
Wi-Fi over Coax - rubatuga
https://en.wikipedia.org/wiki/Wi-Fi_over_Coax
======
segfaultbuserr
What the article didn't mention is the possibility to build a "wired wireless"
network using leaky feeder [0]. Unlike a normal coax feedline, a leaky feeder
is a special coax feedline that is designed to leak a bit of RF along its way,
thus providing wireless coverage on everywhere the coax runs. It's useful in
areas when using antennas at fixed points is difficult to provide a throughout
coverage, such as a building complex or a tunnel.

[0]
[https://en.wikipedia.org/wiki/Leaky_feeder](https://en.wikipedia.org/wiki/Leaky_feeder)

~~~
jimmySixDOF
That's also how you get mobile phones working in elevators

~~~
nerdbaggy
I recently installed an access point in an elevator. All that we did was put a
cat6 into the umbilical cord that contains the elevator power, controls, etc.

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OrangeMango
Back in the day, almost all cell phones had a miniature coax port on them.
They were "hidden" in that you probably had to pry off some nondescript part
or open the battery cover up or something.

This is how we tested mobile networks. It works great. The only problem is
when someone forgets to make a tight connection or attach a signal attenuator.
Since the first thing you always test is emergency calls, once in a while your
call ends up on the real mobile network and the fire department shows up in
your parking lot. This happened about once or twice a month where I worked.
They knew what our business was so they were never mad at us, but we had to
pay a fine each time.

~~~
dogma1138
It’s not only how they tested mobile networks.

The old Nokia and Ericsson phones as well I think as some Motorola’s had a
rubber plug in the back just below the external antenna with a coax port that
you had to remove when you plugged the phone into the a car speaker phone
system the phone would then hook to an external or a larger antenna glued to
the windshield.

~~~
OrangeMango
Interesting, that must be why the coax port survived from engineering to
production.

~~~
dogma1138
The external port was a requirement for kits like this

[https://images-na.ssl-images-
amazon.com/images/I/411PQXERG6L...](https://images-na.ssl-images-
amazon.com/images/I/411PQXERG6L._AC_.jpg)

But signal testing kits also used it.

This survived until 2G the 5110 still had it.

When 3G came out the coverage was good enough and I’m guessing that issues
with signal strength in cars was more or less solved.

~~~
ThePowerOfFuet
What a blast from the past... I remember installing that exact kit (plus
antenna) in my own car some twenty years ago. It worked great.

------
hamtastic
The possibilities here with sufficiently sophisticated software and tunable
hardware (deep learning of RF propagation characteristics in a building using
data from portable sensors) are fascinating! But honestly, MOCA already solves
this problem. The MOCA 2.5 standard
([https://en.wikipedia.org/wiki/Multimedia_over_Coax_Alliance#...](https://en.wikipedia.org/wiki/Multimedia_over_Coax_Alliance#MoCA_2.5))
already delivers 2.5gbit total (time-multiplex) throughput which is more than
enough for real-world no-compromise bidirectional gigabit ethernet on isolated
runs or distributed backhaul to multiple wifi APs. I'm using this in my (115
y.o.) house now where new cable runs are impractical and the coax plant is
unused.

~~~
variaga
Happy to see MoCA get a shout- out. I spent most of 2003-2015 working on the
design of MoCA chips, starting before the v1.0 spec release up through v2.5.
AMA

~~~
wolrah
Why is MoCA still so rare and expensive? Is there a technical reason or is it
just some combination of business reasons?

It seems like it's been the obvious choice for no-new-wires home networking
for years but you basically can't find it at retail and have to look for it
even online.

~~~
Unklejoe
For what it’s worth, you can buy the DirecTV DECA adapters for like $20 on
Amazon. They’re actually just MoCA adapters, but they run at a lower frequency
than regular MoCA so they don’t interfere with the DirecTV signal. They work
fine by themselves.

The only downside is that they’re 100 Mbit/s.

That said, they really do give you a rock solid 100 Mbit/s.

If you need more bandwidth, I think Verizon sells a MoCA 2.5 adapter for like
$60 which should give you GigE.

