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Fibre is being installed into homes and business around the country at an unprecedented rate. Although current technologies like VDSL2 and G.fast can cover current broadband needs, communication providers who want to be ready for the future must continue to invest in modern fibre optic technologies. All major and many smaller network operators are pursuing this route with Gigabit Passive Optical Network (GPON) being adopted as the preferred technology.
The advantage is that GPON supports high data rates (up to 2.5Gbit/s) over long distances (theoretically up to 20km) while ensuring a relatively simple and low-cost rollout as GPON does not require any active intermediate components (e.g. switches) that need their own power supplies. The signal is distributed passively in a point to multipoint topology. GPON is unique in that it functions as a shared medium, which means that all subscribers on a GPON branch must share the data rate and it's this characteristic that presents some challenges when it comes to commissioning and troubleshooting.
Measurement challenges
The point to multipoint topology makes commissioning and troubleshooting such a GPON network somewhat more complex certainly when compared with traditional xDSL in which each subscriber is connected via a modem exclusively to their own DSLAM port. In GPON the analogous devices are Optical Line Terminals (OLT) on the central office side and Optical Network Terminals or Units (ONT/ONU) on the subscriber side. Thus in GPON, multiple ONT's connect to a single OLT. The downstream data to the individual ONT's is transmitted at a wavelength of 1490nm and distributed to the subscribers with the aid of passive splitters. In practice most PON deployments have a split ratio of 1:32 but in theory it could be up to 1:128. Each subscriber can have a different line length however the OLT only transmits with a fixed optical power, eg +3.6 dBm.
Attenuation
Historically during commissioning of a PON branch each end point was tested to ensure sufficient optical power could be delivered. This type of measurement typically required a calibrated power meter with suitable accuracy. Furthermore an optical source of known power had to be connected to the correct PON branch at the headend. This was time consuming and cumbersome.
Modern GPON testing equipment should be able to communicate with the OLT and derive not only its transmit power but various other parameters such as its power class and the PON-ID. From this information a receive power threshold can be set and the engineer is able to simply move from end point to end point to obtain a power measurement not only as an absolute value but also as a Pass/Fail reading.
This test provides a quick overview of the fibre and its end point attenuation with a precise power measurement and a display of the PON-ID to verify that one is connected to the correct PON branch and that there's sufficient signal level to feed the GPON ONT.
Establishing a baseline
Once the PON branch has been installed it highly desirable to a establish baseline measurement across the entire branch spanning out to the end points. A test set that encompasses a workflow manager can assist by presenting the operator with a series of questions that establishes the PON topology. Data such as the PON location, PON-ID, number of end points, splitters, ONT type, etc. can be ascertained to define a precise measuring sequence that guides the engineer through testing of the entire PON branch.
At the end of the process, the test set generates a detailed installation report that precisely matches the commissioned PON branch. In addition to determining the attenuation to each end point, the PON-ID that is individually assigned to the PON branch by the OLT can be verified to ensure a match thus avoiding registering multiple different PON branches at once. Some instruments can even interpret the often highly customer specific PON-ID's and extract important information from them, for instance the assigned IP address or the port to which the end point is connected. An automated query of job data by the workflow manager is must-have feature. These data complete the handover report and provide baseline measurements prior to IP service roll-out.
GPON terminal mode
Once IP services have been deployed over the GPON the ability to test and verify service levels is of paramount importance. Customers may experience problems after set up of the ONT perhaps because the installation ID was not correctly transmitted, the PPP password is incorrect or the IP network behind the OLT is causing problems, test sets that only ascertain attenuation and PON-ID quickly reach their limits.
A test set that can completely replace the customer ONT and one that provides all the tools needed to verify and measure service levels is required. Just as for xDSL services a protocol must be initiated and then a services test performed. The foundation for customer satisfaction does not exist until adequate measurable service levels are attained.
By connecting with the OLT, the test set can acquire and display parameters and save them to a measurement report. It can register the OLT transmission level, the resulting attenuation and the PON ID along with the vendor ID, the own ONT ID and the power class. A GPON status trace can highlight which step of the authentication process the GPON is currently executing and whether there are any problems.
Performance tests
Once authenticated performance testing can occur perhaps in the form of a speed test to a designated remote service. eg a suitable HTTP, FTP, Ookla or iperf server. The performance could be measured at the Ethernet interface of the customer ONT, ie on the copper Ethernet interface, but also directly on the optical fibre too. The test set should be capable of providing the bandwidth actually promised to the customer, where necessary simultaneously with the other tests.
A test set must be agile enough to ascertain how much downstream bandwidth remains while one IPTV stream and multiple VoIP calls are in process at the same time. Triple play tests must be possible at the end of a GPON branch just as they are in an xDSL deployment. A multipurpose test set can simulate all devices needed, whether ONT, PC, set-top box or telephone, without having to utilise multiple devices on site.
Conclusion
In addition to a test sets measurement capabilities, which should provide a wide array of tools necessary to test and evaluate all aspects of service delivery, any test set should assist and guide the engineer in gathering and interpreting results through the use of both 'wizard' style workflow managers and easy to use 'quick tests'.
Test reports should be generated in a standard file format and should be easily distributed to cloud services via integrated WiFi or directly via the GPON interface. The test set should take the form of an all-in-one instrument, so that all tests are always at hand reducing the need to utilise multiple pieces of equipment when performing different tests.
Argus is a brand name of Intec GmbH. For over 30 years Intec have been successfully developing products for international telecom markets. Specialising in high quality telecommunication measuring devices Intec are a leading manufacturer of next generation PON, xDSL, IP and fibre optic measuring equipment.
All Intec Argus testers are developed and manufactured in Germany. Argus testers combine high user friendliness with all the features needed for on-site testing. They simplify in service qualification and troubleshooting for both copper and optical networks and provide SLA verification of Triple Play services such as VoIP, IPTV and IP data. These features make Argus testers an indispensable tool for the maintenance of modern service centric networks.
Intec’s new ARGUS 260 is a next generation multipurpose tester. It unites in a single instrument the functions of both a GPON & xDSL testers providing carriers and network operators the ability to commission and maintain copper and fibre networks with a single comprehensive test solution.
The generous touch display permits in-depth tests such as the optical fault finder and fibre inspection tool. The ARGUS 260 is additionally equipped with an optical light source that transmits a defined ID on a selected wavelength, and can be expanded using the optical power meter (OPM) to form an extremely precise (± 0.25dBm) optical loss test kit without having to cart around a full bag of gear.