| Thread Links | Date Links | ||||
|---|---|---|---|---|---|
| Thread Prev | Thread Next | Thread Index | Date Prev | Date Next | Date Index |
|
Hi Ali,
I am in full agreement with a number of your final conclusions.
We should certainly set realistic expectations, not over promise, and not under deliver. Four channel and ten channel I/O will be hard, and will require a careful development of a new 10G electrical specification, which will be a pre-requisite to writing the MMF optical specifications. The SFP+ interface specification is a good starting point and the link you provided is a good reference. Other SFP+ MSA work will also be valuable material for the 40GE and 100GE multi-lane 10G I/O effort. However, I am in disagreement with some of your intermediate conclusions. The optical link for multi-channel 40GE or 100GE MMF is easier then SFP+, not harder. That is because SFP+ has to satisfy the 10GE-SR 300m MMF standard, and the HSSG 40GE and 100GE objectives are only for 100m of OM3. If SFP+ had to only support a single 10G channel over 100m of OM3, you would have a much easier job of closing the jitter budget. That is why HSSG picked 100m as the objective last fall, so that we would have margin with respect to the optical budget. A signal from a properly manufactured and assembled modern 10G VCSEL transmitter does not need help from a linear receiver+EDC to propagate over 100m of OM3 MMF. And it doesn't matter whether that is single channel or multi-channel. The nice thing about fiber is that it doesn't care if there are 11 or 23 other fibers in close proximity. And the VCSEL in an array will be the same VCSEL as in a single channel application, so it will have similar (not worse) optical performance. The serious problems in this multi-channel application arise from crosstalk in the transceiver optical interfaces, transceiver connectors, host PCB traces, and ASIC I/O. All of these cross-talk terms are additive, for which an equalizer is of no use, with which you agree. And that was my original point, a linear interface+EDC is not a solution that at this time has an identified problem to solve. We may find problems in the future that need a linear interface+EDC, but propagation over 100m of OM3 is not one of them. What we know today is that TX and RX I/O host equalizers are helpful with the host to transceiver electrical interface, and if included, should be optimized for this problem. It is great that your company is making cross-talk measurements on 10G data lanes. I would encourage you to present this data, with the most valuable being multi-lane 10G ASIC I/O cross-talk. That will permit us to start quantifing all the cross-talk penalties that we will have to deal with. Thank you Chris
From: Ali Ghiasi [mailto:aghiasi@broadcom.com] Chris It would be difficult to explain everything in an email, so I am going to be short and probably do a presentation in July. Please see my comments below in blue. Chris Cole wrote: As the editor of SFP+ I would say we are working very hard to solve the 10G-SR and LRM SFP+. I can also tell you the limiting interface is the hardest oneAli, It is not clear what problems are solved by your proposal to define the 40G and 100G MMF optical specifications around linear interfaces and host EDC. Fist, SFP+ 10GE-SR optics are a solved problem today, and the SFP+ 10GE-SR specification, while not optimum, is complete and manufacturable. The original 300m meter 802.3 10GE-SR specification has a number of issues which affect yield and therefore delayed the availability of low cost 10GE. However, as has been seen from numerous emails on this reflector, multiple manufacturers have resolved these issues. due to jitter budget. You can also download the latest SFP+ specifications from the SFF website. ftp://ftp.seagate.com/sff/SFF-8431.PDF LRM uses a linear interface and some of the difficulty is due to TWDP limits and has nothing to do with linear interface. You need to separate linearOn the other hand, the SFP+ 10GE-LRM linear specification still has a number of difficult issues to resolve. So you are proposing to consider as a starting point a spec which is still under discussion and therefore not done, instead of a specification which is complete and verified to work. interface benefit from LRM difficulties. I did not say solve the XTALK issue with EDC or possibly use echo-cancelers. I was saying due to XTALK, additional channel impairments when routingThe reason for going to a lower distance for 40G and 100G is to provide additional margin/yield. Further, 300m multi-ribbon applications are highly unlikely, so it makes little sense to have the 40G or 100G specifications driven by an insignificant fraction of the applications. We may re-visit whether 100m is the right distance (150m has been mentioned as an alternative,) but 300m would be of little value as was commented on by multiple-end users during the HSSG discussion of the MMF objective. Second, I am at loss as to how an EDC solves the additional penalty issue of cross-talk in a multi-lane application. Cross-talk problems are solved through echo-cancellers, not EDCs. So if we wanted to reduce cross-talk effects through signal processing techniques, a solution resembling 1000BASE-T or 10GBASE-T would be required. 