A direct MPO trunk plugs straight into parallel-optic transceivers and adds no mated pair between them. A pass-through panel adds two MPO pairs and allows patching. An MPO cassette converts the trunk to duplex LC for SFP-type ports and adds both MPO and LC pairs. Choose by the module type, the loss budget and how often the links change.
The decision in one sentence
Use a direct MPO trunk to the transceiver when both ends of the link are parallel-optic modules and the cable run is fixed. Use an MPO cassette when the equipment ports are duplex LC and the fibre between racks is carried as MPO. Use a pass-through MPO adapter panel when you want patching flexibility with parallel optics but no conversion to LC. Everything else in this guide is how to test that sentence against your own links, with loss figures, polarity and upgrade paths.
The question matters because the three layouts have different numbers of connector pairs, and the loss budget of a 40G, 100G or 400G parallel link is very small, often under 2 dB. Each added MPO pair costs a large share of it. The background on the connector is in the MPO connector guide, and the family of cables is under MTP and MPO cables.
Three ways to cable one link
Picture a link between two switches in different rows. The three layouts use the same trunk cable but finish it differently.
| Layout | Path from switch A to switch B | Mated pairs in the channel | Fits |
|---|---|---|---|
| Direct trunk to transceiver | QSFP port, MPO trunk, QSFP port | None between the module interfaces | Fixed point-to-point links of 40G, 100G, 400G parallel optics |
| Pass-through MPO panels | QSFP port, MPO patch cord, panel, MPO trunk, panel, MPO patch cord, QSFP port | 2 MPO pairs (one at each panel) | Parallel optics where patching and moves are frequent |
| Cassettes to duplex LC | SFP port, LC patch cord, cassette, MPO trunk, cassette, LC patch cord, SFP port | 2 LC pairs and 2 MPO pairs | Duplex optics (10G, 25G, 100G LR) over MPO backbone |
A cassette is a small module that has an MPO adapter at the back and a row of LC duplex adapters at the front, with fibres wired between them inside. A 12-fibre cassette usually gives six duplex LC ports. A pass-through panel has MPO adapters on both sides and no wiring inside. The parallel-optic modules of 40GBASE-SR4 and 100GBASE-SR4 use fibre positions 1 to 4 for transmit and 9 to 12 for receive on a 12-fibre MPO, leaving the four middle positions idle. The detail is in the guide to 100G SR4, PSM4 and CWDM4 cabling and the guide to 40G SR4 and BiDi cabling.
Worked example 1: a 100 m parallel-optic link at 100G
Assumed values: OM4 trunk of 100 m, fibre attenuation 3.0 dB/km at 850 nm (the standards allow up to 3.5 dB/km), a 100GBASE-SR4 type channel with an insertion loss limit of about 1.9 dB, which should be checked against the clause and module data sheet for your exact equipment. Compare two MPO grades: a standard grade at 0.5 dB per mated pair, and a low-loss grade at 0.35 dB.
| Layout | Fibre loss | MPO pairs | Total, standard 0.5 dB | Total, low-loss 0.35 dB |
|---|---|---|---|---|
| Direct trunk | 0.30 dB | 0 | 0.30 dB | 0.30 dB |
| One pass-through panel at each end | 0.30 dB | 2 | 1.30 dB | 1.00 dB |
| Two panels at each end, cross-connect | 0.30 dB | 4 | 2.30 dB | 1.70 dB |
The result is plain. A direct trunk uses less than a fifth of the budget. A single pass-through panel at each end passes in either grade. A cross-connect with four MPO pairs fails with standard-grade connectors and passes with low-loss, though only just, with 0.2 dB of margin and none for dirt or ageing. The lesson is to count mated pairs before choosing the layout, and to ask for the insertion loss grade of the MPO connectors in the quote. The guide to MPO low-loss versus standard grades goes into the difference.
Worked example 2: a 10G duplex link over an MPO backbone
Now the same trunk feeds 10GBASE-SR optics with duplex LC ports at each end. Assumed values: OM4, 150 m, 3.0 dB/km fibre loss, LC pair 0.25 dB, MPO pair 0.35 dB. A commonly published limit for 10GBASE-SR on OM4 is about 2.9 dB at 400 m reach, which is the right ballpark for the check.
