High-capacity cabling systems for data centres will come with more fibres than ever before. Thorsten Punke explains.

Multi-fibre push-on (MPO) fibre cabling has become the main connectivity technology in data centres, which have different requirements to those of commercial buildings.

Fibre optic systems play a dominant role, as they off er high bandwidth and low energy dissipation when transmitting data.

Data centres have always had special infrastructure requirements. Owners must choose the most suitable medium for the combination of short distances, high-speed applications and the greatest scalability and flexibility. For this reason most use preterminated cabling systems based on MPO (IEC 61754-7) connectivity.

The requirements change much faster than in other areas – 10Gbps data transmission speed technology is already common and 40/100Gbps is knocking on the door.

IEEE 802.3 has defined a parallel transmission with eight fibres for 40Gbps and 20 fibres for 100Gbps when multi-mode systems are used. In the next couple of years, customers will migrate to 40/100Gbps.

The industry has recognised that the current 12-fibre connector is not optimal; more than 12 fibres in one connector would better support future applications.

The MPO technology has a long heritage and is in principle available from 12-74 fibres per connector. The difficulty is the combination of many fi bres and extreme low loss requirements – down to 1.5dB for a 150m channel with OM4 fibres.

Even today not all 12-fibre systems meet the necessary low loss requirements. The next level, to 24 fibres, is therefore a huge technical challenge. The key is precise termination technology with an accuracy in the micro-millimetre (μm) area.

To give some idea of the fine tolerances required, here are some aspects of MPO technology:

the area where all fibres are located is about 3cm x 0.6cm;
the distance between fibres is 250 μm;
each fibre has a protrusion of 1-2μm above the ferrule;
the accuracy of all fibres in x and y directions is only 1μm;
the distance between the rows is 2 μm; and
all fibres need a controlled end face.

To use the fibres in the best way possible, and reduce the number of cables, 24-fibre MPO cabling systems are essential.

The obvious benefits are less cable congestion and rack space, full fibre utilisation with 40Gbps, reduced complexity with 100Gbps and design capability for new applications.

In larger data centres the fibre core count can be easily in the thousands, so 24-fibre systems need less space or offer more fibres per rack.

This is especially important for data centre operators in capital cities, as real estate prices remain high in Australia and New Zealand. Another major consideration is the operational and financial effect.

Economic pressure on data centre owners results in operational efficiency, which requires systems made for the purpose.

Therefore, 24-fibre systems employing a modular approach and real ‘plug and play’ capability keep the time for moves, adds and changes to a minimum.

The financial advantage lies in ‘lifetime’ and scalability. Any future upgrade can be done by simply changing the cassettes and patch cord. In addition, 40Gbps and 10Gbps can be operated via single mode. In this case either 4 x 10Gbps (IEEE 802.ba 40GBASE-LR4) or 4 x 25Gbps transmissions (IEEE802.ba 100GBASELR4) are possible. This results in full system utilisation to 6 x 40Gbps or 10Gbps channels.

An IEEE working group is working on a 4x25Gbps approach for 100Gbps Ethernet over multi-mode. The advantages would be the same as mentioned above for singlemode systems and would increase the number of useable channels per cable.

In any case, 24-fibre systems are prepared for any utilisation – even down to two fibres.

MTP AND MPO

People often get confused when the initials MPO and MTP appear.

The MPO connector family is defined by two Standards: international, IEC-61754-7; in the United States, TIA-604-5 (also called FOCIS 5).

The MTP multi-fibre connector is a USConec Ltd trademarked name for an improved quality MPO connector and is fully compliant with FOCIS 5 and IEC-61754-7.

There are many MPO connectors on the market, but the MTP parts are distinctly different. The MTP connector is in fact a high-performance MPO.

However, MTP is still not an indicator of the final quality of a product. The key to low loss connectivity is knowing the termination process in detail, from fibre cleaving to polishing the end faces. It is therefore important to check the specification sheet for maximum insertion loss, minimum return loss and guaranteed number of connections in a channel.

CLEANING

As with 12-fibre systems, these systems are sensitive and should always be handled with care. Cleaning and inspection are crucial during installation – and still underappreciated. Even though these systems come pre-terminated and with protective caps, it is essential to follow the ‘clean, check, connect’ procedure at all times.

CONCLUSION

The next generation of low-loss MPO systems will come with 24 fibres per connector, allowing the greatest flexibility for forthcoming services and applications.

The MPO technology has a lot of potential for future applications and will be the common interface for cabling systems as well as transceivers.

In any case, it is a high-precision technology that requires the same level of attention applied today when it comes to cleaning, testing and operation.