10GBASE-T power numbers in the many watts have been reported on this reflector. For 40G, we would expect linear scaling in power, and quadratic scaling for full-cross ten-lane echo-cancellation. We will be well served to view this as a solution of last resort, not as a starting point. 10 channels, and SerDes with more jitter you will need to either put a retimer in the module or use a SerDes with build in EDC as better compromise. I was not saying to use the power of EDC to solve fibre dispersion, but rather to solve higher transmit jitter contribution, laser BW limitation, some fiber dispersion,Third, I do not see the motivation to have an EDC for an objective which explicitly states OM3 as the fiber. OM3 does not have dispersion problems over a distance of 100m or 150m. LRM EDC was developed for legacy OM1 fiber, already deployed within buildings, for example between floors. I have heard no application identified in any HSSG presentation for 40G or 100G which would use ribbon-fiber that had dispersion problems like OM1. So we would burden 40G or 100G hosts with an EDC per channel, so that we can use optics that do not meet SR specs on the speculatively assumption that they are lower cost. and the electrical receive chain. I looked at this presentations, on the SFP+ Module Compliance Boards Broadcom is building for the industry with lots of care theFourth, cross-talk for connectors and PCB traces has been simulated and quantified (see for example page 12 of cole_01_1106.pdf.) There is no indication that the cross-talk magnitude is anywhere near requiring the drastic measures of an EDC/Echo Canceller. A careful re-allocation of the SFI (SFP+ interface) jitter budget between the host and optics will permit tolerance of these levels of cross-talk. NEXT measurements are attached. We do measure more crosstalk than your theoretical limits. Chris it does not matter how careful you are here is the facts: - 10X SerDes transmitter will have more jitter - More NEXT - 10 channel optics performance may not be as good as single channel - 10X SerDes receiver will have less jitter tolerance with limiting interface you only have 1 UI to work with but using linear interface you can stretch your 1 UI! I am telling you single channel SFP+ is hard and doing the same with 10 channel will only be harder.Missing is the measurement data for cross-talk in multi-lane 10G I/O CMOS ASICs. Until we have solid data for this, we will not be able to complete the specification of MMF PMDs that do not require CDRs. We also need to set a realistic expectation and not to over promise and under deliver! Thanks, Ali I would encourage all IC vendors participating in the HSSG, who have developed silicon that implements 10G I/O, to bring in multi-lane 10G I/O cross-talk data so that we can base the 40G and 100G specifications on measurement results. Chris ---------------------------------------------------------------------------- "Ali Ghiasi" <aghiasi@broadcom.com> 06/27/2007 12:33 PM To: "Jack Jewell" <Jack.Jewell@PICOLIGHT.COM>, "Paul Kolesar" <pkolesar@systimax.com> cc: STDS-802-3-HSSG@listserv.ieee.org Subject: Re: [HSSG] The List Jack and Paul The question is not whether SFP+ can achieve 300 m SR reach similar to XFP, but how do we get to 10G SFP+ at 2.5x the cost of 1G classic SFP for DCE (Data Center Ethernet) with max reach of 100 m. If we can get to 10G SFP+ at 2.5x the cost of 1G at 300 m then the 10G PAR objective is complete, but how long do we wait the need is know. But I do know the combination of lower cost optics with EDC can deliver the 2.5x cost objective for DCE applications near term. To get to these cost the transmitter very likely will not be fully SR compliant and in that case it does not matter if the reach is 100 or 300 m. The current assumption in the HSSG is that you can achieve SFP+ limiting performance with 4 or 10 channels without the use of CDR in the module, with more crosstalk, less optimum layout, SerDes having more jitter and less tolerance compare to small port count PHYs, optics ??? You will get small benefit from reducing fiber reach to 100m but not enough to close the link budget. As Dan mentioned EDC is becoming a standard feature on PHYs and we definitely need to leverage it for 40G/100G. Use of linear interface is an approach that can close the link budget without the use of CDR in the module, relax the optics specifications, and the same interface can support passive copper Twin-ax up to 10m. Ali |