Cisco 1140 Series Access Point Deployment 802.11 n

Cisco 1140 Series Access Point Deployment Guide

November 2008

802.11n represents the next evolutionary step in standards-based wireless networking and a sea change to the level of mobility on enterprise networks. Featuring numerous enhancements to the physical and MAC layers of 802.11, it is a complete overhaul of previous Wi-Fi technologies and will facilitate the mobilization of bandwidth intensive applications across all industries. Historically, careful planning and point optimizations were required to deploy high throughput applications over bandwidth constrained 802.11a/g networks. Now with the advent of 802.11n and its data rates upwards of 300Mbps, bandwidth is increased roughly 6X over today’s 54Mbps WLANs offering breathing room for even the most demanding use cases. Cisco has made 802.11n Draft 2.0 capabilities available to enterprise-class customers with its Aironet 1140-series Access Point. Sporting 802.11n in both the 2.4GHz and 5GHz frequency bands, this Access Points permits mobility for high throughput applications and increases the reliability and predictability of WLANs.

The Cisco Aironet 1140 series access point caters to indoor environments that demand outstanding wireless performance while blending in seamlessly with the aesthetics of the enterprise environment. In order to meet these set of requirements, the Aironet 1140 was designed with integrated antenna elements similar to its predecessor, the Cisco 1130AG. This access point includes six integrated Omni-directional antenna elements segmented into three discrete elements for each frequency band. Ideal for indoor environments, this access point has a low-profile design that blends into enterprise, health care or educational environments seamlessly. Featuring two integrated 802.11n Draft 2.0 radios, the AP can simultaneously serve 802.11b/g/n and 802.11a/n clients in the 2.4GHz and 5GHz bands, respectively. In addition to a Gigabit Ethernet uplink, the 1140 also supports full functionality while being powered by 802.3af Power Over Ethernet.

It's About More Than Speed

The new standard is fast-really fast, but 11n is not just another acceleration of business as usual, and it's far more than a step forward in performance. The dramatic advances of 11n come in two areas that have challenged many wireless network administrators: reliability and predictability.

Achieving the reliability of wired transmissions with a wireless medium is inherently difficult, and designers have grown accustomed to architecting WLANs around this. For example, bandwidth considerations notwithstanding, most implementers are comfortable planning legacy access point (AP) placement with restrictive voice-over-IP client count limitations due to the increase in 802.11 retries

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So, What Exactly Is 11n?

(and resultant latency) as more voice handsets are introduced to a legacy cell. Such limitations erode with 11n. The media access layer of 802.11n offers more reliability-even to legacy 11a/b/g devices-thanks to the benefits from the physical layer enhancements, multiple antennas, and support for additional spatial streams that come from 11n's new radio technology. All this results in transmissions getting where they need to go the first time they're sent. Cisco tests show that, with both 11n and legacy 11a/b/g clients, 802.11 re-transmissions are reduced by half on Cisco’s 802.11n Access Points compared to legacy transmissions on legacy access points.

Note In this document, the term "legacy" refers to all devices that do not support the high throughput rates of 802.11n. This includes all clients and access points that support 802.11, 802.11b, 802.11g, and 802.11a PHY types.

The ability to provide reliable connectivity consistently across the entire network is an integral part of the radio enhancements of 802.11n. This consistent reliability across time and motion, even in places not before reachable (due to nulls, signal muddling, and other negative effects of multipath), means that 11n finally brings predictability to the Wi-Fi world. In-house testing shows that the Aironet 1140 Series access point reduces variation in client latencies and retries by half.

So, What Exactly Is 11n?802.11n comprises a number of features that come together to give the spec its increased performance, reliability, and predictability. Put basically, these gains are achieved via radio enhancements, MIMO antenna technology, and enhancements to the 802.11 MAC.

Radio EnhancementsThere are two important parts to the PHY (physical layer) augmentations of 802.11n that give the new standard the ability to deliver better raw data rates and, thus, higher application speeds. Both parts are conceptually straightforward. The most obvious enhancement is that the new 802.11n PHY supports much faster data rates, along with support for legacy 802.11a/b/g speeds. The 11n PHY actually supports a very wide array of different speeds, called modulation coding scheme (MCS) rates.

2Cisco 1140 Series Access Point Deployment Guide

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Figure 1 MCS Rates

All things being equal, the 802.11n PHY is able to push more data than previous 802.11 iterations. But to reach even higher connection rates, clients must support a mix of the following two radio enhancements including channel bonding and MIMO.

To further its PHY-speed cause, the 802.11n standard supports the traditional legacy 20 MHz channels and outlines extensions for 40 MHz operation, as well. By adding provisions to the standard to support channel bonding, unlike older, proprietary methods, 11n will make use of the spaces reserved between nonoverlapping channels to achieve increased spectral efficiency in the used bandwidth. In some environments, more than double the effective throughput of the two bonded channels alone may be realized.

