Optical Traffic Analysis Points (TAPs) and
Their Effect on Fiber Link Budgets
Introduction
Networks comprised of switches, servers, and storage elements must work in unison to transport
important information in the form of data traffic. Networks need to be properly designed and maintained
to prevent transport delays and failures even when burdened with very large amounts of data traffic. The
need for real-time network visibility of data traffic becomes vital for effective performance monitoring,
robust security and functional business success. The method used to obtain real-time network visibility
must not disturb a network’s operations while providing precise, high detailed dimensional data statistics.
The deployment of optical Traffic Analysis Points (TAPs) within mission-critical networks is an effective
technique for obtaining real-time network visibility. But consideration must be given to the impact upon
the overall optical budget when TAPs are deployed. This Technology Brief will serve to provide guidance
with respect to TAP deployment.
What is a Traffic Analysis Point (TAP)? A passive optical Traffic Analysis Point (TAP) is a physical access point installed in networks that
provides real-time monitoring of a signal on an individual network link. It is a minimally-invasive
approach to monitor network link signals without causing disruption to the signal.
TAPs are commonly positioned in the network link between two network devices. Each TAP provides
two distinct paths of the same network signal by splitting the optical signal that is carrying the network
link. One path flows through the TAP onto the linked network device and the other path provides an
optical output monitoring port.
TAPs are designed to divert a portion, anywhere from 10% to 50%, of the optical signal via an optical
splitter to a monitoring port. Depending on the ratio of light diverted and manufacturer of the product,
TAPs introduce additional insertion loss into the network. With these additional insertion losses, the
overall fiber budget needs to be considered and carefully assessed in order to compensate for this
additional loss in the link.
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Although all TAPs are designed to complete the same task, they can exhibit different optical performance
characteristics.
When field testing multimode TAPs deployed in a network it is important to validate that the test
equipment source complies with Encircled Flux (EF) specifications in accordance with TIA-492AAAC-A
and IEC 60793-2-10 or IEEE 802.3 clause 52 (reference [23]).
Lower loss will be present in a channel when using a Vertical Cavity Surface Emitting Laser (VCSEL)
instead of a Light Emitting Diode (LED) due to a smaller spot size and numerical aperture (NA) exhibited
by a VCSEL. An under-filled launch that is not compliant to the EF standard requirements may alter the
TAP optical split ratio.
Figure 1 illustrates how a 10 Gb Fiber LC Tap is connected and how it integrates into a multimode fiber
network.
Figure 1. Fiber TAP Network Connection Path and TAP Cassette Ports.
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Fiber Link Budgeting
Per the IEEE 802.3ae 10G standard, the total power budget of a 10GBASE-SR Ethernet transmission
signal running over OM3 cabling (2000MHz*km bandwidth) is 7.3dB. Of the 7.3dB power budget,
1.68dB is not useable by the fiber channel and is reserved for transceiver characteristic penalties that
are controlled by the transceiver manufacturers. The other 5.62dB is available to be controlled by the
fiber channel designer (2.6 for total channel insertion loss and 3.02dB for inter-symbol interference [ISI];
a fiber cable characteristic).
The diagram below illustrates a typical 10G cross-connect channel comprised of LC-LC patch cords
connecting the switch and equipment with an MPO based horizontal cabling infrastructure with MPO to
LC cassettes at the endpoints and in the cross connect.
Figure 2. Typical 10G Cross‐Connect Channel Connectivity in this link is designed to perform at 0.5dB per connection; a cassette is considered one
connection (highlighted in grey) resulting in a total connector insertion loss of 2.0dB. The maximum
channel length allowable per the standard is 266 meters when applying this designed connector loss and
using OM3 fiber cabling.
When a TAP is inserted into the same link (shown in Figure 2); the overall connector insertion loss
increases due to the insertion loss of the TAP. The amount of insertion loss that is introduced by the
TAP varies depending upon the amount of light diverted from the network port to the monitor port, or split
ratio of the TAP.
