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WHITE PAPER




802.11n: Look Before You Leap
Key considerations for moving to 802.11n
80211n look before_wp
The WLAN world is changing
With the introduction of 802.11n, the field of wireless local area networks (WLANs) is undergoing a paradigm shift as
phenomenal as the beginning of WLANs themselves. The extraordinary data rates that the final 802.11n standard will
support solidify the realization of an all-wireless enterprise network. Rich multimedia applications will be seamlessly
streamed to every point in a facility at better performance than possible with legacy 802.11a/b/g technologies.

But the question remains, “How do you plan a high performance 802.11n network?” Despite continual attention from
WLAN equipment vendors and endless discussion about the benefits of 802.11n, no one has adequately explained how
802.11n network planning works and, perhaps more importantly, how it doesn’t work! “How can I reap the maximum
benefit from a clean-slate 802.11n implementation?”, “What happens if I simply rip-and-replace access points (APs)
when migrating my existing network to 802.11n?”, “How would I best pursue a phased-migration to 802.11n?” In this
paper, we will explain the 802.11n basics necessary for answering these questions, and provide guidance to help you
choose the best strategy for your organization.




How 802.11n affects the                               This form of wireless congestion results in
                                                      dropped packets, slower networks, and reduced
three C’s of network planning                         capacity. In addition to traditional co-channel
                                                      and adjacent-channel interferences, 802.11n
Network planning should always be approached          5GHz band deployments must also consider
from the perspective of Context, Coverage, and        potential interference from radar systems.
Capacity (the three C’s of network planning). The     Context also includes the site-specific structure
technological advances of the 802.11n standard        of the environment containing the WLAN, which
affect all three of these. With respect to Context,   dramatically affects the performance of 802.11n’s
network planners must consider how interference       MIMO technology.
from new 40MHz channels and the complex site-
specific dependencies of Multiple Input Multiple      Interference and channel planning
Output (MIMO) technology affect channel planning
and AP placement. When considering Coverage,          Providing a more than two-times improvement in
designers must understand coverage differences        data rate performance over the 20MHz of frequency
between 802.11n and legacy systems and correctly      bandwidth used per channel in 802.11a/b/g, the new
define coverage requirements based on network         40MHz channels of 802.11n are a must for truly high
demands. Finally, Capacity is improved due to the     performance wireless networks. Unfortunately,
802.11n standard’s increased transmit data rates      bigger channels mean greater potential for interference
and MAC layer efficiencies. However, these capacity   and reduced spectrum for channel planning.
improvements can only be fully utilized when the
network client distribution is properly planned.      In the United States, if a 40MHz channel is used in
                                                      the 2.4GHz band, only one other non-overlapping
                                                      20MHz channel is available. The result is a greater
Context                                               likelihood for adjacent-channel interference in
                                                      the 2.4GHz band. Since channel planning in
The context of the environment in which a WLAN        2.4GHz was already a difficult task with only
is deployed is critical. Interference can be caused   three non-overlapping channels in 802.11a/b/g,
by neighboring APs or other wireless transmitters     the use of 40MHz channels is not recommended
broadcasting within the same frequency band.          for 2.4GHz deployments utilizing 802.11n.




1	 WHITE PAPER: 802.11n: Look Before You Leap
Fortunately, the 5GHz band frees 802.11n users          Site-specific effects of MIMO
from the tight spectrum constraints of the 2.4GHz
band. In the United States, the 5GHz band allows        MIMO is inherently site-specific
for 11 non-overlapping 40MHz channels if the AP         In legacy systems, interference caused by
is fully compliant with the dynamic frequency           reflections and diffractions of the transmitted signal
selection (DFS) restrictions (more is mentioned         (called multipath) was viewed as a hindrance to
on DFS in the next section). The abundance of           system performance and was compensated for by
non-overlapping 40MHz channels in the 5GHz band         including large fade margins in the system design to
allows an 802.11n deployment to take advantage          improve signal quality in areas with heavy multipath
of this performance gain and is the recommended         interference. In contrast, in MIMO systems
deployment strategy for high performance WLAN           multipath is the cornerstone of improving system
networks. See Table 3 for 802.11n channel overlap.      performance! By utilizing complex signal processing,
                                                        a MIMO system is capable of sending multiple data
The effect of radar avoidance on 5GHz                   streams at the same time. Effectively, this means
band channel planning (DFS)                             the received signal strength (RSSI) alone is no longer
                                                        sufficient for predicting system performance. Given
As stated previously, to achieve the maximum            the site-specific nature of MIMO, the use of site-
number of non-overlapping 40MHz channels in             specific planning and management tools for 802.11n
the 5GHz band an AP must be fully DFS compliant.        networks is highly recommended.
As defined in section 15 of the FCC rules and
regulations (47 CFR§15), this means that if a device    Contrasting performance of MIMO in dense
detects in-band interference from a nearby radar        office vs. long hallway environments
system it must immediately stop all transmission        What is the best environment for optimal MIMO
within that band for 30 minutes and switch to           performance? To use an academic term, multipath
another, non-interfering channel. Clearly, compliance   rich environments are the best scenarios for MIMO
with this federal regulation will have an effect on     performance. A multipath rich environment is one
5GHz channel planning since it requires the AP          in which the received signal is evenly distributed
channel change dynamically.                             among multiple different paths from the transmitter
                                                        to the receiver. The amount of difference between
Though the inclusion of DFS into a 5GHz 802.11n         individual paths is a basic metric for the richness
deployment may cause issues, the best practices         of the multipath. To best explain this, consider
for planning in a DFS band (5.25-5.35GHz and            two frequently occurring deployment scenarios: a
5.47-5.725GHz) do not change much from                  complex office building and a long, straight hallway.
planning in a non-DFS band. The first step in the
deployment process should involve a site-survey         In an office building, an AP is usually centralized
to determine whether any radar interferers exist in     within the desired coverage area. The room with the AP
the environment. Secondly, the network channel          is usually surrounded by other rooms, which may be
plan should be designed to avoid operation on any       connected by short winding hallways. In general, the
channels where DFS has been detected. Lastly,           environment is dense with obstacles to the signal
since the DFS standard requires the operating           path (typically walls) and there are very few, if any,
channel change dynamically, empty channels              Line-of-Sight (LOS) reception paths which aren’t in the
should be made available for device utilization if      room containing the AP. The complexity of this
radar interference is detected. A good rule of          environment generates many different paths for the
thumb is to provide at least one unused channel         transmitted signal and MIMO systems will perform very
in a non-DFS band.                                      well. This is the preferred MIMO deployment scenario.




