{"id":101631,"date":"2026-08-24T10:25:08","date_gmt":"2026-08-24T10:25:08","guid":{"rendered":"https:\/\/optics.carritech.com\/?p=101631"},"modified":"2026-08-24T10:28:01","modified_gmt":"2026-08-24T10:28:01","slug":"choosing-400g-optics","status":"publish","type":"post","link":"https:\/\/optics.carritech.com\/it\/choosing-400g-optics\/","title":{"rendered":"Choosing 400G optics: SR, DR, FR and LR, and what each one costs you in fibre"},"content":{"rendered":"<p class=\"wp-block-paragraph\">Choosing 400G optics looks like a distance problem. You know how far the link has to go, you pick the part that reaches that far, and you order it.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">It is not a distance problem. It is a <strong>fibre<\/strong> problem wearing a distance problem\u2019s clothes \u2014 because the four letters on the end of the part number decide how many strands the link consumes, what connector it terminates in, and whether it can be broken out into smaller links later. Two optics that both \u201cdo 500 metres\u201d can differ by eight strands and a completely different cabling plant.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">This is the reach ladder, what each rung actually costs you, and the three orders that go wrong most often.<\/p>\n\n\n\n<h2 id=\"h-the-one-thing-to-get-straight-before-choosing-400g-optics-the-names-are-not-all-ieee-names\" class=\"wp-block-heading\">The one thing to get straight before choosing 400G optics: the names are not all IEEE names<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Start here, because it is the source of more confusion than reach ever is.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Some 400G interface names come from IEEE 802.3. Some come from a multi-source agreement between vendors. And at least one common name comes from nowhere at all \u2014 it is a marketing label for a part that a standards body later defined under a different name.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>IEEE 802.3 has never defined a \u201c400GBASE-LR4\u201d or a \u201c400GBASE-ER4.\u201d<\/strong> Its 10 km part is <strong>400GBASE-LR8<\/strong> and its 40 km part is <strong>400GBASE-ER8<\/strong>, both using eight wavelengths on a duplex pair. The only IEEE name containing \u201cLR4\u201d is <strong>400GBASE-LR4-6<\/strong>, added by <a href=\"https:\/\/standards.ieee.org\/ieee\/802.3cu\/7328\/\">IEEE 802.3cu<\/a> \u2014 and its reach is <strong>6 km, not 10<\/strong>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">So what are the modules sold as 400G-LR4 and 400G-ER4? They come from the <a href=\"https:\/\/100glambda.com\/specifications\">100G Lambda MSA<\/a>, which specifies <strong>400G-LR4-10<\/strong> at 10 km and <strong>400G-ER4-30<\/strong> at 30 km. Real specifications, written by a real group, widely implemented \u2014 just not IEEE clauses. Four-wavelength parts are also built to reaches beyond what the MSA documents, which is a vendor implementation rather than a specification, and worth confirming rather than assuming. <a href=\"https:\/\/www.snia.org\/technology-communities\/sff\/specifications\">SFF-8024<\/a>, the document that assigns the identifier codes a module reports about itself, lists them separately from the IEEE PMDs for exactly that reason.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">And <strong>400G-DR4+ \/ 400G-XDR4<\/strong>, the 2 km version of DR4, has no IEEE clause and no MSA specification at all. It is a vendor extension. The standards-track equivalent arrived later as <strong>400GBASE-DR4-2<\/strong>, added to Clause 124 by <a href=\"https:\/\/standards.ieee.org\/ieee\/802.3df\/11107\/\">IEEE 802.3df<\/a> in 2024.