The 5-GHz band boasts more spectrum than the 2.4-GHz band–enough to make 40-MHz channel bonding possible.


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MIMOMultiple Input, Multiple Output, or MIMO, is at the heart of 802.11n and provides for a given transmission to operate at much higher data rates than the PHY would otherwise be capable.

Spatial Division Multiplexing (SDM) is the ability of an 11n access point to use multiple radios and antennas to transmit different signals to the same recipient. The receiver puts the multiple discrete signals back together, thereby realizing a higher data rate than would otherwise be achieved by a single transmitting radio. In ideal conditions, utilizing Spatial Multiplexing permits 802.11n to nearly double throughput when multiple spatial streams are utilized.

In addition to Spatial Division Multiplexing, MIMO also supports Maximal Ratio Combining (MRC) which allows multiple receive signals from discrete antennas to be combined. Traditional methods of receive diversity, as implemented in 802.11a/g devices, only switched to the antenna with the strongest signal thus disregarding information available on the other antenna. In sharp contrast, by using multiple antennas and advanced digital signal processing, 802.11n devices can decrypt both the original RF transmission and subsequent reflected copies to form a more reliable ‘super’ signal comprised of information gained from all antennas. Put more clearly, a hypothetical person with three ears will hear more information than a person with two. The same goes for Access Points and their respective number of antennas. By having multiple RF ‘viewpoints’ an 802.11n device enjoys increased reliability in comparison to a legacy device meaning packets will reach their destination on the first transmission reducing time wasted via costly retries.


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MIMO radios are referenced using the formula TxR:S. The 'T' stands for transmit antennas, the 'R' for receive antennas and the 'S' for the number of spatial streams. The number of spatial streams is the most important variable as it defines the maximum data rate of the radio. For radios supporting one spatial stream, the maximum data rate is 150Mbps, and a radio supporting two spatial streams tops out at a maximum data rate of 300Mbps (assuming a 40MHz-wide channel).

The Cisco Aironet 1140-series utilizes two 2x3:2 MIMO radios, with one operating in the 2.4GHz band, and the other in the 5GHz band. This means each radio has two transmit antennas, three receive antennas and it supports a maximum of two spatial streams. Similarly, a 3x3:2 radio would utilize three transmit antennas, three receive antennas and support a maximum of two spatial streams. In terms of performance, radio configurations of 2x3:2 and 3x3:2 will be equivalent as their maximum speeds are capped at 300Mbps given the number of spatial steams are identical.

MAC EnhancementsUp the stack, 11n's MAC enhancements allow bandwidth-hungry applications to make more efficient use of available transmission time, further increasing effective throughput.

The MAC layer enhancements to 802.11n are deceptively simple in concept, when compared to the substantial decrease in overhead they offer. Both aggregation and support for block acknowledgments help reduce the protocol overhead typical of legacy Wi-Fi networks, thereby boosting speeds. Aggregation allows data frames to be concatenated so that time wasted for medium contention and interframe spacing gaps may be greatly reduced.

Note Aggregation will only occur with unicast traffic; broadcast and multicast traffic cannot be aggregated.

Block acknowledgments (ACKs) further reduce protocol overhead by allowing a group, or block of frames, to be acknowledged with a single ACK frame. Legacy Wi-Fi networks have always required individual acknowledgment of unicast management and data frames.

Other Incremental ImprovementsThere are some additional augmentations to existing 802.11 standards that have been designed into 11n and which are geared toward differentiated media access services and power savings. 802.11n requires that clients support the WMM/11e quality of service (QoS) protocol. This establishes a foundation for administrators to properly enforce QoS policies, and WMM (Wi-Fi Multimedia) serves as the basis for a portion of 802.11n’s packet aggregation capabilities using Block ACK. 11n also supplements 11e's power save method, not just requiring that devices support the more intelligent, trigger-based Automatic Power Save Delivery (U-APSD), but also giving end client devices the abilities to disable transmitters and receivers as traffic patterns allow, thereby drastically reducing the power drain associated with operating 11n's many radios.

Understanding the parts of 802.11n and how they converge to provide enhanced performance, reliability, and predictability will prove imperative when it comes time to design an 11n-ready Cisco Unified Wireless Network.

