A major task in designing a wireless LAN is identifying
the number and location of access points. This involves performing a
radio frequency (RF) site survey
in order to test specific access point locations as the basis for determining
the range boundaries of access points within the actual facility. The results
of this testing indicates the optimum number and location of access points.
Range boundaries characterize the circumference
of the radio cell that the access point produces. With the boundaries known,
a designer can make certain that the radio cells overlap enough to enable roaming
and an adequate degree of redundancy. This may seem straight forward, but complications
arise because of the nature of radio waves and the 802.11
protocol.
Range Boundaries Based on Connectivity
The range boundary of an access point based on connectivity indicates the point
at which a user device can associate with an access point at a specific data
rate. For example, you might define the connectivity range boundary to be at
11 Megabits per second (Mbps), which means that the boundary is met when the
associated data rate drops from 11Mbps to 5.5Mbps. Within this range boundary,
all users will have 11Mbps associations. Similarly, you may define the boundary
to be at 1Mbps, which is when the associated data rate drops from 1Mbps to the
disassociated state. In general, you can define the connectivity range boundary
to be any data rate that the access point supports.
Even though designers generally specify range boundary based on connectivity,
it’s not sufficient for WLAN systems needing to support higher capacities, many
users, or guaranteed levels of performance. The issue is that data rate alone
does not provide a true metric for user performance. The definition of range
boundaries based on connectivity introduces significant risk when planning and
managing the performance of the WLAN.
One user having an 11Mbps association with an access point may be relatively
free from RF interference and have nearly maximum throughput (approximately
6Mbps for 802.11b). In this case, using range boundaries based on connectivity
would probably be satisfactory, assuming the user conditions don’t change.
A more likely scenario, however, would entail multiple users experiencing RF
interference from adjacent access points. Each user may have an 11Mbps association
with the access point, but the interference (and collisions among the users)
could cause each user to experience 50% packet retransmissions. This could significantly
reduce the total throughput for users to unacceptable levels that don’t adequately
support the wireless applications.
Range Boundaries Based on Performance
As WLANs begin to support more users requiring greater capacity, I’m finding
that designers should utilize a more accurate method of defining access point
range. For example, I’ve been working with Cellular Design Services (CDS) over the past
few months as a consultant and designing a common WLAN infrastructure for the
BAA airports, which includes London Heathrow
Airport and several others located in the United Kingdom.
In addition to lower performance bar code applications, the BAA WLAN will support
travelers requiring broadband wireless connections throughout airport public
areas. A combination of these requirements requires careful planning of access
point locations to ensure bandwidth requirements are met. In this case, we’ve
found that the use of range boundaries of access points based merely on the
ability to connect with an access point at a specific data rate is simply not
good enough.
A critical goal of planning a high performance WLAN is to ensure that the packet
retransmissions stay down to an acceptable level. In addition to simply indicating
association with an access point, the range boundary definition should also
include affects of both interference and noise. This defines the useful service
boundary of an access point, which is the distance at which the signal-to-interference-plus-noise-ratio
(SINR) is just large enough to provide sufficient performance. Designers should
utilize this definition of range boundary when specifying the location of access
points, such as when performing an RF site survey.
This method will generally produce a range boundary that’s closer to the access
point than when using connectivity alone to define the boundary. An acceptable
SINR will depend on the actual application, but 30dB SINR will generally provide
optimum performance for maximum capacity WLANs. In order to make use of this
range boundary definition, however, ensure that the site survey tools you select
have the ability to measure SINR.
Range Boundaries Based on Interference
Of course, the range boundary of an access point
is important to know in order to guarantee a specific level of performance for
mobile users, but another element to consider is potential interference with
other systems. An important point to realize is that the range of an access
point in terms of potential interference with other systems can be much farther
than the range necessary for user association.
For example, a user may lose association with the
access point at 150 feet range, but the access point may still interfere with
another system located 500 feet away. As a result, designers should determine
the range boundary based on interference when performing site surveys to ensure
the adequate spacing of access points to minimize interference with other systems
and access points set to the same channel. The actual signal level that will
cause interference depends on the external system. As a result, be sure to understand
the toleration level of all existing and future systems that could be the recipient
of RF interference, and ensure that these systems fall outside the range boundary
(based on interference) of your access points.
Stay tuned! In future tutorials, we’ll look at
tools and RF site survey processes that take advantage of performance and interference
range boundaries.
Jim Geier provides independent consulting services to companies
developing and deploying wireless network solutions. He is the author of the
book, Wireless LANs and
offers workshops
on deploying WLANs.
Join Jim for discussions as he answers questions in the 802.11 Planet Forums.