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">None of that makes the non-IEEE parts bad. It does mean the four letters are not a guarantee of anything, and that two modules with the same name can be built to different documents. Check the lane structure and the reach, not the label.<\/p>\n\n\n\n<h2 id=\"h-the-reach-ladder\" class=\"wp-block-heading\">The reach ladder<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Here is the whole 400G range in one table. Strand count is per link, counting both directions.<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><td>Interface<\/td><td>Defined by<\/td><td>Reach<\/td><td>Fibre<\/td><td>Strands<\/td><td>Connettore<\/td><td>Lanes<\/td><\/tr><\/thead><tbody><tr><td>400GBASE-VR4<\/td><td>IEEE Cl 167 (802.3db)<\/td><td>50 m<\/td><td>OM4\/OM5<\/td><td><strong>8<\/strong><\/td><td>MPO-12<\/td><td>4 \u00d7 100G PAM4<\/td><\/tr><tr><td>400GBASE-SR4<\/td><td>IEEE Cl 167 (802.3db)<\/td><td>100 m<\/td><td>OM4\/OM5<\/td><td><strong>8<\/strong><\/td><td>MPO-12<\/td><td>4 \u00d7 100G PAM4<\/td><\/tr><tr><td>400GBASE-SR4.2<\/td><td>IEEE Cl 150 (802.3cm)<\/td><td>100 m OM4, 150 m OM5<\/td><td>OM4\/OM5<\/td><td><strong>8<\/strong><\/td><td>MPO-12<\/td><td>8 \u00d7 50G, 2 per fibre<\/td><\/tr><tr><td>400GBASE-SR8<\/td><td>IEEE Cl 138 (802.3cm)<\/td><td>100 m<\/td><td>OM4<\/td><td><strong>16<\/strong><\/td><td>MPO-16<\/td><td>8 \u00d7 50G PAM4<\/td><\/tr><tr><td>400GBASE-SR16<\/td><td>IEEE Cl 123 (802.3bs)<\/td><td>100 m<\/td><td>OM4<\/td><td><strong>32<\/strong><\/td><td>2\u00d716 MPO<\/td><td>16 \u00d7 25G NRZ<\/td><\/tr><tr><td>400GBASE-DR4<\/td><td>IEEE Cl 124 (802.3bs)<\/td><td>500 m<\/td><td>SMF<\/td><td><strong>8<\/strong><\/td><td>MPO-12<\/td><td>4 \u00d7 100G PAM4<\/td><\/tr><tr><td>400GBASE-DR4-2<\/td><td>IEEE Cl 124 (802.3df)<\/td><td>2 km<\/td><td>SMF<\/td><td><strong>8<\/strong><\/td><td>MPO-12<\/td><td>4 \u00d7 100G PAM4<\/td><\/tr><tr><td>400GBASE-FR4<\/td><td>IEEE Cl 151 (802.3cu)<\/td><td>2 km<\/td><td>SMF<\/td><td><strong>2<\/strong><\/td><td>Duplex LC<\/td><td>4 \u00d7 100G, CWDM<\/td><\/tr><tr><td>400GBASE-LR4-6<\/td><td>IEEE Cl 151 (802.3cu)<\/td><td>6 km<\/td><td>SMF<\/td><td><strong>2<\/strong><\/td><td>Duplex LC<\/td><td>4 \u00d7 100G, CWDM<\/td><\/tr><tr><td>400GBASE-FR8<\/td><td>IEEE Cl 122 (802.3bs)<\/td><td>2 km<\/td><td>SMF<\/td><td><strong>2<\/strong><\/td><td>Duplex LC<\/td><td>8 \u00d7 50G, LAN-WDM<\/td><\/tr><tr><td>400GBASE-LR8<\/td><td>IEEE Cl 122 (802.3bs)<\/td><td>10 km<\/td><td>SMF<\/td><td><strong>2<\/strong><\/td><td>Duplex LC<\/td><td>8 \u00d7 50G, LAN-WDM<\/td><\/tr><tr><td>400GBASE-ER8<\/td><td>IEEE Cl 122 (802.3cn)<\/td><td>40 km<\/td><td>SMF<\/td><td><strong>2<\/strong><\/td><td>Duplex LC<\/td><td>8 \u00d7 50G, LAN-WDM<\/td><\/tr><tr><td>400G-LR4-10<\/td><td>100G Lambda MSA<\/td><td>10 km<\/td><td>SMF<\/td><td><strong>2<\/strong><\/td><td>Duplex LC<\/td><td>4 \u00d7 100G, CWDM<\/td><\/tr><tr><td>400G-ER4-30<\/td><td>100G Lambda MSA<\/td><td>30 km<\/td><td>SMF<\/td><td><strong>2<\/strong><\/td><td>Duplex LC<\/td><td>4 \u00d7 100G, LWDM<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">The arithmetic underneath it is simple enough to do in your head, and it is the most useful thing on this page:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Duplex LC is always two strands.<\/strong> Every WDM part \u2014 FR4, FR8, LR4, LR8, ER8 \u2014 puts all its lanes down one fibre pair as separate wavelengths.<\/li>\n\n\n\n<li><strong>A parallel part uses two strands per lane.<\/strong> MPO-12 with four lanes is eight strands. MPO-16 with eight lanes is sixteen. SR16 is thirty-two.<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">That is the entire trade. Parallel optics are cheaper to build and use more fibre. WDM optics are more expensive to build and use less fibre. Which one is cheaper <em>for you<\/em> depends on something the datasheet cannot know: whether you already have the strands.