400G and 800G short-reach optics mostly use parallel fibre: several lanes, each on its own fibre pair, carried by an MPO connector. 400GBASE-DR4 and 400GBASE-SR4.2 use 8 fibres on an MPO-12. 400GBASE-SR8 and eight-lane 800G optics use 16 fibres, usually on an MPO-16. Base-8 and base-16 trunks match these counts without leaving fibres unused.
Parallel optics versus duplex optics
A duplex optic sends all of its traffic over one fibre pair. To reach 400G on two fibres it combines several wavelengths: 400GBASE-FR4, for example, multiplexes four wavelengths onto a duplex single-mode link with LC connectors. The cabling stays simple and the complexity sits in the optic.
A parallel optic gives each lane its own fibre pair. 400GBASE-DR4 runs four 100G lanes on four transmit fibres and four receive fibres. The optic is simpler, the fibre count is higher, and the connector is an MPO. Parallel optics have one further advantage: a single port can be split into several lower-speed links, which is useful when a 400G switch port feeds 100G servers or older switches.
Common 400G and 800G interfaces
| Interface | Fibre | Lanes | Fibres used | Connector | Reach |
|---|---|---|---|---|---|
| 100GBASE-SR4 | Multimode | 4 x 25G | 8 | MPO-12 | 70 m OM3, 100 m OM4 |
| 400GBASE-SR8 | Multimode | 8 x 50G | 16 | MPO-16 | 70 m OM3, 100 m OM4 and OM5 |
| 400GBASE-SR4.2 | Multimode | 4 fibre pairs, 2 wavelengths each | 8 | MPO-12 | 70 m OM3, 100 m OM4, 150 m OM5 |
| 400GBASE-DR4 | Single-mode | 4 x 100G | 8 | MPO-12 APC | 500 m |
| 400GBASE-FR4 | Single-mode | 4 wavelengths on one pair | 2 | Duplex LC | 2 km |
| 800G eight-lane single-mode (DR8 type) | Single-mode | 8 x 100G | 16 | MPO-16 APC, or two MPO-12 APC | 500 m for DR8 |
| 800G eight-lane multimode (SR8 type) | Multimode | 8 x 100G | 16 | MPO-16 | Check the module data sheet |
Reaches for the 100G and 400G rows are the IEEE 802.3 figures. 800G modules come in several form factors and connector options, and some twin-port designs present two separate 400G interfaces. Always confirm the connector, fibre count and reach against the data sheet of the exact module before ordering cabling.
MPO-12 versus MPO-16
Four-lane optics (SR4, DR4, SR4.2) use eight fibres. They sit in an MPO-12 connector with positions 1 to 4 transmitting, 9 to 12 receiving and the middle four unused. Eight-lane optics (SR8, DR8) use sixteen fibres, which fit a single row of an MPO-16 connector.
MPO-16 connectors use an offset key, so they cannot be plugged into MPO-12 adapters or mated with MPO-12 cords. A site that runs both four-lane and eight-lane optics needs either two families of cabling or conversion cords and modules between them. Decide this at design stage, because changing the connector family later means replacing cords and cassettes. For single-mode parallel optics, both families are usually APC polished, and APC connectors must mate only with APC.
Base-8, base-12 and base-16 cabling
"Base" describes the fibre count of the building block in a trunk and cassette system. It decides whether every fibre in a trunk can be used by the optics you plan to install.
| System | Connector | Best match | Note |
|---|---|---|---|
| Base-8 | MPO-8 (MPO-12 ferrule, 8 fibres) | Four-lane optics: SR4, DR4, SR4.2 | Every fibre is used; one MPO per transceiver |
| Base-12 | MPO-12 | Duplex links via cassettes | With four-lane optics, a third of the fibres stay dark unless conversion modules are used |
| Base-16 | MPO-16 | Eight-lane optics: SR8, DR8 | One MPO per eight-lane transceiver |
| Base-24 | MPO-24 | Dense trunking | Used with conversion modules, for example one 24-fibre trunk to three 8-fibre ports |
Many existing data centres are base-12. They can run four-lane optics without new trunks, either accepting the dark fibres or adding conversion modules. New builds aimed at 400G and 800G more often choose base-8 or base-16. Radix makes MTP/MPO trunk cables in the fibre counts and MPO types your design calls for.
