How to Choose the Right 10G SFP+ Module: SR, LR, ER or a DAC Cable

SR / LR / ER comparison table

| Type | Wavelength | Fiber type | Max distance | Approx. price | Use when | | --- | --- | --- | --- | --- | --- | | 10GBASE-SR | 850 nm | Multimode (OM3/OM4) | 300 m (OM3) / 400 m (OM4) | about 590 THB each | Linking within a building, or short runs between floors | | 10GBASE-LR | 1310 nm | Single-mode (G.652) | 10 km | about 890 THB each | Linking between buildings on the same campus | | 10GBASE-ER | 1550 nm | Single-mode (G.652) | 40 km | about 1,890 THB each | Linking between sites, or to an off-site data centre |

The prices above are per module. One link needs two modules of the same type (one at each end), and they should be the same brand, or coded to match the switch at each end, to reduce compatibility problems.

Real distances on OM3 and OM4 fiber

The two multimode grades used with SR modules differ in quality:

• OM3 (aqua jacket) — carries 10G up to 300 metres; it is the multimode fiber most often found in buildings cabled since the Gigabit era
• OM4 (an aqua jacket too, but with higher bandwidth) — carries 10G up to 400 metres; suited to links running close to the OM3 limit, or to buildings that have just been recabled

If you are not sure whether the fiber you have is OM3 or OM4, read the print on the jacket, or measure the received power (Rx Power) after installation and compare it with the figure in the spec. If it is below spec even though the run is under 300 m, the cause is usually a dirty connector or an over-tight bend, not the module.

DAC versus fiber modules inside the same rack

When two switches sit in the same rack or in adjacent racks (roughly 3–5 metres apart at most), a DAC (Direct Attach Copper) cable is the better value:

• No need to buy two modules plus a separate fiber patch cord, because a DAC has the SFP+ ends built into a single cable
• It is a passive cable, drawing less power than optical modules and adding no delay from converting the signal to light
• There is no risk of a dirty fiber connector as there is with glass

The limitation is that DAC is fairly stiff copper cable, unsuited to running between floors or along a route with tight bends, and its practical length for 10G is capped at 3–5 metres. It therefore suits links inside a rack or to a neighbouring rack only. Beyond that distance you have to move to SR modules with fiber instead.

Common mistakes

1. Plugging an SR module into single-mode fiber — the link will not come up, or the Rx Power reads far below normal, because the core size and the beam spread do not match what the module was designed for. Change to LR or ER
2. Using LR/ER with multimode fiber — the opposite of the first: light from a 1310/1550 nm laser spreads out in the larger multimode core, distorting the signal over distance
3. Mismatched types at the two ends, for example SR at one end and LR at the other — the wavelengths do not match and the link will not come up at all
4. Getting the distance wrong, for example assuming 250 metres is comfortably within OM3 while forgetting the spare cable coiled in the racks, which pushes the real length past 300 metres

If you are not sure which type to choose, send us the distance and the fiber type you have on LINE and we will advise you before you order, free of charge. Or look up your switch model in the find parts by device model tool