Is IEEE 802.3af PoE truly downwards compatible?

[Ramon F Herrera]

When I learn of cables that carry mixed analog-digital signals, I tend
to be cautious, to say the least. Take for instance DSL, a technology
that I am currently using for the first time ever. At its inception,
tears ago, when a read that they were planning to send the Internet
digital signal in the same conductor as the regular old-fashioned
POTS, I was pretty skeptical. And, sure enough, my DSL breaks and the
POTS phone stops working every once in a while.

Anyway, my question is about PoE. The latest Ethernet (1GB) uses all 8
conductors of the RJ-45 cable. They somehow manage to modulate the
digital signal on top of it (which conductors are used for DC?). Does
it *really* work? What happens to some Ethernet devices that were
manufatured before PoE? Won't they break?

Hmm, perhaps PoE is strictly 10baseT and 100baseT standard? That way
they can send the DC through the unused conductors.

Thanks for your insight.

-Ramon

 

[Peter]

Hi Ramon,

When I learn of cables that carry mixed analog-digital signals, I tend
to be cautious, to say the least. Take for instance DSL, a technology
that I am currently using for the first time ever. At its inception,
tears ago, when a read that they were planning to send the Internet
digital signal in the same conductor as the regular old-fashioned
POTS, I was pretty skeptical. And, sure enough, my DSL breaks and the
POTS phone stops working every once in a while.

I suspect you are confusing the various elements of 2 different
environments, DSL and Cable, and the bearer encapsulation being used
on either (PPPoA or PPPoE). Also, your use of the term "PoE" is quite
confusing, because to me, PoE means Power over Ethernet, something
quite different.

You may find your situation to be isolated to your specific
environment, because in 7 years of using DSL I have never "lost" the
Phone line that carries DSL (except once when a digger chopped the
cable just down the road....;-)).

The interesting thing with DSL is that its a digital methodology
deployed over what is essentialy an analog environment. Surprisingly,
from a transmission perspective, Cable and DSL have a lot of elements
in common.

Both CABLE and DSL employ a Transmisison environment for the WAN
component, and they only really differ in how that environment itself
is built (the modulation schemes differ). DSL is based on ATM, that is
also deployed in other transmission environments, however Cable uses
what was originally a proprietary, now published for all to use,
completely different scheme The one common factor is that BOTH only
use a SINGLE copper pair (in the general domestic market) for the WAN
part of that delivery. The main difference between them here is the
Modulation scheme they each use to deliver the higher layers (the
higher layers being the PPP part of the composite).

At the copper level, it is well known that when you combine 2
modulation schemes on a single copper pair, there WILL be some
interaction between the 2 signals, that may cause an issue with one of
the environments. That is why full DSL requires a Splitter to be used
to help isolate the environments and allow older POTS hardware to
operate in the presence of the VHF signals that are not "normally"
present on the copper, and why "spitterless" DSL is just a way of
lowering the effect of that interaction on older H/W, but there is
still an interaction taking place.

The major effect that humans MAY notice is an effective REDUCTION in
signal levels for voice communications (POTS) on the Wire. This means
(to me) the voice level received at my home from the caller is
slightly reduced compared to when the phone line did not also have
ADSL present, however this reduction has never been great enough to
completely kill the POTS service.

Anyway, my question is about PoE. The latest Ethernet (1GB) uses all 8
conductors of the RJ-45 cable. They somehow manage to modulate the
digital signal on top of it (which conductors are used for DC?). Does
it *really* work? What happens to some Ethernet devices that were
manufatured before PoE? Won't they break?

Assuming you mean PPPoE, well then PPPoE has nothing at all to do with
that actual transmission mode of the WAN to reach your Home, however
it DOES have something to do with the ENCAPSULATION of the data stream
ONTO that WAN component. So I then suspect you really mean "Cable" as
the WAN component, as you can do either PPPoA or PPPoEoA, for DSL as
the WAN transport, and for Cable you usually find PPPoE or one of the
RFC Bridge styles being used, however the reality is that NEITHER
cable nor DSL (for consumer grade use) are yet capable of 1GB WAN
transport! So if you are looking at Cable, then you are still looking
at a SINGLE Copper cable pair delivering the WAN component to your
home, exactly the same way that DSL uses ATM technology for DSL.

So your provider does NOT deliver Ethernet using UTP cabling to your
home.

The part of your question that reads -

Does
it *really* work? What happens to some Ethernet devices that were
manufatured before PoE? Won't they break?

is actually irrelevant, as this has nothing to do with PPPoE (or
PPPoA) at all. You have to dig LOWER in the ISO standards to find
where DSL (or Cable) comes in.

Hmm, perhaps PoE is strictly 10baseT and 100baseT standard? That way
they can send the DC through the unused conductors.

