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    Questions on the 2017 Ethernet Roadmap for Networked Storage

    January 9th, 2017

    Last month, experts from Dell EMC, Intel, Mellanox and Microsoft convened to take a look ahead at what’s in store for Ethernet Networked Storage this year. It was a fascinating discussion of anticipated updates. If you missed the webcast, “2017 Ethernet Roadmap for Networked Storage,” it’s now available on-demand. We had a lot of great questions during the live event and we ran out of time to address them all, so here are answers from our speakers.

    Q. What’s the future of twisted pair cable? What is the new speed being developed with twisted pair cable?

    A. By twisted pair I assume you mean USTP CAT5,6,7 etc.  The problem going forward with high speed signaling is the USTP stands for Un-Shielded and the signal radiates off the wire very quickly.   At 25G and 50G this is a real problem and forces the line card end to have a big, power consuming and costly chip to dig the signal out of the noise. Anything can be done, but at what cost.  25G BASE-T is being developed but the reach is somewhere around 30 meters.  Cost, size, power consumption are all going up and reach going down – all opposite to the trends in modern high speed data centers.  BASE-T will always have a place for those applications that don’t need the faster rates.

    Q. What do you think of RCx standards and cables?

    A. So far, Amphenol, JAE and Volex are the suppliers who are members of the MSA. Very few companies have announced or discussed RCx.  In addition to a smaller connector, not having an EEPROM eliminates steps in the cable assembly manufacture, hence helping with lowering the cost when compared to traditional DAC cabling. The biggest advantage of RCx is that it can help eliminate bulky breakout cables within a rack since a single RCx4 receptacle can accept a number of combinations of single lane, 2 lane or 4 lane cable with the same connector on the host. RCx ports can be connected to existing QSFP/SFP infrastructure with appropriate cabling. It remains to be seen, however, if it becomes a standard and popular product or remain as a custom solution.

    Q. How long does AOC normally reach, 3m or 30m?  

    A. AOCs pick it up after DAC drops off about 3m.  Most popular reaches are 3,5,and 10m and volume drops rapidly after 15,20,30,50, and100. We are seeing Ethernet connected HDD’s at 2.5GbE x 2 ports, and Ceph touting this solution.  This seems to play well into the 25/50/100GbE standards with the massive parallelism possible.

    Q. How do we scale PCIe lanes to support NVMe drives to scale, and to replace the capacity we see with storage arrays populated completely with HDDs?

    A. With the advent of PCIe Gen 4, the per-lane rate of PCIe is going from 8 GT/s to 16GT/s. Scaling of PCIe is already happening.

    Q. How many NVMe drives does it take to saturate 100GbE?

    A. 3 or 4 depending on individual drives.

    Q. How about the reliability of Ethernet? A lot of people think Fibre Channel has better reliability than Ethernet.

    A. It’s true that Fibre Channel is a lossless protocol. Ethernet frames are sometimes dropped by the switch, however, network storage using TCP has built in error-correction facility. TCP was designed at a time when networks were less robust than today. Ethernet networks these days are far more reliable.

    Q. Do the 2.5GbE and 5GbE refer to the client side Ethernet port or the server Ethernet port?

    A. It can exist on both the client side and the server side Ethernet port.

    Q. Are there any 25GbE or 50GbE NICs available on the market?

    A. Yes, there are many that are on the market from a number of vendors, including Dell, Mellanox, Intel, and a number of others.

    Q. Commonly used Ethernet speeds are either 10GbE or 40GbE. Do the new 25GbE and 50GbE require new switches?

    A. Yes, you need new switches to support 25GbE and 50GbE. This is, in part, because the SerDes rate per lane at 25 and 50GbE is 25Gb/s, which is not supported by the 10 and 40GbE switches with a maximum SerDes rate of 10Gb/s.

    Q. With a certain number of SerDes coming off the switch ASIC, which would you prefer to use 100G or 40G if assuming both are at the same cost?

    A. Certainly 100G. You get 2.5X the bandwidth for the same cost under the assumptions made in the question.

    Q. Are there any 100G/200G/400G switches and modulation available now?

    A. There are many 100G Ethernet switches available on the market today include Dell’s Z9100 and S6100, Mellanox’s SN2700, and a number of others. The 200G and 400G IEEE standards are not complete as of yet. I’m sure all switch vendors will come out with switches supporting those rates in the future.

    Q. What does lambda mean?

    ALambda is the symbol for wavelength.

    Q. Is the 50GbE standard ratified now?

    A. IEEE 802.3 just recently started development of a 50GbE standard based upon a single-lane 50 Gb/s physical layer interface. That standard is probably about 2 years away from ratification. The 25G Ethernet Consortium has a ratified specification for 50GbE based upon a dual-lane 25 Gb/s physical layer interface.