2	 WHITE PAPER: 802.11n: Look Before You Leap
Changes in coverage from Legacy to 802.11n
    Transmit Data Rate                           Difference in Coverage from 802.11a/b/g to 802.11n

                                                802.11a/b/g clients will see very similar range when compared
    Minimum                                     to existing networks
    (1Mbps for 2.4GHz, 6Mbps for 5GHz)          802.11n clients will see very similar range when compared to
                                                existing networks
                                                802.11a/g clients will see some coverage improvement
    802.11a/g Maximum (54 Mbps)
                                                Largest increase in coverage area when using 802.11n clients

Table 1: Coverage improvements of 802.11n networks compared to 802.11a/b/g networks.




In a long, straight hallway scenario, the dominant                    as “communication at the minimum supported
path of the received signal is strongly LOS, and the                  transmit data rate for an AP at a given location.”
main multipath contributions come from reflections                    The following discussion outlines the truth
of the signal along the walls of the hallway. In                      regarding 802.11n coverage differences and is
this environment, a network designer can expect                       summarized in Table 1.
their MIMO performance to drop substantially as
the distance between the AP and the receiver                          Fundamentally, 802.11n radios are still bound by
increases down the hallway. Multipath components                      the same governmental power output regulations
in this scenario are fairly similar, and therefore the                (the Effective, Isotropic Radiated Power, or EIRP)
environment is not multipath rich and the MIMO                        as those in the 802.11a/b/g standards. This means
performance gain (while still available) is not as                    in an “apples to apples” comparison, a signal
large as the complex office scenario. With legacy                     transmitted by an 802.11n access point will go no
hardware, placing an AP to maximize LOS coverage                      farther than a signal transmitted by legacy hardware.
down a hallway was an accepted best-practice;                         Despite a lack of transmit range improvement, the
however, it is a hindrance to 802.11n performance                     best 802.11n implementations will leverage the
and is not an optimal deployment scenario.                            increased number of antennas on an 802.11n AP
                                                                      for increased receiver diversity gains. This allows
                                                                      the AP to hear fainter signals and effectively
Coverage                                                              increases the “visible” coverage area, and thereby
                                                                      reduce hidden node problems1.
Coverage differences between legacy 802.11a/b/g
systems and new 802.11n deployments are an                            Since a transmitted signal from an 802.11n AP
area plagued by much confusion. To truly assess                       travels no further than a legacy AP, the transmit
the difference in coverage between 802.11a/b/g                        data rate performance at a given RSSI becomes a
and 802.11n hardware, the term “coverage”                             vital metric for indicating the differences between
must be clearly defined. Throughout this paper,                       802.11n and legacy coverage. The transmit data
coverage will be defined as “communication at a                       rate indicates the speed at which an individual data
specified minimum transmit data rate at a given                       packet is wirelessly transmitted over the air. With
location.” “Range”, on the other hand, is defined                     respect to coverage differences between 802.11n




1
    H
     idden node problems cause interference issues in WLAN networks when devices with overlapping coverage “talk over each other”
    when communicating to other devices. The problem is minimized when all devices on a network can “hear” all other devices on the
    network and therefore know when it is their turn to communicate.




3	 WHITE PAPER: 802.11n: Look Before You Leap
and legacy networks, it is noteworthy that the                   must sacrifice some of its performance when legacy
coverage area of 802.11n at a transmit data rate of              devices are transmitting over the network. The three
54Mpbs is greater than the 54Mbps coverage area                  main cases of client distribution and their effects on
of 802.11a/g. This observation, combined with the                network performance are indicated in Table 2.
lack of range improvement for 802.11n, outlines the
main truth of 802.11n versus 802.11a/b/g coverage       To help mitigate the effect of legacy clients on a
differences. Specifically, coverage improvement         high-performance 802.11n network, it is important
is greater at higher transmit data rates and the        to note that 802.11n can operate in both the 2.4GHz
improvement is less at lower data rates. Further, it    and 5GHz bands. The 5GHz band has long been
is important to note that the use of 40MHz channels     left relatively empty by the mediocre adoption
will additionally improve the coverage area of an       of 802.11a networks, but this is an ideal scenario
802.11n AP at higher data rates, but it also will not   for the new 802.11n standard. Since so few
increase the transmit range of an 802.11n device.       802.11a clients exist in the 5GHz band, “n-only”
                                                        deployment scenarios can be carried out with
                                                        relative ease in this space without having to worry
Capacity                                                about the network being bogged down by legacy
                                                        clients. Deploying 802.11n in the 5GHz band is the
Due to the increased transmit data rates of the 802.11n recommended deployment scenario for n-only,
standard the capacity of 802.11n APs may be greater high performance WLANs.
than that of legacy APs. However, this improvement
is only available when 11n clients are associated       Targeted Upgrades to the Wired Network
to the 11n AP and within the coverage area of the
higher transmit data rates of the 802.11n standard.     Since the transmit data rates of the 802.11n
Since legacy clients on the network now have            standard have increased significantly, for the first
the potential to actually decrease overall system       time it’s possible that a wireless network could
performance, client distribution planning is a major    routinely out-perform a 100-BaseT network. What
factor for high-performance 802.11n deployments.        results is a need to intelligently upgrade wired
                                                                 network infrastructure to support gigabit Ethernet
Mixed networks                                                   on backhaul connections for 802.11n APs only as
                                                                 necessary. When a well-planned wireless network
A tremendously important feature of 802.11n is                   can deliver more than 100 Mbps and advanced
that it is fully backward compatible to 802.11a/b/g              meshing technology can decrease the demands on
networks and devices. While this provides a smooth               the wireless network in general, upgrading the entire
path for migrating existing wireless networks to                 wired network to support gigabit Ethernet can be an
802.11n, it also means that the 802.11n network                  unnecessary, and costly luxury.



