10GBASE-SR Optics: Short-Reach Multimode Explained

What SR means

SR stands for short reach, and in 10G optics it means a multimode module operating at 850nm, the wavelength of the vertical-cavity lasers used for short links. Short reach is a relative term: it is short compared with single-mode modules that reach kilometres, but it comfortably covers in-building links, floor-to-floor runs and data center rows. The defining feature of an SR module is not a single distance figure but the combination of multimode fibre, 850nm operation and a reach that depends on the fibre grade installed.

That dependence on fibre grade is the single most important fact about SR optics. The same module reaches different distances on different grades of multimode fibre, because the grade decides how far the 850nm signal stays usable. A buyer who quotes an SR distance without naming the fibre grade has not specified the link, and any quotation built on an unnamed grade is built on an assumption that may not match the installed fibre.

Fibre grades and what they change

Multimode fibre grades are distinguished by their bandwidth and by the distance they support at a given wavelength, and the grades most often encountered in existing buildings are OM3 and OM4, with older OM1 and OM2 installations still present in some premises. A newer grade supports a longer reach for the same module, which means that two buildings with the same module can have different maximum link lengths purely because their installed fibre differs. The practical step is to identify the installed grade before ordering, either from the installation record or from the fibre markings, and to state it in the enquiry.

Where the installed grade is unknown, the conservative route is to measure or to plan for the shorter reach, or to ask the supplier to confirm the reach against each candidate grade so the order can be placed once the grade is known. Ordering on the longest published reach and discovering a lower grade at installation is the common failure mode for SR links, and it is avoidable by naming the grade in the first enquiry rather than discovering it at the rack.

Where SR fits and where it does not

SR fits in-building and data center links where multimode fibre is installed and the distance is within the multimode range for the installed grade. It fits top-of-rack to aggregation links, floor distribution links and short inter-row links, and it is usually the lowest-cost 10G option for those links because multimode fibre and vertical-cavity lasers cost less than single-mode equivalents. Cost is a genuine advantage here, but it is only an advantage when the distance fits; a link beyond the multimode range needs a different module family regardless of cost.

SR does not fit campus links, inter-building links or any link whose length exceeds the multimode range for the installed grade, and it does not fit links where only single-mode fibre is installed. In those cases the correct module is a single-mode module, such as the 10G single-mode module listed on this site, and the SR discussion is irrelevant. The fibre installed in the building decides the family before any module is chosen, which is why the first question in an SR enquiry should be about the fibre, not about the module.

SR titles on this site - a boundary note

Short-reach titles appear in the source material for this site at title level only, including 10G SR titles with distance and wavelength figures in the title text. Those titles have no detailed specification behind them on this site, so no SR product page exists here and no SR figure is published from a title. This article is therefore a guide to the SR idea, not a product page, and any SR requirement should be sent as an enquiry naming the rate, the installed fibre grade, the distance and the connector, so the figure can be confirmed per model.

The boundary matters because title figures are easy to mistake for specifications. A title that mentions a distance does not state the fibre grade that distance belongs to, the connector, the temperature range or the host compatibility, and a buyer who orders against a title figure is ordering against an incomplete specification. The enquiry route exists precisely for lines like this, where the idea is well understood but the specific module figure has not been published.

What to ask when buying SR optics

A complete SR enquiry names the rate, the installed fibre grade, the distance, the connector and the host platform, in that order. The fibre grade comes before the distance, because the distance is meaningless without it. The host platform matters because host acceptance is tested per module, and a module that matches optically can still be rejected by a host that does not accept it. State the platform when compatibility matters, and do not assume acceptance from the form factor.

Also state the quantity and the environment. Quantity affects the quotation, and environment matters because the listed temperature range for a module covers ordinary indoor conditions; links in unconditioned spaces fall outside that range and should be raised explicitly. An enquiry that covers rate, fibre grade, distance, connector, host, quantity and environment can usually be answered without a second round of questions, which is the fastest route to a confirmed order.

SR against LR - the short version

The SR against LR choice is really a fibre choice in disguise. SR means multimode at 850nm for short links, and LR means single-mode at 1310nm for longer links, and the installed fibre decides which family is even possible. A building with multimode fibre uses SR for its short links, a campus with single-mode fibre uses LR for its longer links, and a mixed estate uses both, with the boundary drawn by the fibre in each run rather than by preference.

Cost follows the same boundary. Multimode SR links cost less per link where they fit, because the fibre and the lasers are cheaper, but they do not fit where the distance exceeds the multimode range, and no cost saving justifies a link that does not reach. The correct question is never which family is cheaper in general, but which family reaches on the fibre that is actually installed, at the distance that is actually required.

Before you send the enquiry

A good enquiry for any topic in this article names the rate, the wavelength, the fibre type, the distance, the connector, the quantity and the application, plus the host platform when compatibility matters and any certification document needed by name. Each item removes a round of questions, and together they usually allow a quotation and a compatibility confirmation without further correspondence. An enquiry that names only a rate, or only a module-style name, will need at least one further round, because none of the other items can be inferred from the rate alone.

Environment and quantity also belong in the first message. Quantity affects the quotation and the delivery schedule, and environment matters because the listed temperature range for a module covers ordinary indoor conditions; links in unconditioned spaces, in roof voids or in outdoor enclosures fall outside that range and should be raised explicitly. Stating the environment when it is not an ordinary indoor data center or wiring closet lets the temperature grade be confirmed as part of the order rather than discovered as a failure in summer or winter.

Finally, check whether the line you need has a product page on this site or only a guide page. A product page carries a listed specification for a specific model, and an enquiry against it can confirm price, stock and compatibility for that model. A guide page explains the line and routes you to an enquiry, because no listed specification exists for that line here. Knowing which of the two you are reading prevents the most common buying mistake on this site, which is treating a guide figure, a title figure or a standard figure as if it were a specification for a specific module.

Frequently asked questions

How do I get a quotation?

Send an enquiry naming the model, the rate, the wavelength, the distance and the quantity you need, plus the application. Quotations are confirmed per order, and specifications are confirmed per model before payment. This site does not list fixed selling figures, and no price is shown for any model.

What certification documents can you provide?

The company states ISO 9001, CE, FC, 3C and RoHS names. Certificates are available upon request. If a buyer needs a certificate for a specific model, name the model in the enquiry so the correct document can be checked. No certificate numbers are published on this site.

Do the specifications on one page apply to the whole range?

No. Specifications are shown per model and must not be read across the whole range. Where a value is not listed for a model, the page says to confirm per model rather than filling in a generic figure, and where the source pages disagree, the page flags the conflict instead of choosing one value as fact.

What are the minimum order quantity and lead time?

At company level both are unavailable on this site and are confirmed per model in the enquiry. A single-model note, where one exists, is marked as applying to that model only and must not be read as a company-wide term.