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Why Wi-Fi ? Why Indeed
This month we are going to look at what is known as Wi-Fi. If you
have or are interested in VSAT, Wi-Fi can really help make the
monthly cost of running your VSAT much more cost effective. Last
month we finished covering the basics on VSAT and what it actually
is, the terms and acronyms that surround VSAT and the concepts of
satellite footprints.
Now we will look at the next piece of the jigsaw puzzle, namely
Wi-Fi, that makes all this technology work for you in a cost
effective manner. So how do Wi-Fi and VSAT complement each other?
You can use Wi-Fi to transmit your VSAT signal to other computer
users thereby sharing your VSAT among several users thus making the
overall cost per user per month quite low. However be careful you
don’t inadvertently become an ISP and fall foul of the
Communications Authority of Zambia. You do know an ISP licence in
Zambia is currently around US$33,000 don’t you?
The name Wi-Fi is simply a trade name owned by an organisation known
as the Wi-Fi Alliance. This is a group made up of the big computer
companies who have come together to agree on a technical standard
that all computer manufacturers adhere to when they manufacture
radio equipment for the purpose of transmitting the Internet (more
correctly, data) from one computer to another.
The Wi-Fi standard is technically known as IEEE 802.11. There are
several versions of IEEE 802.11. These are 802.11b, 802.11g and
802.11a. The two standards known as “b” and “g” are interoperable
and work on the same frequency which is 2.4Ghz. This means that all
“g” equipment can work with “b” equipment. It’s not always the case
that the earlier “b” standard can work with “g” however, so you will
need to be careful if you want to use “b” equipment with “g”
equipment. The 802.11b standard gives theoretical data throughput
of 11Mbps but in “real world” settings it maxes out around 5 Mbps.
The newer (although not necessarily better) 802.11g standard gives a
theoretical through put of 54 Mbps but in the real world achieves
nowhere near that. You can expect around 20Mbps with a strong link
between two “g” devices. Something to be aware of with those
seemingly glamorous “g” speeds; if you mix “b” and “g” on the same
network then even the “g” equipment will only work at the “b” speeds
of 5Mbps. Yes it gets complicated fast!
The 802.11a standard works on a different frequency altogether to
the “b” and “g” standard. 802.11a is in the 5 Ghz frequency
spectrum. This has significant benefits. As you go higher in
frequency you get more “bandwidth” on a given frequency channel
which are typically 20Mhz wide. With the “a” standard you can expect
“real world” speeds of around 12 to 15Mbps over long distances,
something which is not easy to achieve with the 2.4Ghz frequency
bands. More importantly is the concept of the “noise floor”.
Although humans can’t hear it, the environment is a very noisy place
in radio terms. As more and more radios transmit on a given
frequency the background noise the radio has to transmit over
becomes louder and louder. Think of it this way; if you’re sitting
in an empty restaurant talking to a friend it’s very easy to hear
each other talking quite softly. As the restaurant fills up you have
to talk louder and louder to hear each other over the background
noise which becomes quite distracting. (Lusaka restaurants take note
- when you squeeze too many tables in a room it’s hard to hear your
dinner partner talking to you when all those tables are full) This
is quite similar to how radios hear the environment with many radios
all transmitting at the same time. Because there are few users of
802.11a 5.8 Ghz equipment in Zambia (until we published this article
:) ) you generally find that the “noise floor” for 5.8Ghz is low and
so there is little interference. Little interference directly
translates to vastly superior performance, which directly translates
to much faster data speeds. Other important benefits of 802.11a are
that the 5.8Ghz radio signals penetrate walls better and suffer from
“signal scatter” much less. A very important benefit of using
802.11a over long distances is that a thing called “Fresnell Zone”
becomes less of an issue and means we can use smaller towers as you
will see further down.
What is Fresnell Zone ?
Basically this means a radio signal from one antenna to another does
not travel in a straight level line. The signal actually dips down
low towards the ground until it gets half way to the other antenna
it is trying to reach where upon it starts to come back up again.
You can think of it as the shape two funnels would make if they were
joined together at their wide ends. All radio signals in the
microwave band behave in this manner. The higher the frequency the
narrower the size of this cone or funnel shape. This is important
because if that cone (fresnell) touches an object, be it a tree, a
building or the ground, you loose much of your signal. Thus the
narrower the cone the less chance of the signal hitting an object on
the path it is travelling along in order to hit the other antenna on
the other side. Because of the “fresnell effect” when you want to
“shoot” a wi-fi signal to another radio that is then further
attached to a computer, you have to get your antennas up high. The
longer the distance you want to “shoot” your signals, the higher
above ground you must, and we do mean *MUST*, get your antenna. This
is exactly why all those micro wave towers all over the country that
are so common are so high- it’s because of the fresnell effect on
radio signals over distance.