Table 1 shows typical insertion loss values at the network and monitor ports of a 10G TAP for a
10GBASE-SR signal being transmitted over OM3 (50/125um - 2000MHz*km) fiber cable. Note that the
insertion loss associated with the network and monitor ports are dependent upon the split ratio. For
example, the first row of the table shows a split ratio of 90/10, which means that 90% of the optical signal
will travel straight through the TAP and continue to the end point of the link and 10% of the optical signal
will be diverted to the monitor port.
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Table 1. Insertion Losses by Split Ratios Network Port
Split Ratio Insertion Loss 90/10 1.3dB 80/20 1.8dB 70/30 2.4dB 60/40 3.1dB 50/50 4.5dB Monitor Port Insertion Loss 11.5dB 8.1dB 6.3dB 5.1dB 4.5dB The length of the fiber link will greatly decrease from the 266 meters previously calculated using 2.0dB of
connectivity in the channel with the additional loss associated from the TAP network port.
Example: Add a 90/10 split ratio TAP into the link at the cross connect, as shown in Figure 3.
Figure 3. Network Diagram of a 90/10 Split Ratio TAP Inserted at the Cross Connect
Adding a 90/10 split ratio (1.3dB insertion loss) into the fiber link increases the connector insertion loss of
the previous 2.0dB channel to 3.3dB. The maximum channel length allowable per the standard
decreases to 117 meters due to this connector loss increase while using the same OM3 fiber cabling.
The maximum link length when installing a TAP with the 90/10 split ratio decreases from 266m to 117m
based on the IEEE 802.3ae 10G standard.
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Extending the Reach of the Transmission Signal
Knowing that TAPs decrease the channel length, the way to compensate for this introduced loss is to
substitute the design with another type of fiber cable. Included in the power budget is the inter-symbol
interference (ISI) value of 3.02dB. This insertion loss value is dependent on the bandwidth of the fiber
cable that is installed in the link. For example, OM3 fiber is rated at 2000 MHz*km and OM4 fiber is
rated at 4700 MHz*km. Although the fiber cable attenuation is rated the same (3.5dB/km), the OM4
higher bandwidth fiber allows the total ISI in the power budget to decrease, allowing for a longer length of
cable plant for the same amount of connector loss.
Tables 2 and 3: Maximum 10GBASE‐SR channel lengths (for OM3 and OM4) with the maximum total channel connector loss. (The connector insertion loss minus the TAP insertion loss is highlighted in blue) Table 4. EXAMPLE USE OF TABLE; a link with a total connector loss of 3.4dB (1.0dB of connectors and 2.4dB of insertion loss on the 70/30 split TAP) has a maximum channel length of 98m utilizing OM3 cabling and 146m utilizing OM4 cabling 23 April 2013
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Conclusion
The deployment of optical Traffic Analysis Points within fiber paths in the network is an effective method
for obtaining real-time network visibility. Consideration should be taken so that the method used to obtain
real-time network visibility does not disturb network functionality.
Assessing and understanding the effect upon overall channel optical performance, with respect to
increased losses due to TAP deployment, has become an essential requirement in fiber network design.
Best network design practices should account for the effect upon the optical budget when deploying
TAPs into network fiber channels, which will help to ensure that a channel’s optical performance is within
specification.
About Panduit
Panduit is a world-class developer and provider of leading-edge solutions that help
customers optimize the physical infrastructure through simplification, increased agility and
operational efficiency. Panduit’s Unified Physical Infrastructure (UPI) based solutions give
Enterprises the capabilities to connect, manage and automate communications, computing,
power, control and security systems for a smarter, unified business foundation. Panduit
provides flexible, end-to-end solutions tailored by application and industry to drive
performance, operational and financial advantages. Panduit’s global manufacturing, logistics,
and e-commerce capabilities along with a global network of distribution partners help
customers reduce supply chain risk. Strong technology relationships with industry leading
systems vendors and an engaged partner ecosystem of consultants, integrators and
contractors together with its global staff and unmatched service and support make Panduit a
valuable and trusted partner.
www.panduit.com · cs@panduit.com · 800-777-3300
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