100G-SR4 uses 8 multimode fibres on an MPO-12 connector, to 70 m on OM3 and 100 m on OM4. PSM4 uses 8 single-mode fibres on an angled MPO-12, to 500 m. CWDM4 sends four wavelengths over one single-mode pair on LC duplex, to 2 km. Match the cabling to the module, not to the form factor.
Why the same QSFP28 cage needs different cabling
Most 100G optics fit a QSFP28 cage, so on the front of a switch the modules look the same. Inside, they use different ways of carrying four lanes of 25 Gb/s. SR4 and PSM4 send each lane down its own fibre, which is parallel optics. CWDM4 puts four wavelengths on one fibre and is duplex. The cable plant follows from that choice: fibre count, fibre type, connector and polarity are all different.
The other 100G types you may meet, such as 100GBASE-LR4 (10 km on single-mode duplex with LC) and 100GBASE-DR (500 m on single-mode duplex), use the duplex plant described under CWDM4 and follow the same cabling rules for LC. The focus here is the three names that cause most ordering errors. For the wider view of how cabling changes as speeds rise, see the 400G and 800G parallel optics guide, and for the basics of parallel connectors, the MPO cable guide.
The three optics at a glance
| Item | 100G-SR4 | 100G PSM4 | 100G CWDM4 |
|---|---|---|---|
| Defined by | IEEE 802.3bm | Multi-source agreement (MSA) | Multi-source agreement (MSA) |
| Fibre type | Multimode: OM3, OM4, OM5 | Single-mode (OS2) | Single-mode (OS2) |
| Wavelength | 850 nm | 1310 nm | Four wavelengths near 1271, 1291, 1311 and 1331 nm |
| Fibres used | 8 (4 transmit, 4 receive) | 8 (4 transmit, 4 receive) | 2 (one pair) |
| Connector at the module | MPO-12 | MPO-12, angled (APC) | LC duplex |
| Reach | 70 m on OM3, 100 m on OM4 | 500 m | 2 km |
| Can break out to 4 x 25G | Yes, with a suitable module on the far side | Yes, to single-mode 25G, check vendor support | No, the wavelengths are multiplexed |
The reach figures are the standard or MSA values for the optic. A particular module may do more over good fibre, and vendors publish their own supported distances. The MSA names are used by several module makers, so a module label should be checked against the vendor's data sheet.
100G-SR4: multimode, MPO-12, 8 fibres
SR4 is the multimode choice for short links inside a data hall or between adjacent halls. It runs four lanes at 25 Gb/s on 850 nm VCSELs and uses eight fibres of a 12-fibre MPO connector. The four transmit lanes use fibre positions 1 to 4 and the four receive lanes use positions 9 to 12, when the connector is held key up. The middle four positions (5 to 8) carry no signal in a standard SR4 link. Cabling vendors sell the same MPO-12 cable for it as for other 12-fibre uses, and the unused fibres are simply spare.
Reach depends on the multimode grade. The IEEE standard sets 70 m on OM3 and 100 m on OM4. OM5 is specified as at least as good as OM4 for this optic. The fibre grades themselves are described in the OM3 vs OM4 vs OM5 guide. The channel insertion loss allowed by the standard is small, under 2 dB for the whole channel including connectors (check the clause for the exact figure for your grade), so the number of MPO connections in the path is the item to watch. See the MPO loss budget examples.
Polarity for SR4 is Type B in the TIA scheme: the key-up connector at one end maps to the key-up connector at the other, so that fibre 1 at one end goes to fibre 12 at the other. Transmit lanes then arrive at receive positions. The details are in the MPO polarity guide.
100G PSM4: single-mode, angled MPO-12, 8 fibres
PSM4 is the single-mode parallel option, defined by an MSA and not by IEEE. It uses four lanes at 25 Gb/s at 1310 nm, each on its own fibre, so eight fibres on an MPO-12, with the same lane positions as SR4. The reach is 500 m, which covers most of a campus or a large data hall. The point of PSM4 was to avoid the cost of wavelength multiplexing in the module, at the price of using eight fibres instead of two.
Single-mode parallel optics need an angled polish to keep reflections low, so PSM4 modules present an APC MPO interface. Cables for them are OS2 with APC MPO connectors at the module end, as in the OS2 MPO APC trunk cables. A UPC MPO will not mate with an APC module port without high loss and reflection. Confirm the polish on the data sheet before ordering, and read the notes in MPO APC vs MPO UPC.
