802.11n wireless network access points (APs) can be used to create bigger, faster networks that support more demanding...
applications -- even replacing wired Ethernet. But 802.11n APs -- with all of their technical improvements -- must be placed, configured, tuned and monitored differently than other APs. In this tip, we offer best-practices advice on how to get the most from your 802.11n AP deployment.
Room to grow: How to use extended 802.11n airspace
In WLANs, spectrum and channel assignment have a direct impact on capacity and performance. 802.11n WLANs can function on expanded spectrum and many more channels than 802.11 a/b/g networks. Legacy 802.11b/g APs were limited to just three non-overlapping channels in the 2.4 GHz ISM band. 802.11a APs could use 12 non-overlapping channels in the less-crowded 5 GHz UNII band. However, APs must compete for these unlicensed channels with other nearby devices, including cordless phones, Bluetooth peripherals, and APs owned by neighbors and metro-area WLANs.
WLANs that use 802.11n APs play in a much bigger "sandbox." All 802.11n APs can be configured to operate at either 2.4 GHz or 5 GHz. Some even support the UNII-2e extension to the 5 GHz band, adding another 11 channels. Furthermore, most 802.11n APs can optionally combine two 20 MHz-wide channels to form a single 40 MHz channel. Together, these advances enable denser AP deployments that deliver far more capacity to be shared by all users, along with higher throughput for each individual user.
To benefit from this expanded airspace, maximize use of the larger 5 GHz band. Assign APs in brand new coverage areas to UNII channels wherever possible; use ISM channels only where necessary to support legacy b/g clients. Avoid 40 MHz channels in the crowded ISM band altogether. Instead, use them to support new 11n clients in the UNII band, only for high-bandwidth applications that really need faster data rates. And turn on dynamic frequency selection, letting APs automatically change channels within their configured band as needed to avoid co-channel interference.
802.11n access point placement strategies
It may be tempting to deploy 802.11n using a simple one-for-one legacy AP replacement strategy. After all, reusing AP mount points, power supplies and Ethernet drops will reduce install costs. But doing so may not yield the best return on your investment, and in some (uncommon) cases it may not even deliver acceptable coverage.
Most old 802.11a/b/g WLANs were constructed using micro-cell architecture. APs were physically distributed throughout desired coverage areas, creating roughly circular "cells" that covered perhaps 3,000 feet apiece. Channels were assigned to maximize distance between APs using the same frequency, and transmit power levels were adjusted to create just the right amount of overlap. If there was too much cell overlap, clients wasted time bouncing between APs. Too little overlap, and roaming clients dropped connections in weak or dead spots.
With 802.11n, more channels simplify interference avoidance between cells. Cell shape and size change dramatically, however, because 802.11n APs leverage multi-path reflections to multiple "spatial streams" in different directions. Upon receipt, 802.11n APs may recombine diverse spatial streams (to increase total throughput) and/or compare redundant spatial streams (to improve signal-to-noise ratio). As a result, 802.11n cells are jagged and significantly impacted by the surrounding environment. A given AP placed in an open cubicle area may produce a completely different cell size and shape when placed 50 feet down the hall.
So when planning 802.11n AP placement, it's best to use RF planning tools and actual site plans to accurately predict the area covered by each AP. Let those tools suggest optimal positioning to meet your specified coverage, capacity and availability needs at minimum cost. Use "what if" analysis to visualize the impact of placing 11n APs on old mount points, and make desired adjustments before installation to reduce field retrofits.
After installation, conduct site surveys to validate your predictive design, keeping in mind that a stronger 802.11n signal does not necessarily imply higher throughput. For meaningful results, measure actual application performance using both old and new client devices, from many diverse locations and orientations. To facilitate future growth, survey all APs using 5 GHz channels, even if those APs will be used to support 2.4 GHz clients in the near term.
Tapping 802.11n PHY and MAC layer capabilities
802.11n APs support many new PHY and MAC layer capabilities. As noted above, 802.11n APs can be configured to multiplex from two to four spatial streams to increase maximum data rate. Furthermore, 802.11n APs with extra transmit antennas can use them to send the same information redundantly. Because corruption encountered along one path may not occur at the same places along a second path, 802.11n receivers can then take the best of both streams to reduce packet loss and sustain higher data rates despite the presence of interference.