~~~
rubatuga
They're actually ~91Mbit/s, but yes, very, very stable. They also seem to add
about 4ms of latency.

~~~
variaga
The 4ms of latency is a product of the MOCA protocol.

As a fully scheduled network, each packet must wait for a timeslot to send a
reservation packet, wait for the schedule to be updated (map packet), and then
wait for the actual scheduled time.

The reservation timeslots and mail packets are on a fixed schedule
(approximately- there are cases where the timing changes if you have a poor
link) with a consequence of an unloaded MOCA network having a transit time of
~2ms going from the master node, or ~2.5ms going anywhere else (master node
transmit is faster since it gets to skip the reservation step. Times are
averages, as the exact time depends on the alignment of the time of arrival to
the scheduling period). Round- trip ping times should go up by roughly 4.5ms.

Under ideal conditions, it is possible to get 100Mbps UDP throughput on a pair
of MoCA nodes (1518 byte packets). The physical media can support up to
110Mbps (MOCA 1.0), 140Mbps (MOCA 1.1) or 450Mbps (MOCA 2.0) user throughput
per channel (up to 5 channels in MOCA 2.5), but that's shared bandwidth (all
traffic summed together). Throughout will fall off in bad channels (minimum
40Mbps), or if you use smaller packets (higher scheduling requirements per
packet) so YMMV

------
bcoates
Everybody hates punching holes in walls and running new cables, but this is
almost certainly the wrong solution for almost every problem, outside of the
weirdest of edge cases.

For starters, you're going to lose MIMO, so you've substantially downgraded
your network's ability to handle multiple clients. It's also really unlikely
that your weird old coax system is clean and not a rat's nest of splitters and
unterminated ends (=interference).

Attaching multiple antennas to a single AP in a non-MIMO configuration is not
a cost effective way of doing DAS with current technology, if it ever was.
More, lower-powered APs all over the place is where you want to be spending
your budget. If you want to save money on install, do PoE.

~~~
dogma1138
Ethernet over power line (basically DSL) has become quite good these days, you
can get fairly cheap 1gbe units today and unless you have some really crappy
wiring with noisy appliances they don’t have many issues.

~~~
guenthert
Latency. Sadly, Ethernet over powerline (IEEE 1901) made the same unfortunate
choice as WiFi and the (analog phone line) modem V.4x standards in including
error correction. This is not what you want or need for TCP/IP. It's better to
drop a packet than to deliver it late. IEEE 1901 tries to mitigate this by
using forward error correction, but succeeds in that only to a degree.

Bandwidth is alright-ish at about estimated 2MiB/s here (single pair, about
50ft apart), but RTT of ~4ms with occasional spikes in excess of 1s (at a
different location, I've seen spikes in excess of 5s). This is only barely
noticeable for accessing the WWW, but unpleasant for interactive GUI work
(e.g. via NX or VNC) and a deal-breaker for cluster communication protocols.

~~~
aidenn0
1\. There are still some devices designed for lan-only use that assume packet
loss will be extremely rare.

2\. TCP throughput suffers when non-congestion related packet-loss exceeds a
fairly low value

~~~
gitgudnubs
Only an idiot would design a device to assume no packet loss, unless they were
_also_ allowed to design the network. If you buy such a device and put it on a
janky power network, then you have only yourself to blame.

Standards decisions should never be made to accommodate designers that want to
assume there's effectively no packet loss.

Some degree of error correction is totally reasonable, but your first point is
irrelevant to discussion.

------
preek
So.. WiFi cables do exist?

I feel a little bit bad about some discussions I’ve had over the years. They
could have even been more complicated(;

~~~
EarthIsHome
Defining it as a Wi-Fi cable is a bit narrow. It's just a coaxial cable
between an antenna and the router. Just make sure your cable is matched to
your antenna and using the correct connectors.

~~~
dboreham
And an attenuator.

~~~
himinlomax
You'd only need an attenuator if you're connecting two WiFi adapters directly
through coax. In what is discussed here, waves always travel through the air
at some point, which does the attenuation.

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EarthIsHome
This is just extending the antennas using 50-ohm cables.

By using couplers and multiple cables, you can connect multiple antennas to
the same access point.

It's a way to physically separate the antennas from the router by using a coax
cable.

~~~
q3k
Home TV coax has a characteristic impedance of 75ohm. If you just plug
consumer devices in directly you will get a pretty significant SNR loss from
reflections and other impedance mismatch effects.

