SFP carries one lane over two fibres on duplex LC, QSFP carries four lanes and often uses an MPO-12, and QSFP-DD and OSFP carry eight lanes for 400G and 800G. The cable depends on the optical type, such as SR4, DR4 or LR4, not on the form factor. Choose the port, then the module, then the cable.
Cage, module and cable are three separate choices
SFP, QSFP and OSFP are names for the physical shape of a pluggable transceiver and of the cage on the switch or network card that receives it. They say how big the module is and how many electrical lanes it can carry. They do not say how far the signal goes or what cable it needs. That is set by the optical type inside the module, such as SR, DR, FR or LR, and each optical type needs a particular fibre and connector.
So a purchase has three layers, and each one has its own question.
- Cage. Which form factor does the port accept, and at what speed does the host run it?
- Module. Which optical type is plugged in: multimode short reach, or single-mode at 500 m, 2 km or 10 km?
- Cable. Which connector, fibre type, polarity and length does that module need?
Buyers who start from the cable and work backwards to the switch tend to make wrong orders. Start from the port, then the module, then the cable. The sections below follow that order. The cable side is built by Radix Fibrotech to the buyer's drawing, from fibre optic patch cords for duplex ports to MTP and MPO cables for parallel ones.
The SFP family: one lane, two fibres
An SFP port has a single transmit lane and a single receive lane, which means two fibres and a duplex LC connector for the common optical types. Each generation raised the lane speed and kept the cage size.
| Name | Lanes | Line speed | Typical multimode optic | Typical single-mode optic |
|---|---|---|---|---|
| SFP | 1 | 1 Gb/s | 1000BASE-SX, 550 m on OM2 | 1000BASE-LX, 10 km |
| SFP+ | 1 | 10 Gb/s | 10GBASE-SR, 300 m on OM3, 400 m on OM4 | 10GBASE-LR, 10 km |
| SFP28 | 1 | 25 Gb/s | 25GBASE-SR, 70 m on OM3, 100 m on OM4 | 25GBASE-LR, 10 km |
| SFP56 | 1 | 50 Gb/s (PAM4) | Short-reach optics from vendors | Vendor and MSA optics |
| SFP-DD | 2 | Up to 100 Gb/s | Vendor and MSA optics | Vendor and MSA optics |
The reach figures are the IEEE 802.3 values for the named optical type on the fibre grade shown. A module that is not an IEEE type, such as a vendor extended-reach part, has its own data sheet. The wider table of Ethernet reaches is in the IEEE 802.3 fibre Ethernet reach table.
An SFP cage is built to accept lower generations of module in the same shape, and most hosts run an SFP+ module in an SFP28 cage at 10 Gb/s. Whether a given switch accepts a given module is a host matter, because the host can restrict the modules it accepts. Check the switch data sheet.
The QSFP family: four lanes, usually a parallel cable
QSFP stands for quad small form-factor pluggable, and the four refers to lanes. A QSFP28 module carries four lanes of 25 Gb/s for 100 Gb/s in total. The module may combine the four lanes onto one fibre pair with wavelength division multiplexing, or send each lane down its own fibre pair, which is called parallel optics. That choice decides the connector.
| Name | Lanes | Total speed | Lane rate | Common optical types |
|---|---|---|---|---|
| QSFP+ | 4 | 40 Gb/s | 10 Gb/s | 40GBASE-SR4 (MPO-12), 40GBASE-LR4 (duplex LC) |
| QSFP28 | 4 | 100 Gb/s | 25 Gb/s | 100GBASE-SR4 (MPO-12), 100GBASE-LR4 (duplex LC), 100G-CWDM4 (duplex LC), 100G-PSM4 (MPO-12) |
| QSFP56 | 4 | 200 Gb/s | 50 Gb/s PAM4 | 200GBASE-SR4 (MPO-12), 200GBASE-DR4 (MPO-12), FR4 (duplex LC) |
| QSFP-DD | 8 | 400 Gb/s | 50 Gb/s PAM4 | 400GBASE-SR8 (MPO-16), 400GBASE-DR4 (MPO-12), 400GBASE-FR4 (duplex LC) |
| QSFP-DD800 | 8 | 800 Gb/s | 100 Gb/s PAM4 | 800G SR8 and DR8 types (MPO-16 or dual MPO-12) |
Look at the connector column. "SR4" means four parallel lanes and an MPO-12 connector, with 8 of the 12 fibres in use. "LR4" and "CWDM4" mean four wavelengths on two fibres and a duplex LC. Two modules with the same shape and speed therefore need completely different cables, which is why the optical type belongs on the order. The difference between the two designs is explained in the guide to parallel optics versus WDM duplex, and the 100G cases in 100G SR4, PSM4 and CWDM4 cabling.
