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Published on May 6, 2015

Dispersion Testing…Demystified!

Are you in charge of a mobile backhaul network? Are you faced with dispersion challenges that are increasing your bit error rates (BERs)? Do you know what dispersion tests work best for mobile backhaul deployments?

Although dispersion testing was traditionally performed in metro and core networks, it is now, more than ever before, moving closer to the network edge in mobile backhaul deployments or client-side rollouts. But why?

Before we get started, let’s recall the two main types of dispersion: Chromatic dispersion (CD) and Polarization mode dispersion (PMD)—both which can lead to an increase in bit error rates (BERs). Chromatic dispersion (CD) is pulse broadening due to different wavelengths inside a pulse of light traveling at different speeds. Polarization mode dispersion (PMD) is pulse broadening caused by the differences in propagation velocities for different polarization states.

Although bandwidth demand is increasing throughout the entire network, growth is occurring fastest in the mobile backhaul and

fronthaul, driven to a large extent by the extensive use of smartphones and tablets. Dispersion issues have started to arise in mobile backhaul for two main reasons: the increase in distances and faster data rates (10G).

In the past, at 1 Gbit/s or 2.5 Gbit/s, CD and PMD tolerances were very high and manageable. However, with 10G becoming ever-more present, thanks to data-hungry applications, such as video streaming and web browsing, we are beginning to see more and more dispersion related outages. Moreover, the distance in mobile backhaul deployments is increasing; while some are short (less than 5 km), others can span up to 120 km. 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). It therefore makes sense to test dispersion on longer mobile backhaul networks or the metro rings that feed them.

For optimal dispersion testing, there are two solutions available on the market: dual-ended testers, which require a source at one end and a detector at the other, and single-ended testers, in which the source and detector are contained within a single instrument. EXFO’s FTB-5700 is the ideal solution as operators only require one technician since the source and detector are included together in the solution. This can substantially reduce manpower and operational costs. In fact, whether mobile backhaul topology is point-to-point, star or ring, operators can save between 50% and 75% on costs thanks to single-ended dispersion testers.

If you are looking for more information on dispersion analysis and testing methods for any backhaul topology using EXFO’s FTB-5700, download the free Dispersion Testing in Mobile Backhaul Deployments whitepaper.