the same location as the AP (within
one meter). However, if the antennas are remotely located from the APs as may be the case when APs
are located in a closet with RF extension cables to the antennas, the distance between the APs in the
closet can be important to consider in addition to the spacing between the remote antennas.
AP and Antenna Spacing – Overhead and Underfloor Strategies
For overhead and floor-level picocell coverage strategies, the wireless designer should distribute APs
evenly around the auditorium for optimal performance. Ensure that the minimum physical separation
distance listed below is observed.
Figure 23 shows a conference center auditorium, and circles are used to display even AP spacing in the
coverage area. (The circles are a tool used to assist the designer with spacing only and are not the
actual RF coverage for individual APs).
Figure 23 Example Conference Center AP Layout
HD_258
Main
Entrance
16,500 ft2
(1,533 m2
)
400 user capacity

In this case, we consider 2.4 GHz as the worst case due to increased free space propagation in that
band. Table 9 lists the minimum required separation for two APs with 20 MHz minimum center
frequency separation (that is, 1 to 6 or 6 to 11). This provides an additional 15-dB reduction in coupling.
The interference target is typically recommend to be -85 dBm to ensure that no channel bandwidth
degradation occurs and all data rates are available. However, in HD WLANs this may not be possible
depending on the number of channels in use, so -75 dBm is sometimes used as a compromise between
increased capacity and reduced peak performance.
AP and Antenna Spacing – Side Coverage Strategy
In general, wall-mounted deployments on the sides of an auditorium should evenly distribute APs along
the length of each wall being used to maximize the physical separation between APs.
Of somewhat greater concern is a wall-mount deployment where only one wall is available, such as the
back of the auditorium in an audio/visual room where the APs will be colocated and connected to
external directional antennas on the back wall.
In the case of wall-mounted antennas, the gain of the antennas in the direction of other antennas can be
significantly lower than for the ceiling-mounted case. For example, the maximum gain of the AP-ANT-
18, which is a 60-degree sector is 7 dBi in the direction of the clients. However, the side-to-side gain in
the direction of other antennas mounted on the same wall is -10 dBi.
Use Table 10 when the APs are mounted with their antennas on the same wall.
Table 9 Interfering AP to AP Minimum Mounting Distance (Five 802.11BG Channel Separation)
Transmit Power
(dBm)
Interference Target
-85 dBm
Interference Target
-80 dBm
Interference Target
-75 dBm
15 200 ft / 61 m 114 ft / 35 m 65 ft / 20 m
12 144 ft / 44 m 82 ft / 25 m 46 ft / 14 m
9 98 ft / 30 m 58 ft / 17 m 32 ft / 9.8 m
6 72 ft / 22 m 39 ft / 12 m 22 ft / 6.9 m
N O T E
See Appendix C, “Basic Picocell Design” on page 113 for a detailed explanation of the math behind
this table.
Table 10 Adjacent Channel AP spacing (Channel 1 to 6 or 6 to 11),
Wall-Mounted Antenna AP-ANT-18
Transmit Power
(dBm)
Interference Target
-85 dBm
Interference Target
-80 dBm
Interference Target
-75 dBm
15 12.8 ft / 3.9 m 7.2 ft / 2.2 m 3.9 ft / 1.2 m
12 9.1 ft / 2.8 m 5.2 ft / 1.6 m 2.9 ft / 0.9 m
9 6.2 ft / 1.9 m 3.6 ft / 1.1 m 1.9 ft / 0.6 m
6 4.6 ft / 1.4 m 2.6 ft / 0.8 m 1.3 ft / 0.4 m
N O T E
See Appendix C, “Basic Picocell Design” on page 113 for a detailed explanation of the math behind
this table.

