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  •                                    關於25G乙太網的25個問題

                           25 Questions to Ask About 25G Ethernet
     
                                                                     (2015/12 by Ken Van Orman)

     

    25G以太網已經以突出其來的速度衝擊著網絡世界,並且可能最終成為迄今為止最快完成的一項IEEE以太網標準。以下的25個問題將幫助您梳理這一技術的來龍去脈:

     
    Timing for 25 GE and 50 GE
     
    1. 25G以太網?是否就是100G以太網所用的25G電氣通道?    非也!這就是25G以太網。雖然100G以太網中存在25G芯片至模塊電氣能力,但我們所說的是真正的25G以太網“管道”。我們可以將它看作加速2.5倍的10G以太網。 Nope. This is 25G Ethernet. It has 25G chip-to-module electrical capability to thank for its existence, but this is a true 25G Ethernet "pipe." Think of it as 10G Ethernet sped up 2.5 times.
     
    2. 這種技術的來源是什麼?為什麼我並沒有在最新的IEEE 802.3標準中看到這一技術?   25G以太網最初是若干數據中心行業的重量級企業推動的一種技術。目前也已成為IEEE下802.3by任務組的活  動之一。   25G Ethernet was initially an effort by a handful of data center industry heavyweights to move the technology forward. It is now also an IEEE task force under 802.3by.
     
    3. 有兩種規格?它們之間是否能夠互用?   這些規格還沒有形成終稿,但業界對其所提供的最高水平的互用性抱有強烈的期待。此外,很可能大多數設備 都將支持這兩種規格。   The specifications are not final but the industry has a strong desire to provide interoperability to the greatest degree possible. Also, it's likely that most devices will support both specifications.

    4. 為什麼選擇25G?   從10G到40G的跨越是一大步,但事實表明,與10G芯片相比,25G芯片增加的成本並沒有多少。Moving from 10G to 40G is a big jump and it turns out the incremental cost of 25G silicon over 10G is not that great. 
     
    5. 我不久前才將服務器間的連接提速為10G以太網。我真得需要25G以太網嗎?   初期的多數用戶都是超大規模雲服務商,以及那些需要依賴雲或網絡中極端性能才能生存的企業(例如,高頻 率交易)。 Probably not for a while. Most of the early adopters will be Hyperscale Cloud providers and those companies that make their living in the cloud or that require extreme performance from their networks (e.g., high-frequency trading).
     
    6. 我是否應當在購買下一台廣域網路由器時要求它具備25G接口?   不一定。多數早期的應用領域都在數據中心中。Not really. Most of the early applications are in the data center.
     
    7. 這種技術是否支持銅線?    是的,但不支持BASE-T。還有一個獨立的25GBASE-T規格。協會和802.3by的規格都支持最長5米的DAC。  Yes, but not BASE-T. There is a separate effort for 25GBASE-T. DACs up to 5m in length are part of the Consortium and 802.3by specifications.
     
     
    8. 它是否支持光纖?     是的,802.3by規格就包含光PMD。Yes, the 802.3by specification includes an optical PMD.
     
    9. 它是否支持短距和遠距光傳輸?   是和否。 IEEE的光PMD為短距(最長100米)。不過,有些廠商也在積極研發在單模光纖上實現2千米光傳輸 的技術。   Yes and no. The IEEE optical PMD is short reach (up to 100m). However, there are some vendors working on 2km reach optics over single mode fiber.
     
    10. 所用的接口規格是哪一種?    4x25G以太網端口用的是QSFP28,單個25G以太網端口用的是SFP28。  QSFP28 is used for 4x25GE and SFP28 is used for a single 25GE port.
     
    11. 這些規格與QSFP+和SFP+有關聯嗎?   是的。這些新規格在尺寸上與老一代相同,但其設計用途是28G通道速度,而老版本只適用於10G。  Yes. The new form factors are the same size as their older counterparts but designed for 28G lane speeds vs. 10G for the older form factors.
     
    12. 28G? 是打錯字了吧?   非也。以太網並不是使用這些規格的惟一技術,因此可能還有速度更高的其它接口也要使用這些尺寸規格。Nope. Ethernet is not the only application for these form factors so there may be other higher-speed interface uses for them.
     