<\/p>\n\n\n\n<h2 id=\"h-where-choosing-400g-optics-gets-expensive\" class=\"wp-block-heading\">Where choosing 400G optics gets expensive<\/h2>\n\n\n\n<figure class=\"wp-block-image size-large\"><img fetchpriority=\"high\" decoding=\"async\" width=\"1024\" height=\"538\" src=\"https:\/\/optics.carritech.com\/wp-content\/uploads\/2026\/08\/choosing-400g-optics-strand-count-1024x538.png\" alt=\"choosing-400g-optics-strand-count\" class=\"wp-image-101639\" srcset=\"https:\/\/optics.carritech.com\/wp-content\/uploads\/2026\/08\/choosing-400g-optics-strand-count-1024x538.png 1024w, https:\/\/optics.carritech.com\/wp-content\/uploads\/2026\/08\/choosing-400g-optics-strand-count-300x158.png 300w, https:\/\/optics.carritech.com\/wp-content\/uploads\/2026\/08\/choosing-400g-optics-strand-count-768x403.png 768w, https:\/\/optics.carritech.com\/wp-content\/uploads\/2026\/08\/choosing-400g-optics-strand-count-1536x806.png 1536w, https:\/\/optics.carritech.com\/wp-content\/uploads\/2026\/08\/choosing-400g-optics-strand-count-2048x1075.png 2048w, https:\/\/optics.carritech.com\/wp-content\/uploads\/2026\/08\/choosing-400g-optics-strand-count-18x9.png 18w, https:\/\/optics.carritech.com\/wp-content\/uploads\/2026\/08\/choosing-400g-optics-strand-count-600x315.png 600w\" sizes=\"(max-width: 1024px) 100vw, 1024px\" \/><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">Choosing 400G optics gets expensive in three specific places, and none of them is the module price.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Going from 100G SR4 to 400G SR8 doubles the fibre.<\/strong> A 100GBASE-SR4 link is eight strands on an MPO-12. A 400GBASE-SR8 link is sixteen, on an MPO-16 \u2014 a different connector, on trunk cabling sized for the old one. That is not a transceiver upgrade. It is a cabling project, and it belongs in the same business case as the optics rather than turning up as a change request three months later.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Going from 100G SR4 to 400G SR4 does not.<\/strong> The newer <a href=\"https:\/\/standards.ieee.org\/ieee\/802.3db\/10283\/\">IEEE 802.3db<\/a> parts \u2014 VR4 at 50 m and SR4 at 100 m \u2014 put 100G on each of four lanes instead of 50G on each of eight. Same eight strands, same MPO-12, four times the capacity. If your multimode plant is already MPO-12, this is the rung that costs you nothing in fibre.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Crossing from multimode to single-mode changes the plant, not the module.<\/strong> DR4 at 500 m is single-mode on an MPO-12. It looks like a small step up from SR4 at 100 m \u2014 same connector, same strand count \u2014 but it is a different fibre type end to end. In a building already pulled with OM4, \u201cjust go DR4\u201d is a re-pull.<\/p>\n\n\n\n<h2 id=\"h-breakout-what-400g-optics-can-and-cannot-be-split-into\" class=\"wp-block-heading\">Breakout: what 400G optics can and cannot be split into<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">This is the part most reach comparisons leave out, and it is often the reason to choose one rung over another.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Breakout works when the 400G optic\u2019s individual lanes are electrically and optically identical to a smaller standalone PMD. It is not a feature the module has; it is a consequence of how the standard was written.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>400GBASE-DR4 breaks out to four 100GBASE-DR links.