CD/PMD TESTING IN MOBILE BACKHAUL DEPLOYMENTS

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Although capacity is expanding throughout the network, mobile backhaul is where growth is occurring the fastest. Mobile backhaul refers to the part of the network that connects the cell site base stations (also called baseband unit or EnodeB) to the metro network. With metro network spans sometimes exceeding 100 km, and mobile backhaul ranging from a few kilometers up to 120 km in length, these long distances can create dispersion issues which greatly degrade signal quality—since dispersion increases with distance. In addition, data rates now often reach and above 10 Gbit/s in mobile backhaul deployments; a data rate at which dispersion issues start to appear. It is therefore recommended to thoroughly test dispersion after construction or prior to upgrades.

Dispersion consists of two main phenomena, both of which ultimately lead to BER:

• Chromatic dispersion (CD), which is pulse broadening due to different wavelengths inside a pulse of light traveling at different speeds.
• Polarization mode dispersion (PMD), which is pulse broadening caused by the differences in propagation velocities for different polarization states.

Dispersion issues have started to appear in the mobile backhaul for two main reasons: the increase in distances and faster data rates (10G). Table 1 shows the typical tolerances for CD and PMD as a function of data rate, i.e., the maximum values that a system can support without failures.

The second reason why dispersion can now become problematic in mobile backhaul networks is due to the increase in distances. Although some mobile backhaul deployments are short (less than 5 km), others can stretch up to 120 km. On the theoretical side, CD increases linearly with distance (e.g., doubling the distance doubles the CD), and PMD increases with the square root of the distance (e.g., multiplying the distance by four doubles the PMD). A good rule of thumb is to test dispersion in spans longer than 15 km or 20 km. To prevent CD/PMD-related failures, it therefore makes sense to test dispersion before commissioning long mobile backhaul networks or the metro rings feeding them.
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Today’s Challenges

The massive deployment of ROADMs in metro mesh networks. The explosion of 10 Gbit/s—and even 40 Gbit/s—transmission. Widespread triple-play services. All of these technological developments, along with several other market factors, are putting tremendous pressure on installed networks. However, in today’s highly competitive industry landscape, capital expenditures (CAPEX) and operational expenditures (OPEX) are what make it or break it for network service providers.

This added pressure on networks translates into new technology challenges, namely when it comes to chromatic dispersion (CD) and polarization mode dispersion (PMD). More specifically, since they are an even bigger hindrance at higher transmission speeds, CD and PMD must be measured more regularly than ever. Yet, more than ever, budgets to do so are limited. The need to test grows much faster than testing budgets. What is required is an efficient, affordable means to accurately testing both dispersion types.

One way to make testing more efficient and affordable is to carry it out from a single end of the link, since this reduces CAPEX (one test instrument instead of two) and OPEX (one technician instead of two, fewer truck rolls).

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EXFO Solution : SIngle-Ended Dispersion Analyzer EXFO FTB-5700

EXFO FTB-2 Pro with FTB-5700 Single Ended Dispersion Analyzer Module

The FTB-5700 is a single-ended, one-box dispersion analyzer that measures both PMD and CD and delivers valuable benefits: no test delays because of equipment shipping or technician synchronization, shortened test cycles, increased accuracy and efficiency and, ultimately, reduced CAPEX and OPEX. The bottom line: precise network qualification does not compromise the budget, and vice versa.

Testing Topology

In addition, a single unit means a single software to learn and use, simplifying training and minimizing human errors. The output is also a single test file to manipulate instead of two separate files that need to later be combined to produce test reports.

What’s more, using an all-dispersion analyzer avoids having to disconnect the fiber between CD and PMD tests. Since more than 75% of network problems are caused by dirty or damaged connectors, less manipulation is always good.

( For more information about FTB-5700, click here )