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80211n look before_wp

  1. 1. WHITE PAPER802.11n: Look Before You LeapKey considerations for moving to 802.11n
  2. 2. The WLAN world is changingWith the introduction of 802.11n, the field of wireless local area networks (WLANs) is undergoing a paradigm shift asphenomenal as the beginning of WLANs themselves. The extraordinary data rates that the final 802.11n standard willsupport solidify the realization of an all-wireless enterprise network. Rich multimedia applications will be seamlesslystreamed 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 fromWLAN equipment vendors and endless discussion about the benefits of 802.11n, no one has adequately explained how802.11n network planning works and, perhaps more importantly, how it doesn’t work! “How can I reap the maximumbenefit 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 thispaper, we will explain the 802.11n basics necessary for answering these questions, and provide guidance to help youchoose the best strategy for your organization.How 802.11n affects the This form of wireless congestion results in dropped packets, slower networks, and reducedthree C’s of network planning capacity. In addition to traditional co-channel and adjacent-channel interferences, 802.11nNetwork planning should always be approached 5GHz band deployments must also considerfrom 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 structuretechnological advances of the 802.11n standard of the environment containing the WLAN, whichaffect all three of these. With respect to Context, dramatically affects the performance of 802.11n’snetwork planners must consider how interference MIMO technology.from new 40MHz channels and the complex site-specific dependencies of Multiple Input Multiple Interference and channel planningOutput (MIMO) technology affect channel planningand AP placement. When considering Coverage, Providing a more than two-times improvement indesigners must understand coverage differences data rate performance over the 20MHz of frequencybetween 802.11n and legacy systems and correctly bandwidth used per channel in 802.11a/b/g, the newdefine coverage requirements based on network 40MHz channels of 802.11n are a must for truly highdemands. 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 interferenceand MAC layer efficiencies. However, these capacity and reduced spectrum for channel planning.improvements can only be fully utilized when thenetwork 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 greaterContext likelihood for adjacent-channel interference in the 2.4GHz band. Since channel planning inThe context of the environment in which a WLAN 2.4GHz was already a difficult task with onlyis 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 recommendedbroadcasting within the same frequency band. for 2.4GHz deployments utilizing 802.11n.1 WHITE PAPER: 802.11n: Look Before You Leap
  3. 3. Fortunately, the 5GHz band frees 802.11n users Site-specific effects of MIMOfrom the tight spectrum constraints of the 2.4GHzband. In the United States, the 5GHz band allows MIMO is inherently site-specificfor 11 non-overlapping 40MHz channels if the AP In legacy systems, interference caused byis fully compliant with the dynamic frequency reflections and diffractions of the transmitted signalselection (DFS) restrictions (more is mentioned (called multipath) was viewed as a hindrance toon DFS in the next section). The abundance of system performance and was compensated for bynon-overlapping 40MHz channels in the 5GHz band including large fade margins in the system design toallows an 802.11n deployment to take advantage improve signal quality in areas with heavy multipathof this performance gain and is the recommended interference. In contrast, in MIMO systemsdeployment strategy for high performance WLAN multipath is the cornerstone of improving systemnetworks. See Table 3 for 802.11n channel overlap. performance! By utilizing complex signal processing, a MIMO system is capable of sending multiple dataThe effect of radar avoidance on 5GHz streams at the same time. Effectively, this meansband channel planning (DFS) the received signal strength (RSSI) alone is no longer sufficient for predicting system performance. GivenAs 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 802.11nthe 5GHz band an AP must be fully DFS compliant. networks is highly recommended.As defined in section 15 of the FCC rules andregulations (47 CFR§15), this means that if a device Contrasting performance of MIMO in densedetects in-band interference from a nearby radar office vs. long hallway environmentssystem it must immediately stop all transmission What is the best environment for optimal MIMOwithin that band for 30 minutes and switch to performance? To use an academic term, multipathanother, non-interfering channel. Clearly, compliance rich environments are the best scenarios for MIMOwith this federal regulation will have an effect on performance. A multipath rich environment is one5GHz channel planning since it requires the AP in which the received signal is evenly distributedchannel change dynamically. among multiple different paths from the transmitter to the receiver. The amount of difference betweenThough the inclusion of DFS into a 5GHz 802.11n individual paths is a basic metric for the richnessdeployment may cause issues, the best practices of the multipath. To best explain this, considerfor planning in a DFS band (5.25-5.35GHz and two frequently occurring deployment scenarios: a5.47-5.725GHz) do not change much from complex office building and a long, straight hallway.planning in a non-DFS band. The first step in thedeployment process should involve a site-survey In an office building, an AP is usually centralizedto determine whether any radar interferers exist in within the desired coverage area. The room with the APthe environment. Secondly, the network channel is usually surrounded by other rooms, which may beplan should be designed to avoid operation on any connected by short winding hallways. In general, thechannels where DFS has been detected. Lastly, environment is dense with obstacles to the signalsince 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 theshould be made available for device utilization if room containing the AP. The complexity of thisradar interference is detected. A good rule of environment generates many different paths for thethumb is to provide at least one unused channel transmitted signal and MIMO systems will perform veryin a non-DFS band. well. This is the preferred MIMO deployment scenario.2 WHITE PAPER: 802.11n: Look Before You Leap