<\/strong> IEEE\u2019s own comparison of Clause 124 against Clause 140 shows matching optical modulation amplitude, transmit power and extinction ratio per lane \u2014 <a href=\"https:\/\/www.ieee802.org\/3\/df\/public\/23_0523\/dawe_3df_01_230523.pdf\">the two are the same PMD, counted differently<\/a>. One MPO-12 trunk fans out to four duplex LC links.<\/li>\n\n\n\n<li><strong>400GBASE-SR4 breaks out to four 100GBASE-SR1 links<\/strong>, on the same basis \u2014 both live in Clause 167.<\/li>\n\n\n\n<li><strong>400GBASE-SR8 breaks out to eight 50G links.<\/strong> Clause 138 carries 50GBASE-SR alongside it at the same lane rate.<\/li>\n\n\n\n<li><strong>No WDM part breaks out at all.<\/strong> FR4, LR4, LR8, ER8 and the rest carry their lanes as wavelengths on a single fibre pair. There is nothing to fan out \u2014 <a href=\"https:\/\/www.ieee802.org\/3\/ad_hoc\/ngrates\/email\/msg00827.html\">IEEE\u2019s own scoping discussions exclude them from breakout for exactly this reason<\/a>.<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">Worth knowing: <strong>IEEE does not standardise breakout itself.<\/strong> It defines the PMD; the connectors and cable assemblies that make a fan-out physically possible come from the form-factor MSAs. So breakout is enabled by identical per-lane PMDs, not promised by a standard, and it is worth confirming your platform supports the port-splitting configuration before you buy the cassette.<\/p>\n\n\n\n<h2 id=\"h-the-three-most-common-mis-orders\" class=\"wp-block-heading\">The three most common mis-orders<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Ordering a longer reach \u201cto be safe.\u201d<\/strong> Optical receivers have a maximum input power as well as a minimum. Putting a 10 km transmitter on a 40-metre patch can overload the far end, and the failure looks like a bad module rather than an obvious own goal. If a link is short, the short-reach part is not the risky choice \u2014 it is the correct one. Where a longer part genuinely is all you can get, that is an attenuator conversation.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Ordering a duplex part for a parallel plant, or the reverse.<\/strong> This is the mis-order we are asked about most when choosing 400G optics. Both will seat happily in the cage and neither will link. FR4 and DR4 are both single-mode and both cover a couple of hundred metres comfortably \u2014 one wants a duplex LC pair, the other wants an eight-strand MPO-12. The datasheet says so; the four letters do not.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Ordering \u201c400G-LR4\u201d and receiving a 6 km part, or ordering LR4-6 and expecting 10 km.<\/strong> This is the naming problem from the top of the article turning into a real delivery. If the requirement is 10 km on four wavelengths, the specification you want is the MSA\u2019s 400G-LR4-10. If you will accept eight wavelengths, IEEE\u2019s 400GBASE-LR8 does 10 km as well. <strong>400GBASE-LR4-6 does 6 km and is a different part.<\/strong> Ask which document the module is built to, not what the label says.<\/p>\n\n\n\n<h2 id=\"h-cost-briefly-and-honestly\" class=\"wp-block-heading\">Cost, briefly, and honestly<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">The cost side of choosing 400G optics follows a fairly predictable shape: parallel multimode parts sit at the bottom, parallel single-mode above them, four-wavelength WDM above that, and eight-wavelength long-reach at the top. More lasers and tighter wavelength control cost more.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">We are not going to put figures on that, because module pricing moves and any number published here would be wrong within a quarter. What does not move is the arithmetic underneath: <strong>the module is a line item and the fibre is a project.