Nope, PPPoE or PPPoA just ride on the back of DSL or Cable or Ethernet
itself. The reality is that these are just ENCODING schemes to package
up data, and have NOTHING to do with the actual transmission media
being used.

Pehaps a good place to start is to try and understand the bottom 3
layers of the ISO 7 layer protocols and see exactly where DSL/CABLE/
PPP all sit in that structure.

One other thing to consider, is that because DSL is based on ATM
technology, and DSL v1 was approx 8Mb, DSl 2 go up to around 24Mb adn
DSL 2+ even further, then consider that ATM itself goes way above that
to 500+Mb then speed enhancments to DSL technologies look to be far
more likely than when using Cable technologies, which require a
re-write of the Cable protocols to go anywhere near these speeds. Then
again, things can change pretty quick in this world....;-)

I hope this helps..............pk.

 

[Doug McIntyre]

When I learn of cables that carry mixed analog-digital signals, I tend
to be cautious, to say the least. Take for instance DSL, a technology
that I am currently using for the first time ever. At its inception,
tears ago, when a read that they were planning to send the Internet
digital signal in the same conductor as the regular old-fashioned
POTS, I was pretty skeptical. And, sure enough, my DSL breaks and the
POTS phone stops working every once in a while.

I've got many customers that have probably have had years of zero
problems over all time of them using shared DSL/POTS service...
It usually does work flawlessly, as much as the LEC will do it.

Anyway, my question is about PoE. The latest Ethernet (1GB) uses all 8
conductors of the RJ-45 cable. They somehow manage to modulate the
digital signal on top of it (which conductors are used for DC?). Does
it *really* work? What happens to some Ethernet devices that were
manufatured before PoE? Won't they break?

There are several modes of operation for 802.3AF. The standard is free
to get as part of the 802 package from the IEEE (well, at least at one
point in time it was). One mode, operating with Gigabit Ethernet, does
use all 4 pair for signal, as well as PoE. It does really work with
the proper gear.

If a device isn't 802.3af aware, it doesn't complete the initial handshake,
and the head-end unit won't send down more than miliamps of current needed
just to start talking. The end device has to request how much juice is
sent down its way through the protocol negotiation. Normal devices should
be able to deal with the initial startup protocol query probes just fine.
I've never seen any issue.

Hmm, perhaps PoE is strictly 10baseT and 100baseT standard? That way
they can send the DC through the unused conductors.

There are other modes of operation pushing the current over just the unused
pairs in the older/slower speeds of ethernet. They still generally won't
do it (especially in 802.3af, or in cisco pre-standard implementations)
until the protocol handshakes are complete (ie. CDP in cisco pre-standard).

Other non-standards based gear might not necessarily follow any rules,
and just blast juice down the unused pairs. (ie. older Inter-Tel VOIP
phone setups).

Get the standards to learn..

 

[Hal Murray]

Hmm, perhaps PoE is strictly 10baseT and 100baseT standard? That way
they can send the DC through the unused conductors.

You don't need any "unused" conductors.

Consider a simple case of 2 pairs, one for transmit and one
for receive.

You can also send DC over the same wires by making the receive
pair positive relative to the transmit pair. The downstream
station picks the power off the center taps of the isolation
transformers.

 

[Jeff Liebermann]

When I learn of cables that carry mixed analog-digital signals, I tend
to be cautious, to say the least.

Most such arrangements have no interaction between the signals on
different pairs.

Take for instance DSL, a technology
that I am currently using for the first time ever. At its inception,
tears ago, when a read that they were planning to send the Internet
digital signal in the same conductor as the regular old-fashioned
POTS, I was pretty skeptical. And, sure enough, my DSL breaks and the
POTS phone stops working every once in a while.

Actually, I've seen DSL without dialtone. It's rare but still
possible. The only thing that is shared between the CO (central
office) and the subscriber equipment is the copper cable. The low
frequency audio (300-3000Hz) is separated from the DSL carriers
(150KHz to perhaps 1.5Mhz) with simple low pass filters. It enters
the CO at the wiring board, and is immediately separated into the POTS
(plain olde telephone service) audio, which goes to the telco switch,
and the DSL carriers, which go to the DSLAM (digital subscriber line
access mess).

Anyway, my question is about PoE. The latest Ethernet (1GB) uses all 8
conductors of the RJ-45 cable. They somehow manage to modulate the
digital signal on top of it (which conductors are used for DC?).
Does it *really* work? What happens to some Ethernet devices that were
manufatured before PoE? Won't they break?
Hmm, perhaps PoE is strictly 10baseT and 100baseT standard?
That way they can send the DC through the unused conductors.

Thanks for your insight.

