|MadSci Network: Computer Science|
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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