Looking back on an excellent (now historical) article from 2010 reminds us how network integrators in the late 80s were pulling Fiber Distributed Data Interface (FDDI) grade optical cable to “future-proof” installations against the predicted demise of copper. Go figure! This precursor to OM1 used a wide bandwidth 1300nm LED-based light source, but it turned out that it was inadequate for next-generation Gigabit Ethernet (GbE) which emerged in the 90s.
OM1 was the same size as the previous FDDI cable with a 62.5μm core and 125μm cladding (designated as 62.5/125), but was optimized for the new, superior light source at 850nm wavelength. OM2 went to a smaller 50/125 while increasing its bandwidth capability at 850nm.
The image below is a screen grab from the Wikipedia entry on multimode optical fiber. The second column shows bandwidth at both 850nm and 1300nm, the latter being legacy FDDI, which is now long lost to history.
OM3 and OM4 are the same 50/125 configuration as OM2, but are optimized for the newer laser light sources, which in time rendered OM1 and OM2 redundant. To this day, OM3 and OM4 are still optimized for a single wavelength — 850nm. In application this means that each fiber represents a single optical lane. Back then, lasers peaked at about 10Gbps, so for 10GbE Ethernet, two parallel fibers were required for 10Gbps full duplex — i.e.; 10Gbps downstream, 10Gbps upstream. For 40GbE, eight fibers were needed — 4x 10Gbps down, 4x 10Gbps up.
Question time - if you’ve been running OM3 or OM4 to “future-proof” projects, how many have you been running to each point? Two is about normal, right? Before you start to sweat, the good news is that is enough, even for HDMI 2.1 at 48Gbps downstream, and then some for upstream signalling. How so?
New technology has pushed laser speeds up to 25Gbps, but what’s even more compelling is the ability to now adapt multiple optical lanes on a single fiber. This is done through a process called Wavelength Division Multiplexing (WDM), which is something we’ll all be hearing about a lot more. So instead of a single wavelength at 850nm, multiple lasers clustered into a single module can shoot beams at different wavelengths (colors), targeting four main wavelengths from 850-953nm, at up to 25Gbps each (depending on the product).
In practice this could result in full-duplex 100GbE on a single fiber pair, or in some leading examples even a single fiber! But remember we said that OM3 and OM4 are optimized for 850nm? That’s just one wavelength. So how does that work? That’s where the next generation of fiber comes in. OM5 fiber, also nown as Wide-band Multimode Fiber (WBMMF), has been ratified internationally under standards including TIA492AAAE, ISO/IEC11801-3 and ANSI/TIA-568.3-D. It’s essentially OM4 which has been optimized for WDM with a 850-953nm range.
But does that mean that installations with OM3 or OM4 are now redundant? Thankfully, no. OM3 and OM4 will still support WDM, but as they’re not necessarily optimized to do so, bandwidth and range may be more limited compared to using a new OM5 cable. But generally still plenty for residential applications.
CEDIA recommended practices are at this stage unchanged — 2x OM3 or OM4 fibers to each location in star configuration, along with CATx etc. Where OM5 is available, it is certainly a consideration and will be more scalable, but that may be an economic choice as much as a technical one. In time it will become the new normal.
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