MTP OM3 48 fibre assemblies

OM3 is laser-optimised 50/125 micron multimode fibre with an effective modal bandwidth of 2000 MHz.km at 850 nm. Maximum attenuation is 3.5 dB/km at 850 nm. Those numbers set the reach of each parallel interface, as listed in IEEE 802.3.

OM3 therefore suits 10G and 40G links of up to 100 m, and rows of racks within a single hall. For 100G and above, the OM3 reach falls to 70 m, and OM4 is the better grade for any run near that limit. The grades are compared in the OM3, OM4 and OM5 guide, and there is a ready-made comparison of OM3 for the 25G case in OM3 vs OM4 for 25G.

What a 48-fibre MTP OM3 assembly is

A 48-fibre MTP OM3 assembly carries forty-eight 50/125 micron multimode fibres of OM3 grade between two points, ending in MTP connectors at each end. No common single connector holds 48 fibres, so the assembly is built from several connectors. The usual layout is one 48-fibre cable that splits at each end into four 12-fibre legs, each ending in an MTP-12. Another layout uses two 24-fibre legs and two MTP-24 connectors per end. The right layout depends on the equipment ports at each end, so it starts with the drawing and not with the fibre count.

This page is buyer education for these assemblies: how the layouts differ, what OM3 can and cannot carry, how to work out polarity and leg lengths, and what to put in an enquiry. Radix builds fibre assemblies to the buyer's drawing in Pune. For a 48-fibre OM3 build, send the drawing and the reply states what can be built. For the product family, see MTP and MPO cables, and for the related page on the product itself, see MTP OM3 48F patch cords.

Layouts for 48 fibres

The count of lit fibres matters for cost. A 40G-SR4 port uses fibres 1 to 4 and 9 to 12 of the 12 (eight in total) and leaves the four middle fibres dark. Four such ports on a 48-fibre cable leave 16 fibres unused. If your design is all 40G, a base-8 cabling plan may suit better, with the cost of a plan that is specific to 40G. The comparison is explained in MTP-24 vs two MTP-12, and the 40G fibre usage is in 40G SR4 and BiDi cabling.

Polarity for parallel optics

Parallel optics send light on four fibres and receive on four others, so the cable has to flip the fibre positions between ends. In the TIA polarity scheme the usual method for this is Type B, where fibre position 1 at one end lands at position 12 at the other and the connector keys sit on opposite sides (key-up to key-down). Each of the four 12-fibre legs in a 48-fibre assembly has its own polarity, and all four must follow the same method or a link in the middle of the set will not light.

• State the polarity method on the drawing, once, for all legs.
• If the far end goes to LC breakout or a cassette, say which cassette type and which method the cassette expects.
• Mark each leg with a number (1 to 4) at both ends, so that a leg that goes to port 3 at one end arrives at port 3 at the other if that is your plan.

The full set of rules is in the MPO polarity guide, and the cassette wiring maps are in MPO polarity in cassettes. A fibre polarity test is the cheap way to confirm the build, as described in fibre polarity testing.

Leg layout, length and labelling

The cable splits into legs at a furcation body, and the legs carry the connectors. The leg design affects installation more than the fibre does.

The leg lengths and types are examined in MPO fanout lengths and leg types. For pulling and bends in a duct, use the MPO bend radius and pulling tension guide, and do not exceed the limits on the data sheet of the cable you are quoted.

Worked example: twelve 40G ports between two rows

The numbers are assumed for illustration. A design needs 12 links of 40G-SR4 between a leaf row and a spine row, 60 m by cable route. Each link is one 12-fibre MTP with eight lit fibres.

The channel passes. The connector value is the part to question: a standard-loss MPO may be rated at 0.5 dB or more per pair, which would give 1.21 dB and still pass. Adding a patch panel with another pair of connectors on each side doubles the connector loss and leaves little margin. The method is in the fibre loss budget guide. The 33 per cent dark-fibre figure is also a cost to weigh. If every port is 40G for the life of the cable, an eight-fibre plan removes the waste, whereas 48-fibre cabling pays off when the same cable may be re-used for duplex 10G, which lights all 48 fibres.

Choosing OM3 or OM4 for a new 48-fibre build

1. List the longest channel in metres, including slack and patch cords, not the straight-line distance.
2. List the interface speeds now and planned: 40G, 100G SR4 or 400G SR8.
3. Find the reach of each on OM3 and OM4 from the table above. If the longest channel is within about 70 per cent of the OM3 reach, OM3 gives a margin.
4. If any planned speed has an OM3 reach shorter than the channel, choose OM4, or single-mode if the distance is greater than 150 m.
5. Ask the quote to show both grades on the same drawing, so that you can weigh the price difference against the extra reach OM4 gives. The grades are compared in the OM3 MPO trunk page and the MTP-12 OM4 trunk page.

OM3 makes sense when the equipment is already OM3-cabled, when the rack is a copy of an existing one, or when channels are short and the speed will not rise. It is a poor choice for a long-term backbone: a grade that gives 100 m at 40G gives only 70 m at 100G.

When 48-fibre OM3 is the wrong tool

• High-speed links over 70 m. At 100G SR4 or 400G SR8, OM3 stops at 70 m.
• Few ports. If you need two 40G ports, a pair of 12-fibre trunks costs less and is easier to pull than a 48-fibre cable.
• Long cable runs through tight ducts. A 48-fibre cable with four connectors is awkward to pull. Install the trunk without connectors and splice or terminate on site, or use a pulling eye with protected ends.
• Unclear port plan. If you do not know what the legs plug into, fix that first. Gender and polarity guesses cost more than the cable.
• Single-mode designs. Single-mode parallel links need OS2 fibre and angled connectors, and the OM3 page does not apply. See OS2 MPO APC trunk cables.

Request a quote

Send the drawing, the fibre count split, the polarity method, the length and leg lengths, and the equipment ports at each end through the request a quote page. Radix works to the buyer's drawing and BOM in Pune, with quotes by form, phone or WhatsApp. The reply states what can be built against your drawing, along with the loss values and the test report that the quote covers. Radix tests each cord before dispatch.