    13. 這些是惟一使用的尺寸規格嗎?   業界正在對這些規格加以執行標準化,但隨著交換結構的容量提升到QSFP28可支持的交換機面板密度之上,可能還會有其它的解決方案。  The industry is standardizing on these but there may be other solutions as switching fabric capacity increases beyond what QSFP28 can support in switch faceplate density. 
     
    14. 100G以太網SR4和CR4規格需要用到RS-FEC,25G以太網是否需要FEC?   前者是,後者否。目前有兩種FEC模式(BASE-R FEC和RS-FEC),且兩者都為可選擇的銅線互聯(DAC)。   Yes and no. There are two FEC modes (BASE-R FEC and RS-FEC) and both are optional copper interconnects (DACs).
     
    15.我是否需要FEC?它的優勢和劣勢都是什麼?   在使用FEC時會遇到時延的問題。如果您正在設計HFT應用,這對您就非常重要。FEC的好處在於,它能大幅  減少介質中未修正錯誤的數量,並有助於延長這些介質的可用傳輸距離。There is a latency penalty when using FEC. If you are designing for HFT applications this will be important to you. The benefit of FEC is it greatly reduces the number of uncorrected errors across the media and helps to extend the useable reach of those media.
     
    16. 您說的是哪一類的時延問題?   根據估計,BASE-R和RS-FEC分別會產生80納秒和250納秒的時延。It's estimated to be 80ns or 250ns for BASE-R and RS-FEC respectively.
     
    17. 除時延外是否還有其它的不利因素?  還會對電力消耗產生非常小的影響,估計在每個端口25毫瓦至數百毫瓦之間。單獨看雖然不大,但如果客戶需要連接數百萬台服務器,這一問題絕對不容忽視! There is a very small power consumption hit estimated between 25mW and hundreds of mW per port. Not a lot but it may matter to someone with over a million servers to connect
     
    18. 能夠在我的25G網絡中使用10G或40G DAC?  不能。40G DAC的設計不適合運行在每通道25G速度下(它們只能以10G速度運行)。 No. 40G DACs are not designed to run at 25G per lane speeds (they run at 10G speeds).
     
    19. 能否將我的100G以太網接口分成4個25G?  從設計上來說,許多機架頂(ToR)交換機都可以在100、50、40、25和10G速度下運行,如果您擁有這樣的交換機,那麼每個QSFP28端口就可以分成4個25G模式運行。  Many top of rack (ToR) switches are designed for operation at 100, 50, 40, 25 and 10G speeds, so if you have one of those switches it will run in 4x25G mode from each QSFP28 port.
     
    20. 50G?我們什麼時候能用上50G?  我可能忘記說了,協會的規格支持由2條25G通道組成的50G以太網接口。Did I forget to mention the Consortium specification supports a 50G Ethernet interface made up of 2x25G lanes?
     
    21.那麼是否有IEEE的50G以太網規格呢?  沒有,至少現在沒有。但他們可能在今年年底前組建一個這方面的研究組。Nope. Not yet. But they may take this up as a study group before the end of this year.
     
    22. IEEE的50G規格是否也是2x25G的設計? 不是的。業界正在定義一系列的50G系列標準,目的就是實現更高的效率。 No. The industry is trying to get to the next level of efficiency by defining a set of 50G serial standards.
     
    23. 所有這些標準什麼時候能夠完成?  IEEE的目標是在2016年上半年完成802.3by。 50G系列標準中的任何一項最高可能於2018年推出(見上圖)。  The IEEE is targeting to complete 802.3by in the first half of 2016.   Any 50G serial standard is likely out in 2018 at the earliest (see Figure below)
     
    24. 目前市場上有哪些類型的25G端口?  首批正式上市的產品現已開始出貨。這些產品包括ToR交換機、服務器網卡和DAC。 The first GA products are starting to ship right now. These are ToR switches, server NICs and DACs.
     
    25.我們能否提出涉及25G和50G技術的50個問題?  我們還是把另外25個問題留到下一次吧! Let's save the extra 25 questions for another time! But if you do have more questions please put them in the comments below.