Breakout: running one port as several links
Parallel optics let a high-speed port run as several independent lower-speed links through a breakout cable or harness. Common examples:
- 400GBASE-DR4 to four 100G single-lane single-mode links on duplex LC, using an MPO-12 APC to four LC duplex harness.
- 100GBASE-SR4 to four 25GBASE-SR links on multimode, using an MPO to four LC duplex harness.
- 40GBASE-SR4 to four 10GBASE-SR links, the older form of the same arrangement.
The switch and the optic must both support breakout mode, and the lane-to-leg mapping of the harness must match the module. State the module part number in the order so the cable maker can confirm the mapping. See MPO breakout cables for the harness types we build.
Loss and polarity at 400G and 800G
Short-reach multimode interfaces at these speeds have small loss limits. IEEE 802.3 sets 1.9 dB for 100GBASE-SR4 over 100 m of OM4, with 1.5 dB of that allocated to connections, and the multimode 400G interfaces are similarly tight. At the TIA-568 allowance of 0.75 dB per mated pair, two MPO pairs use the whole connection allocation. Designs with cassettes or cross-connects therefore need low-loss MPO connectors with a stated maximum per pair.
Polarity must reverse the fibre order once end to end, so that each transmit fibre reaches a receive fibre. A Type B cord does this for a direct link. Through trunks, follow one of the TIA-568 methods described in our MPO polarity guide, and remember that key-up to key-up adapters cannot be used with APC single-mode connectors.
Planning cabling for upgrades
- Choose trunk fibre counts that divide by 8, 12 and 16, such as 48, 96 or 144, so the trunk can serve any building block later.
- Decide early whether eight-lane optics are likely, and choose MPO-16 or a clear conversion plan if they are.
- Pick one polarity method for the site and keep it.
- For single-mode parallel optics, use APC MPO connectors throughout.
- Specify low-loss connectors where the channel has more than two mated pairs.
- Leave panel space and pathway capacity for higher fibre counts.
Send your switch and module list with your design when you request a quote, and we will confirm fibre counts, MPO types and polarity on the drawing.
Questions buyers ask
Does 400G need MPO-16 connectors?
Only for eight-lane optics such as 400GBASE-SR8. 400GBASE-DR4 and 400GBASE-SR4.2 use eight fibres on an MPO-12 connector, and 400GBASE-FR4 uses duplex LC. Check the module data sheet for the exact connector.
Can I use my existing MPO-12 trunks for 400G?
Yes, for four-lane optics such as DR4 and SR4.2, which use eight of the twelve fibres. Eight-lane optics need sixteen fibres, so on MPO-12 cabling they require conversion cords or modules, or two MPO-12 connections where the module supports it.
Can an MPO-16 cable plug into an MPO-12 port?
No. MPO-16 connectors use an offset key and a sixteen-fibre layout, so they do not mate with MPO-12 adapters or ports. Use a conversion cord or module designed to join the two.
What polarity do 400G parallel optics need?
The link must reverse the fibre order once end to end, so transmit fibres reach receive fibres. A Type B MPO cord does this for a direct transceiver-to-transceiver link. For channels through trunks, follow a TIA-568 method consistently.
Is OM5 needed for 400G?
Not usually. OM5 extends reach only for multi-wavelength optics, such as 400GBASE-SR4.2, which reaches 150 m on OM5 against 100 m on OM4. For 400GBASE-SR8, OM4 and OM5 give the same 100 m.
What is base-8 cabling?
Base-8 is a trunk and cassette system built in blocks of eight fibres, with MPO connectors that carry eight fibres each. It matches four-lane parallel optics exactly, so no fibres are left unused in the trunk.