  4. 4. 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 clientsTable 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 supportedpath 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 truthof the signal along the walls of the hallway. In regarding 802.11n coverage differences and isthis environment, a network designer can expect summarized in Table 1.their MIMO performance to drop substantially asthe distance between the AP and the receiver Fundamentally, 802.11n radios are still bound byincreases down the hallway. Multipath components the same governmental power output regulationsin 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 meansperformance gain (while still available) is not as in an “apples to apples” comparison, a signallarge as the complex office scenario. With legacy transmitted by an 802.11n access point will go nohardware, 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, thehowever, it is a hindrance to 802.11n performance best 802.11n implementations will leverage theand 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 effectivelyCoverage increases the “visible” coverage area, and thereby reduce hidden node problems1.Coverage differences between legacy 802.11a/b/gsystems and new 802.11n deployments are an Since a transmitted signal from an 802.11n AParea plagued by much confusion. To truly assess travels no further than a legacy AP, the transmitthe difference in coverage between 802.11a/b/g data rate performance at a given RSSI becomes aand 802.11n hardware, the term “coverage” vital metric for indicating the differences betweenmust be clearly defined. Throughout this paper, 802.11n and legacy coverage. The transmit datacoverage will be defined as “communication at a rate indicates the speed at which an individual dataspecified minimum transmit data rate at a given packet is wirelessly transmitted over the air. Withlocation.” “Range”, on the other hand, is defined respect to coverage differences between 802.11n1 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
  5. 5. and legacy networks, it is noteworthy that the must sacrifice some of its performance when legacycoverage area of 802.11n at a transmit data rate of devices are transmitting over the network. The three54Mpbs is greater than the 54Mbps coverage area main cases of client distribution and their effects onof 802.11a/g. This observation, combined with the network performance are indicated in Table 2.lack of range improvement for 802.11n, outlines themain truth of 802.11n versus 802.11a/b/g coverage To help mitigate the effect of legacy clients on adifferences. Specifically, coverage improvement high-performance 802.11n network, it is importantis greater at higher transmit data rates and the to note that 802.11n can operate in both the 2.4GHzimprovement is less at lower data rates. Further, it and 5GHz bands. The 5GHz band has long beenis important to note that the use of 40MHz channels left relatively empty by the mediocre adoptionwill additionally improve the coverage area of an of 802.11a networks, but this is an ideal scenario802.11n AP at higher data rates, but it also will not for the new 802.11n standard. Since so fewincrease 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 worryCapacity about the network being bogged down by legacy clients. Deploying 802.11n in the 5GHz band is theDue 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 improvementis only available when 11n clients are associated Targeted Upgrades to the Wired Networkto the 11n AP and within the coverage area of thehigher transmit data rates of the 802.11n standard. Since the transmit data rates of the 802.11nSince legacy clients on the network now have standard have increased significantly, for the firstthe potential to actually decrease overall system time it’s possible that a wireless network couldperformance, client distribution planning is a major routinely out-perform a 100-BaseT network. Whatfactor for high-performance 802.11n deployments. results is a need to intelligently upgrade wired network infrastructure to support gigabit EthernetMixed networks on backhaul connections for 802.11n APs only as necessary. When a well-planned wireless networkA tremendously important feature of 802.11n is can deliver more than 100 Mbps and advancedthat it is fully backward compatible to 802.11a/b/g meshing technology can decrease the demands onnetworks and devices. While this provides a smooth the wireless network in general, upgrading the entirepath for migrating existing wireless networks to wired network to support gigabit Ethernet can be an802.11n, 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 clientsTable 2: Relative throughput performance of an 802.11n network under various client distributions4 WHITE PAPER: 802.11n: Look Before You Leap