Note For more detailed technical information on 802.11n, please see the “802.11n: The Next Generation of Wireless Performance” whitepaper at this URL:http://www.cisco.com/en/US/partner/solutions/collateral/ns340/ns394/ns348/ns767/white_paper_c11-427843_v1.html


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http://www.cisco.com/en/US/partner/solutions/collateral/ns340/ns394/ns348/ns767/white_paper_c11-427843_v1.html

http://www.cisco.com/en/US/partner/solutions/collateral/ns340/ns394/ns348/ns767/white_paper_c11-427843_v1.html

Designing for 11n

Designing for 11n

Design for CapacityAs WLANs have evolved from a standards prospective, they have additionally evolved in how they are utilized. Originally, WLANs were deployed using Access Points at maximum power to ensure every area was provided some level of usable signal. While this design worked fine for traditional bandwidth limited applications, the move to high client densities and multimedia applications calls for a new approach in designing WLANs.

When designing for capacity rather than coverage, aim to have APs closer together at lower power output settings (this can be handled dynamically via Radio Resource Management) and globally disable any legacy data rates that you can afford to disable. In this manner, fewer clients will be loaded onto each Access Point, thus allowing each client to utilize a greater portion of the wireless medium.

The problem with clients operating at low data rates (example, 1Mbps) is that each packet takes up more ‘airtime’ compared to clients utilizing high data rates such as 36Mbps to 54Mbps. In simpler terms, a client utilizing a 1Mbps rate is essentially ‘talking’ slower than other clients and therefore drags down aggregate performance. Since WLANs operate with the guideline that only one device, be it AP or client, can utilize the channel at any one moment, overall system performance drops when a large percentage of low data rate frames monopolize the airtime. By designing Access Point density for capacity and disabling lower data rates, aggregate system capacity can be increased.

Note You may not always be able to disable legacy data rates, depending on the mix of clients you have in your environment. For example, you will likely not want to disable all 802.11b rates, or those legacy clients won't be able to connect at all, though it may be prudent to consider disabling some lower 802.11b rates (such as 1, 2, and 5.5 Mbps).

Leverage 5GHzAlthough the 802.11n standard can operate in both the 2.4GHz and 5GHz frequency bands, specific characteristics of 5GHz make it especially advantageous for 802.11n operation. The value of 11n operations in 5 GHz is threefold. First, the install-base of legacy 11a clients is quite simply not nearly as widespread as 11b/g support is. This means that your 11n 5-GHz clients will contend less often with legacy clients and spend more time operating at high throughput rates. Next, the number of non-802.11 interferers that operate in the 5GHz band is a mere fraction of the multitude of cordless phones and Bluetooth devices that cause performance issues in the 2.4GHz band. Last, and the most important characteristic of 5 GHz, is the exponentially greater available bandwidth compared to 2.4 GHz. In the United States, there are 21 nonoverlapping channels you can use, which means you can deploy with density and capacity in mind and let RRM handle your channel (and power output) plan accordingly.

Note Where possible, plan to support legacy devices you still have in 2.4 GHz, and look to leverage the 5GHz band for clients supporting 802.11n.


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Designing for 11n

5GHz and DFSWhen deploying 802.11n in the 5GHz band, one should be aware of the DFS (Dynamic Frequency Selection) requirements placed on portions of this band. In particular, the UNII-2 and UNII-2-Extended frequency bands require DFS. The channels which require DFS include 52 through 64 and 100 through 140. The DFS requirement stems from that fact that incumbent radar systems use portions of the 5GHz band and precautions must be utilized to prevent harmful interference with these systems. Since the exact portion of the 5GHz band in use by radar varies from locale to locale, the means of Dynamic Frequency Selection was created to facilitate unlicensed sharing of this band with existing radar installations. When radar is detected on a particular 5GHz channel, the AP will automatically change to another channel as required by FCC regulations. As per FCC regulations, when radar is detected on a channel, the AP cannot communicate on that perspective channel for at least 30 minutes. It should also be noted that it may be the case that none of the 5GHz spectrum is utilized by radar installations allowing the entire band to be leveraged for wireless LAN services. Cisco’s Spectrum Expert should be utilized during the survey phase of a deployment to determine if radar sources are present in the environment and if channel plans should be adjusted accordingly to avoid these channels.

Switch port SpeedIn order to fully tap the performance of 802.11n, the wired infrastructure feeding the Access Point must be capable of sustaining the level of throughput this standard can deliver. The Aironet 1140-series features a Gigabit Ethernet (10/100/1000)-capable uplink port which performs at its peak when paired with a Gigabit switch. Although this AP can negotiate down to slower 10/100 speeds on non-Gigabit capable switches, this deployment model introduces the wired network as the bottleneck thus capping peak performance.

The speed of the link to the AP isn't the only consideration here: be mindful of the APs' links back to their controller. Also make sure that you adjust your throughput expectations accordingly depending on which controller you deploy. The 4400 Series wireless LAN controller, the 3750 Series Integrated wireless LAN controller, and the Catalyst 6500 Series Wireless Services Module (WiSM) are likely primary choices for Aironet 1140 Series AP deployments when higher throughputs are desired.

