
Propagation Delay and Its Relationship to Maximum Cable Length - andrelaszlo
http://www.wildpackets.com/resources/compendium/ethernet/propagation_delay
======
powertower
> Electrical signals in a copper wire travel at approximately 2/3 the speed of
> light.

An interesting fact - in those copper wires (or any type of wires) the
electron velocity itself is less than 1 centimeter / second (and often in the
mm/s range). Whether it's DC or AC, it does not matter much.

Think of the wire as a tube with a bunch of ping-pong balls inside - filling
it up tightly from start to end. If I stick my finger into one end of the
tube, a ball will come out at the other end almost instantaneously (regardless
of how long the tube is), but the balls themselves only moved a small distance
- _and only at the speed/velocity I pushed my finger at_. But the "signal"
"traveled" at a much higher velocity.

So in DC - electrons only travel a few centimeters / second (not anywhere near
light speed), and in AC - you're pushing/pulling the same electron back and
forth for eternity at about the same speed as it travels in DC.

Now it's of course a bit more complicated (the electrons rattle around at much
greater speeds, and the mentioned velocity is a net average), and there are
some edge cases, but for the most part the above is accurate.

I'm just not 100% sure of the speed of electrons in a superconducting loop
(Cooper-pair electrons) - if they are truly relativistic or not.

And if this does not blow your mind, you should not be reading about
electricity. PHDs in the electrical field don't even know this stuff.

<http://en.wikipedia.org/wiki/Drift_velocity>

TL;DR; If you raced a snail and an electron in a wire together, the snail
would win - <https://www.youtube.com/watch?v=jbi7gJTPSXk>

~~~
scoot
So if my tube of ping-pong balls is infinitely long, am I sending my signal
infinitely fast?

~~~
Myrth
You're forgetting about propagation of force within the ping-pong ball. It is
not immediate.

~~~
scoot
For which (TIL) the speed of propogation is limited by the speed of light -
except (maybe) gravity:
<http://www.metaresearch.org/cosmology/speed_of_gravity.asp>

------
jacquesm
Cat6 and cat5 are recommended to go no further than 90 meters:

<http://en.wikipedia.org/wiki/Category_6_cable>

<http://en.wikipedia.org/wiki/Category_5_cable>

Links are full duplex nowadays, the bus architecture is long gone and hubs are
rare (usually just old), switches are dirt cheap so no reason for hubs.

The limiting factor is simply the ability of the electronics to recover the
signal reliably. You can go way beyond the specs if you are willing to accept
ever increasing packet losses due to degradation of the signal, one way to
improve throughput over such links is by shortening your MTU.

Another simple solution to make longer runs is to use a fibre-optic repeater
(for instance this ancient 10 mbit device:
[http://www.blackbox.com/resource/files/productdetails/10542....](http://www.blackbox.com/resource/files/productdetails/10542.PDF)
but there are tons of others, more modern gear:
[http://www.signamax.com/mediaconverters/13-10100basettx-
to-1...](http://www.signamax.com/mediaconverters/13-10100basettx-to-100basefx-
single-fiber-converters) as an additional advantage, fibre optics don't suffer
from electrical interference and act as an optical barrier for electrical
signals which is very handy in industrial environments).

Or to forego cabling entirely and to use wireless with directional antennae.

~~~
jallmann
> switches are dirt cheap so no reason for hubs

Hubs are nice to packet-capture from a small device without a managed switch
in between, so it's a bit of a bummer that they're hard to find these days.

~~~
jacquesm
Yes, that's a good trick. And splicing the RX pair to two different cards only
works in rare cases, usually you have to lock the ports to a certain
rate/duplex mode.

~~~
jcr
If you value your time, and hair, the best answer is to just buy a good
network tap.

<http://www.networktaps.com/>

------
tlb
None of this applies to modern gigabit ethernet installations, which are full-
duplex and switched so that collisions don't occur. Maximum cable length is
limited by signal degradation, so the limit is soft. With good quality cables
you can go a little beyond the rated maximum.

~~~
ajross
Exactly. The switches buffer packets. The article is talking about a CSMA
limitation of the old unbuffered hub architecture (and the "real electrical
bus" coax networks before that) which have been irrelevant for over a decade
now.

And my experience is that you can actually get _well_ beyond the official
lengths for most signal types even with comparatively cheap cables as long as
you're willing to test and throw out the odd cable or connector. Standards are
written with conservatism in mind.

~~~
jcr
> Exactly. The switches buffer packets.

In most cases, you're right. The vast majority of switches use a sore-and-
forward design (buffering), but when you get into latency-critical
applications, another type of switch design is used, typically called a cross-
bar switch.