Polarity follows the same Type B logic as SR4. Because the fibre is single-mode, loss per connector is the most important item in the budget, and the loss limits are set in the MSA and vendor documents. Ask the module vendor for the maximum channel loss, and do not assume the SR4 figure applies.
100G CWDM4: single-mode duplex with LC
CWDM4 multiplexes four 25 Gb/s lanes onto one fibre using four coarse wavelengths in the 1271 to 1331 nm range, spaced 20 nm apart. A second fibre carries the return. The result is a standard duplex single-mode link with LC connectors, which can run 2 km. The module is more complex than PSM4, but the cable plant is simple and uses the same duplex fibre as a 10G or 25G single-mode link.
For this reason CWDM4 is attractive where fibre is already installed: an existing single-mode LC duplex plant can often carry 100G with no recabling. The checks are fibre type (OS2, not OM), connector polish (UPC is usual for LC duplex single-mode modules, but follow the data sheet), and total loss against the module budget. The guide to single-mode vs multimode fibre explains why multimode plant cannot be reused.
CWDM4 cannot be broken out into four 25G channels with a passive cable, because the four lanes share one fibre as wavelengths. To feed four 25G ports from a single 100G port you need a parallel optic (SR4 or PSM4) and a breakout cable, such as the OM4 MPO to LC breakout cables for multimode.
Fibre count and utilisation
A big difference between the three is fibre use. Parallel optics use 8 fibres per link and duplex uses 2. In a trunk of 12 fibres, SR4 or PSM4 uses 8 of them and leaves 4 dark. Some designs use conversion harnesses to pack three 8-fibre links onto 24 fibres, which removes the dark fibres at the price of extra connections and loss (check the vendor design). The decision between parallel and duplex plants therefore depends on how many fibres the pathway can carry. When the pathway is full, duplex is more efficient. When the pathway is large and the reach is short, parallel optics save module cost.
| Plant | Fibres per 100G link | Links per 144-fibre cable |
|---|---|---|
| SR4 or PSM4 on MPO-12 | 8 of 12 fibres used, 12 allocated | 12 links if one MPO-12 per link |
| CWDM4 on LC duplex | 2 | 72 links |
The example in the next section shows the effect on a real design, using assumed values.
Worked example: a 60 m link, two ways
The values here are assumed. A switch in row A links to a switch in row C, with a route length of 60 m, via two patch panels, one at each end.
Option 1, SR4 on OM4. Cabling is an MPO-12 trunk of 60 m with a short MPO patch cord at each end. There are two mated MPO connector pairs in the channel at the panels, plus the transceiver ports. Assume 0.35 dB per mated MPO pair and 3.0 dB per km for OM4 at 850 nm. Fibre loss is 0.06 km x 3.0 = 0.18 dB. Connector loss is 2 x 0.35 = 0.70 dB. Total channel loss is about 0.88 dB, which is inside a sub-2 dB allowance with about 1 dB of margin. The reach of 60 m is inside the 100 m for OM4. It would not be inside the 70 m allowance for OM3 if the route grew to 80 m.
Option 2, CWDM4 on OS2. Cabling is an LC duplex link with two panels. Assume 0.3 dB per LC mated pair (two pairs) and 0.4 dB per km for fibre. Fibre loss is 0.06 km x 0.4 = 0.024 dB. Connector loss is 0.60 dB. Total is about 0.62 dB, which is inside any CWDM4 budget by a wide margin. The reach is not a limit.
| Item | SR4 on OM4 | CWDM4 on OS2 |
|---|---|---|
| Fibres used | 8 of 12 | 2 |
| Assumed channel loss | 0.88 dB | 0.62 dB |
| Margin to a 2 dB allowance | About 1.1 dB | About 1.4 dB |
| Headroom for upgrade to 400G | Via SR8 or DR4 with a different plan | Via FR4 or DR4 on single-mode |
| Module cost | Lower | Higher |
The cable works either way. The choice rests on module cost, fibre supply and the future path. The 2 dB figure in this example is a round number used only to show the margin. The module data sheet sets the true budget.
How to choose and specify: steps
- List the module type on each end by its full name and vendor part number, not by "100G" alone.