802.11n Network performance with various client distributions
 Client Distribution                        Performance in terms of throughput

                                            802.11a/b/g clients will perform as well or slightly better than if
 802.11a/b/g clients only                   the network was 802.11a/b/g
                                            802.11a/b/g clients will perform as well or slightly better than if
                                            the network was 802.11a/b/g
 802.11a/b/g and n clients
                                            802.11n clients will perform better than 802.11a/b/g clients,
                                            but not as well as if it was only 802.11n clients
 802.11n clients only                       Best performance for 802.11n clients

Table 2: Relative throughput performance of an 802.11n network under various client distributions




4	 WHITE PAPER: 802.11n: Look Before You Leap
client segregation strategy has been highly-touted
                                                        by many industry professionals for good reason.
                                                        This methodology adapts into any migration
                                                        strategy, since a dual band 802.11n AP can support
                                                        both the 2.4GHz and 5GHz bands simultaneously,
                                                        while maintaining n-only segregation. Assuming
                                                        the vast majority of legacy clients are operating in
                                                        the 2.4GHz band, 40MHz channels can be used in
                                                        the 5GHz band without much trouble. Further, as
                                                        802.11n clients are introduced into the 2.4GHz band
               40MHz                       20MHz        it should not cause too much trouble for the 11n
                                                        network, since there will likely be a higher capacity
                                                        for 11n clients than for legacy clients. However,
                                                        due to the coexistence of legacy clients, 11n clients
Table 3: 802.11n Channel Overlap                        in the 2.4GHz band cannot operate at the same
                                                        performance as in the 5GHz band.

                                                       Additionally, AP placement for 802.11n networks
                                                       should consider the effects of multipath on MIMO
Recommendations for                                    system performance. APs should be placed in
                                                       locations which both maximize NLOS paths to
11n network migration                                  the receiver and minimize the signal path loss.
                                                       This results in a design decision depending on the
Until this point, this paper has primarily focused on  environment the network is being deployed. For
how the basics of 802.11n technology relate to the     example, in a building with thick, highly attenuating
three C’s of network planning, Coverage, Context,      walls, it may still be more beneficial to place APs
and Capacity. Now, we will consider three strategies in hallways despite its detrimental effect on MIMO
to migrate a legacy network to 802.11n in a way        system performance gain since the signal will not
that considers the three C’s, namely Clean-Slate,      propagate far enough via the NLOS paths for a
Rip-and-Replace, and Phased Migration.                 MIMO system to effectively utilize the rich
                                                       multipath present in the NLOS link. Due to MIMO
Clean-slate design                                     link design complications it is recommended a
                                                       network planning and simulation tool be used
Clean-slate network migration is based on the removal during AP placement since it drastically reduces
of existing network infrastructure and the subsequent the required calculation time.
deployment of new network infrastructure into
locations that are chosen based on maximization of     Ultimately, a clean-slate 802.11n design should
network performance. Despite the increased costs       produce a highly optimized network maximizing
of performing a clean-slate design for an 802.11n      performance and minimizing hardware costs. In
network, the clean-slate design migration strategy     addition to client distribution planning and AP
provides the advantage of a wireless network that can placement issues, upgrades to the wired backhaul
be specifically designed from the ground up to take    for the wireless network still need to be included,
full advantage of 802.11n’s unique qualities. The      but this is the case for all 802.11n deployments and
clean-slate design is discussed first, since it is the migrations. While a clean-slate migration may be the
model for an optimally performing 802.11n deployment. most expensive option from a planning perspective,
                                                       it is also the most likely to achieve a robust, reliable
To minimize the adverse effect of legacy clients on    network prepared to meet the current network
the network, use the 5GHz band for 802.11n clients     requirements and scale as those requirements
and high bandwidth applications. The 5GHz n-only       increase in the future.




5	 WHITE PAPER: 802.11n: Look Before You Leap
Rip-and-replace migration                                  In the end, the problems associated with a rip-and-replace
                                                           migration strategy simply highlight that planning
Rip-and-replace migration involves the one-to-one          will always be an integral step for any change to a
replacement of existing APs for new 802.11n APs.           wireless network. As such, even when the change
The rip-and-replace migration strategy is often            is as simple as a one-for-one device replacement the
discussed within 802.11n circles. The attractiveness       best practice for network planning is still to assess
of this strategy is that it does not require any           your network performance requirements, assess the
additional cabling or installation costs beyond            capability of your hardware and plan carefully.
simply exchanging existing APs for new 802.11n
units. Unfortunately, the simplicity of the rip-and-       Phased migration
replace migration strategy also faces drawbacks,
since the segregation of 802.11n clients into the          The main goal of a phased migration is to supplement
5GHz band may not provide sufficient coverage for          an existing 802.11a/b/g network with 802.11n APs in
some legacy systems. Additionally, legacy network          order to satisfy the demands of the existing network
design practices can conflict with best practices for      without incurring the cost associated with a full
maximizing MIMO system performance.                        network migration. Phased migrations are useful
                                                           if a portion of your facility is already adopting 11n
The problem with 5 GHz segregation within a                clients or if high bandwidth applications like large file
802.11n network is existing networks designed for          transfers are needed in specific locations.
maximum coverage at 2.4 GHz may have nodes too
far apart to satisfy the coverage requirements of a        The first step of any network migration should be to
5GHz deployment. Since networks designed on the            identify how the existing network is failing to satisfy
transmit range to maximize an AP’s coverage area           current demand. This is of particular importance for
are common among 802.11b/g deployments, this               a phased migration, since the stated goal is to meet
can be a persistent problem for rip-and-replace            the demands of specific portions of the existing
migrations wishing to provide 5GHz segregation.            network. A comprehensive network management
This is because, for the same transmit power, signals      system is very beneficial in this initial, requirements
do not travel as far in the 5 GHz band as they do in the   gathering, stage of the network migration.
2.4 GHz band. Since first-round 802.11n hardware
does not have an increased range over existing 5 GHz    Once performance requirements have been
APs, a network which was designed based on the          determined, if an existing legacy client network is
maximum range at 2.4 GHz will likely have numerous      already operating at maximum capacity, simply
coverage holes in its 5GHz 802.11n network.             replacing existing APs with 802.11n APs is not likely
                                                        to satisfy capacity requirements since legacy clients
As discussed previously, MIMO system performance on the migrated 802.11n network still only operate at
is maximized in multipath rich environments. In rip-    legacy speeds. As mentioned previously, if a portion
and-replace migrations (from an existing 802.11a/b/g of the network is already adopting 802.11n clients
network), it is likely there are several APs positioned and capacity requirements are being met, then a
to maximize coverage in hallways, as this was a         one-to-one exchange of a legacy AP for 802.11n
widely accepted best-practice for legacy network        hardware would be a prudent, short-term option.
deployments. A rip-and-replace migration of these       Otherwise, Motorola recommends AP density
“long hallway” APs reduces performance gains of         be increased by providing additional 802.11n APs
the 802.11n migration.                                  into the capacity-strapped region of the network.
                                                        These new APs should be given priority in terms of
Further, the issues presented above are for a network coverage area per AP via judicious power-planning
where it is assumed performance requirements            since the end goal of a phased migration is to have
have not increased from its initial deployment. If, as  the 802.11n network as the only network.
most likely the case, the performance requirements
have increased, a rip-and-replace strategy may be       Unless network requirements can be met through
impossible since increases in AP density may be         a rip-and-replace of a network’s subset, it is
necessary even for an 802.11n network. In contrast, recommended that new 802.11n APs be placed
an existing high-performance 802.11a/b/g network        independently with respect to existing legacy
may be able to migrate to 802.11n with fewer APs,       AP locations. This may also necessitate small
since there are known coverage improvements for         adjustments to the existing AP placements, but it
802.11n at higher data rates.                           will allow the phased migration to maximize the




