open fiber control in PAM-5
Hello 10G'ers,
I would like to eliminate another misconception
regarding the serial at 5 Gbaud vs the 4-WDM
at 1.25 Gbaud approaches in PAM-5: the supposed
signal power advantage of the serial approach
due to eye-safety limits.
I will show below that the launched power
PER CHANNEL in 4-WDM can be safely set at the
same level as the total launched power in the
serial approach, due to the redundancy in
its receiver (no single point of failure).
---> Since the launched power level per
transmitter of the serial and 4-WDM
approaches will be the same, 4-WDM will
enjoy a 12 dB advantage in SNR, due to
its receiver bandwidth being 4 times
smaller (same signal power, much smaller
noise power).
The key is in the "open fiber control" method.
How do I envision this method in the PAM-5
specific case ?
In a serial approach using, for example, 850 nm
lasers, the maximum launched power is -4 dBm.
In a 4-WDM approach we could use the following
procedure to keep the launched power PER CHANNEL
at -4 dBm (for a total launched power of +2 dBm)
and still ensure a safe-eye environment:
1) When a near-end node is connected to a link
and powered-up it will transmit a signal at -4 dBm
using only ONE transmitter and keep the other
three transmitters off. In this way the eye-safety
limit is satisfied.
2) The near-end node will remain in this state
for as long as it does not sense a signal in
any of its four receivers.
3) Also the far-end node, when it is powered-on,
will do the same: transmit on only one channel
using the maximum -4 dBm allowed for eye-safety
considerations.
It makes sense to use only one transmitter
after power-up to send a life signal to a
potential partner on the other side of the
link, just to conserve power consumption
as long as there is no answer from the other
side.
Which transmitter should be used for sending
this life signal ? For reasons that will
become obvious later, the best choice is
the transmitter that sends the PAM-5 encoded
TA symbols, following the nomenclature of
the 1000BASE-T standard (*)
For the following, remember that each
receiver has four channels, named RA, RB,
RC, RD.
4) If any of the two partners senses and recognizes
a signal in its RA-receiver, it will go
to the next state of its state machine: it
will switch-on the other three transmitters and
begin transmitting IDLES, with each transmitter
using a full -4 dBm launched power.
The advantage of using only the TA-transmitter
during the first step of establishing a link
becomes now obvious: in the 1000BASE-T,
the RA-receiver has the capability to
synchronize the receiver descrambler to
the transmitter scrambler, by just using
only the information embedded in the
transmitted TA-symbols.
Hence, there is no danger that the receiver
will confuse a spurious signal with the real
signal: it has to be able to synchronize
its descrambler and verify that the
synchronization is indeed correct using just
the embedded information in the transmitted
TA-symbols, in order to make a positive
identification. Without this identification
it will not switch 'on' the other three
transmitters.
5) During normal operation, if any partner suddenly
ceases to receive signals on ANY of its four receivers
(that are tuned to four different wavelengths)
during more than, say, 1 millisecond, it will switch
back to its previous state, that is, shut off
three transmitters and send a life signal
using only one transmitter at - 4 dBm. This
might be the case, for instance, when a
technician opens the link at any point
between the two partners.
(the loss of signal is easier to detect than
the existence and validation of a real signal,
hence, a very simple and robust no-signal-detect
circuit may be used).
Notice that the four receivers, that are tuned
to four different wavelengths, must malfunction
in order to miss the "open link" event. It is
this redundancy in a 4-WDM system that allows
the use of a total + 2 dBm launched power during
normal operation.
The above procedure could also be a replacement of the
PHY Control State Diagram, Figure 40-15, of the
1000BASE-T standard (with further details to be added
later), since in 10 GbE we do not need the concept of
"master" and "slave" and loop timing used in 1 GbE
(that was used there to eliminate the Echo and NEXT
interferers).
(*) during normal operation, after the link has been
established, a transceiver sends through its four
transmitters quartets of PAM-5 symbols, {TA,TB,TC,TD},
with TA = {+2,+1,0,-1,-2} and similar for TB,TC,TD.
Jaime
Jaime E. Kardontchik
Micro Linear
San Jose, CA 95131
email: kardontchik.jaime@xxxxxxxxxxx