<\/strong> A part that saves you a re-pull is cheap even when it is the more expensive module, and the reverse is true just as often.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">If you want the rest of that model \u2014 power, port density, and the switches you do not have to buy \u2014 it is in <a href=\"https:\/\/optics.carritech.com\/it\/400g-transceiver-cost\/\">the real TCO of 400G<\/a>. And if you are weighing a single-fibre option lower down the range, <a href=\"https:\/\/optics.carritech.com\/it\/bidi-transceivers\/\">the strand arithmetic for BiDi<\/a> runs the same way in reverse.<\/p>\n\n\n\n<h2 id=\"h-before-you-order-a-checklist-for-choosing-400g-optics\" class=\"wp-block-heading\">Before you order: a checklist for choosing 400G optics<\/h2>\n\n\n\n<ol class=\"wp-block-list\">\n<li><strong>Establish what is in the ground first.<\/strong> Fibre type, strand count, connector type, and whether the trunk is MPO-12 or MPO-16. This constraint overrides preference every time.<\/li>\n\n\n\n<li><strong>Then pick the shortest reach that clears the distance.<\/strong> Over-reaching is the single most common error in choosing 400G optics \u2014 allow margin for patching and future re-routes, but do not reach for the longest part on the shelf.<\/li>\n\n\n\n<li><strong>Check the lane structure, not the letters.<\/strong> Four lanes or eight, parallel or WDM, and which document the part is built to.<\/li>\n\n\n\n<li><strong>Decide about breakout now.<\/strong> If the port might become four 100G links later, that decision belongs at purchase, because it rules out every duplex WDM option.<\/li>\n\n\n\n<li><strong>Check the part number against what your platform actually accepts<\/strong> before you commit to a quantity.<\/li>\n<\/ol>\n\n\n\n<p class=\"wp-block-paragraph\">You can check any 400G part number against a compatible equivalent \u2014 form factor, reach, media and connector side by side \u2014 in <a href=\"https:\/\/optics.carritech.com\/it\/optical-transceivers\/compatibility-check\/\">the compatibility checker<\/a>. If the list you are working from is a quote or a bill of materials rather than a single part, <a href=\"https:\/\/optics.carritech.com\/it\/quote-list-upload\/\">send us the list<\/a> and we will price it line by line.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Reference material worth keeping: the <a href=\"https:\/\/www.thefoa.org\/tech\/ref\/testing\/test\/testing.html\">Associazione per la fibra ottica<\/a> on fibre testing and loss budgets, and the <a href=\"https:\/\/www.tiafotc.org\/ieee-802-3-ethernet-standards-update\/\">TIA Fiber Optics Tech Consortium<\/a> standards tables, which track which IEEE amendment added what.<\/p>","protected":false},"excerpt":{"rendered":"<p>Choosing 400G optics looks like a distance problem. You know how far the link has to go, you pick the part that reaches that far, and you order it. It is not a distance problem. It is a fibre problem wearing a distance problem\u2019s clothes \u2014 because the four letters on the end of the [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":101633,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[227,171],"tags":[],"class_list":["post-101631","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-400g","category-optical-transceivers"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v28.2 (Yoast SEO v28.2) - https:\/\/yoast.com\/product\/yoast-seo-premium-wordpress\/ -->\n<title>Choosing 400G Optics: SR vs DR vs FR vs LR<\/title>\n<meta name=\"description\" content=\"Choosing 400G optics: the reach ladder from SR4 to ER8, what each one costs you in fibre strands, which parts break out \u2014 and the names IEEE never defined.\" \/>\n<meta name=\"robots\" content=\"index, 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