Build Your Own 10Mbit/sec Optical Data Link

redcliffe writes: "This website has complete plans to build a 10 megabit per second optical data link that can work over up to 1 kilometre. It uses fairly cheap components, such as standard LED's instead of laser diodes. This also makes it a lot safer to work with, i.e. you won't burn your eyes out if you accidently look into it."

Comments

free from interference?

[Karmageddon]

From the website, The operation is very reliable and immune to interference.

um... immune to interference? So, you've got a link going to the divorcee down the block and the two of you are getting naked on-cam, and the blinds are open (of necessity) and you don't think the little crowd of neighborhood kids at your window is going to interfere?

Sigh...

[cperciva]

It uses fairly cheap components, such as standard LED's instead of laser diodes. This also makes it a lot safer to work with, i.e. you won't burn your eyes out if you accidently look into it.

The mere fact lasers are used in most fiber optics does not immediately render them dangerous. Typical power levels are on the order of a few mW, far too low to cause any permanent damage.

Re:Sigh...

[dattaway]

I wish this were true. Many years ago, I was fascinated by lasers; unfortunately, infrared isn't too visible. Several years later, I now have dark stuff floating around in my eyes. For a person 33 years of age, I would not recommend looking at concentrated sources of energy, no matter how small.

It may be a milliwatt, but its still heat: focused smaller than the head of a pin. It may burn. Damage in the eyes shows up many years later.

Re:laser diode

[Drakula]

The laser diodes in laser pointers cannot be used for a number of reasons, here are a couple: 1) Wrong wavelength: you need lasers with a wavelength near either 1.3 microns or 1.55 microns to take advantage of the disperison and attentuation minimums of optical fiber. Laser pointers have wavelengths in the visible, near 650nm. 2) Very narrow linewidth is need to reduce pulse spreading . Laser pointer devices are relatively broad.

There are others but I can't think of them right now.

Just fyi...

Laser?

[mindstrm]

The article quite clearly states that for 10M, they use a laser diode...

Got AUI?

[Uller-RM]

Has anyone else bothered to read this enough to notice that it will not work with twisted pair Ethernet? It requires an AUI connection. They even say in the FAQ, redesigning it to work with TP would be a pain in the arse.

In order to use the circuit, you have to either buy an AUI->TP transciever, or set up a bridging machine.

Just saving a bit of time for some people who are no doubt running out to Ripoff Shack grabbing l33t bl00 leds.

Re:Got AUI?

[neonstz]

Well, getting used NICs with AUI should be a problem I think. I've bought quite a few at large garage sales. If it is a problem getting one however, just get a Sun Sparcstation 5 or some other machine with built-in AUI. :)

Re:Got AUI?

[Uller-RM]

Ethernet over TP implements a link integrity signal. AUI does not. That's the main barrier to using this particular rig - you'd have to come up with an emulator for that signal, or the cards would refuse to transmit.

(And FYI, you're partly right. Ethernet over TP uses Manchester encoding, which means that it watches for changes in logic rather than states - i.e. transitioning from ground to 5V is a 0, and 5V to ground is a 1. Hard drives actually also use this method.)

Re:Got AUI?

[Zog]

It's not the end of the world. AUI adapters are easy to find and are pretty cheap (mine was ~$10 from a store that sells random computer cables/adaptors; they can be had online for about the same).

Also, using AUI makes a fair amount of sense - it *is* simpler, so you don't have to worry about the signals as much, so it's easier to make at home :)

Now, compare the price of parts plus the AUI adapter (I'll let you do that one) to the price of a cheap (though crazy fast) commercial link, and it should make sense why this is good. Also, as someone else already stated, there's the simple fact that you're using your very own homemade optical datalink ;)

Re:laser diode

[Anonymous Coward]

hello, good morning! this article is about point to point transmission through air. They use red LEDs but you can also use others. Linewidth and doesn't matter at 10 Mbit/s - the pulses are very long anyway. The 625 nm LEDs they use have a linewidth of 100 nm (huge!).

Re:VISIBILITY

[Anonymous Coward]

hint: they are using a fresnel lens to collimate the beam

Re:Why not 802.11b?

[autopr0n]

He's talking about directed 802.11, not Omnidirectional. omnidirectional WiFi dosn't get nearly that range

Eye burn?

[rew]

This also makes it a lot safer to work with, i.e. you won't burn your eyes out if you accidently look into it."

Ehmm. Modern High efficiency LEDs also carry the "don't look directly into this" warnings. And those things are BRIGHT.

Roger.

Re:Shortwave

[billstewart]

Close, but nobody smokes cigars any more.
The most popular stuff seems to be 1200 baud on 2-meter, which is line-of-sight plus repeaters, though there's some 300 baud HF stuff that has more chance of going city-to-city, and some fancier 9600 baud stuff.

Here's some text snagged from The FAQ at TAPR.ORG

TNC (terminal Node Controller)
A TNC contains a modem, a computer processor (CPU), and the associated circuitry required to convert communications between your computer (RS-232) and the packet radio protocol in use. A TNC assembles a packet from data received from the computer, computes an error check (CRC) for the packet, modulates it into audio frequencies, and puts out appropriate signals to transmit the packet over the connected radio. It also reverses the process, translating the audio that the connected radio receives into a byte stream that is then sent to the computer.

Most amateurs currently use 1200 bps (bits per second) for local VHF and UHF packet, and 300 bps for longer distance, lower bandwidth HF communication. Higher speeds are available for use in the VHF, UHF, and especially microwave region, but they often require special (not plug-and-play) hardware and drivers.

Computer or Terminal
This is the user interface. A computer running a terminal emulator program, a packet-specific program, or just a dumb terminal can be used. For computers, almost any phone modem communications program (i.e. Procomm+, Bitcom, X-Talk) can be adapted for packet use, but there are also customized packet radio programs available. A dumb terminal, while possibly the cheapest option, does have several limitations. Most dumb terminals do not allow you to scroll backwards, store information, upload, or download files.

A radio

For 1200/2400 bps UHF/VHF packet, commonly available narrow band FM voice radios are used. For HF packet, 300 BPS data is used over single side band (SSB) modulation. For high speed packet (starting at 9600 bps), special radios or modified FM radios must be used. 1200 bps AFSK TNCs used on 2-meters (144-148Mhz) is the most commonly found packet radio.

Re:One kilometre.

[redcliffe]

AFAIK, it either works or it doesn't. Like you get 10mb/sec or you get nothing. There is no link quality control on it from what I can see.