  6. 6. 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 clientsTable 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 MIMORecommendations for system performance. APs should be placed in locations which both maximize NLOS paths to11n network migration the receiver and minimize the signal path loss. This results in a design decision depending on theUntil this point, this paper has primarily focused on environment the network is being deployed. Forhow the basics of 802.11n technology relate to the example, in a building with thick, highly attenuatingthree C’s of network planning, Coverage, Context, walls, it may still be more beneficial to place APsand Capacity. Now, we will consider three strategies in hallways despite its detrimental effect on MIMOto migrate a legacy network to 802.11n in a way system performance gain since the signal will notthat considers the three C’s, namely Clean-Slate, propagate far enough via the NLOS paths for aRip-and-Replace, and Phased Migration. MIMO system to effectively utilize the rich multipath present in the NLOS link. Due to MIMOClean-slate design link design complications it is recommended a network planning and simulation tool be usedClean-slate network migration is based on the removal during AP placement since it drastically reducesof existing network infrastructure and the subsequent the required calculation time.deployment of new network infrastructure intolocations that are chosen based on maximization of Ultimately, a clean-slate 802.11n design shouldnetwork performance. Despite the increased costs produce a highly optimized network maximizingof performing a clean-slate design for an 802.11n performance and minimizing hardware costs. Innetwork, the clean-slate design migration strategy addition to client distribution planning and APprovides the advantage of a wireless network that can placement issues, upgrades to the wired backhaulbe 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 andclean-slate design is discussed first, since it is the migrations. While a clean-slate migration may be themodel for an optimally performing 802.11n deployment. most expensive option from a planning perspective, it is also the most likely to achieve a robust, reliableTo minimize the adverse effect of legacy clients on network prepared to meet the current networkthe network, use the 5GHz band for 802.11n clients requirements and scale as those requirementsand high bandwidth applications. The 5GHz n-only increase in the future.5 WHITE PAPER: 802.11n: Look Before You Leap
  7. 7. Rip-and-replace migration In the end, the problems associated with a rip-and-replace migration strategy simply highlight that planningRip-and-replace migration involves the one-to-one will always be an integral step for any change to areplacement of existing APs for new 802.11n APs. wireless network. As such, even when the changeThe rip-and-replace migration strategy is often is as simple as a one-for-one device replacement thediscussed within 802.11n circles. The attractiveness best practice for network planning is still to assessof this strategy is that it does not require any your network performance requirements, assess theadditional cabling or installation costs beyond capability of your hardware and plan carefully.simply exchanging existing APs for new 802.11nunits. Unfortunately, the simplicity of the rip-and- Phased migrationreplace migration strategy also faces drawbacks,since the segregation of 802.11n clients into the The main goal of a phased migration is to supplement5GHz band may not provide sufficient coverage for an existing 802.11a/b/g network with 802.11n APs insome legacy systems. Additionally, legacy network order to satisfy the demands of the existing networkdesign practices can conflict with best practices for without incurring the cost associated with a fullmaximizing MIMO system performance. network migration. Phased migrations are useful if a portion of your facility is already adopting 11nThe problem with 5 GHz segregation within a clients or if high bandwidth applications like large file802.11n network is existing networks designed for transfers are needed in specific locations.maximum coverage at 2.4 GHz may have nodes toofar apart to satisfy the coverage requirements of a The first step of any network migration should be to5GHz deployment. Since networks designed on the identify how the existing network is failing to satisfytransmit range to maximize an AP’s coverage area current demand. This is of particular importance forare common among 802.11b/g deployments, this a phased migration, since the stated goal is to meetcan be a persistent problem for rip-and-replace the demands of specific portions of the existingmigrations wishing to provide 5GHz segregation. network. A comprehensive network managementThis is because, for the same transmit power, signals system is very beneficial in this initial, requirementsdo 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 hardwaredoes not have an increased range over existing 5 GHz Once performance requirements have beenAPs, a network which was designed based on the determined, if an existing legacy client network ismaximum range at 2.4 GHz will likely have numerous already operating at maximum capacity, simplycoverage holes in its 5GHz 802.11n network. replacing existing APs with 802.11n APs is not likely to satisfy capacity requirements since legacy clientsAs discussed previously, MIMO system performance on the migrated 802.11n network still only operate atis maximized in multipath rich environments. In rip- legacy speeds. As mentioned previously, if a portionand-replace migrations (from an existing 802.11a/b/g of the network is already adopting 802.11n clientsnetwork), it is likely there are several APs positioned and capacity requirements are being met, then ato maximize coverage in hallways, as this was a one-to-one exchange of a legacy AP for 802.11nwidely 