Migrating to 11n in StagesMoving your 11a/b/g Unified access point installation wholesale over to an 802.11 11n-capable network might seem a bit daunting. Instead of opting for a rip-and-replace upgrade, there's plenty of flexibility built into the controller architecture to allow you to plan your upgrade in phases.

There are a number of ways to move to an 802.11n-based Cisco Unified Wireless Network, but a common method is swapping out legacy LWAPP APs in favor of an 1140. You could also simply supplement existing coverage by adding in 1140 Series APs where you need additional capacity. For example, replacing an 1130-series with an 1140-series in conference rooms or high density areas will add extra capacity. Cisco's controller-based architecture provides the flexibility for such scenarios. A single controller can manage both legacy and 802.11n-capable APs, and RRM will continue to properly architect an optimal RF configuration even in such environments where 802.11n-capable and legacy access points are installed in the same vicinity.


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Designing for 11n

Coexistence in an 802.11a/b/g/n Network

The beauty of in 11n is not just the increased performance, reliability, and predictability made available to clients that support the new standard. It's also that 11n provides backward compatibility with legacy devices-and they get to share the performance boost.

Note Testing has shown that legacy devices can realize up to a 10 percent increase in throughput and draw on the benefits of the AP's multiple antennas and radios to drastically reduce retries.

802.11n allows legacy 11a/b/g clients to connect to an 11n infrastructure, albeit at rates allowed by the lesser standards. Backward compatibility is achieved using what's called "protection mechanisms" in order to allow higher throughput 11n devices to realize faster rates alongside their slower legacy cousins. Aggregate performance in such mixed-mode environments will be lower than with just 11n devices, but regardless of the client mix on an 802.11n AP, the overall throughput will exceed that of the slowest legacy standard.

11n protection mechanisms for legacy devices operate very similarly in the way 11g interoperates with 802.11b, though with differing throughput implications. The protection mechanism, which typically employs CTS-to-self frames (though some clients may use the whole RTS/CTS exchange) to alert others of an impending transmission, allows legacy devices to understand when 802.11n clients are using the medium, even though the slower clients can't decipher the 802.11n data rates.

The throughput impact of coexistence will vary based on the number of clients, the mix of those clients (11a/b/g/n), specific traffic load characteristics, the varying distances of clients from the AP (which means slower links as they move farther away), and individual client rate shifting peculiarities.

Note Since all current Voice over WLAN phones operate using the 802.11a/b/g standard, the total voice capacity of an 1140 Access Point is not increased over an 1130-series.

There's no clear answer to the question of how legacy device coexistence will affect overall 11n throughput other than to say that performance will vary. If nothing else, we can be sure of a few things:

• You will always realize some performance gain with 11n APs because all 11n devices will be able to transmit at 11n rates, even if legacy devices slow down aggregate performance when they operate in their legacy rates.

• The higher your percentage of 5GHz devices (even if they're legacy 802.11a clients) compared to 2.4-GHz devices, the better your overall performance will be.

• As time goes by, we'll see more 11n devices out there. This means that coexistence concerns will gradually diminish as existing legacy clients are swapped out through regular refresh cycles to 11n capable clients.

11n Client ConsiderationsLike 802.11a/b/g deployments, the overall performance of the deployment is governed not only by the infrastructure but also by the client adapter. To this end, ensure that WLAN client adapters used in an 802.11n deployment are appropriately selected and configured to deliver superior performance.

In terms of selecting client adapters, those that are integrated into the notebook often perform better than add-on cards. Always ensure that a client selection ensures that the device is certified by the Wi-Fi Alliance in regards to supporting 802.11n Draft 2.0. Cisco has collaborated with Intel in extensive


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Deploying 11n

interoperability testing and as a result, optimal performance can be expected when Intel 4965AGN or 5100/5300AGN client chipsets are utilized. It’s also imperative that client driver version be kept current as performance will increase as subsequent hardware drivers are released.

The maximum throughput performance will be realized when notebooks are connected to local AC power and any power saving mechanisms are disabled. Keep this in mind when benchmarking the performance of the 802.11n infrastructure as notebooks will often throttle CPU and motherboard power consumption to save battery life which can negatively impact throughput.

Deploying 11n

AP Placement and Site SurveyingThe recommended density for deploying 802.11n-capable Access Points is nearly identical to the recommendations put forth for existing 802.11a/g infrastructures. For a Greenfield installation of an 802.11n infrastructure, a site survey will be required to assess the multipath propagation characteristics and determine optimal AP density. In the case that the deployment will be an upgrade of an existing wireless network, a 1 for 1 replacement strategy will provide better coverage than the existing infrastructure. This will also allow the existing wire drops to be reused for the infrastructure upgrade thus forgoing costly cable runs.