Because each AP has only so many multiple-input multiple-output (MIMO) antennas, you must choose which benefits are most important to your network. For high-throughput applications that can tolerate some loss (e.g., streaming video), use more antennas for spatial multiplexing. For interactive, latency-sensitive applications (e.g., voice), sacrifice speed for quality.
Some of the biggest efficiency improvements in 802.11n stem from MAC layer enhancements. In legacy 802.11a/b/g WLANs, APs had to explicitly acknowledge every single frame. But 802.11n APs can use Block Acknowledgement, whereby several frames are sent without delay and then acknowledged with a single reply. In WLANs that support streaming applications, configure 802.11n APs to use Block Acknowledgement to cut inter-frame delay. For APs that support new 802.11n clients, enable the new Short Guard Interval (SGI) to further reduce the pause between transmitted symbols from 800 to 400 ns.
Even more airtime efficiency can be gained by using 802.11n frame aggregation options. 802.11a/b/g APs can send just 2,304 payload bytes per frame, but 802.11n APs can bundle smaller frames together for transmission to reduce MAC layer overhead. MAC service data unit (MSDU) aggregation can group payloads with the same WMM priority into frames up to 7,935 bytes long, while MAC protocol data unit (MPDU) aggregation bunches complete frames into a single transmission up to 65,535 bytes. The longer each transmission, the less airtime wasted on MAC layer overhead (e.g., preambles, inter-frame spacing), increasing total application throughput. The best choice for your WLAN depends on application/client device mix and load, so monitor in-situ performance and then optimize these AP settings.
Deploying 802.11n access points in a legacy network
Unless deploying a brand new WLAN from scratch, most companies will require new 802.11n APs to coexist with legacy APs and/or clients. Today, most new notebooks ship with 802.11n adapters, but it will take two or three years before 11n-capable notebooks dominate. Some legacy clients may persist even longer -- especially devices like printers, handheld barcode scanners, and voice handsets that tend to be slower to use new chipsets and remain in use longer.
802.11n-capable APs can operate in three standard modes: high-throughput, non-high-throughput, and mixed. Non-high-throughput (non-HT or legacy) mode makes a new AP operate just like an 802.11a/b/g AP. Choose this mode when installing a new AP for future growth, but dedicate one of its 11n-capable radios to support legacy clients only. This mode is intended only to ease migration -- it cannot deliver any 11n improvements in data rate, capacity or efficiency.
In the near term, most new 11n APs should be set to mixed mode to apply protection techniques that ensure peaceful coexistence with nearby 802.11a/b/g devices (clients and/or APs). At the PHY layer, 802.11n APs start each transmission with a new "high throughput" preamble that legacy devices would not understand. To avoid collisions while still using new 11n features, mixed-mode APs must send the old a/b/g preamble, followed by the new preamble. Furthermore, because legacy devices cannot understand new 11n transmissions' content, mixed-mode APs are required to transmit legacy format CTS-to-Self or RTS/CTS (ready to send/clear to send) frames, to let everyone around them know how long the channel will be busy.
Of course, these extra preambles and frames increase overhead and reduce operating efficiency. Eventually, most 802.11n APs will disable legacy protection and operate in high-throughput (greenfield) mode. Doing so when legacy devices are still present would have an adverse impact on everyone. Some 11n APs can safely operate in high-throughput mode immediately, however. Specifically, a WLAN with no existing 802.11a APs or clients may configure new 11n APs to use 5 GHz channels in high-throughput mode.
Creating an 802.11n migration plan
In fact, planning for migration is an important part of 802.11n AP rollout. Some companies may choose to deploy 802.11n as an overlay WLAN, leaving legacy APs to support old clients while dedicating new APs to support only new clients, applications and coverage areas. Alternatively, some companies will retire legacy APs and replace them with new APs that simultaneously support both old and new clients. In this case, it is common to deploy dual-radio 802.11n APs, configuring one radio to support 802.11a/b/g/n clients and the other to support 11n clients only.
No matter how you decide to use 802.11n initially, it is a good idea to establish a plan for when and how legacy APs and mixed mode will be phased out of your new WLAN. Doing so will not only set expectations but will help you phase in new 802.11n capabilities to optimize overall WLAN performance and maximize your return on investment.
About the author:
Lisa A. Phifer is vice president of Core Competence Inc. She has been involved in the design, implementation and evaluation of data communications, internetworking, security and network management products for more than 20 years and has advised companies large and small regarding security needs, product assessment and the use of emerging technologies and best practices.
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