Report on proposing an 802.11 Solar Powered Repeater
Radioactive Networks often gets asked to produce proposals on various
topics. One of the things that we were asked to write about was a solar
powered repeater. I the WORD document here we
discus how we would install a repeater at a specific site.
Active 802.11 Repeaters
When building an 802.11 system you will find that there are many cases where
a line of sight point to point link is impossible. There will be times where
a non-line of site link may work at reduced speed, but most of the time the
whole thing will be a disaster. As the path gets longer, the chances of
failure are increased.
All is not doom and gloom in the wireless world however, thanks to the
Linksys WAP-11 Access Point. This unit is not only cheap at about $120 from
here.
This device becomes the building block of a number of possible solutions.
Where most suppliers attempt to create a hierarchy of products through the
features that are included in each model, LinkSys have included as many
features as possible in their base model.
Access Points Connecting Together
The feature that we are particularly interested here is the ability for an
Access Point to act as a station on another access point. That is, for an
Access Point to behave as if it was a normal wireless device connecting to
an Access Point, BUT with the bonus of any traffic on the LAN connected to
the Access Point being transmitted over the wireless network.
This makes it particularly easy to link two networks. All it takes ia a
WAP-11 Access Point at each end, connected to their repective LAN's. After
configuring one unit as a master, and the other as a slave, everything
should work as if the access points were just simple switches or hubs
connected by a piece of Ethernet cable.
Smart Repeater
That does not really help us where we do not have line of sight though. In
order to implement a link with a repeater we need to add more access points.
To understand why, you need to understand how normal repeaters work, and how
802.11 works.
Normal repeaters, in the radio world operate by receiving signals on one
frequency and transmitting on another frequency at the same time. This is
possible because most transmissions are based on FREQUENCY DIVISION
MULTIPLEXING. This means that the transmissions actually change frequency as
they go through the repeater, and seperate frequencies are used for
receiving and transmitting.
In 802.11 the same frequency is used for transmitting and receiving, using a
system known as TIME DIVISION MULTIPLEXING. A repeater using this scheme
must use some type of Store And Forward, in which packets to be repeated are
recorded and played back a fraction of a second later.
Three Access Point Repeater
The Three Access Point Repeater is quite simple. It exists of an ACCESS
POINT at the Repeater Site operating as a master, with the units at each end
attaching to it as slave units. This is a simple solution, but does has some
disadvantages. The worst is that of the throughput of the link. Because
every packet is being recieved and then retransmitted on the same frequency
the maximum availability of bandwidth on the link is halved.
If you had 11 MBit/sec connections, with encruption and overhead this comes
to about 8 MBit/sec. Now we need to halve this because the link is only
available half the time, so we get about 4 Mbit/Second through this link.
This is respectable, but nothing like our original speed. If the base
bitrate is 512 kBit/Sec things are a lot worse.
Four Access Point Repeater
Because of the frequency use problems of the three access port solution, an
alternate solution can be used. It operates throuh the use of Frequency
Diversity on top of the normal 802.11 TDM.
The main idea is that each link to the repeater operates on a different
channel or frequency. One link might operate on Channel 1, and the other on
Channel 12. At the repeater site we have two Access Points rather than one.
Each of these access points is operating on a seperate frequency, they are
connected through the use of a crossover ethernet cable.
To get the best performance, the system should be designed with more gain
than would normally be required such than any interference from the other
access point at the repeater site is cancelled out.
For security purposes it is probably best to have the access points on the
repeater to be slave units for each end of the link, as the access points
have the greatest coverage, and if we are placing these for commercial use,
we want to limit other users from accessing these stations.
Powering the Repeater
The Linksys WAP-11 units require about 550 mAmps at 5 Volts, or just over
2.5 Watts during use. This is not much, until you need to supply this
ammount of power to an un-powered repeater site. What some users are
suggesting is a 60 watt solar panel for each WAP-11 with a 17 AH Gel Cell
Battery. Reports suggest that this will keep the link operational in almost
any weather conditions.
Consulting
Radioactive Networks is always on the lookout for Consulting work in this
area. Please support us if you are able. Another way to help us is by
purchasing your WAP-11 through AMAZON.COM by clicking here.
Thank You
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