This section will sum up the results of our research into wireless LAN.
The research was done by a collaboration of ACCU,
Digital,
Lucent, Surfnet
and the computational physics
workgroup of Utrecht University. One of the aims of the project is to advise
the board of the University College on the
suitability of a wireless LAN for the College campus. In the near future,
the focus of the research will shift to simulating the MAC layer behaviour
of the 802.11 wireless LAN standard. This will allow us to gain a more
fundamental insight in the possibilities and restrictions of wireless LAN.
Scaling
One of the major concerns about wireless LAN was whether or not the throughput
would scale with the number of clients. Under extreme loads, collissions
are very likely to occur. At a certain collission rate, the system will
drown in retries, that in their turn collide and generate a retry. This
effect, that is known to occur in ethernet, would render a very low throughput
in situations where you need the most.
In order to investigate this effect, a throughput measurement tool
was developed in our workgroup and two test sessions were organised in
November and December. It allows to have clients sending as much garbage
data as they can to a server application, which calculates the throughput.
The aggregate throughput of up to 25 clients was seen to remain stable
at 1.5-1.8 Mbit/second, depending on block size. These results are backed
by independent measurement using existing tools. Further, two operating
system/protocol combinations (TCP-IP under Windows and IPX under Novell)
were seen to produce similar results. Therefore, we conclude that wireless
LAN is able to deliver reliable communication for a large number of users.
Detailed testplans and results of the two scaling tests are published here;
Testplan
and results
of the November test.
Testplan
and results
of the December test.
Multicast
For future applications in education multicast might prove to be very useful
to reduce the necessary bandwidth. Therefore, it is important that the
wireless LAN truly supports multicast. We tested this feature by having
one PC broadcast a video to three clients via an access point. The clients
each received up to 1.3 Mbit/s. This is only possible using multicast,
as the aggregate throughput would otherwise be nearly 4 Mbit/s.
Architecture
According to the 802.11 standard, the frequency band from 2.400 to 2.483
GHz has been assigned for wireless LAN use world wide. It is possible to
operate more than one overlapping cell at different frequencies. However,
if the frequencies are too close, interference will distort both signals
and lead to much lower throughputs.н Thus, the placement of the access
point on the campus determines the network capacity in each area.
In order to estimate the number of access points needed and the possibility
to raise the bandwidth in designated areas, a number of tests were performed.
A first test consisted of throughput measurements for clients communicating
to two access points at different frequencies in the same room. It showed
that the frequency difference should be at least 30 and preferably 40 MHz
to avoid interference. With the product under investigation, two overlapping
cells are allowed. When the full band will be used it should be possible
to operate three independent cells, yielding a total throughput of 24 Mbit/s
(Later this year, new cards with up to 8 Mbit/s will be launched).
On the other hand, the range of each access point is an important parameter.
It depends mainly on the amount and sort of material obstructing the signal
and might differ significantly from building to building. Therefore, the
quality of the signal was measured in a future University College building,
having placed one access point. Horizontally, the range was found to be
of the order of 50 meters. Vertically, good signal is basicly confined
to one floor. In most areas, the range of the signal is not going to be
the limiting factor, as too many users in one cell will result in low service
speed.
The results mentioned in this section are published here;
The range measurements.
The interference
measurement.
Roaming
For three months, a wireless LAN has been used as production network at
two locations in Utrecht University. During this period, it has proved
to be a stable, easy-to-manage and mobile networking solution. Roaming
within one subnet is secure and fast - the connection being lost for milliseconds.
For a change of frequency, a simple user intervention is still required.
If one wants to allow roaming between subnets, mobile IP will have to be
implemented.
Conclusion
Generally, wireless LAN proved to be a reliable and reasonably fast mobile
networking solution. For most purposes, the bandwidth it provides should
suffice, although cabled LANs offer more possibilities to extend the bandwidth.
In situations where mobility is as good a criterium as bandwidth, a wireless
LAN should certainly be considered a serious option.
dr.ir. C.Th.A.M. de Laat,
Faculty of Physics and Astronomy, Utrecht University, Princetonplein 5,
NL-3584CC Utrecht, The Netherlands.