[MLB-WIRELESS] 802.11g Starts Answering WLAN Range Questions

[Matt Pearce]

Very interesting reading.  One thing I did manage to extrapolate from the
whole lot is that the xMbps is really a useless figure.  I wonder if there
is anyway to force the companies that make all the gear to rate actual
kB/sec at set gains so we could actually sort through all the crap and
really get to the crux of the matter without having to buy first ??

It would definately be a big step in the right direction, although I can see
the marketing divisions up in arms if the are forced to tell people that
this equipment is really only capable of 1mB/sec (random figure) instead of
the much hyped 54Mbps.  Perhaps some standardization of actually data
transmitted by wire or wireless could even be the way to go, and the figures
rated in bytes per second of actually data that the end user wants.

my 2 cents, anyone care to comment ??

Matt.

----- Original Message -----
From: "John Dalton" <john.dalton at bigfoot.com>
To: "Jason Brice" <Jason.Brice at kiandra.com>
Cc: <melbwireless at wireless.org.au>
Sent: Thursday, January 16, 2003 10:26 PM
Subject: Re: [MLB-WIRELESS] 802.11g Starts Answering WLAN Range Questions


> Technically, the correct name for the OFDM modulation used
> in 802.11a is COFDM.  COFDM stands for Coded Orthogonal
> Frequency Division Multiplexing.  Coding plays a central
> role in how 802.11a works.  The theory goes something like this...
>
> The 802.11a system contains an FFT (Fast Fourier Transform).  Think
> of the FFT as something which divides the frequency band of the
> 802.11a signal into lots of smaller frequency bands, like radio
> stations next to each other (there are 64 of these 'radio stations'
> in 802.11a).  The transmitter distributes the bits to be transmitted
> across these 64 different narrow channels (each corresponding to a
> 'radio station'), which then zing off through the antenna to the
> receiver.  On the way a significant percentage of these signals
> get trashed by the environment in which the radio signals travel.
> It's a tough world out there for radio signals.  The end result
> is the receiver does not receive the data from some of the 'radio
> stations', so we have missing data.  We need some way to fill these
> gaps.
>
> This is where coding comes in.  To code a stream of bits we pass
> it through a black box which introduces REDUNDANCY.  This means more
> bits come out then go in.  If this coding is done properly, we can
> remove some of the bits in the output from the black box, then
> use the remaining bits to recover the data which went into the black box,
> effectively 'filling in the holes'.  (The thing which
> does this recovery is called a decoder.)  In the first 802.11a
> prototypes, the coder produced twice as many bits as it received.
> (the final standard allows other rates as well).  This meant the
> data rate going into the coder was 27Mbit/s while the date rate
> coming out was 54Mbit/s.
>
> So what we do is pass the 27Mbit/s data from the user through a coder.
This
> doubles the number of bits so the bit rate goes from 27Mbit/s to 54Mbit/s.
> We then use the FFT to transmit the date.  In the process a number of bits
> may get lost.  We then pass the received data through a decoder, which
> drops the bit rate back to 27Mbit/s and in the process fills in the
'holes'.
> There are other 'inefficiencies' in the system, but this is the main one.
>
> See now how 54Mbit/s is a funny number?
>
>
> Compare this with 802.11b...
>
> Here we have a data stream running at 11Mbit/s.  We pass this
> data stream through a device called a spreader.  This makes the
> data take up the same amount of frequency spectrum as a 22Mbit/s
> signal.  We then transmit the signal.  In the receiver we pass the
> received signal through a thing called a despreader (duh?) which
> reduces the data stream back to the original spectrum (equivalent
> to 11Mbit/s) and in the process reduces the 'crudiness' due to the
> environment through which the radio signal had to pass to get
> from the transmitter to the receiver.
>
>
> Anyway to summarise.  Saying 802.11a is 54Mbit/s is like saying
> 802.11b is 22Mbit/s.  (and I'm not referring to '802.11b+', which
> in itself is a misleading use of terminology as such equipment
> is non-standard.)
>
>
> A disclaimer:  This is the best plain English explanation I could
> write in the time available.  There are lots of holes and questionable
> simplifications in it.  If you recite the above in your information
> theory exam you will fail.  Having said that I think it's an okay
introduction.
> Feel free to improve on it under the terms of the Free Documentation
License.
> For the full story, go and download the 802.11a standard from the
> IEEE's web site (and get a good book on communications theory from
> the library).
>
> Please let me know what parts of this don't make sense, so I can rewrite
it.
> I don't mean to insult anyone's education with the above, but I tried to
> keep it simple so everyone could understand it.
>
> For the technically minded, from memory the standard specifies
> either rate 1/2 or rate 2/3 codes.  I think these are convolutional
> codes.  I can't recall the waterfall curves from memory.  If I get
> a chance I'll try to post some numbers from the standard, but no promises.
> As you can gather from the rates, these is some serious error correction
> going on here, compared to what you have on wires.
>
> Coding is distinct from framing.  There are further losses in 802.11a
> (and 802.11b) due to framing.
>
> Regards
> John Dalton
>
>
> Some keywords for the interested to read up on:
>
> Forward Error Correction
> Multipath Fading
> Fast Fourier Transform
> Direct Sequence Spread Sprectrum
> Quadrature Amplitude Modulation
>
>
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