Why are we fixated on Multimode fiber for high bandwidth communication?

Phil Bedard bedard.phil at gmail.com
Tue Dec 31 19:58:44 UTC 2013

Money, really.  The optics and fiber cost is cheaper than SM.  The
standards around SM optics are to reach relatively long distances, so the
transmitters and receivers are more expensive and they use way more power.

That being said, I see MM in modern datacenters being used in-rack or very
short distances due to the reasoning you mentioned, having to run at least
4 pair for 40G or 12 pair in the case of 100GBase-SR10.  I know there are
structured cabling solutions for handling the bundles but it sure seems
like a pain.  QSFP28 will bring 100G back down to 4 pairs of fibers at
least for those who want to use MM.  There was significant push by Google
and others to come up with a shorter-reach 100G SM standard (LR10) because
people don't want to use 12-pair MTP cables around their datacenter and
LR4 wasn't a good fit.

We are pretty much all SM for anything 10G and above as a standard, but
have looked at 100GBase-SR10 for short-reach 100G interconnects due to the
significant reduction in cost and power compared to 100GBase-LR4.


On 12/31/13, 2:00 PM, "eric clark" <cabenth at gmail.com> wrote:

>I've been working with 40 gig for a few years. When I first ordered a
>switch, one of the first publicly available with full 40 gig, I was
>appalled that I was going to have to use 4 pair of multimode fiber for
>of my connections. I had planned on using single mode because I can do
>with 1 pair.
>Even today, we're still looking at MM fiber instead of SM, even with the
>horrendous limitations and cost issues of MM. For instance, if you need to
>go 301 meters or more, you've got to go OM4 which  is very expensive. You
>have to lay 4 times the number of pairs as SM and when we move to 100G,
>it'll be even worse because they're still doing things in 6,12,etc... SM
>can do 100G easily, up to 1K with the lower grade fiber, so in the SM 100G
>world, you'd be installing 1/12 the strands as you would in multi mode. I
>just can't figure where this makes sense....
>I am aware that single mode has more expensive optics, and I know how much
>they cost when I first looked at this, but if this were the standard, that
>price would drop enormously.
>Anyone know why the industry has their head stuck on MultiMode?

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