When considering the outcome of deploying an 802.11n solution, a few key points can summarize the expectations.

1. The maximum range of each Access Point will exhibit only limited improvements

2. Legacy (802.11a/g) clients will exhibit enhanced coverage at existing rates

3. 802.11n clients will exhibit enhanced coverage and performance leveraging high-throughput data rates.

Differences in Range

At each power level, the Cisco 1140-series provides a 10-15% increase in linear range over its 802.11a/b/g counterpart, the 1130-series. While this can be beneficial, it does not mean that Access Points should now be spaced farther apart and run at higher power levels. Just like 802.11a/g, 802.11n is a shared medium and designing a wireless network for maximum range will limit per client throughput. It is also important to keep in mind that these access points will provide higher data rates over the course of the entire coverage area.


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Deploying 11n

Comparison of 2.4GHz Range

Figure 2 Cisco 1130


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Deploying 11n

Figure 3 Cisco 1140

In the 2.4GHz band, the maximal range of the 1140-series AP is marginally better than the 1130-series, but the true advantage to deploying the 1140 can be found in the enhanced coverage. As depicted in the above images, an 1140-series Access Point supports a greater area of coverage utilizing high-bandwidth (depicted in green) data rates.


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Deploying 11n

Comparison of 5GHz Range

Figure 4 Cisco 1130


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Deploying 11n

Figure 5 Cisco 1140

Similar to the 2.4GHz frequency band, the maximal range of an 1140 in the 5GHz frequency band is only marginally more than that of an 1130 but the coverage of high-bandwidth data rates is far superior.

Coverage for Legacy ClientsThe primary improvement for legacy clients realized by deploying 802.11n Access Points will be the enhanced ability to support high data rates over a larger portion of the Access Point’s entire coverage area. By leveraging the MRC capabilities of the MIMO radios, Cisco’s 1140-series Access Point will be even by able deliver measureable improvements to legacy clients. While the maximum obtainable rate of 802.11a/g legacy clients is still capped at 54Mbps, the ability to obtain this and other high speed data rates at distance will be increased.


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Deploying 11n

In order to quantify exactly how much improvement a legacy client can extract from a connection to an 802.11n capable Access Point, an ‘Active Iperf’ site survey was performed using AirMagnet’s Surveyor 6.0 product and a Cisco CB-21AG client card. This form of survey used Iperf to push active traffic through the wireless link while the survey was being performed thus increasing the accuracy of data rate measurement.

2.4GHz – 802.11g Coverage

As depicted in the below images, the range at which the 48Mbps data rate can be obtained is measured. In the case of a legacy client interoperating with a Cisco 1130-series Access Point, this range came to 86ft from the AP. When the same test was performed with an 1140-series Access Point, the 48Mbps rate could be obtained all the way to 102ft. This represents an 18% increase in coverage for legacy clients in the 2.4GHz band. In addition to the increased range of the 48Mbps cell, the predictability of coverage is also demonstrated as the 1130-series survey features numerous rate shifts (depicted in yellow) where as the 1140-series sports consentient (green) coverage with no drops in data rate.

Figure 6 1130 Series 802.11g – 48Mbps Coverage

Figure 7 1140 Series 802.11g – 48Mbps Coverage


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Deploying 11n

5GHz – 802.11a Coverage

In addition to coverage improvements for legacy clients in the 2.4GHz band, the 1140-series also provides enhanced coverage in the 5GHz band. As depicted in the below images, the coverage of an 1140-series Access Point is greater than that of the 1130-series. For the 1130’s 802.11a radio, the range in an indoor office environment extends to a maximum of 86ft for the data rate of 48Mbps. As for the 1140 series, the 48Mbps data rate is obtainable all the way to 97ft. This represents a coverage improvement of 12% compared to the 1130 series. Once again, the predictability of the 1140-series is also demonstrated in the images showcasing a coverage pattern that is less ‘spotty’ than the 1130-series.

Figure 8 1130 Series 802.11a – 48Mbps Coverage

Figure 9 1140 Series 802.11a – 48Mbps Coverage


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Deploying 11n

Coverage for 11n ClientsAlthough an 802.11n access point deployment can deliver marked improvements when interoperating with legacy clients, the high-performance standard performs at its peak when paired with 802.11n-capable clients.

2.4GHz - 11n Coverage

In the 2.4GHz band, the recommended mode of operation is to utilize a traditional 20MHz wide channel which provides a maximum data rate of 144.4Mbps. Below is a coverage map of an 1140 when paired with an 802.11n client. As depicted in the image, the data rate of 100Mbps or more is maintained for a large portion of the Access Points coverage area. Once signal strength begins to decrease, the access point will rate shift to roughly 802.11g speeds and then finally to 802.11b speeds out at the extreme edge.