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 APsthe 802.11n migration. into the capacity-strapped region of the network. These new APs should be given priority in terms ofFurther, the issues presented above are for a network coverage area per AP via judicious power-planningwhere it is assumed performance requirements since the end goal of a phased migration is to havehave not increased from its initial deployment. If, as the 802.11n network as the only network.most likely the case, the performance requirementshave increased, a rip-and-replace strategy may be Unless network requirements can be met throughimpossible since increases in AP density may be a rip-and-replace of a network’s subset, it isnecessary even for an 802.11n network. In contrast, recommended that new 802.11n APs be placedan existing high-performance 802.11a/b/g network independently with respect to existing legacymay be able to migrate to 802.11n with fewer APs, AP locations. This may also necessitate smallsince there are known coverage improvements for adjustments to the existing AP placements, but it802.11n at higher data rates. will allow the phased migration to maximize the6 WHITE PAPER: 802.11n: Look Before You Leap
  8. 8. Using LANPlanner to Define Network Requirements802.11g_(1) 802.11n_(1) 802.11g_(2) 802.11n_(3) 802.11n_(2) 802.11g_(3) Original Network Clean-Slate802.11n_(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
  9. 9. MIMO system performance of 802.11n hardware efficiently determine not only the best migration pathand makes channel planning for the final hybrid for their network, but also the costs associated withnetwork much simpler. the upgrade. Comparison of 802.11a/b/g vs. 802.11nHow can Motorola With LANPlanner 11.0, 802.11n performanceLANPlanner® assist in the improvements are no longer left to complicated spreadsheets and non-intuitive numbers. When802.11n planning process? planning an 802.11n network, users are able to clearly visualize the improvements they areAs various impediments to 802.11n planning have receiving from their 802.11n deployment withinbeen discussed throughout this paper, it has become a site-specific heatmap. Combined with theincreasingly apparent that properly planning an exceptional, automated reporting capability of802.11n network is a daunting task and is quite LANPlanner, the benefits of 802.11n migrationdifficult without the use of site-specific network can be effectively communicated with ease.modeling techniques. Fortunately, Motorola has toolsto assist network planners in this process. Whether 802.11n enabled survey capabilitythe project requirements involve a clean-slate design,a rip-and-replace network migration, or simply a Due to the channel planning issues surroundingpre-deployment survey to assist in requirements 40MHz channels, DFS compliance in the 5GHzgathering for a phased migration, LANPlanner 11.0 band, and the effect of client distribution on networkhas 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/gSite-specific performance predictions networks. As such, the LANPlanner survey tool Motorola SiteScanner™, provides native support for802.11a/b/g planning has always been a computationally 802.11n surveys using widely available, industryintense challenge. Now that 802.11n includes standard 802.11n WLAN cards.additional non-intuitive multipath factors, site-specificmodeling is even more important. As previously In addition to pre-deployment surveys, this capabilitydiscussed, 802.11n performance will change depending allows customers to verify the performance of theiron the environment surrounding the AP. LANPlanner 802.11n APs once they have been deployed. Since11.0 supports 802.11n planning, and has the capability characterization of MIMO system performance requiresto include site-specific effects into its calculations client association and data transfer from the AP, theof the transmit data rate. These predictions are a AP Performance mode of SiteScanner has been improvedmust 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 of802.11n network simulate their deployment plans in performance predictionsLANPlanner prior to making an upgrade decision. As always, LANPlanner provides users with powerfulLegacy-to-11n Migration Wizard tools for optimizing their LANPlanner networkMany industry discussions have indicated that planning models. Performance data collected fromthe most likely scenario for first-round 802.11n SiteScanner measurement surveys can be used todeployments will be migrations of existing close the feedback loop for 802.11n performance802.11a/b/g networks to support 802.11n. Since predictions. Through optimization, the user canthis is the case, LANPlanner 11.0 provides a achieve the maximum possible prediction accuracyNetwork Migration Wizard for this very purpose. for their 802.11n AP model.Based on user-defined constraints for the migrationsuch as region of interest, client distribution, and LANPlanner — part of the 802.11n solutionmigration strategy, LANPlanner provides a suggestedbaseline deployment for the specific needs of each LANPlanner is part of Motorola’s comprehensiveenvironment. Network planners can easily input 802.11n solution, which also includes Motorola’sthe performance requirements of their network AP-7131 802.11n access point and the RFS6000and then simulate multiple migration scenarios to and RFS700 802.11n ready WLAN switches.8 WHITE PAPER: 802.11n: Look Before You Leap
  10. 10. motorola.comPart 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 servicenames are the property of their respective owners. ©2008 Motorola, Inc. All rights reserved. For system, product or services availability and specific information within yourcountry, please contact your local Motorola office or Business Partner. Specifications are subject to change without notice.