Singlemode or Multimode Fibre? Choosing Fibre for a Singapore Building

Once a run is too long for copper, or has to cross between floors or buildings, the cable is fibre. The next question on the drawing is which kind: singlemode or multimode. Both work. The difference is how far they carry at a given speed, what the equipment at each end costs, and how much room they leave for whatever the network needs in ten years.

What is the difference between singlemode and multimode?

The difference is the size of the glass core that carries the light. Singlemode has a core of about 9 microns, so light travels down it in essentially one path. Multimode has a 50-micron core, so light travels down it in many paths that arrive at slightly different times. That spreading is what limits how far multimode can carry a fast signal.

The reaches for multimode are the standard figures in IEEE 802.3 for each Ethernet type. They fall as the speed rises, which is the point to remember: a multimode riser that is comfortable at 10 Gb/s can be too long at 100 Gb/s.

Where does each one belong?

• Between buildings and on campus backbones: singlemode. The distances are long, and singlemode makes no electrical connection between the two buildings.
• To a telecom carrier or a data centre: singlemode. Carrier and data-centre hand-offs in Singapore are singlemode as a rule.
• Risers inside a building: either, priced both ways. A riser of a hundred metres or so is within multimode's reach today. Whether it stays within reach depends on the speeds the building will run later.
• Inside a data room or between racks: often multimode. The runs are short and the equipment is cheaper, and that is where multimode earns its place.

What about OM1, OM2 and OM5?

OM1 and OM2 are older multimode grades, usually in orange jackets, still found in buildings cabled in the 1990s and 2000s. OM1 has a larger 62.5-micron core and carries 10 Gb/s only about 33 metres. If an existing building has OM1 or OM2, it is worth knowing before a network upgrade is planned around it.

OM5 is a newer multimode grade, in a lime-green jacket, designed to carry several wavelengths down one fibre in some data-centre equipment. For most office and commercial buildings, it offers little over OM4.

What mistakes cost the most?

Mixing the two. Singlemode and multimode fibre cannot be joined in one link. A multimode patch cord plugged into a singlemode link, or the other way round, loses most of the light. The fault is often intermittent rather than total, which makes it slow to find. Patch cords, adapters and transceivers all have to match the fibre in the wall.

Mixing connector polish. Singlemode connectors come with a flat (UPC, usually blue) or angled (APC, usually green) end. The two do not mate properly. Carriers often use APC, so the patch cords at the hand-off have to suit it.

Choosing on today's speed alone. The cheapest fibre for the network as it is now can be the most expensive one in ten years, when the riser has to be pulled out and replaced.

No records. Unlabelled cores and no test results mean the next contractor has to test everything again before they can work on it.

How do you know the fibre performs?

By testing every link and keeping the results. A light source and power meter measures end-to-end loss against a budget. That tells you whether the link works. An OTDR walks the length of the fibre and shows where loss sits at each splice and connector. That tells you why a link fails and where to look. The difference is set out in OTDR or power meter testing, and reading an OTDR trace explains what the trace shows.

Put three things in the specification before award:

1. The fibre type and core count, per link: OS2, OM3 or OM4, and how many cores, including spares
2. The standard and test method, under ANSI/TIA-568.3 or ISO/IEC 11801, so there is no argument about what a pass means
3. The handover: loss results and traces for every link, the core labelling schedule and an as-built drawing

Where it fits

Where copper stops and fibre starts is covered in does structured cabling include fibre. On a fit-out, the fibre decisions are among those to settle before the ceiling closes. And the people who splice and test it matter as much as the fibre itself, as set out in choosing a fibre splicing contractor.

Where Fiber Tech sits

Fiber Tech Solutions Pte Ltd installs, splices, terminates and certifies singlemode and multimode fibre across Singapore. Work has ranged from 288-core cables in live telecom exchanges to a 5 km, 96-core fibre backbone with 1,000 terminations, and directly terminated OM3 links at a StarHub data centre certified to TIA-568. Testing is done with company-owned OTDRs and power meters, and the company is FOA-certified for splicing, termination and testing. It holds IMDA Wiring Contractor (Class) Licence No. IW105464 and has been a Singapore specialist contractor since 2007. To discuss a riser, a backbone or a data room, get in touch.

— Written by the team at Fiber Tech Solutions Pte Ltd, IMDA-licensed wiring contractor and FOA-certified fibre specialists, Singapore.