Download Tapping Low Optical Budget Links

Tapping Low Optical Budget Links
Technical Note
Tapping of fiber optic links has long been the recommended method, over network switch mirror or SPAN ports, of
obtaining copies of network traffic for the purpose of performance and security monitoring (see “Tap vs Span” whitepaper
for more on this topic). The generally accepted and preferred method of tapping these networks is using a passive
fiber optical splitter TAP, which diverts a percentage of the light to a monitor port and the remainder continues on
into the network.
Monitoring Device
Tx Tx
Passive Fiber TAP
30%
30%
70:30 Splitters
70%
100%
70%
100%
Tx Rx
Tx Rx
Router
Switch
T HE CHALLENGE
Passive optical tapping is a great solution in that it provides 100% copy of all traffic on the network, is low-cost, and
ensures fail-safety of the network link, should power at the site fail. However, it also means that when installing these
TAPs into the network links, careful consideration of the optical characteristics of the network equipment and monitoring
infrastructure needs to be given; transmit power, receive sensitivity, and cable, connector, & splitter losses.
This tends to be more of a concern for Multimode fiber, where Fast Ethernet (100baseFX and Gigabit Ethernet
(1000baseSX) generally are not a major issue, even with OM1 (62.5 micron core) fiber, but it does become significantly
more of an issue for rates above Gigabit. The reasons for this are relatively simple:
ƒƒ Multimode is a lower grade of fiber compared to single mode fiber due to how this fiber propagates the optical signals
along the fiber; which becomes exacerbated when more than one wavelength is used simultaneously on the same fiber
ƒƒ Most multimode fiber deployed is OM1 or OM2 (lowest grade with 50 micron core), whereas OM3 and OM4 can improve
the situation somewhat but are not commonly used to-date
ƒƒ Multimode transceivers have worse power characteristics
ƒƒ A split ratio beyond 70:30 is not recommended, and 50:50 is the preferred, for multimode
Tapping Low Optical Budget Links; TECHNICAL NOTE
ƒƒ Insertion losses accumulates based on cable lengths and
number of connections in the path
ƒƒ Loss characteristics become much greater as the
transmission rate increases or more than one
wavelength is used simultaneously
To illustrate this, 10G Ethernet (10GbaseSR) is not
recommended to be run over OM1 fiber if passive tapping
is going to be used, and neither 40G (40GbaseSR4) nor
100G (100GbaseSR10) are even supported with OM1
fiber. Plus, the Cisco 40G Multimode BiDi (40GbaseSR2)
technology’s power budget is very limited and it is not
recommended to passively tap it at all.
It should be noted that the problem can just as easily
impact a single mode network, if there are sufficient losses
in the network due to cable lengths and intermediate
connections or inappropriate split ratios are used.
When deploying a network and a monitoring solution,
with selected transceivers, the transceivers will start off
performing close to the best-case characteristics, which
means the Tx power and Rx sensitivity. At this point,
whatever passive tapping solution is installed may have
and monitor sides. The main drawback of this solution is
that if and when the power goes down on the active TAP
device, and the power is later returned, there will be a
break in the network links.
The characteristics needed for a successful active tapping
solution are:
ƒƒ Layer 1 Fail Open state for when power is lost to the
active TAP
ƒƒ Fast switchover times from power-on to power-down,
and vice versa, with insignificant break period in the link
ƒƒ Support the full range of optic technologies that can run
into issues with passive tapping (10G MM, 40G MM, 40G
MM BiDi, 40G SM, etc.)
ƒƒ Manually force TAP into Layer 1 Fail Open state while
power is on
ƒƒ Close integration with the network packet brokers that
are being fed the tapped signals
ƒƒ Detection and notification of when the power goes down
and is restored to an active TAP
a good chance of working, as long as cables lengths,
interconnections, and splitter ratio are selected and
managed properly. However, transceivers degrade over
their life, which means that the Tx power will reduce
and the Rx sensitivity will worsen, and so possibly 1 to 2
years after deployment, the network and/or monitoring
solution will begin to have reliability problems because of
insufficient light.
VSS’ Solut ion
VSS Monitoring’s vBroker Series of products offers a
range of active tapping options, starting from 1G right up
to 40G. These are called PowerSafe™ chassis modules,
which on their own provide the first three characteristics
listed above, and since they are used within a vBroker
chassis they also support the latter three characteristics.
Cisco’s 40G BiDi is one of the worst cases, where the
T he Fix
The solution to the problem is also a relatively simple one,
although may not very popular due to perceived risks and
increased cost of the solution.
Instead of passively tapping the fiber links, you can deploy
active optical taps, which do not employ any splitters, and
available optical budget between transceivers, even
excluding any cabling, is 2.5 dB between the minimum
Tx power and the least Rx sensitivity. The 40G PowerSafe
chassis module, available on the vBroker 400 family of
NPBs, has been acknowledged by Cisco as a great solution
to the 40G MM BiDi dilemma.
will actually regenerate the signal on both the network
For more information please contact us at [email protected]
VSS Monitoring is a world leader in network packet brokers (NPB), providing a visionary, unique systems approach to integrating
network switching and the broad ecosystem of network analytics, security, and monitoring tools.
VSS Monitoring, the VSS Monitoring logo, vBroker Series, Distributed Series, vProtector Series, Finder Series, TAP Series, vMC, vAssure,
LinkSafe, vStack+, vMesh, vSlice, vCapacity, vSpool, vNetConnect and PowerSafe are trademarks of VSS Monitoring, Inc. in the United
States and other countries. Any other trademarks contained herein are the property of their respective owners.
www.vssmonitoring.com
© Copyright 2003 – 2014. VSS Monitoring Inc. All rights reserved.
TN-20140627