Some 802.3af implementations works quite nicely with Gigabit ethernet.
See general description of 802.3af at:
<http://en.wikipedia.org/wiki/Power_over_Ethernet>
"A "phantom power" technique is used so that the powered
pairs may also carry data. This permits its use not only
with 10BASE-T and 100BASE-TX, which use only two of the
four pairs in the cable, but also with 1000BASE-T (Gigabit
Ethernet), which uses all four pairs for data transmission.
This is possible because all versions of Ethernet over
twisted pair cable specify differential data transmission
over each pair with transformer coupling; the DC supply
and load connections can be made to the transformer
center-taps at each end. Each pair thus operates in
"common mode" as one side of the DC supply, so two pairs
are required to complete the circuit. The polarity of the
DC supply is unspecified; the powered device must operate
with either polarity or pair 45+78 or 12+36 with the use
of a bridge rectifier."

Note that there are many PoE devices and adapters that will only work
with dedicated power wires (10base-T and 100base-TX only) as well as
those that will also work with 1000base-TX. There are also adapters
that function as a PoE (power over ethernet) device with specific
devices, but are not 802.3af compliant. Check the specs first,
especially if you mix and match.

 

[John Larkin]

When I learn of cables that carry mixed analog-digital signals, I tend
to be cautious, to say the least. Take for instance DSL, a technology
that I am currently using for the first time ever. At its inception,
tears ago, when a read that they were planning to send the Internet
digital signal in the same conductor as the regular old-fashioned
POTS, I was pretty skeptical. And, sure enough, my DSL breaks and the
POTS phone stops working every once in a while.

Anyway, my question is about PoE. The latest Ethernet (1GB) uses all 8
conductors of the RJ-45 cable. They somehow manage to modulate the
digital signal on top of it (which conductors are used for DC?). Does
it *really* work? What happens to some Ethernet devices that were
manufatured before PoE? Won't they break?

Hmm, perhaps PoE is strictly 10baseT and 100baseT standard? That way
they can send the DC through the unused conductors.

Thanks for your insight.

Ethernet is transformer-isolated on both ends of the twisted pairs. To
do PoE, they center-tap the isolation transformers on both ends and
use those taps as a DC path. The PoE hub applies about 50 volts DC
between the ct's of the transmit and receive pair (at a random
polarity!) and the end device picks it off, rectifies it for luck, and
drives an isolated dc-dc converter to get local power. There is a
simple dc-current handshake that lets the end gadget inform the hub
about its PoE capabilities.

Standard PoE is good for something like 15 watts. There's a PoE+ or
some such that uses 4 twisted pairs to get more power.

There are a number of PoE management chips, and their datasheets and
appnotes have details. National has some good ones.

As far as I know, PoE works fine. The balanced DC running down each
pair doesn't disturb the ac-coupled data or annoy the magnetics.

http://en.wikipedia.org/wiki/Power_over_ethernet


John

 

[Thrill5]

Your analog phone line from the phone company supplies DC power as well, as
well as just about every PBX on the market to power the phone at the end.
Supplying DC power does not affect the signal that is being transmitted over
it.

 

[Sam Wilson]

Hi Ramon,


I suspect you are confusing the various elements of 2 different
environments, DSL and Cable, and the bearer encapsulation being used
on either (PPPoA or PPPoE). Also, your use of the term "PoE" is quite
confusing, because to me, PoE means Power over Ethernet, something
quite different.

He's talking about Power over Ethernet - IEEE 802.3af - see the subject
line.

Sam

 

[Sam Wilson]

When I learn of cables that carry mixed analog-digital signals, I tend
to be cautious, to say the least. Take for instance DSL, a technology
that I am currently using for the first time ever. At its inception,
tears ago, when a read that they were planning to send the Internet
digital signal in the same conductor as the regular old-fashioned
POTS, I was pretty skeptical. And, sure enough, my DSL breaks and the
POTS phone stops working every once in a while.

Doesn't happen here in the UK, or at least not that I've noticed. DSL
certainly works successfully in many places which suggests it's not
fundamentally flawed.

Anyway, my question is about PoE. The latest Ethernet (1GB) uses all 8
conductors of the RJ-45 cable. They somehow manage to modulate the
digital signal on top of it (which conductors are used for DC?). Does
it *really* work? ...

Yep, it works. If you read up on how Gigabit Ethernet works over copper
(5-level trellis coded signals using each pair in both directions
simultaneously - I believe it was developed by people with experience in
modem design) you'll find that adding DC offset is trivial by

... What happens to some Ethernet devices that were
manufatured before PoE? Won't they break?

The standard goes to some lengths to ensure they don't. Non-standard
implementations may break things.

Hmm, perhaps PoE is strictly 10baseT and 100baseT standard? That way
they can send the DC through the unused conductors.

Both are options.

Sam