Analysis of EPON optical link diagnosis problems

In recent years, major operators have made remarkable achievements in the construction of FTTx access networks . However, with the continuous expansion of the PON network, the number of access nodes has also increased dramatically. The problems of power supply guarantee, link interruption, equipment failure, etc. brought by the huge number of access nodes have brought considerable pressure to the daily maintenance and troubleshooting of FTTx. How to improve the maintenance efficiency of FTTx and reduce OPEX will be one of the key issues in the future FTTx construction. In particular, as the access points move downward, the access equipment is moved out of the machine room and installed in various natural scenes. The temperature, humidity, power supply, and network point enclosure have dropped sharply compared to the original equipment environment. The maintenance problem of EPON optical links is particularly prominent.

EPON optical link diagnosis issues

In terms of link maintenance, the maintainability protocol definition of GPON was originally stricter and more complete than that of EPON. However, in recent years, thanks to the large-scale commercial development of EPON, at the equipment level, EPON's OAM and protection switching capabilities have surpassed GPON equipment.

This paper focuses on the EPON optical link protection and remote diagnosis management functions and the implementation of UTStarcom's corresponding management system.

In the PON system, it is an important OAM function to detect EPON optical path anomalies introduced by ONU. This is because in the PON system, upstream data transmission is realized by TDMA time division multiplexing, and each ONU sends data to OLT after obtaining an available time slot. The PON upstream service flow allocation process during normal operation is shown in Figure 1.

The ONU3 distribution fiber may be faulty, causing abnormal optical power received on the OLT side, as shown in Figure 2.

If an ONU has abnormal upstream data transmission, such as occupying too many time slots or continuously sending data, the faulty ONU will affect all other ONUs under the same PON port, as shown in Figure 3.

In actual operation, the transmission quality of the optical fiber of the PON link may gradually decline due to aging. At this time, the system should be able to detect the problematic optical fiber in advance to avoid affecting the user's business. Therefore, in the EPON system, it is very important to detect the optical path anomaly introduced by the ONU. The EPON maintenance system needs to be able to find and isolate the faulty ONU or link to ensure the normal operation of the system. On this basis, UTStarcom provides powerful fault analysis and early warning capabilities to provide further convenience for system maintenance.

Maintenance of EPON Optical Links

As the next generation FTTx access technology that supports wide-area optical path coverage, the maintainability of the PON system ODN is the key to reducing overall OPEX. The EPON system supports complete EPON optical link and ONU optical performance diagnostic functions, including:

From the OLT strategy, the optical power of each ONU on the PON optical link;

When ONU loses power, it reports a "Dying Gasp" power failure event;

Diagnose PON line anomalies, identify and isolate faulty ONUs from the PON link.

The following takes the relevant design and implementation of UTStarcom EPON system as an example to discuss and analyze the relevant key technologies of the optical link measurement and diagnosis functions of the EPON system.

OLT received optical power detection

UTStarcom EPON OLT supports the measurement function of the average upstream optical power received from each ONU. The measurement accuracy in the range of -30dBm to -10dBm is no less than ±1dB, and the minimum measurement sampling time is less than 600ns (i.e., no longer than the signal duration of a standard 64-byte minimum length message. In some cases, the ONU only sends OAM and MPCP messages of the minimum length to the OLT. At this time, the OLT can still accurately detect the received optical power from the ONU).

When the upstream optical power received by the OLT from a certain ONU is too low (lower than the upper limit of OLT sensitivity specified in the standard) or too high (higher than the lower limit of OLT overload optical power specified in the standard), the EPON OLT generates a corresponding optical power over-limit alarm. The OLT also supports the measurement of the upstream optical power of the ONU under the PON interface to realize the fault diagnosis and performance prediction of the optical link.

Fault diagnosis: Analyze the attenuation of the optical link and other indicators based on the optical power of each ONU received on the PON interface to see if they are normal.

Performance prediction: By regularly measuring and analyzing the upstream optical power received from each ONU, it provides a prediction of the optical link attenuation or optical module performance.

OLT also provides monitoring of its own optical module operating temperature, supply voltage, bias current, transmit optical power and other parameters.

Measurement of diagnostic parameters of optical transceivers in ONU

UTStarcom EPON ONU supports the optical module digital diagnostic monitoring interface function based on SFF-8472, providing monitoring of optical module operating temperature, power supply voltage, bias current, transmit optical power, receive optical power and other parameters.

The EMS and OLT of the UTStarcom EPON system support real-time measurement of the above parameters of specific ONUs, and also support policy-based optical link measurement and digital diagnostic functions for specific or all ONUs, such as measuring optical module parameters for specific ONUs or specific ONU groups in a specific area at a specific time to achieve regular monitoring and analysis of optical modules.

ONU optical transceiver parameter out-of-limit alarm

When one or more parameters of the ONU's optical transceiver are too low (lower than the set threshold) or too high (higher than the set threshold), the ONU can send a corresponding over-limit alarm (Alarm) or over-limit warning (Warning) to the OLT through the event notification mechanism.

Powerful UTStarcom integrated network diagnostic system

Combining the above optical link measurement and diagnosis technologies, UTStarcom's integrated network diagnostic system CNDS (Centralized Network Diagnostics System) can gradually deploy functions such as fault warning, service statistics, and user capability assessment while realizing intelligent diagnosis, thereby continuously improving the operation and maintenance efficiency of the EPON network, reducing OPEX, and improving user satisfaction.

UTStarcom CNDS is a powerful integrated end-to-end diagnostic system for UT NGN and access networks, supporting functions including network scanning, line quality monitoring, user service information, node/module/port diagnosis, network connection health diagnosis, task management, optical link diagnosis, etc. CNDS can detect the service quality of user lines and report user line faults that may cause user data or voice service quality problems. CNDS also provides expert fault diagnosis and analysis functions to help operators find the root cause of the fault and give final fault handling suggestions based on the analysis results. CNDS has a friendly web-based operation interface for operators to handle different fault types. As a subsystem of CNDS, UTStarcom's PON optical link diagnostic function can provide powerful fault location functions and remote troubleshooting support, representing an important development direction of FTTx network maintenance technology.