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Fig. 2. 802.11 relative rate and range comparison: Many network administrators assume that they can use fewer<br />

IEEE802.11n APs than with 802.11abg. However, most networks will still see many 802.11abg stations present, so<br />

similar numbers of 802.11 APs will be needed for adequate 802.11n and 802.11abg stations coverage.<br />

none, WEP, and TKIP. Using no encryption is<br />

not a realistic option for today’s networks. WEP<br />

and TKIP provide some security, but are not<br />

nearly as robust as AES. Besides inferior<br />

security, WEP and TKIP limit data rates to<br />

54Mbps, so the increased speed of 802.11n<br />

cannot be realised.<br />

Greater encryption power<br />

While AES is more secure, it does create an<br />

increase in required encryption power on the<br />

core network. Since 802.11n supports approximately<br />

six times the data throughput of<br />

802.11ag, six times as much traffic may need<br />

to be encrypted. Many back-end controllers<br />

cannot handle this increase in encryption. In<br />

fact, some vendors state an 80% drop in<br />

throughput capability when using encryption.<br />

A <strong>wireless</strong> network should have a continuously<br />

operating network threat sensor. If the<br />

network is migrated to 802.11n, the monitoring<br />

tool needs to be migrated as well.<br />

Using 100baseT wired switches in the network<br />

is inefficient with 802.11n. Two-radio 802.11n<br />

APs can generate up to 250Mbps of data traffic,<br />

oversubscribing a 100baseT connection. This<br />

effect is more marked when four- and eightradio<br />

arrays are used. Gigabit Ethernet switch<br />

ports are required to support the data traffic<br />

from 802.11n APs and arrays.<br />

Core switches will also be affected by<br />

802.11n, particularly when using controllerbased<br />

<strong>wireless</strong> networks. With a controller, all<br />

<strong>wireless</strong> traffic must go from the APs to the<br />

controller for processing and then back to the<br />

APs (‘tromboning’). To minimise tromboning,<br />

traffic should be processed at the edge where<br />

possible (Fig. 3).<br />

802.11n equipment typically requires too<br />

much power to be used with 802.3af provided<br />

over a wired Ethernet connection (PoE), so<br />

<strong>special</strong> power injectors are required. Some<br />

vendors have a low power mode that allows<br />

their devices to be run with standard 802.3af<br />

PoE ports; however many of the features of<br />

802.11n must be turned off and/or performance<br />

reduced. It is better to plan for full power and<br />

all of the functionality by using higher-powered<br />

injectors.<br />

RF design considerations<br />

An 802.11n network will always perform better<br />

than an 802.11abg network, but there are<br />

design decisions that can be made to increase<br />

the performance even more.<br />

802.11n is the first Wi-Fi standard that can<br />

operate in both the 2.4GHz and the 5GHz bands.<br />

5GHz has many advantages over 2.4GHz and<br />

every effort should be made to move the network<br />

to primarily 5GHz. Most enterprise class<br />

equipment can support 5GHz today – however<br />

there are some handheld devices (e.g. Blackberrys<br />

and iPhones) and net<strong>book</strong>s that are primarily<br />

2.4GHz, and so it is prudent to leave some<br />

2.4GHz APs in place to support handheld Wi-Fi<br />

devices. In buying net<strong>book</strong>s, care should be<br />

taken to ensure that both 5GHz and 2.4GHz<br />

operation is supported (Fig. 4 over page).<br />

Some APs are fixed with one radio operating<br />

at 5GHz and one radio at 2.4GHz. As the<br />

<strong>wireless</strong> network transitions to predominately<br />

5GHz with this type of equipment, the 2.4GHz<br />

radios will go unused. Better alternatives are<br />

APs and arrays that have software-selectable<br />

frequencies which may be used with both 5GHz<br />

and 2.4GHz, convertible to 5GHz as a majority<br />

of the stations become 5GHz-capable.<br />

Network administrators often assume that<br />

they can put more users in the higher<br />

throughput 802.11n network than they could<br />

Fig. 3. Wi-Fi architecture comparison: Minimise traffic load from 802.11n networks on the core by processing as much<br />

traffic as possible at the edge. However, the core must be capable of dealing with higher 802.11n network traffic volumes<br />

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11

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