6	 WHITE PAPER: 802.11n: Look Before You Leap
Using LANPlanner to Define Network Requirements



802.11g_(1)



                                                                                                 802.11n_(1)




                       802.11g_(2)                                                                                   802.11n_(3)




                                                                                   802.11n_(2)


                                        802.11g_(3)




          Original Network                                                              Clean-Slate




802.11n_(1)                                                               802.11g_(1)




                       802.11n_(2)                                                                  802.11g_(2)




                                                                                                                  802.11n_(3)


                                802.11n_(3)
                                     = 130.00 Mbps
                                     = 117.00 Mbps
                                     = 104.00 Mbps
                                     = 78.00 Mbps
                                     = 65.00 Mbps
                                     = 58.00 Mbps
                                     = 54.00 Mbps
                                     = 48.00 Mbps
                                     = 36.00 Mbps
                                     = 24.00 Mbps
                                     = 18.00 Mbps
                                     = 11.00 Mbps
                                     = 6.00 Mbps
                                     = 5.50 Mbps
                                     = 2.00 Mbps
                                     = 1.00 Mbps
                                     = 1.00 Mbps




              Rip-and-Replace                                                       Phased Migration


                    Transmit Data Rates



                           = 130.00 Mbps              = 36.00 Mbps    = 130.00 Mbps
MIMO system performance of 802.11n hardware                 efficiently determine not only the best migration path
and makes channel planning for the final hybrid             for their network, but also the costs associated with
network much simpler.                                       the upgrade.

                                                            Comparison of 802.11a/b/g vs. 802.11n
How can Motorola                                            With LANPlanner 11.0, 802.11n performance
LANPlanner® assist in the                                   improvements are no longer left to complicated
                                                            spreadsheets and non-intuitive numbers. When
802.11n planning process?                                   planning an 802.11n network, users are able to
                                                            clearly visualize the improvements they are
As various impediments to 802.11n planning have             receiving from their 802.11n deployment within
been discussed throughout this paper, it has become         a site-specific heatmap. Combined with the
increasingly apparent that properly planning an             exceptional, automated reporting capability of
802.11n network is a daunting task and is quite             LANPlanner, the benefits of 802.11n migration
difficult without the use of site-specific network          can be effectively communicated with ease.
modeling techniques. Fortunately, Motorola has tools
to assist network planners in this process. Whether         802.11n enabled survey capability
the project requirements involve a clean-slate design,
a rip-and-replace network migration, or simply a            Due to the channel planning issues surrounding
pre-deployment survey to assist in requirements             40MHz channels, DFS compliance in the 5GHz
gathering for a phased migration, LANPlanner 11.0           band, and the effect of client distribution on network
has the capability to meet the demands of 802.11n.          performance, pre-deployment surveying for 802.11n
                                                            is even more important than it was for 802.11a/b/g
Site-specific performance predictions                       networks. As such, the LANPlanner survey tool
                                                            Motorola SiteScanner™, provides native support for
802.11a/b/g planning has always been a computationally      802.11n surveys using widely available, industry
intense challenge. Now that 802.11n includes                standard 802.11n WLAN cards.
additional non-intuitive multipath factors, site-specific
modeling is even more important. As previously              In addition to pre-deployment surveys, this capability
discussed, 802.11n performance will change depending        allows customers to verify the performance of their
on the environment surrounding the AP. LANPlanner           802.11n APs once they have been deployed. Since
11.0 supports 802.11n planning, and has the capability      characterization of MIMO system performance requires
to include site-specific effects into its calculations      client association and data transfer from the AP, the
of the transmit data rate. These predictions are a          AP Performance mode of SiteScanner has been improved
must when planning new 802.11n deployments or               to seamlessly facilitate these new requirements.
migrating 802.11a/b/g networks to support 802.11n.
Motorola recommends any company deploying an                Automated tuning of
802.11n network simulate their deployment plans in          performance predictions
LANPlanner prior to making an upgrade decision.
                                                            As always, LANPlanner provides users with powerful
Legacy-to-11n Migration Wizard                              tools for optimizing their LANPlanner network
Many industry discussions have indicated that               planning models. Performance data collected from
the most likely scenario for first-round 802.11n            SiteScanner measurement surveys can be used to
deployments will be migrations of existing                  close the feedback loop for 802.11n performance
802.11a/b/g networks to support 802.11n. Since              predictions. Through optimization, the user can
this is the case, LANPlanner 11.0 provides a                achieve the maximum possible prediction accuracy
Network Migration Wizard for this very purpose.             for their 802.11n AP model.
Based on user-defined constraints for the migration
such as region of interest, client distribution, and        LANPlanner — part of the 802.11n solution
migration strategy, LANPlanner provides a suggested
baseline deployment for the specific needs of each          LANPlanner is part of Motorola’s comprehensive
environment. Network planners can easily input              802.11n solution, which also includes Motorola’s
the performance requirements of their network               AP-7131 802.11n access point and the RFS6000
and then simulate multiple migration scenarios to           and RFS700 802.11n ready WLAN switches.




