MadSci Network: Computer Science
Query:

Re: Is it possible to use a modem over a wireless link?

Date: Tue Jul 11 00:06:46 2000
Posted By: Abtin Spantman, , Electrical Engineering, L. S. Research, Inc.
Area of science: Computer Science
ID: 959764340.Cs
Message:

Response to:
Is it possible to use a modem over a wireless link? (ID = 959764340.Cs) 


Hi:

The answer to the first part is 'YES', you can use a modem over a wireless 
link.
The answer to the second part is more complicated. Theoretically, if you 
stick a length of wire instead of the phone-jack, you will automatically 
have a simple wireless link, assuming both modems are identical.  
In other words, they operate at the same frequency, with the same 
modulation, and the same coding techniques if they are digital.  I did say 
theoretically!   Constructing a "good" wireless link is much more involved. 
I will try to provide some insight into what is needed.

The word modem is actually made up of two words:  "MOdulator and 
DEModulator." 
A modulator takes the data (or voice) and imposes that data on a carrier. 
A carrier is anything that can transfer information (data or voice).
A demodulator takes the (carrier + modulation) and extracts the data back 
out.

Let me explain with a loose example. When I talk to you, I am actually 
acting as a modem, just as you are acting as a modem when you listen.  When 
I talk, my thoughts (data) go to my vocal muscles (modulator) which 
vibrates my vocal cords (antenna) which sends a sound (carrier) out with a 
recognizable word (modulation).into the air (medium).  This sound with a 
word, then goes into your ear and vibrates your ear drum (antenna) which 
vibrates the ear receptor muscles (demodulator) which then transfers the 
thought (data) to you.

Data=my thoughts
Modulator=my vocal muscles
Carrier=sound
Carrier + modulation=words
Demodulator=ear muscles
Data=my thoughts        and the cycle is complete. We started with data, 
and we ended with data.

The modem in a computer, for instance, already does the work needed to go 
wireless.  It takes the data from the computer, and modulates it and 
imposes it on a carrier which is a high frequency AC signal.  All we need 
to do is provide an antenna, and we have a wireless FM transmitter.  That 
is why I jokingly suggested sticking a wire in the phone jack on the modem 
(but don't do that - it may damage the modem). And the second modem would 
act as an FM receiver.  A "good" wireless link would have more transmit 
power, probably a different modulation method, probably operate on higher 
carrier frequencies, and probably have some sort of error detection method. 
 Actually, most of the computer modems already have error detection and 
error correction methods built in their communication techniques.  

Lastly, it would be inefficient to go through a modem, then into a separate 
wireless link.  I would take the data directly into the wireless link, if I 
had one.  I certainly would not try to make one -especially if the data is 
critical.  If this is just for learning, then yes, we can come up with 
something.  There are a number of commercially-available IC's from National 
Semiconductor, and Motorola that are suited for wireless links.  The 
companies provide free samples of those IC's, if you ask for them, and, 
along with the data sheets for those IC's.  In the data sheets, you will 
find sample circuits to build simple wireless links. That would be a 
simplex (or one-way) link.  A full duplex (two way) link would require some 
serious circuitry and work.  I suspect the tranceiver that you had in mind 
would need at least a duplex setup 

The good news is that there are many commercially available wireless modems 
and communication links on the market.  Intel makes a wireless home network 
tranceiver pair for the PCs.  If you need higher performance, a company 
called "3COM" would be my first search, another company would be "Linksys". 
 Both of these companies make modems, as well as some wireless (cellular) 
modems. There are many many wireless link companies out there also, at all 
different performance levels and costs.

Let me know if you have follow-up questions.  I would be happy to answer 
them - if I can.

Abtin Spantman
SPANTMAN@EXECPC.COM




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