OMG! What’s happening to multimode fibre?
VIDEO UPDATED when OM5 became official: see here
If you’re in a hurry watch the video. If you’d like the full story, then read on…
OM1, OM2, OM3, OM4, OM5? WB-MMF…
Most of us are familiar with the OMn labels that we use to talk about the different performance grades of multimode fibre. But things are changing! It’s time to finally say goodbye to some old faithfuls and time to introduce the new kid on the block, as it looks as though he might be staying around a while… So let’s review where we’ve come from with multimode fibre and where we’re heading.
Multimode fibre for telecoms…
Some of the earliest telecoms systems operated on multimode fibre and the ITU that makes the Recommendations for telecoms systems even produced a Recommendation for it that they called G.651. However, in telecoms the requirement to go over much longer distances meant that singlemode fibre rapidly took over, and the ITU went on to produce a whole series of Recommendations for different types of singlemode fibre (G.652… G.657) but that’s another story…
Multimode fibre for datacoms
In Europe and Japan the 50/125 fibre described in G.651 became the standard multimode fibre type that was used for datacoms links in premises networks, whilst in America they went their own way and developed 62.5/125 fibre.
LEDs for 10 & 100Mb/s
These early fibres were OK for the systems of the day that operated at low data rates such as 10Mb/s and 100Mb/s. They could support links up to 2km or more, plenty far enough for most office and even campus networks. That 62.5/125 fibre even had a benefit that the larger core could catch more of the light that came out of the LED light source.
The Gigabit era
However, as the data rates climbed into the Gigabit range, then LEDs ran out of steam and were replaced by high speed lasers, these could send much higher date rates and concentrated the light energy into a much smaller spot than LEDs could. So 62.5/125 no longer had a benefit, and it also has a number of downsides. Also the performance of the 50/125 fibre could be optimised for laser transmission. At this stage the maximum distance that you could go down the fibre changed from being limited by the power level in the fibre, to being limited by the distortion of the signal as it travelled along the multimode fibre. This distortion was primarily due to something called modal dispersion. So a new generation of 50/125 fibre was developed that had much better modal dispersion to support high data rate laser transmission over longer distances.
Now that we had two multimode fibres of the same size with different performance standards we needed a naming scheme and so the OMn categorisation came about in the premises cabling standards. So OM1 became associated with the old style 62.5/125 fibre and OM2 with the old style 50/125 fibre. The new ‘laser optimised’ multimode fibre with much better modal dispersion gave us the OM3 performance grade, capable of supporting low cost 10G Ethernet over distances of 300m.
As time went by, the manufacturers of multimode fibre got better and better at optimising their fibres for high performance laser transmission at 850nm and after a while of OM3+, OM3super, OM3-ER (extended range) etc. they got together and standardised on the performance required for OM4.
Goodbye to OM1 & OM2
A major revision of the premises cabling standards that defined the OMn categories is underway. This is quite an epic task and so is taking some time to sort out, but one thing is for sure, it’s time to say goodbye to OM1 and OM2. These will have no part in the standard for new cabling infrastructure. OM3 becomes ‘entry level’ multimode fibre. OM1 & OM2 will only have grandfather rights.
However, at data rates beyond 10G it gets tricky and so the first generation 40 & 100G Ethernet solutions for multimode are using parallel fibres each operating at 10G. The second generation 100G on multimode uses four parallel fibres for transmit and another four for receive, each operating at 25G, but it would be really nice if we could do 100G on just a pair of multimode fibres…
So this is where Wide Band Multimode Fibre comes in: ‘the new kid on the block’. Whereas OM3 and OM4 are optimised for 850nm, WBMMF gives good performance across the wavelength range from 850nm up to 950nm. Performance that is good enough to support 4 SWDM channels each carrying 25G of data. That can deliver 100G Ethernet, or even 128G FibreChannel using just a pair of multimode fibres. This is really attractive for the data centre environment and so we think this fella is going to stay around a while. He might even join the OMn family and take on the name OM5; after all, he can do everything that OM4 can do and more besides. But he is a bit different and so maybe something like OM4W might suit him better. WBMMF even matches the bend tolerant characteristics of the bend-loss insensitive versions of OM4.
But isn’t singlemode fibre taking over everywhere?
“I thought that all multimode fibre was going to be obsolete…” I hear you say…
Whilst it’s true that singlemode fibre is best for longer distances, the cheapest singlemode optical interfaces at 1310nm are 2-3 times the cost of multimode for the same data rate, and some singlemode interfaces can get very expensive, long distance ones at 1550nm may be 10x the price of multimode interfaces.
So for the short distances that are in datacentres where most links are less than 100m and the number of links is multiplying like crazy, the cost of the transceivers at each end of the fibre is really important.
So multimode has some life left in it yet, and with the new kid on the block the future of the multimode family looks secure for another generation! I’ll be talking more about him in my next blog.