8	 WHITE PAPER: 802.11n: Look Before You Leap
80211n look before_wp
motorola.com

Part number WP-802.11nPNM. Printed in USA 09/08. MOTOROLA and the Stylized M Logo are registered in the US Patent  Trademark Office. All other product or service
names are the property of their respective owners. ©2008 Motorola, Inc. All rights reserved. For system, product or services availability and specific information within your
country, please contact your local Motorola office or Business Partner. Specifications are subject to change without notice.

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80211n look before_wp

  • 1. WHITE PAPER 802.11n: Look Before You Leap Key considerations for moving to 802.11n
  • 3. The WLAN world is changing With the introduction of 802.11n, the field of wireless local area networks (WLANs) is undergoing a paradigm shift as phenomenal as the beginning of WLANs themselves. The extraordinary data rates that the final 802.11n standard will support solidify the realization of an all-wireless enterprise network. Rich multimedia applications will be seamlessly streamed to every point in a facility at better performance than possible with legacy 802.11a/b/g technologies. But the question remains, “How do you plan a high performance 802.11n network?” Despite continual attention from WLAN equipment vendors and endless discussion about the benefits of 802.11n, no one has adequately explained how 802.11n network planning works and, perhaps more importantly, how it doesn’t work! “How can I reap the maximum benefit from a clean-slate 802.11n implementation?”, “What happens if I simply rip-and-replace access points (APs) when migrating my existing network to 802.11n?”, “How would I best pursue a phased-migration to 802.11n?” In this paper, we will explain the 802.11n basics necessary for answering these questions, and provide guidance to help you choose the best strategy for your organization. How 802.11n affects the This form of wireless congestion results in dropped packets, slower networks, and reduced three C’s of network planning capacity. In addition to traditional co-channel and adjacent-channel interferences, 802.11n Network planning should always be approached 5GHz band deployments must also consider from the perspective of Context, Coverage, and potential interference from radar systems. Capacity (the three C’s of network planning). The Context also includes the site-specific structure technological advances of the 802.11n standard of the environment containing the WLAN, which affect all three of these. With respect to Context, dramatically affects the performance of 802.11n’s network planners must consider how interference MIMO technology. from new 40MHz channels and the complex site- specific dependencies of Multiple Input Multiple Interference and channel planning Output (MIMO) technology affect channel planning and AP placement. When considering Coverage, Providing a more than two-times improvement in designers must understand coverage differences data rate performance over the 20MHz of frequency between 802.11n and legacy systems and correctly bandwidth used per channel in 802.11a/b/g, the new define coverage requirements based on network 40MHz channels of 802.11n are a must for truly high demands. Finally, Capacity is improved due to the performance wireless networks. Unfortunately, 802.11n standard’s increased transmit data rates bigger channels mean greater potential for interference and MAC layer efficiencies. However, these capacity and reduced spectrum for channel planning. improvements can only be fully utilized when the network client distribution is properly planned. In the United States, if a 40MHz channel is used in the 2.4GHz band, only one other non-overlapping 20MHz channel is available. The result is a greater Context likelihood for adjacent-channel interference in the 2.4GHz band. Since channel planning in The context of the environment in which a WLAN 2.4GHz was already a difficult task with only is deployed is critical. Interference can be caused three non-overlapping channels in 802.11a/b/g, by neighboring APs or other wireless transmitters the use of 40MHz channels is not recommended broadcasting within the same frequency band. for 2.4GHz deployments utilizing 802.11n. 1 WHITE PAPER: 802.11n: Look Before You Leap
  • 4. Fortunately, the 5GHz band frees 802.11n users Site-specific effects of MIMO from the tight spectrum constraints of the 2.4GHz band. In the United States, the 5GHz band allows MIMO is inherently site-specific for 11 non-overlapping 40MHz channels if the AP In legacy systems, interference caused by is fully compliant with the dynamic frequency reflections and diffractions of the transmitted signal selection (DFS) restrictions (more is mentioned (called multipath) was viewed as a hindrance to on DFS in the next section). The abundance of system performance and was compensated for by non-overlapping 40MHz channels in the 5GHz band including large fade margins in the system design to allows an 802.11n deployment to take advantage improve signal quality in areas with heavy multipath of this performance gain and is the recommended interference. In contrast, in MIMO systems deployment strategy for high performance WLAN multipath is the cornerstone of improving system networks. See Table 3 for 802.11n channel overlap. performance! By utilizing complex signal processing, a MIMO system is capable of sending multiple data The effect of radar avoidance on 5GHz streams at the same time. Effectively, this means band channel planning (DFS) the received signal strength (RSSI) alone is no longer sufficient for predicting system performance. Given As stated previously, to achieve the maximum the site-specific nature of MIMO, the use of site- number of non-overlapping 40MHz channels in specific planning and management tools for 802.11n the 5GHz band an AP must be fully DFS compliant. networks is highly recommended. As defined in section 15 of the FCC rules and regulations (47 CFR§15), this means that if a device Contrasting performance of MIMO in dense detects in-band interference from a nearby radar office vs. long hallway environments system it must immediately stop all transmission What is the best environment for optimal MIMO within that band for 30 minutes and switch to performance? To use an academic term, multipath another, non-interfering channel. Clearly, compliance rich environments are the best scenarios for MIMO with this federal regulation will have an effect on performance. A multipath rich environment is one 5GHz channel planning since it requires the AP in which the received signal is evenly distributed channel change dynamically. among multiple different paths from the transmitter to the receiver. The amount of difference between Though the inclusion of DFS into a 5GHz 802.11n individual paths is a basic metric for the richness deployment may cause issues, the best practices of the multipath. To best explain this, consider for planning in a DFS band (5.25-5.35GHz and two frequently occurring deployment scenarios: a 5.47-5.725GHz) do not change much from complex office building and a long, straight hallway. planning in a non-DFS band. The first step in the deployment process should involve a site-survey In an office building, an AP is usually centralized to determine whether any radar interferers exist in within the desired coverage area. The room with the AP the environment. Secondly, the network channel is usually surrounded by other rooms, which may be plan should be designed to avoid operation on any connected by short winding hallways. In general, the channels where DFS has been detected. Lastly, environment is dense with obstacles to the signal since the DFS standard requires the operating path (typically walls) and there are very few, if any, channel change dynamically, empty channels Line-of-Sight (LOS) reception paths which aren’t in the should be made available for device utilization if room containing the AP. The complexity of this radar interference is detected. A good rule of environment generates many different paths for the thumb is to provide at least one unused channel transmitted signal and MIMO systems will perform very in a non-DFS band. well. This is the preferred MIMO deployment scenario. 2 WHITE PAPER: 802.11n: Look Before You Leap
