RE: 850 nm solutions
Thanks for appreciating my comments.
I agree that for the low cost technology, the combination of the installed
MM fiber and 850 nm VCSEL is the best choice. Both CWDM 10GbE-SX and
850nm-4WDM-1.25 Gbps will achieve 100 meter, or more depending on which
modal bandwidth, OFL (over filled launch) 160 MHz-km, or RML (restricted
mode launch) 380 MHz-km is used.
If the RML bandwidth is used, the CWDM and 4WDM technologies can reach 160
meter and 380 meter, respectively. Theoretically, 4WDM (1.25 Gbps) has
twice the distance of CWDM (2.578 Gbps).
It is true PAM-5 is new to the optical link application and it has to be
proved at the actual link. It is also true 1000BASE-T has developed 1.25
Gbps PAM-5 codec which is available in the market.
I believe we can use 1000BASE-T driver to drive 1000BASE-SX transceiver for
evaluation of the 850nm-4WDM-1.25 Gbps approach. Of course, the test setup
has to be carefully connected for a high frequency test; otherwise the
distortion from the connections will mislead the test result.
I am pretty sure, I will set it up some time to evaluate the PAM-5
Edward S. Chang
NetWorth Technologies, Inc.
Edward and all,
Thank you Edward for your insight.
There are have been in Albuquerque excellent presentations
using 850 nm VCSELs by J. Yorks, P. Kolesar et al, F. Peters,
R. Colla et al and J. Jewell. They were all targetted towards
the new 2200 MHz*km MMF using 10 Gbaud lasers. Unfortunately,
at this speed they cannot support the installed MMF (the
eye closes at about 25 meters).
However, there are two proposals using 850nm - 4WDM that
do support both the installed MMF and the new 2200 MHz*km
MMF. See the Albuquerque presentations:
B. Wiedemann et al: "Evaluating CWDM 10GBASE-SX"
J.E. Kardontchik et al: "850nm-4WDM-1.25Gbaud transceiver
over MMF for 10 GbE"
Both provide ample support for the new MMF: the first
proposal, at 2.578 Gbaud, has a target of 550 meters, and
the second proposal, running at 1.25 Gbaud, has a target
of 1,000 meters. Hence, both meet with ample margin
the minimum objective of the HSSG regarding the new
2200 MHz*km MMF, 300 meters.
The 2.578 Gbaud proposal has a target of 100 meters on
installed MMF. The 1.25 Gbaud proposal has a target of
160 meters on installed MMF. Hence, both meet also the
minimum objective of the HSSG regarding the installed
MMF, 100 meters.
The 2.578 Gbaud proposal has the advantage of using
on-off optical modulation. Its PCS, using 64/66, is new
and needs still some work and debugging in the coming
months. Work is being presently done in both areas to
validate this proposal. It is a very good proposal.
I happen to co-sponsor it ... :-)
The 1.25 Gbaud proposal uses PAM-5 modulation, that
is new to the optical community. However, it only needs a
very small dynamic range (18-level ADC in the receiver),
corresponding to an SFDR (Spurious Free Dynamic Range)
SFDR = 20*log(18) + (2/3)*10*log(0.625e+9)
= 84 dB*Hz^(2/3)
This SFDR is well within the present VCSEL technology
capabilities. (*) This proposal has the advantage that it reuses
the 1000BASE-T PCS, that has already been debugged and
standardized and some companies are already offering it
in commercial products. It uses the same symbol rate as
the 1 GbE optical transceivers, 1.25 Gbaud, simplifying
the coexistence of 1 and 10 GbE transceivers on the same
board and minimizing packaging and PCB costs and EMI.
It could be the lowest cost 10 GbE system solution on MMF,
both installed and new.
The only area that needs detailed work, in order to meet
the July 2000 schedule-milestone, is in providing actual
5-level pseudorandom stimulus to VCSELs at 1.25 Gbaud
and measuring the actual performance of the optical link.
My company has shown a lot of support towards developing
the whole proposal. However, it does not have the means
and optical expertise in this area and we will welcome
any optical companies interested in performing these
measurements and presenting them to the Task Force.
These are the 850 nm choices.
(*) See, for example:
"Dynamic Range of VCSELs in Multimode Links"
by: H.L.T. Lee, R.V. Dalal, R.J. Ram and K.D. Choquette
IEEE Photonics Tech. Letters, vol 11, pp 1473-75, Nov 1999
Jaime E. Kardontchik