Description
What is a Passive Optical Network?
A Passive Optical Network or PON is a network in which fiber-optic cabling (instead of copper) brings signals all or most of the way to the end-user. It is described as passive because no active equipment (electrically powered) is required between the central office (or hub) and the customer premises. Depending on where the PON terminates, the system can be described as an FTTx network, which typically allows a point-to-point or point-to-multipoint connection from the central office to the subscriber’s premises; in a point-to-multipoint architecture, a number of subscribers (for example, up to 32) can be connected to just one of the various feeder fibers located in a fiber distribution hub, dramatically reducing network installation, management and maintenance costs.
Passive Optical Network (PON) challenges
The bandwidth boom is driving communications service providers (CSPs) to upgrade fiber-to-the-home (FTTH) architectures with next-gen passive optical networks (PON) in view of delivering gigabit broadband. However, next-gen PON is often overlaid on legacy PON, making the network more complex and adding challenges for field crews when it comes to testing and ensuring quality of service and quality of experience.
With multiple wavelengths at the customer premises, how can frontline technicians test through the complexity?
Points to PONder:
Measuring upstream and downstream power
Data from the optical network termination (ONT) is hard to measure because:
- During installation, an ONT is typically in keep-alive mode and emits in short bursts. The upstream transmission is therefore not continuous. Standard power meters cannot measure the power coming from an ONT.
- An ONT will transmit upstream only if it receives a downstream signal from the optical line termination (OLT) in the central office. To measure the power in both directions, the ONT must receive enough power and emit properly. Standard power meters cannot measure power from the ONT, because it stops emitting when not receiving a downstream signal from the OLT.
To overcome this difficulty, the test and measurement industry began developing pass-through PON-specific power meters (PPM) in the early 2000s capable of measuring power in both directions simultaneously.
Testing more wavelengths
With legacy GPON this is quite simple, as there is only one wavelength coming from the customer premises, and one or two coming from the OLT. Wavelength filtering is therefore not required upstream, and only minimally required downstream to separate the two wavelengths.
Next-generation PON, however, complicates things by adding several possible wavelengths, both upstream and downstream.
The wavelength filtering offered in standard PPMs is limited. If more than one wavelength or signal reaches a detector, either upstream or downstream, that detector provides the total measurement for both. This means that the technician cannot know if a specific wavelength is within parameters and if the service is being correctly delivered.
NG-PON2 and multiplexing
NG-PON2, a sophisticated PON technology, complicates matters even further with a process known as wavelength division multiplexing PON (WDM-PON). This process uses a specific but very narrow bandwidth to send multiple wavelength-dependent signals, which can significantly increase the number of wavelengths downstream. And again, each one needs to be measured individually with a pass-through device.
Passive Optical Network (PON) testing solutions
What’s needed is a testing device that discriminates between various wavelengths and technologies, detects several burst modes and has pass-through capabilities. The tester must cover both next-gen and legacy PON technologies.
EXFO’s PPM-350D next-generation PON power meter is one such innovative tool—designed specifically to test both legacy and next-generation PON technologies (EPON, GPON, XG-PON, XGS-PON, 10G-EPON, NG-PON2). It is also optimized for the keep-alive burst signal on the ONT side, providing power/loss measurement both for downstream and upstream signals.
How to choose a Passive Optical Network (PON) tester?
When selecting a PON tester, consider the benefits of smart automation. Look for the capacity to automatically detect the PON technology in use—legacy or next-generation—and the smart application of thresholds. EXFO’s patented PON-aware™ technology provides such built-in expertise. This will eliminate human errors and provide visibility into the complexity of today’s passive optical networks.
