Re: [802.3EEESG] 10BASE-T question
Hi Pat,
Here is my two pennies.
I am guessing that the reason to lower down the output voltage of 10BaseT is
to reduce its power consumption so that higher speed phy can be downgrade to
10BaseT. However, even though we modify the standard to allow lower output
voltage for 10BaseT, we probably will end up a 10BaseT phy which has
comparable power consumption of 100BaseT. It will lose the advantage of
speed change. The benefit of changing the spec could turn out to have a new
lower power 10BaseT when it drives longest CAT 3 cable thus only 10Mbps can
be negotiated successfully.
By the same token, so far no one considered adding a Power Back Off mode on
1000BaseT and 100BaseT for shorter cable length because the saving of power
may be very marginal.
I am afraid that the incentive of changing 10BaseT spec is not as great as
devising an electrical idle mode so that all phy modes (10, 100, 1000, 10G)
can be switched to it.
By the way, in order to meet the template of Fig 14-9 the transmitter
normally needs to pre-emphasize the waveform for fat bit (20ns or 2.5MHz
carrier). I don't have simulation result at hand so I am not sure for the
same amount of pre-emphasis (preset in IC design) used for CAT 3 model test
it still fits the template when we use CAT 5 cable model. We may need to
consider the attenuation difference between 2.5MHz and 5MHz for both cable
models.
Best Regards,
-Joseph Chou
-----Original Message-----
From: Pat Thaler [mailto:pthaler@BROADCOM.COM]
Sent: Wednesday, March 28, 2007 6:37 PM
To: STDS-802-3-EEE@listserv.ieee.org
Subject: Re: [802.3EEESG] 10BASE-T question
Mike,
I think that some adjustment to the 10BASE-T transmit voltage would be
entirely appropriate.
The 10BASE-T output voltage spec (IEEE 802.3-2005 14.3.1.2.1) currently
requires that the driver produce a peak differential voltage of 2.2 to
2.8 V into a 100 Ohm resistive load - a very normal output voltage when
the standard was written in the late 80's, but pretty high nearly 20
years later. This voltage allowed 10BASE-T to coexist in bundled Cat 3
cable with analog phone ringers. The transient when an analog phone
ringer goes off-line in that situation could produce over 250 mV.
That high output voltage is not necessary over Cat 5 or better cable.
The simple change would be to add a differential output voltage spec for
operation over Cat 5 or better cable. In that case, remove the minimum
voltage spec for peak differential voltage into a 100 Ohm resistive
load. One still would keep the maximum voltage spec of 2.8 V or perhaps
substitute a lower maximum. Change the requirement for the Figure 14-9
output voltage template to be the signal produced at the end of a
worst-case Cat 5 cable instead of at the end of the (Cat 3) twisted-pair
model.
This should be fully backwards compatible with existing 10BASE-T
compliant PHYs over Cat 5 cable. The newly specified transmitters will
produce a signal over Cat 5 cable that is within the range of signal
that the original 10BASE-T produces over the Cat 3 cable channel it
specified. That template provides a minimum eye opening of 550 mV. If I
plugged the numbers into my calculator correctly, the attenuation
difference between Cat 5 and Cat 3 cable at 10 MHz is more than 4 dB so
this should allow the transmit voltage to drop by that. It should be
very little work to do this change.
A more aggressive change that would require real work would be to
determine what receive voltage could be tolerated by today's receivers
which probably can tolerate a smaller eye-opening especially if they are
a 1000BASE-T receiver operating in a slowed down mode. But in that case,
one would either need to only use the lower eye-opening when stepped
down by EEE or add negotiation for low voltage 10BASE-T to auto-neg
because it wouldn't ensure backwards compatiblity with classic 10BASE-T
receivers.
I think the fully-backwards compatible change would be pretty easy to
justify.
To summarize, for operation over the channels specified by 100BASE-TX,
1000BASE-T and 10GBASE-T, delete the spec for minimum voltage into a 100
Ohm load and change the test condition for the Figure 14-9 voltage
template to be over a worst case 100BASE-TX channel.
Regards,
Pat
At 01:46 PM 3/28/2007 , Mike Bennett wrote:
>Folks,
>
>For those of you who were able to attend the March meeting, you may
>recall we had a discussion on 10BASE-T (in the context of having a low
>energy state mode) and what we might change to specify this, which
>included possibly changing the output voltage. Concern was raised that
>the work required to specify a new output voltage for 10BASE-T would
>far outweigh the benefit. Additionally, there was a question regarding
>the use of 100BASE-TX instead of doing anything with 10BASE-T. Would
>someone please explain just how much work it would be to change
>10BASE-T and what the benefit would be compared to using 10BASE-T with
>the originally specified voltage or 100BASE-TX for a low energy (aka
"0BASE-T" or "sleep") state?
>
>Thanks,
>
>Mike