[ale] Bridge for LAN/Wifi

[Alex Carver]

On 2021-01-09 12:28, Steve Litt via Ale wrote:
> On Sat, 9 Jan 2021 12:45:16 -0500
> Boris Borisov via Ale <ale at ale.org> wrote:
> 
>> Back in the days we did it with couple wifi 11Mbps devices in bridge
>> mode. I thinks we're one of these early Linksys devices. That was the
>> backup of the ethernet cable. Wasn't 150 feet more like width of six
>> lanes.
>>
>> The other backup was leased line with couple modems.
>>
>> Networks was more exciting back then. You go with whatever can build
>> or afford.
>>
>> Firewals, servers, long range wifi antennas, internet billing systems
>> all hand made and coded :)
> 
> If you really have reliable every-time line of sight, I'd imagine a
> green laser from one building to a solar cell at the next building
> would do the trick. The laser would need to be high off the ground to
> avoid blinding anyone.
> 
> At 17 I did the analog equivalent. I placed the speaker output of an
> amp in series with a flashlight with batteries, shined it into a solar
> cell several feet away, amplified the output of the solar cell, and got
> the audio loud and clear, if a little treble-challenged.
> 
> By the way, if you strike a match in front of an amplified solar cell,
> the sound coming out of the amplifier sounds just like the sound a
> stricken match makes without an amplifier. In other words, the light
> patterns resemble the sound patterns.

The solar cell and the laser don't respond fast enough for data rates 
over a few hundred kbps.  The laser modules used in fiber networks use a 
continuous laser emitter and a crystalline optical modulator that splits 
the laser into two paths and recombines them at the other end.  One of 
the paths is modulated to change the phase delay which will cause 
destructive or constructive interference at the output.  The detectors 
are usually not silicon based because silicon detectors are slower to 
respond and have higher noise levels.  The common type of receiver is an 
InGaAs PIN diode which only works in the IR band but has excellent noise 
and speed performance.