Five critical planning steps for wireless LANs, Step 2: Moving to 802.11a

Part two covers 802.11a and its ability to mitigate problems of interference caused by too many devices using the same radio channels.

This article is part two in a five-part series from contributor Michael Finneran. Read Step 1: Planning for capacity, not just coverage here.

Critical Step 2: Moving to 802.11a

For 2003, the big story in wireless LAN technology was the introduction of the 54 Mbps 802.11g radio link. While 802.11g delivers roughly five times the raw capacity of 802.11b's 11 Mbps transmission rate, the euphoria ignored one major deficiency in 802.11g -- it still works in the unlicensed 2.4 GHz Industrial, Scientific, and Medical (ISM) radio band.

The two major problems with 2.4 GHz systems are the limited amount of radio spectrum available and the potential for interference from other users. The FCC has allocated 83.5 MHz of radio spectrum to the ISM band, and as each 802.11b WLAN channel requires roughly 25 MHz, only three non-interfering channels can be accommodated. Even though it supports a higher data rate, an 802.11g channel requires only 20 MHz. However, to provide interoperability with 802.11b systems, 802.11g uses the same three channels.

* In November 2003 the FCC increased the frequency allocation in the U-NII band from 300 MHz to 555 MHz. Initially, there were twelve 802.11a WLAN channels defined in the original 300 MHz, and the IEEE has yet to determine how many additional channels will be assigned in the new allocation.
U.S. unlicensed frequency bands
Name Frequency range Bandwidth Bandwidth/WLAN channel WLAN channels
Industrial, Scientific, Medical (ISM) 902 M to 928 MHz 26 MHz Not used Not used
Industrial, Scientific, Medical (ISM) 2.400 G to 2.483.5 GHz 83.5 MHz 802.11b to 25 MHz
802.11g to 20 MHz
3
3
Unlicensed National Information Infrastructure (U-NII) 5.150 G to 5.850 GHz (non-continuous) 555 MHz 802.11a- 20 MHz 12 (potential 24*)

While much has been made of the interference from other 2.4 GHz devices including cordless phones, baby monitors, garage door openers, and microwave ovens, in actuality, the biggest source of interference is other 802.11 wireless LANs. At BCR's Next Generation Networks Conference in November 2003, Richard Eckard of Verizon Laboratories noted that when his company began to install WLAN hot spots in Manhattan, they often found as many as twenty other WLANs operating within range of their planned locations. Any wireless LANs operating on the same channel in the same area will create interference and degrade the performance of your network. With only three channels to work with, it's hard to get out of the way.

The answer is 802.11a that operates in the less congested 5 GHz band, and it will quickly become the preferred option for commercial users. In the U.S., the 5 GHz Unlicensed National Information Infrastructure (U-NII) band was initially allocated 300 MHz of non-contiguous bandwidth between 5.150 and 5.585 GHz. With each 802.11a channel occupying 20 MHz, they could accommodate 12 non-interfering channels. In November 2003, the FCC allocated an additional 255 MHz to the U-NII band (5.470 to 5.725 GHz), which could provide an additional 10 to 12 channels; the IEEE has yet to decide how many channels they will define.

It has also been noted that there are fewer devices currently operating in the 5 GHz band, and hence, less interference. However, the 5 GHz U-NII band is also unlicensed and so it is available to all. However, while other applications might eventually find their way into the 5 GHz band, with 12 to 24 channels available, it should be easier to avoid the interference. The downside of 802.11a is that the 5 GHz signal suffers greater loss when passing through obstructions, so upgrading to from a 2.4 GHz network will likely require more access points and a redesign of the radio coverage plan.

One of the least productive developments for commercial users is a non-standard 108 Mbps radio links. Chip manufacturer Atheros has been a major culprit in this with their Super G and Turbo Mode radio links for the 2.4 G and 5 GHz bands respectively. The "magic" here is that they expand the bandwidth of the radio channel to provide the higher data rate. However, expanding the channel bandwidth reduces the number of non-interfering channels in the 5 GHz band from 12 to 6, and in the 2.4 GHz band from 3 to 1 (i.e. it uses Channel 6, but overlaps into channels 1 and 11). This is a great trick for home users, but in commercial environments, we need more not fewer channels. In short, leave this one home.

Go on to part three in the series, Assessing security enhancements.


About the author:
Michael Finneran is an independent telecommunications consultant specializing in wireless networks and technologies. Besides his research and consulting activities, he writes a regular column called "Network Intelligence" for
and teaches their seminars on wireless technologies and wireless LANs. He can be reached at mfinneran@att.net.
A large-scale WLAN will be laid out like a cellular network with different channels used at each access point. With a limited number of channels available, channel reuse is inevitable. The basic rule is that you cannot reuse a channel in an adjacent cell. While 802.11b and g networks dominate today, commercial users must plan their move to the 802.11a radio link to increase the number of available channels and simplify the network layout. Business Communications Review
This was first published in June 2004

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