QSFP-DD and OSFP at 400G and 800G
Both forms carry eight electrical lanes, and both are used for 400 Gb/s (eight lanes at 50 Gb/s) and 800 Gb/s (eight lanes at 100 Gb/s). They differ in size, cooling and compatibility.
| Point | QSFP-DD | OSFP |
|---|---|---|
| Lanes | 8 | 8 |
| Fits QSFP28 and QSFP+ modules | Yes, by design, so one cage can take older QSFP modules | No, it is a different shape. Adapters exist for some uses, so check the vendor. |
| Size and heat | Compact, kept close to QSFP size for port density | Larger, with room for a heat sink on the module and a higher power envelope. The original specification targeted up to 15 W per module; check the current MSA. |
| Typical home | Switches that must keep older 100G ports | New 800G designs and some 400G switches |
| Cables at the front | Same optical types: SR8 on MPO-16, DR4 on MPO-12 APC, FR4 on duplex LC | Same optical types |
The cable does not depend on whether the module is QSFP-DD or OSFP. It depends on the optical type, so a 400GBASE-DR4 module needs an MPO-12 APC single-mode cable in either shape. The optic and cabling detail is covered in the guides to 400G SR8, DR4 and FR4 cabling and the 800G OSFP and QSFP-DD cabling guide. For the 16-fibre connector used by SR8, read the MPO-16 cable assemblies page.
Which cable each module needs
Use this table when you have a module list and need a cable list. The fibre count is the number of fibres in use, which can be fewer than the number in the connector.
| Module type | Fibre | Connector at module | Fibres used | IEEE reach |
|---|---|---|---|---|
| 10GBASE-SR (SFP+) | OM3 or OM4 | Duplex LC | 2 | 300 m OM3, 400 m OM4 |
| 25GBASE-SR (SFP28) | OM3 or OM4 | Duplex LC | 2 | 70 m OM3, 100 m OM4 |
| 40GBASE-SR4 (QSFP+) | OM3 or OM4 | MPO-12 | 8 | 100 m OM3, 150 m OM4 |
| 100GBASE-SR4 (QSFP28) | OM3 or OM4 | MPO-12 | 8 | 70 m OM3, 100 m OM4 |
| 100GBASE-LR4 (QSFP28) | OS2 | Duplex LC | 2 | 10 km |
| 400GBASE-SR8 (QSFP-DD, OSFP) | OM3 or OM4 | MPO-16 | 16 | 70 m OM3, 100 m OM4 |
| 400GBASE-DR4 (QSFP-DD, OSFP) | OS2 | MPO-12 APC | 8 | 500 m |
| 400GBASE-FR4 (QSFP-DD, OSFP) | OS2 | Duplex LC | 2 | 2 km |
OM3 and OM4 reaches shown here apply to the standard link models in IEEE 802.3, which also budget for connectors and fibre. A link with many mated pairs may need to be checked with the fibre loss budget method. Single-mode parallel types use angled-polish connectors, and mixing flat and angled polish at any mating damages both, as explained in the guide to APC to UPC mating mistakes.
Worked example: breaking out a 100G port into four 25G links
A common design uses a leaf switch with 100G QSFP28 ports and 25G servers. Each QSFP28 port is broken out into four 25G links with an MPO-to-LC breakout cable. The numbers below are assumed for the example.
| Step | Working | Result |
|---|---|---|
| Switch ports broken out | 32 QSFP28 SR4 ports | 32 breakout cables |
| 25G server links | 32 x 4 | 128 links |
| Fibres carried per breakout | 4 duplex legs x 2 fibres | 8 fibres |
| Fibres used in total | 32 x 8 | 256 fibres |
| Fibre positions in MPO-12 trunks | 32 x 12 | 384 positions |
| Positions unused | 384 - 256 | 128, which is one third |
| Longest server run | Assumed 40 m including slack | Within 100 m OM4 |
One third of the fibre positions lie idle, because SR4 uses 8 of the 12. Eight-fibre harnesses remove that waste, and the choice is covered on the MPO-8 to LC breakout cables page. Whichever form is used, each leg is an LC duplex pair, and the transmit and receive positions must match the MPO polarity method, as in the MPO polarity guide. The server side is an SFP28 module running 25GBASE-SR, and the leg length decides the fan-out: see MPO fanout lengths and leg types.
Direct attach and active optical cables as alternatives
Every pluggable port also accepts cables that have the module built in. A direct attach copper (DAC) cable carries the electrical lanes over twinax copper for a few metres, and an active optical cable (AOC) has the optics fixed to a fibre. Both are cheaper to buy than a pair of modules and a patch cord, and both lock you to a fixed length. The trade-off is in the guide to active optical cable vs direct attach copper.
As a rule of thumb, copper DAC suits runs inside a rack or to the next rack, AOC suits runs up to tens of metres where weight and bend matter, and separate modules with fibre patch cords suit anything that passes through a patch panel, is likely to be rerouted, or goes past the reach of an AOC. Structured cabling through a panel always uses modules and fibre cords or trunks.