~~~
EarthIsHome
Sure. But I would bet that most Wi-Fi access points use a 50-ohm system.
(Plus, the connectors on a home tv coax cable probably won't mate with the
router connectors.)

If you want to use a 75-ohm cable, you'll have to use impedance converters to
convert to 50 ohms.

Bottom line: just make sure the components you're connecting in your system
are properly matched to avoid a lot of reflections and power loss.

~~~
dboreham
This confused me but I think you mean "Most WiFi Access Points use a..."?

~~~
EarthIsHome
Thanks! Corrected

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punnerud
By turning a Coax cable inside out you get a radiating cable. This is used now
in every train tunnel in Norway to get good 4G coverage.

------
rubatuga
I'm curious to know, at what frequency in a pair of wires is it considered to
be EM waves travelling through it? I know that Ethernet is changing voltage
(electric field) at frequencies of ~30MHz, while WiFi travelling through a
pair is an EM wave at a frequency of 2.4 GHz. At what point does it change to
EM?

~~~
PoachedSausage
At frequencies greater than 0 Hz (DC) structures of two or more conductors can
support transmission of TEM electromagnetic waves. At microwave frequencies
and above it is more efficient to switch to waveguide (a conductive tube).

~~~
rubatuga
Thanks for clearing it up for me!

------
mjlee
[https://en.wikipedia.org/wiki/Leaky_feeder](https://en.wikipedia.org/wiki/Leaky_feeder)
might be interesting too - essentially using a cable to act as the actual
antenna.

------
smitty1e
I'll bite: how is this superior to MOCA?
[https://en.wikipedia.org/wiki/Multimedia_over_Coax_Alliance](https://en.wikipedia.org/wiki/Multimedia_over_Coax_Alliance)

------
bob1029
I think it would be pretty awesome to inject the WiFi RF into the coax of my
home from the distribution panel. I only use the 1 line for my Comcast
internet and ethernet/wifi to connect everything else. All of those other coax
lines to each room are sitting entirely unused right now...

All you would need is a WiFi router that has a bunch of RG6 connectors on it.
Wire in where the coax splitter/amplifier would normally live for
CATV/satellite service. And then in each room where the coax feed ends up,
simply screw on an antenna with matching RG6 connector.

------
js2
Too bad we ripped out all those 50-ohm cables years ago:

[https://en.wikipedia.org/wiki/10BASE5](https://en.wikipedia.org/wiki/10BASE5)

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pedrocr
The article mentions 802.11r for fast roaming between APs and how this avoids
it. I've seen mentions of networks where the APs just forward WiFi packets and
a central router does all the actual negotiation to achieve the same effect.
It's too bad something like OpenWRT doesn't include it.

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tomasGiden
Ericsson has something similar for cellular signals over CAT5 called Ericson
Dot [0]. Though I believe they have it at baseband or intermediate frequency
in the cable and then shift it up in the Dot. Disclosure, I worked at Ericsson
when the first Dot was released, but never with it myself. But they were
really proud of it I remember.

[0] [https://www.ericsson.com/en/portfolio/networks/ericsson-
radi...](https://www.ericsson.com/en/portfolio/networks/ericsson-radio-
system/radio/indoor/radio-dot-system)

------
y04nn
Also, modulation of light waves with radio signals to send them over fiber:
[https://en.wikipedia.org/wiki/Radio_over_fiber](https://en.wikipedia.org/wiki/Radio_over_fiber)

------
friendlybus
Small example [https://hackaday.com/2018/10/19/wired-wireless-over-
coax/](https://hackaday.com/2018/10/19/wired-wireless-over-coax/)

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rietta
I'm struggling to see how this is anything but using feed lines to the
antenna. We've done this for decades to attach rooftop high gain antennas to
an equipment closet or if you are a HAM radio operator and want to have an
outdoor antenna while your rig remains inside.

If this were node-to-node, doesn't this just fit the definition of old school
ethernet over coax?

~~~
pmorici
It is easier and cheaper to use 50ohm compatible WiFi when dealing with
situations like trying to communicate with equipment inside an enclosure
through a slip ring.

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geocrasher
Running 2.4ghz or 5.8ghz over run-of-the-mill coaxial cable will introduce a
lot of losses. It'll have to be low loss stuff similar to LMR400 to really be
useful, I'd think. This would only be feasible if such coax already exists.
Otherwise it's cheaper to run multiple access points.

------
acd
Wifi over coax could have usage for trains.

You would connect wifi access point to fibre from the ground. Wifi signal is
output to coax cable running along side with the power lines or from the
ground next to the track. Wifi is beamed into the train. Local Wifi access
points spreads the wifi signal in the train.