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Deploying 11n

5GHz 11n - 20MHz vs. 40MHz

In the 5GHz band, there are two recommended modes of operation featuring specific tradeoffs and advantages on each side. One option is to utilize the traditional 20MHz channel width in the 5GHz bands which tops out at a data rate of 144.4Mbps. While using a 20MHz channel doesn’t offer 802.11n’s peak throughput of 300Mbps, it does utilize less spectrum which can be beneficial in spectrum constrained installations. For maximum throughput, a 40Mhz channel size should be utilized and this is the recommended mode of operation if peak throughput is a priority.

5GHz (20MHz) - 11n Coverage

Below is a coverage map of an 1140’s 5GHz radio operating in 20MHz-mode when paired with an 802.11n client. As depicted in the image, the data rate of 100Mbps or more is maintained for a large portion of the Access Points coverage area. Once signal strength begins to decrease, the access point will rate shift to roughly 802.11a speeds.


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Deploying 11n

5GHz (40MHz) - 11n Coverage

Below is a coverage map of an 1140’s 5GHz radio operating in 40MHz-mode when paired with an 802.11n client. As depicted in the image, the data rate of 300Mbps is maintained within close proximity of the Access Point. As the client moves farther away, data rates shift down to 100-150Mbps. Once signal strength begins to decrease to minimal levels, the access point will rate shift to roughly 802.11a speeds.

Performing Surveys for New Installs

For a new deployment of an 802.11n infrastructure, the ability to reuse existing mounting locations is inherently impossible. Given that the greatest speeds of 802.11n are available in the 5GHz band using a 40MHz channel, the recommended cell size for new 802.11n deployments is based on that band and mode of operation. In order to assess the coverage of an Access Point operating in the 5GHz 40MHz mode, one should utilize an 802.11n-compatible site survey tool with an 802.11n client. Based on collected site survey data, a general rule of thumb for the recommended RSSI for deploying an 802.11n access point using a 5GHz 40MHz channel is computed.


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Deploying 11n

Figure 10 5GHz 40MHz - Data Rate vs. RSSI

For simplicity, the above graphic depicts the average expected data rate vs. RSSI at the client. This is intended as a guideline to set expectations as to the performance of a 5GHz 802.11n infrastructure as range is increased. For the maximum data rate of 300Mbps, one can expect this cell size to include from right next to the Access Point and extend to signal strength of -55dBm to -60dBm. For an average data rate of 200Mbps, the maximum cell size will extend from -65dBm to -70dBm. These numbers should be utilized to lay the ground working in planning an 802.11n Greenfield deployment but are not an acceptable substitute for performing on-site surveys.

Note The actual fringe of the cell, as measured in RSSI, is dependent on the capabilities of the client card in use. The above data was obtained using an Intel 4965AGN client card. Ensure that each different type of 11n adapter to be used in a deployment is tested during the survey process.


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Deploying 11n

Figure 11 Example 5GHz Survey – 1140 in an Office Environment

In order to quantify the 5GHz 802.11n performance of the Cisco 1140 in a typical office environment, a site survey using AirMagnet Surveyor 6.0 was completed. In this test the Access Point was powered using PoE and configured with power level 1 (the highest setting). The result of the testing showed the 300Mbps cell extended to roughly 30ft from the Access Point. As for 200Mbps, the cell size extended to 66ft as depicted in the above image.

Once a survey such as this is executed in the intended deployment location, one can use this data to deploy additional 802.11n Access Points. If the perspective Greenfield wireless installation will be intended to support a client base of both legacy and 802.11n clients, than a survey must be performed for each client type.

Controller Uplink Capacity Design

Cisco defined how switched and routed campus networks are designed and built using hierarchical network design. This same approach can be directly applied to the scaling of an 802.11n WLAN infrastructure. Specifically, the entire industry understands the simple, multilayer campus design concepts of access, distribution, and core layers because that's how Cisco taught people to build campus networks. Cisco customers and partners have been successfully using these design concepts for years. As such, these design concepts are fully vetted and mature.

Multilayer campus design guidelines for over-subscription of the uplink from the access layer call for an over-subscription rate between 8:1 and 20:1. However, in real world deployments, it's not unrealistic to see Cisco customers deploying at 50:1 or even 100:1 over-subscription of the uplink from the access layer. There are many reasons why Cisco customers have deployed at these over-subscription rates but the germane point is that customers have successfully deployed with these over-subscription rates and been satisfied with network performance.

WLAN access points are, by definition, access layer devices. With a Cisco Unified WLAN Network, these access layer devices are logically uplinked to a WLAN controller. So it's quite natural and appropriate to apply well-established campus design criteria to the Cisco Unified WLAN Network. The below table outlines the over-subscription rates for Cisco WLAN Controllers given the maximum number of supported APs joined to a controller. These table values assume a fully loaded controller. For example, a full loaded WiSM would be populated with 300 Access Points.