Choosing between SFP, QSFP and OSFP: decision steps
- Write down the port speed on the host, from the switch or network card data sheet, and the form factor of the cage.
- Write down the distance of each link, including slack. Group the links under 100 m, under 500 m, under 2 km and over that.
- Pick the optical type for each group: SR for the shortest, DR or FR for the middle, LR for the longest.
- From the optical type, read off the connector and the number of fibres. Use the table above.
- Decide whether any port is broken out to slower links, and which end the MPO is on.
- Choose the fibre grade (OM3, OM4, OM5 or OS2) and the polish. Single-mode parallel links normally take angled polish at the MPO.
- Fix the polarity method for the whole channel, from module to module.
- Check whether the host accepts third-party modules, and write the exact module part number on the cable drawing.
- Send the drawing for a quote, with lengths and labelling.
Common mistakes
| Mistake | Result | Remedy |
|---|---|---|
| Buying a "QSFP cable" without the optical type | An MPO cable arrives for a duplex-LC module, or the other way round | Name the optic: SR4, DR4, LR4, CWDM4, SR8, FR4 |
| Assuming QSFP-DD and OSFP modules swap | They are different shapes | Match the cage to the module form factor |
| Treating all 12-fibre MPO links as the same | A DR4 link on a multimode cable, or a flat-polish MPO on an angled-polish module | Match fibre grade and polish to the module |
| Ignoring polarity on parallel optics | Transmit lanes land on transmit lanes and the link stays dark | Fix the polarity method before ordering trunks |
| Using OM3 for a 100 m SR4 plan | The IEEE reach on OM3 is 70 m for 100GBASE-SR4 | Use OM4, or shorten the link |
| Forgetting host lock-in | The switch rejects a third-party module | Check the vendor policy and the module compatibility list |
Radix Fibrotech makes fibre assemblies to the buyer's drawing in Pune, covering lengths, fibre counts, polarity, breakout and marking. For a mixed order with LC cords, MPO trunks and breakouts, send the module list and link lengths through the quote form and the reply covers the cable list. The data centre side is on the data centre page.
Questions buyers ask
What is the difference between SFP and QSFP?
An SFP module has one lane in each direction, so it needs two fibres, and it runs from 1 Gb/s to 25 Gb/s or more depending on generation. A QSFP module has four lanes, so it carries 40 Gb/s to 200 Gb/s on four times the lane count. QSFP is larger and often uses a multi-fibre MPO connector.
Can I plug an SFP module into a QSFP port?
Not directly, because the cages differ in size. Some switches accept a QSFP-to-SFP adapter, which presents one lane of the QSFP port to an SFP module. This depends on the host and the adapter, so check both data sheets before you rely on it.
Is QSFP-DD compatible with QSFP28?
QSFP-DD was designed so that a QSFP-DD cage also accepts QSFP28 and QSFP+ modules, which lets a switch keep older optics. The port then runs at the speed of the module installed. OSFP is a different shape and does not accept QSFP modules.
What is the difference between QSFP-DD and OSFP?
Both have eight lanes and support 400 Gb/s and 800 Gb/s. QSFP-DD is smaller and accepts older QSFP modules. OSFP is larger and has a bigger thermal allowance, so it suits higher power modules. Cabling does not differ between them, because the optical type sets the cable.
Which fibre does a 100G QSFP28 module need?
It depends on the optic. 100GBASE-SR4 needs multimode OM3 or OM4 on an MPO-12 connector, with reach of 70 m on OM3 and 100 m on OM4. 100GBASE-LR4 and 100G-CWDM4 need single-mode OS2 on duplex LC. 100G-PSM4 needs single-mode on an MPO-12 with angled polish.
How many fibres does a 400G module use?
It depends on the type. 400GBASE-SR8 uses 16 fibres on an MPO-16 connector, 400GBASE-DR4 uses 8 fibres of an MPO-12, and 400GBASE-FR4 uses 2 fibres on a duplex LC. Check the module data sheet and the connector style before ordering the cable.
Can one QSFP port feed four SFP ports?
Yes, in breakout mode. A 100G QSFP28 SR4 port is split into four 25G SFP28 links with an MPO-12 to four-duplex-LC breakout cable, and a 40G QSFP+ port into four 10G SFP+ links. The host must support the breakout mode, and the cable polarity must match.
Do I need OM4 for 25G and 100G multimode?
IEEE reach figures are 70 m on OM3 and 100 m on OM4 for both 25GBASE-SR and 100GBASE-SR4. OM4 gives the extra 30 m and some loss margin. For runs inside a row, OM3 can be enough, and the choice is explained in the guide to OM3 versus OM4 for 25G.