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Avoiding the Critical OM3/OM4 Fiber Pitfall—Micron Mismatching

February 23, 2016

So much networking buzz revolves around 40G and 100G networking speeds these days, but we aren’t forgetting about the vast landscape of 1G and 10G networks.

If you’re building a new 1G or 10G fiber network—or expanding an existing one—there are plenty of things to take into consideration (media conversion being just one).

For your short-reach installations, you’ll have to contend with different optical multi-mode (OM) standards (OM1, OM3, OM4, OM5) and ensure you aren’t falling into the critical visibility pitfall—mismatched fiber core sizes.

The Fundamental Specs for OM1, OM3 and OM4 Fiber

Network architects might be comfortable with the relative simplicity and familiarity of copper gigabit networks with RJ-45 connectors. But the reality is that fiber is far more complicated and you can’t install it without understanding what fits your network. Here are the specs for OM1, OM3 and OM4 fiber that you need to know before undergoing any upgrades or new installations.

OM Type

Fiber Core (in mm)

Maximum Bandwidth (MHz*km)

Link Length for 1G Networks (m)

Link Length for 10G Networks (m)

OM1

62.5

500

300

33

OM3

50

2,000

1,000

300

OM4

50

4,000

1,100

550

Source: American Fibertek Application Note, Practical Multimode Fiber Considerations

At first glance, it’s obvious that as you move down the list, performance improves. Even though some experts urge network architects to avoid new OM1 installations, the fact is that as of 2013, OM1 fiber accounted for over 25% of North American multimode fiber sales. These multi-mode fiber types are coexisting and 1G and 10G networks, leading to a critical pitfall for network architects.

The differing fiber core sizes (62.5 micron for OM1 and 50 micron for OM3/OM4) can cause severe optical loss if not handled correctly. Don’t fall into the critical pitfall. 

Why You Can’t Mix-and-Match OM1 Fiber with OM3/OM4 Fiber

Knowing whether you’re connecting 62.5-micron or 50-micron fiber in your network is essential to ensuring total visibility. It might seem obvious, but connecting a 62.5-micron OM1 fiber to a 50-micron OM3/OM4 fiber means the core sizes don’t match and light transmitted from the OM1 fiber will be lost as it floods the 50-micron core. 

You shouldn’t connect a 62.5-micron network component with a 50-micron component; but there’s another layer to the issue when network TAPs are introduced.

Your network TAPs must also match the fiber core size in your network. We’ve seen customers purchase a whole set of network TAPs only to find that the solutions were built to support 50-micron OM3/OM4 fiber—not the larger OM1 fiber. Connecting 62.5-micron fiber from network elements simply dropped packets and defeated the purpose of deploying network TAPs. 

The bottom line: Before you purchase your network TAPs, check and see which fiber core size you’re dealing with. When you know you have OM1 or OM3/OM4/OM5 fiber to TAP, you can purchase the correct visibility solutions and ensure your appliances see every bit, byte and packet® of traffic.

Ensuring 100% network visibility is about much more than just OM fiber matching.

Looking to add fiber TAPs to your next deployment, but not sure where to start? Join us for a brief network Design-IT consultation or demo. No obligation - it’s what we love to do!

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Heartbeats Packets Inside the Bypass TAP

If the inline security tool goes off-line, the TAP will bypass the tool and automatically keep the link flowing. The Bypass TAP does this by sending heartbeat packets to the inline security tool. As long as the inline security tool is on-line, the heartbeat packets will be returned to the TAP, and the link traffic will continue to flow through the inline security tool.

If the heartbeat packets are not returned to the TAP (indicating that the inline security tool has gone off-line), the TAP will automatically 'bypass' the inline security tool and keep the link traffic flowing. The TAP also removes the heartbeat packets before sending the network traffic back onto the critical link.

While the TAP is in bypass mode, it continues to send heartbeat packets out to the inline security tool so that once the tool is back on-line, it will begin returning the heartbeat packets back to the TAP indicating that the tool is ready to go back to work. The TAP will then direct the network traffic back through the inline security tool along with the heartbeat packets placing the tool back inline.

Some of you may have noticed a flaw in the logic behind this solution!  You say, “What if the TAP should fail because it is also in-line? Then the link will also fail!” The TAP would now be considered a point of failure. That is a good catch – but in our blog on Bypass vs. Failsafe, I explained that if a TAP were to fail or lose power, it must provide failsafe protection to the link it is attached to. So our network TAP will go into Failsafe mode keeping the link flowing.

Glossary

  1. Single point of failure: a risk to an IT network if one part of the system brings down a larger part of the entire system.

  2. Heartbeat packet: a soft detection technology that monitors the health of inline appliances. Read the heartbeat packet blog here.

  3. Critical link: the connection between two or more network devices or appliances that if the connection fails then the network is disrupted.

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