How to: Migrate to 802.11n in the Enterprise
by Jim Geier

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The 802.11n standard, ratified in 2009, is currently the industry standard for implementing wireless LANs. 

802.11n is the way to go

802.11n is a major upgrade to legacy (802.11a, 802.11b, and 802.11g) wireless LANs. If you implement the 802.11n network carefully, it’s possible to achieve a ten fold increase in throughput with as good as or better range compared to 802.11b/g networks. As a result, 802.11n offers support for a much wider range of applications. 802.11n is also backward compatible with legacy client devices, which allows the effective migration to 802.11n from an existing 802.11b/g network. An 802.11b client associating with an 802.11n access point, for example, can interface with the network at 802.11b speeds (1, 2, 5.5, and 11Mbps). 

802.11n makes more efficient use of the available spectrum. For example, there’s support for block acknowledgements. Instead of requiring an acknowledgement frame for every data frame, 802.11n allows a single acknowledgement to acknowledge multiple data frames. This significantly reduces overhead and allows shorter inter-frame spacing, which gives a boost to performance (even for legacy client devices). Other improvements, such as MIMO, enhanced power management, and QoS, also contribute to making 802.11n a possible solution for supporting mission-critical applications. 

If you have an existing wireless LAN (or none at all), now is the time to consider deploying 802.11n.

Preliminary steps

Before migrating to an 802.11n network, complete the following preliminary steps to establish a solid basis for implementing the network:

1. Review requirements for wireless networking. You may have defined wireless requirements for an existing 802.11b/g network, but the higher performance of 802.11n will likely open the door to bandwidth-hungry applications. Give special attention to capacity requirements, types of client devices, and roaming areas.

2. Assess the existing network. Consider beefing up your wired network to support the higher capacity of 802.11n. Predict the flow of traffic and determine whether existing 10/100Mbps switches will support the higher traffic. If not, you may need to eventually upgrade the switches. Cisco recommends 1Gbps switch connections to realize the full capacity of the Cisco 1250 802.11n access point. Based on Cisco testing, the full capacity of the 1250 is roughly 200 Mbps (client throughput) with both access point radios enabled. Also, determine whether you’ll need to upgrade other wireless systems, such as intrusion detection sensors. 

3. Review the existing PoE infrastructure. To achieve maximum performance from 802.11n implementations, you may need to upgrade the existing PoE to an enhanced version. The IEEE 802.3af standard currently defines PoE to deliver 15.4 watts, but many 802.11n access points, such as the Cisco 1250, require 18.5 watts to run both 2.4GHz and 5GHz radios (needed for maximum performance). Cisco sells a proprietary enhanced PoE for this purpose. If you don’t upgrade to 18.5 watt PoE, the access points may only operate a single radio or use both radios with limited 802.11n functionality. IEEE is working on a 30 watt upgrade to PoE, and it may be ratified as soon as 2009.

Design tips

When you’re ready to design the 802.11n network, consider the following tips:

If you have a legacy wireless LAN, check whether it’s possible to upgrade the existing access points to 802.11n. Unfortunately, in most cases, you’ll need to replace the existing access point hardware. Some of the vendors, though, have made their 802.11n access points compatible with the older wireless controllers already in place. So, it’s likely that only the access points will need replacing (of course that is still a big investment). 

Ideally, focus on an 802.11n-only solution. This will provide the greatest performance for wireless applications because legacy client devices won’t bog down the network (due to protection mechanisms) and all client devices will have the capability to operate at higher 802.11n line speeds. An all-802.11n solution, however, may not be feasible. Many of the existing client devices, such as laptops, may already have 802.11b/g radios, and the costs associated with upgrading them to 802.11n may be cost-prohibitive. Do the math to figure out the best way to go for your situation.

Possibly implement mixed mode (legacy and 802.11n clients). This approach is best if it’s not feasible to upgrade all existing wireless clients to 802.11n. The most effective configuration for mixed mode is to install two radios in each access point. Place the 802.11b/g traffic on 2.4GHz radios and the 802.11n-only traffic on the other (ideally 5GHz) radios. Keep in mind that not all countries allow 802.11n 5GHz operation.

Consider using 40MHz channels. 802.11n supports both 20MHz (legacy channels) and wider bandwidth 40MHz channels in both 2.4GHz and 5GHz bands. The 40MHz channels in the 5GHz band will provide the most performance, with up to double the throughput compared to 20MHz channels. Because of limited overall bandwidth, though, you shouldn’t configure 40MHz channels in the 2.4GHz band. Stay with 20MHz channels there. 

Use multiple wireless controllers for redundancy and increased performance. A single controller may be sufficient for 802.11b/g implementations, but it may become a bottleneck for the much greater volume of 802.11n traffic. In addition, the migration to 802.11n often ushers in mission-critical applications, and a single controller represents a single point of failure that you should avoid. Check with what your vendor recommends and ask for independent test documentation indicating needs for multiple controllers. Also, make sure the existing controller is upgraded to an appropriate level to support 802.11n. 

Deployment tips

Consider the following when rolling out an 802.11n network:

Start by performing a wireless site survey. This is even more important with 802.11n as it is with 802.11b/g because propagation of 802.11n signals is more complex. The survey will assess existing network infrastructure and determine the optimum location for mounting access points. A previous site survey completed for an 802.11b/g network will generally not be sufficient. The coverage patterns of 802.11n radios, for instance, can be very different than 802.11b/g. 802.11n uses MIMO and does a better job of handling multipath propagation, which results in better coverage in some parts of a facility, but the use of 5GHz frequencies may cause less range in some areas. 

Keep in mind that the way 802.11n signal values translate to performance are different than 802.11b/g, so you’ll need to re-evaluate the definition of range boundaries before performing the survey. It’s very important to take into account performance of the target client devices. For example, some client radio will likely have lower transmit power and utilize fewer antennas as compared to the access points, making the client uplink signal strength much less than the downlink. 

Also, avoid reliance on predictive planners. They didn’t work too well for 802.11b/g networks. Don’t expect them to be much better for the more complex 802.11n networks. You should always perform enterprise wireless surveys with a real target client device and access point. 

Swap out the legacy access points with 802.11n. Do this in a smaller area first as a pilot test to verify the design, and then complete the replacements in phases throughout the entire facility. You can likely get by at first using a single (possibly existing) wireless controller. As with most implementations, performance needs increase over time. Predict when additional controllers will be needed, and plan their installations. 

Verify acceptable operation. As with any wireless installation, always verify signal coverage and performance after installing the access points using actual target client devices. In mixed mode implementations, ensure that 802.11n devices are in deed operating at 802.11n rates by checking client association tables. Also, check how laptops behave when running on battery power. Some will significantly reduce power consumption and result in lower radio throughput. If you must maximize performance, consider reconfiguring the power saving feature in the laptops (in some cases, you might have to disable it). 

Plan the obsolescence of 802.11a/b/g. As 802.11n products become available, the legacy devices will likely not be strongly supported. In addition, the operation of legacy devices on your 802.11n network can bog down performance of all users. Focus on purchasing new client devices with 802.11n and replacing existing 802.11a/b/g radios with 802.11n as quickly as possible (of course after considering the economics). 

As a final tip, be sure to spend adequate time understanding 802.11n and planning the migration. 802.11n is more complex than legacy wireless LANs, and you need to carefully phase in the new technology to realize its potential. 

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