  • 5. Changes in coverage from Legacy to 802.11n Transmit Data Rate Difference in Coverage from 802.11a/b/g to 802.11n 802.11a/b/g clients will see very similar range when compared Minimum to existing networks (1Mbps for 2.4GHz, 6Mbps for 5GHz) 802.11n clients will see very similar range when compared to existing networks 802.11a/g clients will see some coverage improvement 802.11a/g Maximum (54 Mbps) Largest increase in coverage area when using 802.11n clients Table 1: Coverage improvements of 802.11n networks compared to 802.11a/b/g networks. In a long, straight hallway scenario, the dominant as “communication at the minimum supported path of the received signal is strongly LOS, and the transmit data rate for an AP at a given location.” main multipath contributions come from reflections The following discussion outlines the truth of the signal along the walls of the hallway. In regarding 802.11n coverage differences and is this environment, a network designer can expect summarized in Table 1. their MIMO performance to drop substantially as the distance between the AP and the receiver Fundamentally, 802.11n radios are still bound by increases down the hallway. Multipath components the same governmental power output regulations in this scenario are fairly similar, and therefore the (the Effective, Isotropic Radiated Power, or EIRP) environment is not multipath rich and the MIMO as those in the 802.11a/b/g standards. This means performance gain (while still available) is not as in an “apples to apples” comparison, a signal large as the complex office scenario. With legacy transmitted by an 802.11n access point will go no hardware, placing an AP to maximize LOS coverage farther than a signal transmitted by legacy hardware. down a hallway was an accepted best-practice; Despite a lack of transmit range improvement, the however, it is a hindrance to 802.11n performance best 802.11n implementations will leverage the and is not an optimal deployment scenario. increased number of antennas on an 802.11n AP for increased receiver diversity gains. This allows the AP to hear fainter signals and effectively Coverage increases the “visible” coverage area, and thereby reduce hidden node problems1. Coverage differences between legacy 802.11a/b/g systems and new 802.11n deployments are an Since a transmitted signal from an 802.11n AP area plagued by much confusion. To truly assess travels no further than a legacy AP, the transmit the difference in coverage between 802.11a/b/g data rate performance at a given RSSI becomes a and 802.11n hardware, the term “coverage” vital metric for indicating the differences between must be clearly defined. Throughout this paper, 802.11n and legacy coverage. The transmit data coverage will be defined as “communication at a rate indicates the speed at which an individual data specified minimum transmit data rate at a given packet is wirelessly transmitted over the air. With location.” “Range”, on the other hand, is defined respect to coverage differences between 802.11n 1 H idden node problems cause interference issues in WLAN networks when devices with overlapping coverage “talk over each other” when communicating to other devices. The problem is minimized when all devices on a network can “hear” all other devices on the network and therefore know when it is their turn to communicate. 3 WHITE PAPER: 802.11n: Look Before You Leap
  • 6. and legacy networks, it is noteworthy that the must sacrifice some of its performance when legacy coverage area of 802.11n at a transmit data rate of devices are transmitting over the network. The three 54Mpbs is greater than the 54Mbps coverage area main cases of client distribution and their effects on of 802.11a/g. This observation, combined with the network performance are indicated in Table 2. lack of range improvement for 802.11n, outlines the main truth of 802.11n versus 802.11a/b/g coverage To help mitigate the effect of legacy clients on a differences. Specifically, coverage improvement high-performance 802.11n network, it is important is greater at higher transmit data rates and the to note that 802.11n can operate in both the 2.4GHz improvement is less at lower data rates. Further, it and 5GHz bands. The 5GHz band has long been is important to note that the use of 40MHz channels left relatively empty by the mediocre adoption will additionally improve the coverage area of an of 802.11a networks, but this is an ideal scenario 802.11n AP at higher data rates, but it also will not for the new 802.11n standard. Since so few increase the transmit range of an 802.11n device. 802.11a clients exist in the 5GHz band, “n-only” deployment scenarios can be carried out with relative ease in this space without having to worry Capacity about the network being bogged down by legacy clients. Deploying 802.11n in the 5GHz band is the Due to the increased transmit data rates of the 802.11n recommended deployment scenario for n-only, standard the capacity of 802.11n APs may be greater high performance WLANs. than that of legacy APs. However, this improvement is only available when 11n clients are associated Targeted Upgrades to the Wired Network to the 11n AP and within the coverage area of the higher transmit data rates of the 802.11n standard. Since the transmit data rates of the 802.11n Since legacy clients on the network now have standard have increased significantly, for the first the potential to actually decrease overall system time it’s possible that a wireless network could performance, client distribution planning is a major routinely out-perform a 100-BaseT network. What factor for high-performance 802.11n deployments. results is a need to intelligently upgrade wired network infrastructure to support gigabit Ethernet Mixed networks on backhaul connections for 802.11n APs only as necessary. When a well-planned wireless network A tremendously important feature of 802.11n is can deliver more than 100 Mbps and advanced that it is fully backward compatible to 802.11a/b/g meshing technology can decrease the demands on networks and devices. While this provides a smooth the wireless network in general, upgrading the entire path for migrating existing wireless networks to wired network to support gigabit Ethernet can be an 802.11n, it also means that the 802.11n network unnecessary, and costly luxury. 802.11n Network performance with various client distributions Client Distribution Performance in terms of throughput 802.11a/b/g clients will perform as well or slightly better than if 802.11a/b/g clients only the network was 802.11a/b/g 802.11a/b/g clients will perform as well or slightly better than if the network was 802.11a/b/g 802.11a/b/g and n clients 802.11n clients will perform better than 802.11a/b/g clients, but not as well as if it was only 802.11n clients 802.11n clients only Best performance for 802.11n clients Table 2: Relative throughput performance of an 802.11n network under various client distributions 4 WHITE PAPER: 802.11n: Look Before You Leap
