Don’t say Green – say Aqua!

With Brocade’s recent announcement of a 16 Gbps capable Fibre Channel Switch and Director, the question of which cable type to purchase becomes even more relevant.  Do you buy OM3 or OM4 cable over OM2?

Now if your saying… OM-what?   Let me start at the beginning…

Back when fibre channel was fresh and new and ran at 1 Gbps, the common multi-mode fibre cable that we used had a glass core that was 62.5 microns in diameter.  This became known as OM1 type fibre cable.   We rapidly switched to 50 micron cores because you could get a reliable signal across a longer distance, say 500 meters maximum rather than 300 meters.   The 50 micron cable became known as OM2 type cable.

What has happened since then is that fibre channel speeds have moved from 1 Gbps to 2 GBps to 4 Gbps to 8 Gbps to 16 Gbps.   This is exciting stuff, but with every increase in speed, we suffer a decrease in maximum distance.   This means that something else needs to change…  and that something is the quality of the cables, or more specifically, the modal bandwidth (the signalling rate per distance unit).

With the evolution of 10 Gbps ethernet, the industry produced a new standard of fibre cable which the fibre channel world can happily use.   Its called laser optimized cable, or more correctly: OM3.   Since then OM3  has been joined by an even higher standard known as OM4.

Lets look at the distances we can achieve with different cable types.  You can see in the table below that the modal bandwidth (given in MHz times kilometers), improves as we move to higher quality glass.    You can also see that single mode fibre (with the 9 micron core) has not suffered the same issue with decreasing maximum distances as speeds have increased.  These numbers come from Brocades SFP specification sheets found here and here (so there may be slight variations if you view specs from other vendors).

I didn’t fill in the table for 1 Gbps and 2 Gbps using OM4 cable, simply because I couldn’t find it…  but the distances would be very large indeed.

So how can you tell what sort of cable you have?   The first hint is the colour, the second is the printing on the cable.  Cables that are 50 micron and orange are almost certainly OM2. Cables that are aqua in colour (don’t call them green!)  are either OM3 or OM4.  In the example below I can clearly tell which cable is OM3.

Pictured below is a roll of OM3 cable, all ready for deployment with standard LC connectors.  Note you can also get OM3 cable with a smaller LC type connector used on the mSFPs in the high density 64 port blades in the Brocade DCX.   You can find additional information on identifying cables here.

So should you be buying OM3 cable over OM2?  Or even considering OM4?

The reality is that in many cases, server and storage hardware is often in the same or adjacent racks to the switch hardware.   If this is true for your site, OM2 will satisfy the vast bulk of requirements, because the distances are quite short.  The most common cable I add to configurations is either 5 or 25 meters long.  This is why OM2 is still IBM’s cable of choice, since either length would satisfy 16 Gbps connectivity.   Checking with some local cable vendors,  OM2 cable also remains the cheaper alternative.

Clearly if your computer room is large enough to need cable runs of over 35 meters, then serious consideration should be given to future proofing parts of your cable infrastructure with OM3 (or even OM4).  There is nothing wrong with having a mix of cable types – just don’t join them together.

I would be curious to know how many sites are choosing to move to OM3?  Feel free to comment either way.  I think there will be more to come on this subject, and remember…. OM3 and OM4 cables are aqua  not green or blue.   #;-).

Would love to hear about your sites recent cabling purchases.


And if the word Aqua reminds you of a late 90s Scandinavian pop group, look no further:


About Anthony Vandewerdt

I am an IT Professional who lives and works in Melbourne Australia. This blog is totally my own work. It does not represent the views of any corporation. Constructive and useful comments are very very welcome.
This entry was posted in SAN and tagged . Bookmark the permalink.

11 Responses to Don’t say Green – say Aqua!

  1. rogerluethy says:

    Great work on the OM’s and cable types here ! Thanks !

  2. Pingback: What cable do we need ? | Storage CH Blog

  3. Alex says:

    Killer blog post! These are the type of GEMS that are of such great value!



  4. avandewerdt says:

    Thanks for the feedback. One thing I should have been clear about is that I am talking about pure Fibre Channel. If sites are considering a shift to FCoE then OM3 becomes the cable of choice.

  5. Suad says:

    I’m colorblind (according to my wife) but it doesn’t look like aqua to me.
    How about …

    (and I have been painting the interior of a house so have been recently staring at colour charts trying to figure out how they get the patterns of colours in that specific order)

    • avandewerdt says:

      Back in the late 80’s when I applied to work at IBM they sent me off for a medical, which included a test for colour blindness. Wonder if they still do this? Must ask a recent hire….

  6. xxxtube says:

    Excellent Blog !!!!

  7. Nick says:

    Excellent article. Thank You Anthony!

  8. Pingback: What cable do i need ? « Storage CH Blog

  9. karlochacon says:

    great article. Now I get thw OM# thing

  10. Craig says:

    So I’m still getting this sorted in my head. Is it correct to say the following (Im referring to the table in this article)?

    – If I have a switch and storage unit that are both capable of 8Gbps that are 1m apart its fine to use OM1 cabling and still achieve 8Gbps?
    – If I connect some workstations (with 8Gbps HBAs) to the switch that are 30m away (in the next room) I’d need to use OM2 cabling for them to achieve 8Gbps ?

    Whats throwing me is that everyone refers to OM1 as 1Gbps cabling, does this just means its best performance over its maximum/extreme range?

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