If the antennas are remotely located from the APs, the values of Table 10 apply to the minimum spacing
between antennas and it is a good idea to check that the minimum spacing between APs meets the
values of Table 11, which are computed for the direct coupling between APs that are located in a closet.
Aesthetic Considerations
In many auditoriums aesthetics requirements significantly limit the ability to attach APs in view. The
availability of suitable mounting locations can have a significant impact the performance of the overall
RF design. In the auditorium shown in Figure 24, high and low ceilings, dense users, and tightly
controlled aesthetics severely limit the options available to mount APs.
Sometimes a suitable cover can be utilized to hide the AP, but in most cases it is necessary to mount the
AP in spaces that are not visible. These spaces may include interstitial spaces between floors, drop
ceilings, behind curtains, catwalks, and maintenance areas.
Figure 24 Aesthetics Requirements Vary Between Auditoriums
Aruba recommends the following best practices for installations with restrictions on mounting:
 The small, attractive design of the AP-105 with no antennas makes it resemble a smoke alarm or
other typical ceiling device. The status lights on the AP can be disabled so there is no indication of
activity from the ground. Aesthetics committees are likely to approve the use of the AP-105 in
ceiling-mounted or wall-mounted deployments.
 Another option for wall-mounted installations is to use a flush-mounted panel antenna like the
AP-ANT-18, connected to an AP-124 mounted on the other side of the wall or inside the wall itself.
 For installations that absolutely cannot have any visible network equipment, mounting of AP-124
with AP-ANT-18 in the interfloor space below aiming up is the best solution.
Table 11 AP spacing (channel 1 to 6 or 6 to 11), APs in a closet
Transmit Power
(dBm)
Interference Target
-85 dBm
Interference Target
-80 dBm
Interference Target
-75 dBm
15 1.3 ft / 0.4 m 0.7 ft / 0.22 m 0.4 ft / 0.12 m
12 1.0 ft / 0.3 m 0.5 ft / 0.16 m 0.3 ft / 0.09 m
9 0.7 ft / 0.2 m 0.4 ft / 0.11 m 0.2 ft / 0.06 m
6 0.5 ft / 0.14 m 0.3 ft / 0.08 m 0.1 ft / 0.04 m

General Installation Best Practices
Antenna mounting locations are always important. Here are some suggestions:
 Select mounting locations that have no obstructions between the front of the antennas (or
integrated antenna APs) and the intended wireless clients.
 If external antennas are being used, plan to mount your APs as close to their antennas as possible. If
absolutely necessary, use good-quality, low-loss coaxial cable to connect AP to antenna when
mounting the AP some distance away from the antenna.
Follow these guidelines when aligning antennas:
 Do not mix mounting strategies in the same room. When planning adjacent HD WLANs, use the
same strategy (overhead, side, or picocell) in all rooms.
 Always mount antennas with built-in downtilt flat against the ceiling or floor so that the beam is
exactly vertical.
 Keep a safe distance between your integrated antenna APs and any location where people will be
present. There are Specific Absorption Rate (SAR) distance requirements designed to protect the
human body from coming into too-close contact with wireless devices and wireless energy. In the
U.S. the SAR regulations require at least 6 in (15 cm) of clearance between WLAN antennas and the
human body. Plan to allow at least this much clearance, though more is better.
 When using side coverage with directional antennas on opposite sides of the same room, mount the
antennas using an appropriate amount of mechanical downtilt so that the 3-dB beamwidth of the E-
plane is aimed below the far antennas. (Note that the AP-ANT-17 and AP-ANT-18 have a built-in
downtilt of about 20 degrees).
Managing Adjacent HD WLANs
It is common to find adjacent auditoriums at universities, hotels, and convention centers, either on the
same level or spanning multiple floors. In this case, it’s very possible that auditoriums will interfere
with one another and reduce overall throughput. In this situation, it may be necessary to use APs with
integrated or external directional antennas to preserve network performance.
Managing Clients
We stated earlier that the client devices dominate the CCI/ACI problem in HD WLANs because they
greatly outnumber the AP. Always use very low EIRP on the AP in an high-density deployment. Then,
enabling TPC is critical to getting as many client devices as possible to lower their power to match the
APs. Clients that do not honor TPC and use full power may create interference with adjacent
auditoriums. There is little you can do about it—user education is the key. Provide resources for your
users that identify the best version of driver and its appropriate configuration. Strongly encourage users
to update their drivers—and remind them often.
N O T E
Each strategy is carefully designed to (i) ensure a uniform signal level throughout the auditorium; and
(ii) control both AP-to-AP interference inside and outside the auditorium. Mixing strategies will
reduce performance and increase interference.