  • 7. client segregation strategy has been highly-touted by many industry professionals for good reason. This methodology adapts into any migration strategy, since a dual band 802.11n AP can support both the 2.4GHz and 5GHz bands simultaneously, while maintaining n-only segregation. Assuming the vast majority of legacy clients are operating in the 2.4GHz band, 40MHz channels can be used in the 5GHz band without much trouble. Further, as 802.11n clients are introduced into the 2.4GHz band 40MHz 20MHz it should not cause too much trouble for the 11n network, since there will likely be a higher capacity for 11n clients than for legacy clients. However, due to the coexistence of legacy clients, 11n clients Table 3: 802.11n Channel Overlap in the 2.4GHz band cannot operate at the same performance as in the 5GHz band. Additionally, AP placement for 802.11n networks should consider the effects of multipath on MIMO Recommendations for system performance. APs should be placed in locations which both maximize NLOS paths to 11n network migration the receiver and minimize the signal path loss. This results in a design decision depending on the Until this point, this paper has primarily focused on environment the network is being deployed. For how the basics of 802.11n technology relate to the example, in a building with thick, highly attenuating three C’s of network planning, Coverage, Context, walls, it may still be more beneficial to place APs and Capacity. Now, we will consider three strategies in hallways despite its detrimental effect on MIMO to migrate a legacy network to 802.11n in a way system performance gain since the signal will not that considers the three C’s, namely Clean-Slate, propagate far enough via the NLOS paths for a Rip-and-Replace, and Phased Migration. MIMO system to effectively utilize the rich multipath present in the NLOS link. Due to MIMO Clean-slate design link design complications it is recommended a network planning and simulation tool be used Clean-slate network migration is based on the removal during AP placement since it drastically reduces of existing network infrastructure and the subsequent the required calculation time. deployment of new network infrastructure into locations that are chosen based on maximization of Ultimately, a clean-slate 802.11n design should network performance. Despite the increased costs produce a highly optimized network maximizing of performing a clean-slate design for an 802.11n performance and minimizing hardware costs. In network, the clean-slate design migration strategy addition to client distribution planning and AP provides the advantage of a wireless network that can placement issues, upgrades to the wired backhaul be specifically designed from the ground up to take for the wireless network still need to be included, full advantage of 802.11n’s unique qualities. The but this is the case for all 802.11n deployments and clean-slate design is discussed first, since it is the migrations. While a clean-slate migration may be the model for an optimally performing 802.11n deployment. most expensive option from a planning perspective, it is also the most likely to achieve a robust, reliable To minimize the adverse effect of legacy clients on network prepared to meet the current network the network, use the 5GHz band for 802.11n clients requirements and scale as those requirements and high bandwidth applications. The 5GHz n-only increase in the future. 5 WHITE PAPER: 802.11n: Look Before You Leap
  • 8. Rip-and-replace migration In the end, the problems associated with a rip-and-replace migration strategy simply highlight that planning Rip-and-replace migration involves the one-to-one will always be an integral step for any change to a replacement of existing APs for new 802.11n APs. wireless network. As such, even when the change The rip-and-replace migration strategy is often is as simple as a one-for-one device replacement the discussed within 802.11n circles. The attractiveness best practice for network planning is still to assess of this strategy is that it does not require any your network performance requirements, assess the additional cabling or installation costs beyond capability of your hardware and plan carefully. simply exchanging existing APs for new 802.11n units. Unfortunately, the simplicity of the rip-and- Phased migration replace migration strategy also faces drawbacks, since the segregation of 802.11n clients into the The main goal of a phased migration is to supplement 5GHz band may not provide sufficient coverage for an existing 802.11a/b/g network with 802.11n APs in some legacy systems. Additionally, legacy network order to satisfy the demands of the existing network design practices can conflict with best practices for without incurring the cost associated with a full maximizing MIMO system performance. network migration. Phased migrations are useful if a portion of your facility is already adopting 11n The problem with 5 GHz segregation within a clients or if high bandwidth applications like large file 802.11n network is existing networks designed for transfers are needed in specific locations. maximum coverage at 2.4 GHz may have nodes too far apart to satisfy the coverage requirements of a The first step of any network migration should be to 5GHz deployment. Since networks designed on the identify how the existing network is failing to satisfy transmit range to maximize an AP’s coverage area current demand. This is of particular importance for are common among 802.11b/g deployments, this a phased migration, since the stated goal is to meet can be a persistent problem for rip-and-replace the demands of specific portions of the existing migrations wishing to provide 5GHz segregation. network. A comprehensive network management This is because, for the same transmit power, signals system is very beneficial in this initial, requirements do not travel as far in the 5 GHz band as they do in the gathering, stage of the network migration. 