High-density fibre and modular cabling

Tomasz Wiercinski, the cabling contractor's foreman, stops by Lakehead Signal's office with a sample in a plastic bag: a thin, flat fibre trunk with a rectangular connector on each end. 'Your spec says field-terminated LC everywhere in the data room,' he says. 'For two hundred and forty fibres, that's a lot of splicing in a small room with the servers arriving in six weeks. Have you thought about pre-terminated?' Daniel had not. Roz had, and had been waiting for him to ask. 'Good,' she says. 'Today we find out why data centres stopped splicing.'

Data centres need far more fibre in far less space than any other part of a building, and they need it installed quickly, tested reliably and changed often. Two developments make this possible: array connectors that terminate many fibres at once, and modular, factory-terminated cabling that is plugged together on site rather than built there. This lesson explains both, along with the polarity rules that keep transmitters connected to receivers, and the density and loss trade-offs a designer must check.

Array connectors

An MPO connector, short for multi-fibre push-on, holds a row or rows of fibres in a single rectangular ferrule. The common types hold 8, 12, 16 or 24 fibres; larger counts exist for special uses. A single MPO connector takes up about the same space as one duplex LC connector but carries six or twelve times as many fibres. Alignment is done with two precision guide pins on one connector, called the male or pinned connector, that fit into holes on the other, the female or unpinned one. Two MPO connectors mated together must be one of each.

MPO connectors are used in two ways. As trunk connectors, they join pre-terminated trunk cables to cassettes or breakout modules, which convert them to duplex LC or other connectors at the patch panel. As equipment connectors, they plug directly into parallel optic transceivers, which send and receive on several fibres at once. A 400 gigabit short-reach interface, for example, may use eight fibres: four to transmit and four to receive, each at 100 gigabits per second.

Very small form factor connectors

Newer duplex connectors, often called very small form factor connectors, fit two or more duplex connections into the space of one LC duplex. They let designers put more individually patchable ports in a panel and are designed for transceivers that split one high-speed port into several lower-speed ones. Their names are trademarks of their makers, and the designer should specify them by performance and compatibility rather than by brand alone unless the owner has standardized on a product.

Pre-terminated trunks and cassettes

A pre-terminated trunk is a fibre cable cut to length and terminated with connectors in a factory, then tested and shipped with a test report. On site, it is pulled into place and plugged in. Cassettes, also called modules, sit in patch panels and contain short factory-assembled harnesses that connect an MPO trunk at the rear to duplex connectors at the front. Some systems skip the cassette and use MPO adapter panels, with MPO-to-duplex harnesses at the equipment.

Pre-terminated systems have clear advantages in data centres. Installation is fast, with no field polishing or splicing. Factory termination gives lower and more consistent loss than most field work. Trunks can be replaced or moved without disturbing nearby cables. And the system is easier to change: when the network moves from duplex to parallel optics, the cassettes can be swapped for MPO adapter panels while the trunks stay. The disadvantages are that every trunk must be measured and ordered to length before installation, lead times can be several weeks, and a trunk damaged on site must usually be replaced rather than repaired.

Lengths matter. A trunk that is too short cannot be stretched; one that is far too long leaves coils that fill trays and cabinets. Designers measure each route along the actual pathway, including the rise into each cabinet and the slack needed for future movement, and add a small allowance. Many contractors walk the route with a measuring wheel or use a three-dimensional model of the trays once they are installed, then order trunks.

Fibre counts and utilization

The number of fibres per connector should match the applications. Parallel optics that use eight fibres fit neatly on 8-fibre or 16-fibre connectors, or on 24-fibre trunks split three ways into 8-fibre groups. When an 8-fibre application runs over a 12-fibre MPO connector, four of the twelve fibres are unused, a third of the installed fibre. Systems built in groups of 8 or 16, often called base-8 and base-16, avoid this waste for those applications, while base-12 and base-24 systems match duplex breakouts of six or twelve ports. The designer should ask the network team which interfaces they expect in the next five to ten years and choose a base that wastes the least.

Polarity

Every fibre link must connect a transmitter at one end to a receiver at the other. With duplex patch cords this is easy to see and easy to fix by flipping a connector. With MPO trunks and cassettes, the fibres cross over inside components where nobody can see them, so the designer must choose a consistent polarity method for the whole system. The fibre component standard, ANSI/TIA-568.3, describes three methods, called A, B and C, which differ in how fibre positions are arranged inside the trunk and which type of patch cords is used at each end.

The practical rule is to pick one method, document it in the specification, buy every component from a system whose parts are designed for that method, and label cassettes and trunks accordingly. Mixing methods is the most common cause of links that test perfectly for loss but carry no traffic.

Density and its limits

A 1U patch panel with duplex LC adapters typically holds 48 duplex ports, or 96 fibres. With cassettes and MPO trunks the same space may hold 144 fibres, and with very small form factor connectors or MPO adapter panels much more. High density saves cabinet space, which is valuable in a data centre, but it has limits. Technicians must be able to reach, identify and remove one cord without disturbing its neighbours, so density gains are only useful if the panels, cord management and labelling make that possible.

Loss in modular systems

Every MPO-to-cassette transition adds a connection, and each connection uses some of the loss budget. A channel that passes through a cassette at each end of a trunk has at least four mated pairs: duplex cord to cassette, cassette to trunk, trunk to cassette and cassette to duplex cord. As RCDD-110 showed, high-speed multimode applications may allow less than 2 dB for the whole channel, so standard maximum connector loss would not fit. Designers using modular systems specify low-loss components with guaranteed maximum values, add up the channel, and confirm it against the application limit.

Cleaning, inspection and testing

High-density connectors are only as good as their end faces. A speck of dust on one fibre of an MPO ferrule can add loss or cause reflections, and on parallel optics a single dirty fibre can take down a whole link. Every connector should be inspected with a microscope or automated inspection probe and cleaned if needed before it is mated, every time. The international end-face inspection standard, IEC 61300-3-35, gives pass and fail criteria, and many owners require inspection images to be saved for a sample of ports.

Factory test reports for pre-terminated trunks do not replace field testing of the installed system, because trunks can be damaged during installation and cassettes can be installed incorrectly. Field certification measures insertion loss and verifies polarity for each channel or link, using test equipment with MPO interfaces or with fan-out cords. RCDD-116 covers testing in detail.

The decision

Roz and Daniel revise the specification. Each cabinet will get a 24-fibre, base-8 compatible, pre-terminated OM4 trunk for today's parallel optic uplinks and a 12-fibre singlemode trunk for the duplex singlemode links the network team expects next. Cassettes in the network row will be low-loss, and the polarity method is written into the specification with a requirement that all components come from one documented system. Tomasz measures the tray routes as soon as the trays are installed, and the trunks are ordered four weeks before the cabinets arrive.