Optical network testing in a 5G environment

April 20, 2020
You can’t have 5G without fiber – or at least that’s what the fiber community will tell you. Since they'll be right in most instances, the time to consider the fiber test requirements for this application is now.

You can’t have 5G without fiber – or at least that’s what the fiber community will tell you. And while wireless options and HFC will have roles to play in 5G fronthaul, midhaul, and/or backhaul networks, the fiber folks are likely going to be right more often than not in their assertions. And that means more fiber testing, particularly as the number of fibers involved likely will be greater that in 4G support applications. Whereas operators have grown accustomed to a pair of fibers to each radio unit, some experts suggest that 5G radio units might require as many as 12. And that could mean new cable compositions and connectors, such as MPOs.

Add in the fact that 5G will require significantly more radio units than 4G to cover the same service area, and field technicians will be very busy with fiber as 5G rolls out. So regardless of whether an operator plans to support their own 5G mobile services or supply backhaul/midhaul/fronthaul assistance to others, the time to consider the fiber test requirements for this application is now.

Start with the basics

There are several fiber network topology options coming to market to support 5G requirements. Most are based on either some form of wavelength-division multiplexing (WDM) or passive optical network (PON) technology. Yet some basic aspects of fiber testing will remain constant regardless of the network topology.

The first is fiber inspection. Light should efficiently enter the fiber on one end and exit at the other, naturally. This evaluation process begins with endface inspection; as any fiber veteran will tell you, dirty connections are the root of most problems. Endface inspection tools are widely available from a variety of sources. Several of these vendors have developed MPO-friendly tools that can make inspection easier and more efficient when the technician is confronted with one of the multi-fiber connectors expected to be used more frequently in 5G applications. Many of these tools provide automated pass/fail inspection, which can be save a lot of time and headaches.

Once you’ve determined that connector-related debris won’t get in the way of your signal, it’s time to do Tier 1 and Tier 2 fiber certification. Tier 1 involves the use of a light source on one end of the fiber and a power meter on the other to ensure that the amount of light that makes it across the connection is sufficient for the requirement.

Tier 2 sees the use of an optical time domain reflectometer (OTDR), which further characterizes the fiber and can uncover any issues that might have been detected during the Tier 1 test. OTDRs identify, measure the effects of, and locate just about anything that can degrade an optical signal. These “events” can be as innocuous as a mated connector or splice or something potentially more harmful such as a bend or break. As the unit’s name implies, the OTDR is able to detect light reflecting off of any of these events, which means it can be effective with access to just one end of the fiber.

Measurements a typical OTDR can make include:

  • Attenuation, which is the optical power or signal loss or the rate of loss between two points along the link.
  • Event loss, which is the difference in the optical power before and after an event.
  • Reflectance, which is the ratio of the reflected power to the incident power of an event.
  • Optical return loss (ORL), which is the ratio of the reflected power to the incident power for a fiber link.

While the ability to perform these tests from just one end of the link is certainly efficient, test vendors suggest that field technicians perform an OTDR examination on both ends of the link. Events can have differing effects on optical transmission depending upon the direction from which the transmission originated, they point out. In particular, fiber backscatter coefficient mismatches can cause a splice to appear as gain in one direction and a loss in another. Averaging the gain and loss recorded via bidirectional measurements can provide a more accurate portrayal of a splice’s loss characteristics.

WDM complexities

While these measurements are commonly performed regardless of the architecture choice, WDM and PON each bring their own set of requirements. As WDM is likely to be used more widely for 5G support networks, we’ll start there.

WDM, of course, sees multiple wavelengths transmitted across a fiber, generally (but not always) in the same direction. Again, Tier 1 and Tier 2 type tests are required, and they’re quite similar in philosophy to the tests just described. But they’re necessary because individual wavelengths in a multiwavelength transmission are more susceptible to link events and interference from fiber elements and each other than a single-wavelength signal. And, particularly in DWDM applications, optical power might not be distributed evenly among all the wavelengths, meaning one wavelength might stream safely through a particular link and another might encounter problems.

Therefore, technicians must repeat the Tier 1 testing that combines a light source on one end and a power meter on the other. But, in this instance, the light source must be able to transmit across multiple wavelengths and the power meter (often called a “channel checker”) must detect incoming wavelengths across the same spectrum. The testing requires a degree of coordination between the technicians on each end of the link to make sure the instruments on the two ends are in synch. Tests sets are available that can not only perform wavelength presence and power level measurements, but also detect drift, offset, and channel spacing.

WDM link Tier 2 testing once again calls for an OTDR, but one that can perform the evaluations mentioned above across multiple wavelengths. OTDRs can be used both before multiplexing/demultiplexing is applied to a link as well as afterwards for certification and, when necessary, troubleshooting.

Pondering PON

PON hasn’t been a popular method for backhauling 4G network traffic, principally because GPON and EPON didn’t match well with the latency requirements of CPRI. However, with the advent of eCPRI and time-sensitive Ethernet in 5G schemes on the one hand and 10-Gbps PON approaches on the other, PON may prove a tempting choice for some operators, particularly if the infrastructure also can be used to support residential and business services delivery.

From a testing perspective, the wild card in PON deployments is the splitter, which adds another loss element to the network and can make end-to-end testing tricky. Test vendors recommend that one-sided OTDR tests be conducted from the optical network terminal/optical network unit end, which will be from the radio unit back toward the next hub. However, bidirectional OTDR testing is also recommended, as PONs transmit signals on multiple wavelengths and in each direction. OTDRs specially designed for the complexities of PON infrastructure are available from multiple vendors.

And you’re ready to go

Once you’ve certified that the fiber network should be able to handle 5G traffic, the network operator isn’t done with testing, of course. The transmission health of that traffic and overall network operation needs to be ascertained and monitored (as described in "Fiber Optical Testing Needs for the 5G Network Evolution"). But if the steps described here are taken, the foundation for a successful 5G fronthaul/midhaul/backhaul network will have been laid.

Stephen Hardy is editorial director of Broadband Technology Report.

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