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Configuring Your Network for 11n

As can be clearly seen from the above table, even if designing for peak throughput of 250Mbps per AP, no existing Cisco controller except the 21XX-series and WLCM-series are over-subscribed beyond current campus design guidelines of 20:1. For maximum performance in an 802.11n deployment, utilize the 440X or WiSM controllers.

Note Although the 21XX-series and WLCM-series controllers fully support 802.11n functionality, the uplinks of the devices can become a potential bottleneck with locally switched WLANs. A remedy to this issue is to ensure the AP is connected to a Gigabit Ethernet switch and employ H-REAP to switch traffic locally at the AP.

In a mixed client environment which is indicative of most deployments today, an 802.11n AP will experience maximum throughputs of about 200Mbps, which places high-end controller oversubscription values on-par or better than the recommendation of 8:1.

Scaling the Network

As additional 802.11n clients and more demanding applications place strain on the overall architecture, one can monitor and alleviate this issue using a few simple considerations. First, to assess the utilization of particular wireless LAN controllers, utilize Cisco’s WCS ‘802.11n Scaling’ report to chart throughput over time. If a controller, or set of controllers, are to be found reaching their maximum capacity the entire capacity of the system can be augmented by adding additional controllers. In this manner, a subset of your Access Points will now operate on the new controllers, resolving any bandwidth limitations that may have occurred.

Note Increasing system capacity by adding controllers in this manner is called "horizontal scaling." Horizontal scalability allows you to scale the WLAN infrastructure to meet additional load by augmenting the aggregate capacity and spreading the load evenly across each individual controller.

Configuring Your Network for 11nThe Aironet 1140-series Access Point interoperates with Wireless LAN Controllers like any other lightweight AP. Just make sure you're running 5.2 (or higher) controller code and that your APs can discover and join such a controller.


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Ensuring 802.11n Rates are Enabled

Step 1 Under the ‘Wireless’ tab on the controller GUI, click on the ‘802.11a/n’ or ‘802.11b/g/n’ network setting.

Step 2 Click ‘High Throughput (802.11n)’ to open the configuration page.

Step 3 Ensure that ‘11n Mode’ is enabled. By default, all of the MCS (or 802.11n data rates) are enabled.

Configuring WLANs for 802.11nIn order for the wireless LAN (or SSID) to operate at 802.11n rates, two specific items regarding encryption and quality of service must be configured. As per the requirements of the 802.11n standard itself, all encrypted links must now use AES encryption. The alternative to this is running the WLAN with no encryption, which is entirely possible. However this requirement precludes the use of WEP, TKIP or any other form of legacy encryption when using 802.11n data rates.


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Figure 12 Setting “Layer 2 Security” to “WPA2 AES”

Figure 13 Setting “Layer 2 Security” to “None”

In regards to quality of service, the WLAN must be configured to allow WMM in order for 802.11n rates to be operational. To configure this on the wireless LAN controller, ensure that the ‘WMM Policy’ under the WLAN profile configuration is set to either ‘Allowed’ or ‘Required’.


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Figure 14 Setting “WMM Policy” to “Allowed”

Figure 15 Setting “WMM Policy” to “Required”

Configuring Packet AggregationAggregation is the process of grouping packet data frames together rather than transmitting them separately. The following two aggregation methods are available:

• Aggregated MAC Protocol Data Unit (A-MPDU)

• Aggregated MAC Service Data Unit (A-MSDU).

A-MPDU is performed in the software whereas A-MSDU is performed in the hardware.

You can specify the aggregation method for various types of traffic from the access point to the clients. Table 1 defines the priority levels (0-7) assigned per traffic type.

Table 1 Traffic Type Priority Levels

User Priority Traffic Type

0 Best effort

1 Background


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You can configure each priority level independently, or you can use the all parameter to configure all of the priority levels at once. When you use the enable command, the traffic associated with that priority level uses A-MPDU transmission. When you use the disable command, the traffic associated with that priority level uses A-MSDU transmission. Configure the priority levels to match the aggregation method used by the clients. By default, only priority level 0 is enabled.

Note Link Aggregation is enabled by default.

