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Takai-san’s 7/31/08 email discusses a number of points. Our arguments concerning his first two points (Cost and Time to Market) are unchanged from cole_04_0708, so are not repeated here. The remaining points are addressed below.
Power
The long term power consumption of 40GE CWDM and 40GE Serial is similar. Four 10G un-cooled DFBs and associated Laser Drivers use about the same power as one cooled 40G EML and associated Modulator Driver. The remaining ICs are also about the same if advanced process nodes and new designs are assumed. As was pointed out by Joel Goergen during the Q&A session in Denver, a 40GE Serial block diagram has comparable circuitry to 40GE CWDM block diagram when drawn fairly to permit apples to apples comparison.
There is no basis for a claim at this late stage in the debate that Serial has a power advantage over CWDM, and that CWDM “power reduction plans are invisible.” In jewell_03_0508, p.9 and again in traverso_02_0708 p. 12, ratios of power between an aggressive Serial implementation and CWDM implementation are 0.96 and 0.97, i.e. clear statements in pro-serial presentations that there is no advantage.
Size
For future generation products, CWDM has an advantage over Serial for fitting into a smaller form factor like QSFP because similar to a 10GE-LR SFP+, the re-timing CDRs can be moved outside of the module. Serial always has to have the 4:1 SerDes function in the module. Even with aggressive projections about future component size and power, Serial has a packaging and thermal management design challenge to fit into QSFP.
What is required to fit 40GE CWDM into QSFP is optics integration. This type of technology has been described in numerous presentations to the HSSG and involves flip-chipping lasers onto a PLC with an integrated AWG Mux. The CWDM grid prevents use of a monolithic DFB array and requires flip-chipping discrete DFBs, but that is a yield and cost issue not a feasibility or size issue. The time line for such an advanced development program is lengthy, but is similar to realistic PCB RF-interconnect 40GE Serial development schedules. The investment required to bring this advanced technology to market is high, again similar to one required for low cost 40GE Serial.
In contrast, no advanced technology development is required to quickly bring to market first generation low cost CWDM products based on discrete optics packaged in a larger form factor.
Reliability
There is no current 1310nm 10G DFB failure data that justifies bringing up concerns about the reliability of a 4x10G CWDM PMD. 10G 1310nm PMDs ship in volume today with very high reliability. If there is actual field failure data behind this concern, it would add credibility to have it presented.
Other Aspects
It is no longer possible to simply increase Baud to match data rate, because of fundamental electrical and optical propagation limits. This was recognized during the 100G SMF PMD discussion, with Serial never a viable alternative for the 10km or 40km reach. In the future, all data rates beyond 100G will use some form of multi-lane technology. 40G is the inflection point where cost and difficulty of Serial rises dramatically compared to multi-lane alternatives. Optical communication has reached the point that all other forms of communication (wired or wireless) reached many years ago, where simple modulation format serial solutions are not practical.
Chris
From: Atsushi Takai [mailto:atsushi.takai@xxxxxxxxxx]
TF members
I am supporting 40G serial. My view on 40G Serial and CWDM is as below.
(1) COST
We can achieve less than 4x10G cost using 40G serial while we need some breakthrough technology using 40G CWDM
I agree 40G CWDM will be 4-8 times of 10G as written in "cole_04_0708" page 8. And most likely 6 times. Using current technology, it is difficult to achieve less than 4 because we have to pay for wavelength control. In case of 40G serial, module structure is the same as X2. I believe we can achieve less than 4 times cost for each part in the 40G module comparing 10G serial. We are waiting 3rd generation SERDES and 2nd generation of driver and TIA IC to achieve low cost 40GbE serial. I am sure that industry is working on these devices.
(2) Time
There exists plural 40G serial module vendors today using 1550-nm EA-DFB. 1310-nm EA-DFB is easier because we can neglect dispersion problem. Thus 40GbE serial is possible. And cost reduction plan is visible. I also sure 40G CWDM will be available if market will accept larger than 4 times cost. Cost reduction plan will follow the same as 10G and plan for less than 4 times is invisible today.
(3) Power consumption
We need also breakthrough to achieve less than 4 time power consumption using CWDM. Power consumption reduction plan is invisible We are sure we can achieve less than 4 time power consumption using serial in future.
(4) Size
I am not sure it is too early to talk 40G electrical interface. But 40GbE serial has possibility for XFP or SFP+ size. I did not hear LX4 XFPor SFP+ due to pin constraint and power consumption.
(5) Risk for wavelength failure
WDM technology has always the risk for wavelength failure, while serial does not have. We may resolve some way for 40G CWDM because of high reliability of long wavelength optical devices.
(6) Another aspect
We made speed breakthrough every 4 or 5 years in the past. AND We started to deliver 10G modules in 1997. The 300-pin MSA started in 2000. 10GbE was issued in 2002. 40G is the next milestone for technology evolution and now is a little bit behind the past trend. (We had unhappy period that every progress seemed to stop.) Today 40G is the technology to challenge and overcome for both optical and electrical technology. I think IEEE should not avoid such technology evolution.
I believe IEEE should take 40GbE serial.
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