A
rough rule of thumb is that a 1-2 Km link can be made on about a
10-12 metre high tower, 3-5 Km’s needs a 15-18 metre high tower, 6-8
kms needs a 20-24 metre high tower and 10-15 Kms needs about a 30
metre high tower. Longer distances need higher towers. This is all
assuming no obstacles in the way between towers. 802.11b and 82.11g
suffer from fresnell much more then 5.8Ghz and for this reason we
use 802.11b and 802.11g for local access links and 5.8Ghz 802.11a
radios for long distance “backbone” or “backhaul” links, although
there is nothing to stop you using 5.8Ghz 802.11a for local access
if you want.
How far can a Wi-Fi signal go?
We can go up to a real world limit of about 50 Kms before other
issues get in our way but of course in reality we find the tower
heights need to be so high this is not really feasible…far cheaper
to have another VSAT running in that location 50 Kms away. Distance
is all about power (not that type of power…MMD need not apply) and
power comes from two places (no we don’t mean Lusaka or the
Copperbelt). In the first instance power is generated by the radio
transmitter. And secondly even more power is produced by the
antenna. The sum total of these two power sources lets us design
what is known as a “link Budget” This is the most critical aspect of
making a Wi-Fi link work
A
single wireless link between two points is really quite simple. As
shown in figure 1 below, the basic components are two network
devices attached to radios and antennas at each end.
Figure 1: Simple Wireless Link
For a wireless network to work reliably, sufficient radio power
needs to get across the link in both directions. As most networks
utilise unlicensed spectrum (900MHz, 2.4GHz or 5.8GHz), the maximum
radiated power is limited to certain levels set by the
Communications Authority. These limits allow the unlicensed
spectrum to be re-used many times over. The best way to design a
wireless link is to start by doing a link budget to understand how
the link can be optimised.
Figure 2
shows the main components that should be considered in doing a link
budget. Each of these elements is considered in turn below.
Figure 2: Main Components in Link Budget
A
radio produces power, typically expressed in mW. However as the
output power from a radio gets severely attenuated over distance, mW
is not a good measurement standard. As an example the signal level
reaching the receive antenna is often considerably less than
one/millionth of a mW!
So instead a logarithmic scale is normally used instead – dBm.
Figure 3 shows some
common mW levels expressed in dBm.
Figure 3: Conversation of mW to dBm
For the purposes of link budget it is important to know the gain of
an antenna. Typically this is expressed in dBi. It is a measure of
how much the antenna amplifies the section in a particular
direction.
The combination of:
radio output power - cable losses + antenna gain = EIRP
EIRP stands for Equivalent Isotropically Radiated Power. The reason
it is important is that this is the figure that has limits set by
the Communications Authority. For 2.4GHz and 5.8GHz it is typically
4 watts or 36dBm.
As the signal travels through the air it is attenuated. Increasing
frequency and distance both increase attenuation, as shown in figure
5.
Figure 5: Radio Path Loss Vs Distance
Your choice of antenna is important - if your antenna doesn’t work
properly, your network won’t work properly either. The following is
a brief summary of the main factors to consider in choosing an
antenna.
At the Receive Antenna the process is repeated in reverse. The
receive antenna amplifies the signal by its gain and the resulting
signal is attenuated by any cable losses. Any signal left at the
end of this journey then hits the receive radio. A radio receiver
is designed to receive very weak signals and amplify them so the
data they contain can be extracted. Most radios give a figure for
receiver sensitivity (usually in dBm) – this is the minimum amount
of power the radio receiver requires to operate. So in terms of our
link budget, receiver sensitivity is counted as a positive
number.
So referring back to Figure 2, our total link budget is:
Other things being equal, if the Link Gain is positive the link will
work.
In practice, for most link budget calculations a Fade Margin is also
allowed to allow for a margin of error due to real world
conditions. The amount allowed depends upon a number of factors
including distance and environmental conditions but -10, -15 or -20
dB is often used in practice.
Next Month
Well that’s a lot to swallow for one instalment. Next month we will
look at antennas and the many different types available and how each
type should be used in a given circumstance. We wouldn’t be far
wrong by saying this: the success or failure of your radio links and
thus your network for sharing the Internet is all about the antennas
you choose for your network and believe us, there is a bewildering
array of antennas out there.
If all this is too much don’t worry, just seek out a radio
networking specialist who actually enjoys all this stuff (no snide
comments please) and they will take care of all the details for you.
Just make sure they don’t use slick salesman techniques or wear
shiny shoes ;-). |
August
2006