Overhead or Floor Coverage
If you’ve already selected an overhead or under-floor coverage strategy using downtilt antennas, your
HD WLANs will likely coexist without any further action on your part. Especially in the case of under-
floor coverage, where EIRP levels can be very low, the amount of signal penetrating to the next floor is
likely well below the receive sensitivity of the radios upstairs. The front-to-back ratio of the antennas,
which is a measure of the rejection of signals from the opposite side, will also diminish interference so
long as they are all aligned in the same direction. In general, the higher the gain of a directional antenna,
the greater its front-to-back ratio.
Figure 25 shows an elevation view of a two-story building with wireless installed in all the auditoriums.
An overhead coverage strategy has been selected. Floors generally absorb more RF energy than walls
(10 dB is a typical value).
Figure 25 Using AP-105 Integrated Directional Antenna to Isolate Adjacent HD WLANs
Side Coverage with Directional Antennas in Series
Figure 26 shows the same two-story building using a side-coverage strategy. Wall-mounted directional
antennas help reduce the noise between classrooms (typically 6 dB) on the same floor and also help to
reduce the noise between the upper and the lower floors.
Figure 26 Using AP-105 Integrated Directional Antenna to Isolate Adjacent HD WLANs
HD_266
Classrooms
+3 dBi
-10 dBi
10 dB
loss
Second Floor
Classrooms
HD_259
Classrooms
+3 dBi
-10 dBi
6 dB loss
Second Floor
Classrooms

Side Coverage with Back-to-Back APs and Directional Antennas
Sometimes in older buildings it is not possible to run power or data cabling to every wall. In these cases,
you can place APs with either integrated directional or external directional antennas on opposite sides
of the same wall. However, this is almost certain to increase ACI and CCI levels and must be done with
great care. Figure 27 shows the right and wrong ways to design this.
Never place back-to-back APs or antennas on the same channel. This does not work unless there is a lot
of space between them (at least 2X the adjacent-channel separation distances listed in AP and Antenna
Spacing – Side Coverage Strategy on page 46). Even with 20 dB front-to-back ratios (which would be
very good), interference will be significant. Instead, make sure there are at least 40 MHz of separation in
the channels (36 and 44 for instance).
Figure 27 Back-to-Back Directional Antennas
HD_260
40 MHz frequency isolation between APs
36
36
40
44
Wrong
Right
36 36
20 MHz frequency isolation
and observe adjacent channel
spatial separation distance
Right
Back-to-back APs
on same channel

High-Density Wireless Networks for Auditoriums VRD | Solution Guide Infrastructure Optimizations for HD WLANs | 51
Chapter 5
Infrastructure Optimizations
for HD WLANs
The HD WLAN capacity plan and RF coverage strategy you selected in the last two chapters depend on
a number of very important assumptions. For example, the usable channel count assumes that the AP
radios are optimally assigned and that all clients can use them. The concurrent user target assumes that
all clients in the auditorium are evenly distributed across APs, rather than being clustered together on
just a few of them. In this chapter, you will learn about specific Aruba infrastructure features that help
manage the environment to turn these assumptions into reality so that your design will work as
expected. Along with the capacity plan and RF design, the controller configuration is the third part of
the “recipe” for a successful high-density wireless network.
Essential ArubaOS Features for HD WLANs
ArubaOS can intelligently manage the HD WLAN environment to provide the best possible experience
to all users in the coverage area. To achieve this, the Aruba controller must be configured to
continuously optimize the allocation of channels, clients, power, and airtime. When learning HD WLAN
design, it is useful to think of these optimizations being applied in a specific sequence.
This chapter presents the ArubaOS features behind these optimizations in detail. Some of these features
require that the wireless designer makes certain choices, and these are covered as well.
Achieving Optimal Channel Distribution
To make best use of scarce spectrum, we must optimize the distribution of RF spectrum to APs and
clients. In any HD WLAN, we would like to use as many allowed RF channels as possible, and ensure
that they are properly distributed within the coverage area after accounting for in-band 802.11 and non-
Wi-Fi transmissions outside the room.
 Even distribution of
channels with ARM
 Enable load-aware,
voice-aware, and
video-aware scanning
 Unnecessary
2.4-GHz radios
disabled with Mode-
Aware ARM or static
assignment
 Enable DFS channels
if being used
 Shift all 5-GHz-
capable devices off
2.4-GHz band with
Band Steering
 Even distribution of
clients with Spectrum
Load Balancing
 Restrict the maximum
allowable EIRP with
ARM to minimize cell
overlap
 Control power on
clients with 802.11h
TPC
 Minimize CCI and
ACI with Receive
Sensitivity
Tuning-Based
Channel Reuse
 Ensure equal access
to medium with
Airtime Fairness
feature
 Limit “chatty”
protocols
 Enable Multicast
Rate Optimization
and IGMP Snooping
 Enable Dynamic
Multicast Optimization
for video
 Reduce rate
adaptation by
eliminating low
legacy rates
Optimal
Airtime
Management
Optimal
Power
Control
Optimal
Client
Distribution
Optimal
Channel
Distribution

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