(Cisco Controller)> config 802.11a network disable(Cisco Controller)> config 802.11a 11nSupport a-mpdu tx priority 0 enable(Cisco Controller)> config 802.11a network enable

(Cisco Controller)> config 802.11b network disable(Cisco Controller)> config 802.11b 11nSupport a-mpdu tx priority 0 enable(Cisco Controller)> config 802.11b network enable

2 Spare

3 Excellent effort

4 Controlled load

5 Video, less than 100-ms latency and jitter

6 Voice, less than 10-ms latency and jitter

7 Network control

Table 1 Traffic Type Priority Levels

User Priority Traffic Type


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Confirming Packet Aggregation is Enabled(Cisco Controller)> show 802.11a802.11n Status: A-MPDU Tx: Priority 0............................... Enabled Priority 1............................... Disabled Priority 2............................... Disabled Priority 3............................... Disabled Priority 4............................... Disabled Priority 5............................... Disabled Priority 6............................... Disabled Priority 7............................... Disabled

Disabling 5GHz UNII-2-Extended ChannelsGiven that the most recent allocation of spectrum in the 5GHz band was just introduced in mid-2007, a number of Wi-Fi clients present in production environments do not actually support connections in the UNII-2 Extended Band. This allotment of channels ranging from 100 to 140 is essentially invisible to older Wi-Fi clients created before this band was made available for Wi-Fi by the FCC.

Note As of this writing, the list of clients which do not currently support this band include Cisco’s CB-21AG, Intel’s 4965AGN, Intel’s 3945ABG, Intel’s 2915ABG and Apple MacBook Pros with an Atheros card. Clients which do support this band include Apple’s iMac (with Broadcom 43xx-series wireless chipsets) and Intel’s 5100/5300AGN adapters. Ensure that the latest drivers are installed to leverage these channels. Please check with the vendor to ensure support.

In order to ensure that RRM does not utilize any of the UNII-2 Extended channels for automatic channel selection, navigate to ‘802.11a -> RRM -> DCA -> DCA Channel List’ and uncheck channels 100 through 140.

Figure 16 DCA Channel List


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Starting with Wireless LAN Controller version 5.2, the default configuration of RRM’s dynamic channel assignment algorithm will exclude these UNII-2 Extended channels from the list of automatically selected channels. This frequency sub-band can be re-enabled by means of the following configuration parameter found in the ‘802.11a -> RRM -> DCA -> DCA Channel List’ configuration on the controller. However, before re-enabling this frequency sub-band, ensure that the client base supports UNII-2 Extended channels.

Figure 17 UNII-2 Extended Channels

40MHz OperationThe maximum throughput of 802.11n can only be realized when 40MHz channels are utilized. In this mode, two traditional and contiguous 20MHz channels are bonded together to create a double-wide channel which essentially doubles performance. As a result of the expansive amount of spectrum in the 5GHz frequency band, 40MHz channels are recommended for operation in this band. There are two mechanisms by which this 40MHz operation is configured on the controller, either by using RRM to automatically assign channels, or statically configuring Access Point radios into a specific 40Mhz channel.

40MHz Configuration using RRMRRM (Radio Resource Management) can be utilized to automatically select 40MHz channels amongst 802.11n capable Access Points. The configuration for this capability can be accessed under the ‘802.11a/n -> RRM -> ‘Dynamic Channel Assignment’ configuration page. Change the ‘Channel Width’ configuration from ‘20MHz’ to ‘40MHz’ and click Apply.

Figure 18 40MHz Configuration Using RRM


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40MHz Configuration using Static Assignments40Mhz channels can also be statically configured on the Access Point radio much like the configuration of a 20MHz channel. In the below screenshot, the 802.11a/n radio is configured on channel 36 with a 40MHz channel width. Although the actual channel selected in this configuration is 36, the radio, in fact, is utilizing both channels 36 and 40 to create a 40MHz-wide channel. This is showcased in the ‘Current Channel’ field which specifies both the control and extension channel used by this radio.

Figure 19 40MHz Configuration Using Static Assignments

When selecting a statically assigned channel in 40MHz, the radio will automatically select the appropriate extension (or adjacent channel) to bond with the channel selected. In the above example, channel 36 was selected and channel 40 was automatically utilized as the extension channel. In another example, if channel 44 were selected, channel 48 would be selected as the extension channel. This applies to all channels across the 5GHz band configured for 40MHz operation and follows the convention that only adjacent channels can be bonded.

Confirming 802.11n OperationAfter ensuring that the WLAN infrastructure is appropriately configured for 802.11n operation, confirming that the client is utilizing 802.11n’s high throughput rates is simple. Checking the client’s mode of operation is accessible in the wireless LAN controller GUI by clicking ‘Monitor’ and then ‘Clients’. As depicted in the screenshot below, the protocol field for each client will be populated with the type of connection the client is using. For 802.11n, this can be represented as ‘802.11n(5)’ which means an 802.11n connection on the 5GHz radio or ‘802.11n(2.4)’ which is indicative of an 802.11n connection on the 2.4GHz radio.

Figure 20 Confirming 802.11n Operation


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Cisco 1140 Series Access Point Deployment 802.11 n

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