more problems soon for US 2.4Ghz band
hfeinstein at cox.net
Tue May 30 17:17:17 CDT 2006
The idea of urban hotspot networks in not new. The usefulness of these
networks in undeniable, however,
it is going to present serious problems for continued ham /secondary/
usage. An urban hotspot network consists
of multiple Access Points (AP) spaced very close to one another in order
to provide something like constant coverage within a geographic area.
It is a common thing on a university campus or laboratory grounds. This
idea is now being planned to cover larger urban area such as a downtown
district or an entire small town.
Planning is also underway to install urban hotspot networks in a section
of a city, for example, Manhattan
between Battery Park and Midtown in NYC or downtown Philadelphia. A
wifi user might go anywhere within the wide coverage area and stay
A primitive network design might not support true roaming, enabling a
user to seamlessly move between APs,
however, more sophisticated single vendor systems do support this
feature and their equipment might be
chosen for installation.
The popularity of the 2.4Ghz band makes this frequency a good choice for
an urban hotspot network. An architecture to support this kind of
network will consist of a high speed fiber backbone connecting
periodically spaced APs in a grid formation. Some RF relaying, similar
to packet repeating, might also be used.
A bit of thinking should convince you that a well designed system will
open up all kinds of new
commercial and personal use. The old joke among Arpanet researchers
talked about "toasternet"
a humorous mythical future network so pervasive that even a household
toaster will have its own IP address.
I do not see that as
so much a joke anymore than as a certainty. So many users who will
discover so many uses of a new urban hotspot network will create fast
growing traffic demands. Among 802.11 designers it is well understood
that the 802.11 protocol was never designed nor intended to support this
kind of common carrier access situation. The basic structure can easily
become overloaded and there is no place to go but adjacent frequencies.
Now I want to say a few words about ham usage of the 2.4Ghz band. There
are two ways people use this
band, the first is the transitional ham usage in contests. satellite and
some point-to-point experimental links. Then there is the ham wifi
experimenters who, because of ham rules, can run higher power than their
non-ham co-occupants. It will be very difficult for either of these
groups of hams to use this band in any
location near an urban hotspot network because of interference. The
interference will show up as an increase
in the frequency of retransmissions and lower throughput. This might be
mitigated by using very tight antennas
in a point-to-point link but broad beam widths near these urban centers
will spell problems.
Although urban hotspot networks will happen and there is probably
nothing hams can or ought
do about it, there is one
area that hams can have positive influence. Commercial Wifi operators
are also worried about interference
from ham usage of 2.4Ghz and try to stay informed by reading ham mailing
lists. Alas, hams seem intent
on mudding up the waters when asked a question about their operations.
For their part, the commercial operators are not reassured at
the usual maneuvering on ham mailing lists and seriously talk about
approaching the FCC (or their congressman or senator) for protection or
at the least monitoring and opposing Part 97 NPR that they /think /will
complicate their operations. Gloating over their tertiary status as
having to suffer through whatever interference
secondary or primary users dish out will not lead to an understanding
confidence and trust. This kind of thing could be headed off but we
hams ought to
be better acquainted with the spectrum management issues on 2.4Ghz
before offering up opinions to
concerned commercial operators.
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