2.4 GHz band. Since first-round 802.11n hardware does not have an increased range over existing 5 GHz Once performance requirements have been APs, a network which was designed based on the determined, if an existing legacy client network is maximum range at 2.4 GHz will likely have numerous already operating at maximum capacity, simply coverage holes in its 5GHz 802.11n network. replacing existing APs with 802.11n APs is not likely to satisfy capacity requirements since legacy clients As discussed previously, MIMO system performance on the migrated 802.11n network still only operate at is maximized in multipath rich environments. In rip- legacy speeds. As mentioned previously, if a portion and-replace migrations (from an existing 802.11a/b/g of the network is already adopting 802.11n clients network), it is likely there are several APs positioned and capacity requirements are being met, then a to maximize coverage in hallways, as this was a one-to-one exchange of a legacy AP for 802.11n widely accepted best-practice for legacy network hardware would be a prudent, short-term option. deployments. A rip-and-replace migration of these Otherwise, Motorola recommends AP density “long hallway” APs reduces performance gains of be increased by providing additional 802.11n APs the 802.11n migration. into the capacity-strapped region of the network. These new APs should be given priority in terms of Further, the issues presented above are for a network coverage area per AP via judicious power-planning where it is assumed performance requirements since the end goal of a phased migration is to have have not increased from its initial deployment. If, as the 802.11n network as the only network. most likely the case, the performance requirements have increased, a rip-and-replace strategy may be Unless network requirements can be met through impossible since increases in AP density may be a rip-and-replace of a network’s subset, it is necessary even for an 802.11n network. In contrast, recommended that new 802.11n APs be placed an existing high-performance 802.11a/b/g network independently with respect to existing legacy may be able to migrate to 802.11n with fewer APs, AP locations. This may also necessitate small since there are known coverage improvements for adjustments to the existing AP placements, but it 802.11n at higher data rates. will allow the phased migration to maximize the 6 WHITE PAPER: 802.11n: Look Before You Leap
  • 9. Using LANPlanner to Define Network Requirements 802.11g_(1) 802.11n_(1) 802.11g_(2) 802.11n_(3) 802.11n_(2) 802.11g_(3) Original Network Clean-Slate 802.11n_(1) 802.11g_(1) 802.11n_(2) 802.11g_(2) 802.11n_(3) 802.11n_(3) = 130.00 Mbps = 117.00 Mbps = 104.00 Mbps = 78.00 Mbps = 65.00 Mbps = 58.00 Mbps = 54.00 Mbps = 48.00 Mbps = 36.00 Mbps = 24.00 Mbps = 18.00 Mbps = 11.00 Mbps = 6.00 Mbps = 5.50 Mbps = 2.00 Mbps = 1.00 Mbps = 1.00 Mbps Rip-and-Replace Phased Migration Transmit Data Rates = 130.00 Mbps = 36.00 Mbps = 130.00 Mbps
  • 10. MIMO system performance of 802.11n hardware efficiently determine not only the best migration path and makes channel planning for the final hybrid for their network, but also the costs associated with network much simpler. the upgrade. Comparison of 802.11a/b/g vs. 802.11n How can Motorola With LANPlanner 11.0, 802.11n performance LANPlanner® assist in the improvements are no longer left to complicated spreadsheets and non-intuitive numbers. When 802.11n planning process? planning an 802.11n network, users are able to clearly visualize the improvements they are As various impediments to 802.11n planning have receiving from their 802.11n deployment within been discussed throughout this paper, it has become a site-specific heatmap. Combined with the increasingly apparent that properly planning an exceptional, automated reporting capability of 802.11n network is a daunting task and is quite LANPlanner, the benefits of 802.11n migration difficult without the use of site-specific network can be effectively communicated with ease. modeling techniques. Fortunately, Motorola has tools to assist network planners in this process. Whether 802.11n enabled survey capability the project requirements involve a clean-slate design, a rip-and-replace network migration, or simply a Due to the channel planning issues surrounding pre-deployment survey to assist in requirements 40MHz channels, DFS compliance in the 5GHz gathering for a phased migration, LANPlanner 11.0 band, and the effect of client distribution on network has the capability to meet the demands of 802.11n. performance, pre-deployment surveying for 802.11n is even more important than it was for 802.11a/b/g Site-specific performance predictions networks. As such, the LANPlanner survey tool Motorola SiteScanner™, provides native support for 802.11a/b/g planning has always been a computationally 802.11n surveys using widely available, industry intense challenge. Now that 802.11n includes standard 802.11n WLAN cards. additional non-intuitive multipath factors, site-specific modeling is even more important. As previously In addition to pre-deployment surveys, this capability discussed, 802.11n performance will change depending allows customers to verify the performance of their on the environment surrounding the AP. LANPlanner 802.11n APs once they have been deployed. Since 11.0 supports 802.11n planning, and has the capability characterization of MIMO system performance requires to include site-specific effects into its calculations client association and data transfer from the AP, the of the transmit data rate. These predictions are a AP Performance mode of SiteScanner has been improved must when planning new 802.11n deployments or to seamlessly facilitate these new requirements. migrating 802.11a/b/g networks to support 802.11n. Motorola recommends any company deploying an Automated tuning of 802.11n network simulate their deployment plans in performance predictions LANPlanner prior to making an upgrade decision. As always, LANPlanner provides users with powerful Legacy-to-11n Migration Wizard tools for optimizing their LANPlanner network Many industry discussions have indicated that planning models. Performance data collected from the most likely scenario for first-round 802.11n SiteScanner measurement surveys can be used to deployments will be migrations of existing close the feedback loop for 802.11n performance 802.11a/b/g networks to support 802.11n. Since predictions. Through optimization, the user can this is the case, LANPlanner 11.0 provides a achieve the maximum possible prediction accuracy Network Migration Wizard for this very purpose. for their 802.11n AP model. Based on user-defined constraints for the migration such as region of interest, client distribution, and LANPlanner — part of the 802.11n solution migration strategy, LANPlanner provides a suggested baseline deployment for the specific needs of each LANPlanner is part of Motorola’s comprehensive environment. Network planners can easily input 802.11n solution, which also includes Motorola’s the performance requirements of their network AP-7131 802.11n access point and the RFS6000 and then simulate multiple migration scenarios to and RFS700 802.11n ready WLAN switches. 8 WHITE PAPER: 802.11n: Look Before You Leap
  • 12. motorola.com Part number WP-802.11nPNM. Printed in USA 09/08. MOTOROLA and the Stylized M Logo are registered in the US Patent Trademark Office. All other product or service names are the property of their respective owners. ©2008 Motorola, Inc. All rights reserved. For system, product or services availability and specific information within your country, please contact your local Motorola office or Business Partner. Specifications are subject to change without notice.