- Check the maximum link length against the module data sheet and the fibre grade you have.
- Decide parallel or duplex. For under 100 m with spare fibre, SR4 is common. For longer links or tight pathways, duplex single-mode.
- Write the connector and polish at the module end: MPO-12 flat for SR4, MPO-12 APC for PSM4, LC for CWDM4.
- Decide the gender of each MPO end. Transceiver ports are pinned, so the cord end is female. See MPO pinned vs unpinned.
- Set the polarity: Type B for straight parallel links, and verify it against the cassette or panel design.
- Calculate the loss of the whole channel against the module budget, with the number of mated pairs counted honestly.
- Specify the length from the route, with a service loop, and the jacket type for the pathway.
Common mistakes
| Mistake | What happens | Fix |
|---|---|---|
| Buying OM4 trunks for a PSM4 link | No link, since PSM4 is single-mode | Match the fibre to the optic: OM for SR4, OS2 for PSM4 and CWDM4 |
| UPC MPO to an APC module port | High loss and reflection, damaged faces | Use APC where the module calls for it |
| Assuming SR4 and PSM4 are interchangeable because the connector looks alike | Wrong fibre, wrong wavelength | Check the module name on both ends |
| Passing CWDM4 through a breakout cable | Four wavelengths cannot be split by a passive cord | Use parallel optics for breakout |
| Counting only fibre length in the budget | The MPO connections use most of the SR4 allowance | Add loss per mated pair for every panel and cassette |
| Mixing polarity types in one link | Transmit meets transmit, link is dead | Design the whole channel for one method |
| Ignoring the dark fibres in an MPO-12 | Wasted capacity, or unplanned use | Plan whether the spare four fibres stay dark |
Cables for these links
Radix Fibrotech makes MPO and MTP trunks, patch cords and breakout cables, and duplex single-mode and multimode patch cords, to the customer's drawing in Pune. The cable families that apply to this guide are MTP-12 OM4 trunk cables for SR4, OS2 MPO APC trunk cables for PSM4, and LC to LC OS2 duplex patch cords for CWDM4. The overview page for the whole family is MTP and MPO cables.
For an enquiry, give the module names, the fibre grade, the gender at each end, the polarity method, the length and the jacket. The request a quote page has a form, and the same details can be sent by phone or WhatsApp.
Questions buyers ask
How many fibres does 100G SR4 use?
Eight fibres on an MPO-12 connector: four transmit on positions 1 to 4 and four receive on positions 9 to 12. The four middle fibres carry no signal in a standard SR4 link. Reach is 70 m on OM3 and 100 m on OM4.
What is the difference between PSM4 and SR4?
Both use eight fibres on an MPO-12. SR4 is multimode at 850 nm and reaches 100 m on OM4. PSM4 is single-mode at 1310 nm, uses an angled (APC) connector and reaches 500 m. The two are not interchangeable on the same cable plant.
Does CWDM4 need MPO connectors?
No. CWDM4 uses a single-mode duplex pair with LC connectors, and reaches 2 km. The four wavelengths share one fibre, so only two fibres are needed. It suits existing single-mode LC plant.
Can I use multimode cable for 100G PSM4 or CWDM4?
No. Both are single-mode optics, so they need OS2 fibre. Multimode cable has too large a core and different wavelength behaviour, and the link will not work or will be far below the stated reach.
Which polarity does 100G SR4 need?
Type B, where the key-up connector at one end maps to the key-up connector at the other, so fibre 1 meets fibre 12 and the lanes line up from transmit to receive. Check the cassettes or panels, since they can add a flip that changes the overall method.
Can I break out a 100G SR4 port into four 25G ports?
Yes, with an MPO-12 to four LC duplex breakout cable and 25G SR modules on the far side, if the vendor supports that mode. CWDM4 cannot be broken out this way because it multiplexes four wavelengths on one fibre.
Do 100G optics need APC connectors?
Only some. PSM4 modules use an angled MPO interface. SR4 uses a flat multimode polish. CWDM4 modules normally use UPC LC. The data sheet of the module is the final authority.
What should I tell a supplier when I order cabling for a 100G link?
Give the module names on both ends, the fibre type, connector and polish, gender at each MPO end, polarity method, route length and jacket. Naming the optic avoids the mistake of buying cabling that fits the cage but not the signal.