40G QSFP+ Selection Notes: SR4 and LR4, and What to Confirm

Selecting a 40G QSFP+ module looks simple from the catalogue: a family of titles, each named for its reach or its variant, and a buyer who picks the name that matches the link. In practice the selection needs more care, because the titles in a family do not all carry the same evidence, and the name of a title is not a specification. This note works through the 40G QSFP+ selection honestly, starting from the one model with published figures and explaining what to confirm for the titles without them.

The model with published figures is the 40G QSFP+ SR4, and its detail page states a 40Gb/s rate, an 850nm wavelength, 100m and 300m reach options over multimode fiber, power consumption below 3.5W and a 0 to 70 degree Celsius operating range, with the 300m option belonging to the CSR4 model. Those figures describe a specific job: 40G over multimode fiber inside a building or across a data-centre floor, where the fiber plant is multimode and the links are short. A buyer whose plant is multimode and whose links fit within 100m, or within 300m for the CSR4 model, has a complete answer for that job, and the SR4 is the right selection.

The LR4 title is a different situation, and this note states that plainly. The 40G QSFP+ LR4 appears in the catalogue as a series-level title, which means its name is published but no performance figures have been published for it on this site. The name suggests a long-reach single-mode variant, by analogy with the naming pattern of other families, but this site does not convert that suggestion into a specification: no wavelength, no reach, no fiber type and no power figure is stated for the LR4 title here, because none has been published for it. A buyer who needs the LR4 title should state it by name during the inquiry, together with the link it will serve, so the answer can come from the published specification document for that title.

That distinction — a model with figures against a title without them — is the whole selection method for this family. It applies to the ER4 title as well, which is also listed at series level without published figures, and to the 25G SFP28 title recorded on the same category page, which is also listed without figures. The method is: where figures exist, select on figures; where figures do not exist, select on the title and confirm the figures during the inquiry. Selecting on a name alone, without confirming the figures, is how a buyer orders a long-reach module for a short link, or a multimode module for a single-mode plant, and both mistakes produce failed links.

The plant question decides the family before the title question decides the model. A multimode plant calls for a multimode module, and within this family that is the SR4, whose figures are published. A single-mode plant calls for a single-mode module, and within this family that means a title whose figures must be confirmed during the inquiry, because the single-mode titles in this family are listed at series level. A buyer should therefore state the plant first — multimode or single-mode, and the link length — and let the plant narrow the choice before the title is picked.

The connector question follows the plant question. The SR4 uses an MPO connector, because a 40G parallel-optics module sends four lanes over a multi-fiber connector, and the MPO connector is what carries those lanes in one plug. A buyer planning an SR4 deployment should check that the fiber plant and the patch panels support MPO connections, or plan the conversion as part of the project, because an SR4 module cannot be plugged into a duplex LC plant without a conversion strategy. The connector for a series-level title should be confirmed during the inquiry, together with its figures.

Before ordering, confirm quantity, price and delivery time during the inquiry, together with the plant type, the link length and the title by name. Certificates are available upon request. Where a figure is needed that this site does not publish, ask for it during the inquiry, and the answer will come from the published specification document rather than from an estimate.

MPO polarity is the detail that most often defeats a 40G SR4 deployment, and it deserves a paragraph because it is easy to overlook and hard to diagnose. An SR4 module sends four lanes in parallel over a multi-fiber connector, and the mapping of transmit fibers at one end to receive fibers at the other end — the polarity — must be correct, or the link will not come up. There are three standard polarity methods, differing in how the fibers are ordered and whether the trunk cable is straight or crossed, and the wrong method produces a link where transmit meets transmit. A buyer planning an SR4 deployment should state the polarity method the installation uses, or state the equipment at each end so the method can be confirmed, because polarity is not a property of the module alone.

The migration path from 40G to 100G is the strategic question behind many 40G selections, and it should be asked at selection time rather than at upgrade time. A site deploying 40G today will likely deploy 100G tomorrow, and the choices made today — the fiber plant, the patch panels, the connector types, the spares holding — either ease that migration or obstruct it. A multimode plant built for 40G SR4, for example, will not necessarily carry 100G at the same distance, because higher speeds over multimode fiber reach less far. A single-mode plant, by contrast, carries both speeds with only a module change. A buyer selecting 40G should ask what the plant will carry at 100G, and whether the 40G choice constrains the 100G choice, before committing.

Power and cooling budgets for a 40G row are the third planning question, and they follow from the published figures. The SR4 model states power consumption below 3.5W, and a row of 40G ports multiplies that figure by the port count, plus the switch's own consumption, to give the cooling load. That load must fit the cooling available in the row, and a row planned for 10G will not necessarily cool a 40G deployment, because the per-port power is higher and the port density may be higher too. A buyer planning a 40G row should work out the cooling load before ordering, because a row that cannot be cooled cannot be populated, however good the modules are. Before ordering, confirm quantity, price and delivery time during the inquiry. Certificates are available upon request.