Is 802.11n Worth the Money?By CWNP On 09/12/2007 - 5 Comments
I'm thinking that most 802.11n deployments will initially be in the 5 GHz UNII band using 40 MHz channels. Otherwise, why would anyone roll it out - right? There are a few enhancements other than just throughput with 802.11n equipment, but you should consider if they will be enough to justify the costs. Throughput enhancements will likely have to be a large part of a WLAN upgrade to 802.11n to get organizations to spend the money. Starting with an assumption that using 40 MHz channels in 5 GHz bands will be the norm due to more available channels and less congestion in each channel, let's consider the ramifications of backwards compatibility with 802.11a devices.
802.11a devices use 20 MHz channels. In a 40 MHz environment, this is going to necessitate use of protection mechanisms in 20/40 MHz mode (very likely) or use of the PCO functionality of 802.11n (unlikely, at least for a while). Protection mechanisms, as we all know from 802.11b/g mixed deployments, cause a severe degradation in throughput. With the number of 802.11a capable phones that will be in the average enterprise WLAN when 802.11n is finally rolled out en masse, it's a given that 802.11n will not initially give us the kind of throughput that many newcomers to this industry might be hoping for.
Only a greenfield 802.11n deployment is going to give us a significant throughput boost over 802.11a in the 5 GHz UNII bands, and that's not going to happen until wireless phone manufacturers deliver 802.11n capable handsets. You won't see HT PHY capable handsets for quite a while - especially since we JUST NOW got 802.11a/g handsets. Manufacturers' reasoning seems to be that the HT PHY doesn't give you any performance boost over the OFDM PHY since voice handsets use such small amounts of bandwidth to begin with. My humble opinion is that their justification saves them money, but costs the customer BIG in lost throughput across the WLAN. If the handsets were HT PHY capable (along with everything else on the WLAN), a greenfield deployment would give the end user the significant performance gains they are looking for and would easily justify an 802.11n upgrade of the infrastructure. It's the same scenario we had with upgrading the infrastructure to 802.11g - only this time, 802.11b, 802.11g, and 802.11a all affect 802.11n deployments, depending on the frequencies deployed on 802.11n networks of course.
Other than throughput (since we already know that we're not going to get much of a boost from 802.11n in the average deployment due to various protection mechanisms), what other performance enhancements might make the upgrade to 802.11n worth the money? Use of multiple radio chains and new PHY protocols gives us much better sensitivity and better range. Transmit Beamforming (TxBF) gives us targeted, high-gain transmission (whenever vendors decide to give us this functionality). MIMO gives us resistance to the degrading effects of multipath, eliminating many coverage hole issues. Is it worth the money to upgrade if throughput isn't significantly enhanced? Only you can be the judge of that.
What problems might we encounter when migrating to 802.11n? Consider that RF planning tools - and even manual surveys for that matter - must now take into consideration added coverage due to multipath. With very sensitive radio chains, receivers will have even more problems with co-channel inteference if the site survey isn't adjusted. It's possible that with future feature sets like TxBF, we might have client stickiness problems once again. Isn't 802.11n fun?