- Fibre: 0.15 km x 3.0 = 0.45 dB.
- Two LC pairs, one at each cassette front: 2 x 0.25 = 0.50 dB.
- Two MPO pairs, one at each cassette rear: 2 x 0.35 = 0.70 dB.
- Total: 0.45 + 0.50 + 0.70 = 1.65 dB.
That leaves over 1 dB of margin against 2.9 dB. The cassette layout looks costly in connector pairs, but a 10G channel has room for it. The same cassette layout under a 100G parallel optic would not fit, which is why cassettes are mostly a duplex-optics tool. If the trunk feeds a 40G port that is split into four 10G lanes, use an MPO-to-LC harness or a cassette that maps eight fibres to four duplex ports, see MPO-8 to LC breakout cables.
Where each layout wins
| Question | Direct trunk | Pass-through panel | Cassette |
|---|---|---|---|
| Loss budget | Best | Fair | Worst, but fine for 10G and 25G |
| Rack space | Least | Some | Most; one 1U panel holds several cassettes |
| Moves and changes | Pull and re-run | Re-patch at the panel | Re-patch with LC cords |
| Upgrade path | Trunk stays, change module | Trunk stays, change patch cord polarity | Cassette ties you to the LC map; swap for a new cassette |
| Testing | Needs MPO test gear | Needs MPO test gear | Test at the LC front with ordinary gear |
| Cleaning points | Two (module ends) | Six or more | Eight or more |
A layout is also a policy. Direct trunking says the link is permanent. A patch panel says the cable plant is stable and the equipment moves. A cassette says the equipment ports are duplex LC and the backbone is an infrastructure that outlives them. Choose the policy that matches how the room is run.
Polarity and gender traps specific to cassettes
Polarity is the main way these layouts go wrong. Under the TIA structured cabling scheme there are three methods, A, B and C, set by the trunk type and the cassette wiring. A cassette's internal map is fixed. A Method A cassette set pairs with a straight trunk and a flipped patch cord on one end. A Method B set pairs with a reversed trunk. Mixing cassette types from different makers is the usual cause of links that come up with transmit facing transmit. The guide to MPO polarity types A, B and C explains the three methods, and the guide to MPO polarity in cassettes shows the wiring maps.
For a pass-through layout the same methods apply, but there are no hidden wires to worry about. Polarity is set by the trunk and the two patch cords alone.
Gender is the second trap. Transceiver ports carry guide pins, so patch cords that plug into a module must be unpinned at that end. The cassette and panel adapters have their own conventions, which differ between makers, so check each interface rather than assume. The method is in the guide to MPO pinned versus unpinned.
Testing, cleaning and day-to-day running
More mated pairs means more places for dirt. Every MPO pair has twelve fibre faces that must be clean, and a single speck on one of them can stop one lane. The cleaning method is in the guide to how to clean MPO connectors. A cassette hides the MPO pair inside a panel, which makes it harder to reach and inspect, so inspect the rear connector on installation and leave it alone afterwards.
Testing differs too. A cassette lets a technician test the LC front with a standard light source and power meter. A pass-through or direct layout needs an MPO-capable tester, or a fan-out cord to connect an LC tester, and polarity must be checked. If the site has only LC test gear, keep that in mind before choosing parallel layouts.
Upgrade paths: what happens when the speed changes
The cable plant outlives the switches by several generations, and the layout decides how much of it you can reuse.
- 10G to 40G or 100G parallel. Cassettes are removed and replaced by pass-through adapters or by a direct trunk. The MPO trunk behind them stays if it is Type B or can be flipped by a patch cord, and if the fibre grade supports the new reach.
- 40G or 100G SR4 to 400G. 400G SR8 uses 16 fibres and 400G DR4 uses single-mode; see the 400G SR8, DR4 and FR4 cabling guide. A 12-fibre trunk cannot serve SR8 without changes.