    Q. Are there any parallel options for using 2 or 4 lanes like in 128GFCp?

    A. Many Ethernet specifications are based upon parallel options. 10GBASE-T is based upon 4 twisted-pairs of copper cabling. 100GBASE-SR4 is based upon 4 lanes (8 fibers) of multimode fiber. Even the industry MSA for 100G over CWDM4 is based upon four wavelengths on a duplex single-mode fiber. In some instances, the parallel option is based upon the additional medium (extra wires or fibers) but with fiber optics, parallel can be created by using different wavelengths that don’t interfere with each other.



    What’s Happening with 25GbE

    August 25th, 2014

    In July 2014, IEEE 802.3 voted to form a Study Group for 25Gb/s Ethernet.  There has been a lot attention in the networking press lately about 25Gb/s Ethernet, but many people are asking what is it and how did we get here.  After all, 802.3 already has completed standards for 40Gb/s and 100Gb/s and is currently working on 400Gb/s, so from a pure speed perspective, starting a 25Gb/s project now does look like a step backwards.

    (Warning: the following discussion contains excessive physical layer jargon.)

    The Sweet Spot

    25GbE as a port speed is attractive because it makes use of 25Gb/s per lane signaling technology that has been in development for years in the industry, culminating in the recent completion of 802.3bj, the standard for 100GbE over backplane or TwinAx copper that utilizes four parallel lanes of 25Gb/s signaling to achieve the 100Gb/s port speed. Products implementing 25Gb/s signaling in CMOS technology are just starting to come to market, and the rate will likely be a sweet spot for many years, as higher rate signaling of 40Gb/s or 50Gb/s is still in early technology development phases. The ability to implement this high speed I/O in CMOS is important because it allows combining high-speed I/O with many millions of logic gates needed to implement Ethernet switches, controllers, FPGAs, and microprocessors. Thus specifying a MAC rate of 25Gb/s to utilize 25Gb/s serdes technology can enable product developers to optimize for both the lowest cost/bit and the highest overall bandwidth utilization of the switching fabric.

    4-Lane to 1-Lane Evolution

    To see how we got here and why 25Gb/s is interesting, it is useful to back up a couple of generations and look at 10Gb/s and 40Gb/s Ethernet.  Earliest implementations of 10GbE relied on rather wide parallel electrical interfaces: XGMII and the 16-Bit interface.  Very soon after, however, 4-lane serdes-based interfaces became the norm starting with XAUI (for chip-to-chip and chip-to-optical module use) which was then adapted to longer reaches on TwinAx and backplane (10GBASE-CX4 and 10GBASE-KX4).  Preceding 10GbE achieving higher volumes (~2009) was the specification and technical feasibility of 10Gb/s on a single electrical serial lane. XFI was the first followed by 10GBASE-KR (backplane) and SFI (as an optical module interface and for direct attach TwinAx cable using the SFP+ pluggable form factor).  KR and SFI started to ramp around 2009 and are still the highest volume share of 10GbE ports in datacenter applications. The takeaway, in my opinion, is that single-lane interfaces helped the 10GbE volume ramp by reducing interconnect cost. Now look forward to 40GbE and 10GbE. The initial standard, 802.3ba, was completed in 2010.  So during the time that this specification was being developed, 10Gb/s serial interfaces were gaining traction, and consensus formed around the use of multiple 10Gb/s lanes in parallel to make the 40GbE and 100GbE electrical interfaces. For example, there is a great similarity between 10GBASE-KR, and one lane of the 40GBASE-KR4 four-lane interface. In a similar fashion 10Gb/s SFI for TwinAx  & optics in the SFP+ form factor is similar to a lane of the 40GbE equivalent interfaces for TwinAx and optics in the QSFP+ form factor.

    But how does this get to 25Gb/s?

    Due to the similarity in technology needed to make 10GbE and 40GbE, it has because a common feature in Ethernet switch and NIC chips to implement a four-lane port for 40GbE that can be configured to use each lane separately yielding four 10GbE ports.

    From there it is a natural extension that 100GbE ports being implemented using 802.3bj technology (4x25Gb/s) also can be configured to support four independent ports operating at 25Gb/s.  This is such a natural conclusion that multiple companies are implementing 25GbE even though it is not a standard.

    In some environments, the existence of a standard is not a priority.  For example, when a large-scale datacenter of compute, storage and networking is architected, owned and operated by one entity, that entity validates the necessary configuration to meet its requirements. For the broader market, however, there is typically a requirement for multi-vendor interoperability across a diverse set of configurations and uses. This is where Ethernet and IEEE 802.3 has provided value to the industry for over 30 years.

    Where’s the Application?

    Given the nature of their environment, it is the Cloud datacenter operators that are poised to be the early adopters of 25GbE. Will it also find a home in more traditional enterprise and storage markets? Time will tell, but in many environments ease of use, long shelf life, and multi-vendor interoperability are the priorities. For any environment, having the 25GbE specification maintained IEEE 802.3 will facilitate those needs.