The requirements for testing fiber optic networks will vary depending on the specific type of network as well as the network designer’s overall test requirements. Regardless of type, there are two basic or generic pieces of Optical Test Equipment that will be used; an Optical Time Domain Reflectomer or OTDR, and a pair of optical test equipment pieces that are referred to as a Power Meter & Light Source. These tests are typically measured in “dB”. The term dB is the expression of attenuation or power loss over an optical fiber as it travels from a termination end to a point along the fiber’s path. Once an optical fiber is connected to a piece of active equipment then all tests are in dBm. The active equipment will be transmitting an actual or real optical power at a specific wavelength and referenced at 1mW.
A PON power meter is essential for field technicians installing or maintaining any type of PON network. PON Power Meters are able to simultaneously test upstream and downstream through optical fibers, at 1490nm, 1550nm, and 1310nm wavelengths, as well as estimate signals of the voice, data, and video streams.
The term PON stands for “passive optical network”. A PON is a fiber optic telecommunications network that delivers broadband to transmit data over fiber optic cables to the customer premises. Its architecture implements a point-to-point or a point-to-multipoint arrangement of nodes in a communications network. A Point-to-Multipoint network uses a single fiber to serve multiple endpoints by using unpowered or passive fiber optic splitters. A splitter is used to divide the fiber bandwidth among multiple access points. Passive optical networks are often referred to as the “last mile” between an internet service provider (ISP) and its customers.
Point-To-Point or P-T-P type optical network
A P-T-P network is a network that has two termination points and nothing in-between. As with all fiber optic networks, when it is being constructed the fibers must be terminated to allow for any tests to be performed. So one end of the network is terminated and an OTDR test is performed on each fiber to ensure that the termination and length of fiber beyond does not have any issues. That test result will be stored for future needs and noted in “dB”. If the network requires splicing then after the fibers are spliced, the OTDR again is used to ensure that the splice and added fiber length again meets requirements. The testing with the OTDR continues and is completed after the end of each fiber is terminated. At this point, another set of tests is required which is commonly referred to as an End-To-End test. This test requires the use of a light source and a power meter and again all test results are stored. The optical power meter will be set to “dB” and referenced to a light source which is typically called “Zeroing”. The units are then moved to opposing ends and the field technicians will send and receive wavelengths specified by the designer. Again this is a measurement that will be used against the designer’s overall Link Loss Budget. The P-T-P network will have its termination ends referred to as “A” and “B” and at least two unique wavelengths will be sent and received over each fiber. This is typically required to ensure that any wavelength used by a transmitter can be used between the two that are specified. The network designer will define these wavelengths as well as provide a label for these ends. The technician that is performing the tests will reference these labels in any reporting back to the designer.
Point-To-Multipoint PON type network
Now when a Point-To-Multipoint network is constructed such as a Passive Optical Network or PON many of the tests and test equipment remain the same but will require a few special features. The OTDR testing during construction remains the same, with tests performed each time a fiber is terminated or spliced. Again, this continues to the far ends of the fibers after they are terminated. Once all fibers are terminated, again they will be tested using a power meter and light source. The need for special equipment is required for the activation phase of a PON network and that’s where the similarity between a P-T-P and a P-T-Multipoint ends.
The PON network activation phase begins by connecting a power meter up to an active piece of equipment called an Optical Line Terminal or OLT and set to the appropriate wavelength and the unit is set to “dBm” and this becomes the referenced power.
NOTE: There are several generations of PON network OLTs which use different wavelengths so the optical power meter must have the capability to be set to those wavelengths. GPON is 1490nm, XGPON is 1577nm and NGPON has multiple wavelengths ranging from 1596nm to 1602nm.
As the links are connected out to the far end, the technician repeats the test and ensures that there are no issues. This testing continues out to the far end, which in a PON network there is a piece of active equipment called an Optical Network Terminal or ONT or at times called an Optical Network Unit or ONU. Regardless, that piece of equipment receives light from the OLT transmitter and communicates back to the OLT with its own transmitter. The ONT cannot communicate back to an OLT without first receiving the OLT’s transmitter’s wavelength. At this time, there is an absolute need to use a specialized optical power meter which can measure the OLT’s power and allow that power to pass through and provide the signal to the ONT/ONU so it can send back a signal.
The PON meter has two test ports; one is named DROP and the other ONT or ONU. The technician connects the drop, which is connected via a fiber all the way back to the OLT into the port named DROP and then connects the ONT/ONU connectorized fiber pigtail into the ONT/ONU port. Now the PON meter is in-line between the OLT and ONT/ONU and allows for the OLT to communicate with the ONT/ONU. The technician again will be observing the incoming OLT power level, as well as the outgoing ONT/ONU power level. If all is well, the drop is connected to the ONT/ONU and service activation tasks can continue.
I hope you have found this article informative and that you learned a bit about FTTx networks.