- Duplex to duplex at a higher speed. Cassettes stay in service, because 25G and 100G LR use LC duplex.
If you expect to move to parallel optics soon, it can be cheaper to install pass-through panels now with trunks that support both uses than to install cassettes and replace them. The reasoning behind trunk length and fibre choice is in the guide to how to choose MPO trunk cable length, and the broader view of room design is in MPO connectivity in the data centre.
Decision steps
- List the module type at each end: parallel optic (MPO port) or duplex (LC port).
- If both ends are MPO-port modules and the link is permanent, price a direct trunk first.
- If the link will be patched or moved, count mated pairs and run the loss arithmetic with the real grade of connector.
- If either end is LC duplex, choose cassettes at that end, or an MPO-to-LC harness if the run is short and fixed.
- Decide the polarity method for the whole plant before ordering any cassette.
- Decide gender at each interface.
- Check the test gear you own against the layout.
- Write the layout, the polarity method and the loss grade into the order.
Common mistakes and when cassettes are the wrong tool
A frequent error is installing cassettes in a room that will run 100G SR4, then finding there is nowhere for the parallel fibres to go. Another is mixing makers' cassettes, which can disagree on polarity or on the guide-pin gender of the rear adapter. A third is adding a second patch panel for convenience, which adds two MPO pairs and uses most of the loss budget of a 100G link. A fourth is forgetting that a cassette uses all 12 fibres of the trunk, while a parallel optic uses only 8, so the trunk may be underused.
Cassettes are the wrong choice for links where loss is tight, where the optics are parallel, or where only a few fibres are needed. A harness is simpler when the run is short and the end equipment is permanent. Direct trunks are wrong for rooms that change often. For the cables themselves, see MPO trunk cables, and for the room context, data centre.
What to put in an RFQ
State, for each link: the equipment and module type at each end, the layout (direct, pass-through or cassette), fibre type and count, the polarity method, gender at each interface, length, jacket (LSZH or other), and the insertion-loss grade you want for MPO connectors. Radix builds MPO assemblies to the buyer's drawing in Pune. Send the list through the request a quote page and ask the quote to name the connector grade and the test records.
Questions buyers ask
What is an MPO cassette?
It is a small module with an MPO adapter on the back and several LC duplex adapters on the front, with fibres wired between them. A common 12-fibre cassette gives six duplex LC ports. It lets an MPO trunk feed duplex equipment.
Can I connect an MPO trunk straight to a QSFP transceiver?
Yes, if the trunk has a connector of the right gender at the module end and the right polarity. QSFP modules for 40G and 100G SR4 take an MPO-12 connector directly. A direct trunk adds no mated pair between the modules.
When should I use a cassette instead of a direct trunk?
Use a cassette when the equipment ports are duplex LC and the backbone is MPO. Use a direct trunk when both ends are parallel-optic modules and the link is permanent.
Do cassettes work with 100G SR4?
Not usually. A 100G SR4 module uses a parallel MPO interface, so it needs an MPO connection, not LC. A cassette is for duplex optics. The extra loss of an MPO to LC conversion also uses much of the small 100G budget.
How much loss does an MPO cassette add?
A cassette adds an MPO pair at the rear and an LC pair at the front, plus the short internal fibre. As assumed values, about 0.35 dB for the MPO pair and 0.25 dB for the LC pair give about 0.6 dB per cassette. Ask the supplier for the actual figures.
What is the difference between a cassette and a pass-through panel?
A cassette converts MPO to LC. A pass-through panel only joins MPO to MPO. It has no internal wiring, so polarity is set by the cables alone and the loss is lower.
Why does polarity matter more with cassettes?
The wiring inside a cassette is fixed. Choosing the wrong trunk type or mixing cassettes from different methods leaves transmit facing transmit. Set the polarity method for the whole plant before ordering.
Can Radix make the trunk and patch cords for these layouts?
Radix builds MPO trunk and patch assemblies to the buyer's drawing in Pune. State the layout, polarity method, gender and connector grade in the request and the quote replies with what can be built.