~~~
EquallyJust
Some automated train systems already do this for control such as some of the
Vancouver SkyTrain lines.

From the SelTrac wikipedia page:

"in newer versions of SelTrac, the control signal is transmitted inside the
running rails at radio frequency using IEEE 802.11 (WiFi) access points. "

[https://en.wikipedia.org/wiki/SelTrac#Communication](https://en.wikipedia.org/wiki/SelTrac#Communication)

------
dfamorato
Pardon my ignorance on the topic, but are there any commercially available
(enterprise grade) products for Wi-Fi over Coax?

If this is used for “back-haul”, what is the key differentiator between this
and MoCa (or similarly Ethernet over Powerline)

~~~
vvanders
Well if you want one difference this won't throw RF hash all over the spectrum
like ethernet over powerline does.

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jpizza
This is a very timely post for me. My contractor mentioned this to me last
week. It caught me totally off guard and seemed counter intuitive to my
knowledge of wifi connections. Pretty neat and thank you for sharing.

~~~
mceachen
Wouldn't it be more "future proof" to use something like cat7 rather than
coax?

~~~
jrockway
I think what Wifi-over-Coax is trying to get around is having to coordinate
multiple access points. Wifi breaks down particularly horribly when some of
the stations and APs can't hear each other. By only having 1 AP, you avoid
some of that problem. (Of course, stations can still interfere with each
other. So I'm not sure how useful this is in practice.)

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LockAndLol
That article is only written for experts. It'd be great if someone here that
understands what it's actually about could update the article and contribute a
better description for the laymen.

------
garaetjjte
Alternatively you can use HomePlug protocol over coax cabling. (HomePlug is
normally used to Ethernet-over-mains)

Instead of buying complete mains units there also smaller modules too, eg.
LX200V20

------
dusted
10base2:
[https://www.youtube.com/watch?v=82E7vhtLxNs&t=9](https://www.youtube.com/watch?v=82E7vhtLxNs&t=9)

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globular-toast
I wasn't aware of coax usage other than for TV. I guess I assumed that
twisted-pair cables were far superior for data transmission, but maybe not?

I recently moved in to a home that has been pre-wired with Cat5 and also coax
to each room. The wiring was left unterminated but I assumed the coax was just
meant for TV reception. But I did discover there is a second coax cable that
has been left unterminated at both ends and I wonder if this is meant for
data? I really don't like not knowing but I'm unable to get hold of the people
who did the wiring during lockdown!

~~~
toast0
A typical satellite receiver has two or sometimes three coax connections to
the dish (or to the inhouse distribution platform). They could have built for
that.

Or it's a spare pull in case the cable fails mid span. Or to be able to get
cable + antenna. Or to be able to use a centrallized powered splitter for two
uses at one location, which would be better than using another splitter in the
room, but probably not needed.

------
k33n
Is this just coax as backhaul? Or is signal actually emanating from the
shielded coax cable? I haven't had my coffee yet.

~~~
pjc50
It's sticking the Wifi antennas at the end of a long cable. A "feedline" in
the usual terminology.

How this interacts with beamforming and antenna diversity is .. not specified.

The basic idea seems simple enough: rather than stick an AP on one side of a
wall and accept a 15-20dB loss through the wall, or use two APs, cable one
antenna through the wall so the AP has a clear line of sight on both sides of
the wall.

~~~
chrismeller
That was my interpretation as well. I feel like this is another case of
someone new to the industry creating a “standard” for something that, had they
been around for a few decades, they would have realized already existed in a
different context.

Case in point: we have had “external” uni- and omni-directional antennas, even
for plain old 802.11, for decades.

~~~
Sharlin
Isn't this exactly reusing old technology that has existed for decades (well,
over a century now) in a slightly different context? I mean, a lot of pre-
existing coax is there exactly for the purpose of running a signal between an
antenna and a device…

~~~
chrismeller
Of course. But would you call that “WiFi over coax”? No one I knows would...