[http://www.researchgate.net/publication/220055485_A_low-
late...](http://www.researchgate.net/publication/220055485_A_low-
latency_modular_switch_for_CMP_systems)

<http://www.google.com/patents/US5339396>

I think opinions vary on whether or not crossbar designs "buffer" but it
mostly depends on how you define buffering. None the less, the low-latency
crossbar designs try to eliminate the latency caused by buffering in the older
but more common store-and-forward designs.

~~~
ajross
This is a valid point and I should have been more precise. But AFAICT all
those interconnects are single duplex point-to-point links, and thus not
subject to the CSMA issues of asymmetric collision detection detailed in the
linked article.

------
andrewvc
Cool article. Though it only talks about ancient Ethernet variants. Issues
like packet collision don't happen on most networks because we use switches
instead of vampire taps and hubs.

Can someone more knowledgeable explain the parts that are still correct?

~~~
X-Istence
All of them. Just because Station A and Station B are switched means that we
could now say that the Switch is Station C. So lets say Station A starts
transmitting at the same time as Station C (our switch is sending a packet
from Station B to Station A) we can still get a collision.

We no longer get a collision between the nodes on the switch because of the
fact that the switch will buffer packets as necessary before sending them on
to their intended target, which reduces the amount of collisions that can/do
occur...

~~~
mikeash
I thought this stuff was full-duplex now, with each direction using different
wires. So there's only ever one device that can transmit on a given set of
wires, thus no chance for collisions. Is that not the case?

~~~
mprovost
Yes that is true for gigabit. Gigabit ethernet is ethernet in name only. It
doesn't resemble the original standard in most meaningful ways. But the name
is a brand at this point so whatever the new standard is, it's always called
ethernet even if it's completely different electrically. Or even optically in
the case of fibre, which is something that was never originally anticipated
that it would run over.

~~~
yuhong
Backward compatiblity too.

------
smoyer
Propagation delay is dependent upon a variety of factors and while the numbers
the article quotes may be correct for thick-net and thin-net, the cable we use
for CATV trunk-line is significantly faster at about 0.85c ([http://www.china-
satellite.com/sdp/1385167/4/pd-5853918/8382...](http://www.china-
satellite.com/sdp/1385167/4/pd-5853918/8382534-2286595/RG540_QR540_QR540JCA_QR540JCAM109_Trunk_Line_Cable.html)).

Interestingly, the propagation delay through single-mode fiber tends to be
significantly lower than through copper (well ... again it depends on the
cable), but the 0.66c speed is a pretty good number for fiber.

------
jleader
When was this article written? The last time I worked in an office with
10Base2 cabling (aka "thin ethernet", using RG-58 coax about 5mm thick with
BNC connectors) was 1995. The last time I saw 10Base5 (aka "thick ethernet",
using an extra-stiff variant of RG-8 coax about 10mm thick, in continuous runs
with "vampire taps") was sometime in the late 1980s.

And if the maximum cable length were simply proportional to signal propagation
speed / bit rate, then why does 10Base2 have a shorter maximum length than
10Base5? They're both carrying 1e6 bits per second. Do signals really travel 3
times faster through fat coax than through thin?

------
weichi
Interesting article. A nit that can be picked is that it is not generally true
that electrical signals in copper travel slower than c. The reason the signal
travels slower than c in an ethernet cable is that the copper is surrounded by
dielectric material, and it is the presence of the dielectric that slows down
the signal. If the 10BASE5 coax cable was filled with air instead of foam, the
signal would travel at c.

------
thedufer
Very misleading article title. The entire thing is about collisions, which
only limit the length between ethernet cards, which is 5 times the length in
the title. I suspect the 500m is related to the bandwidth and signal
degradation rather than anything in the ethernet protocol.

------
ghshephard
"You may know that the minimum frame size in an Ethernet network is 64 bytes
or 512 bits, including the 32 bit CRC. You may also know that the maximum
length of an Ethernet cable segment is 500 meters for 10BASE5 thick cabling
and 185 meters for 10BASE2 thin cabling. It is, however, a much less well
known fact that these two specifications are directly related."

Odd, given that the VERY first thing anybody is taught about CSMA/CD ethernet
is how the maximum length of cable and minimum packet size are precisely
related. Indeed, it's almost the only thing people are taught about the
relationship of those two.

------
jpdoctor
> _Since we know that Ethernet operates at 10Mbps or 10,000,000 bits per
> second_

I'm trying to figure out if this essay was written in the late 80s or the
early 90s.

