Fiber to the x (FTTx) is a generic term for broadband network architecture that uses optical fiber to replace the copper cabling usually used for connecting the network to the end user. E.g. fiber-to-the-building (FTTB) is a term that defines the scenario when fiber reaches the boundary of the building, such as the curb or basement in a commercial or residential building. The fiber is there connected to some kind of connection device where the final connection to the tenants is made via alternative means.
An Optical Network Unit (ONU) is an example of such a connection device. When installed close to, or in, a commercial or residential building the connection device is often referred to as a Multi-Dwelling Unit (MDU). The ONU delivers services over Very-high-speed Digital Subscriber Line (VDSL2) or Ethernet from a Giga-bit or Ethernet Passive Optical Network (GPON or EPON) feed. The services are provided to the ONU via a fiber connection coming from an access node (OLT—Optical Line Termination) in a central office. The ONU converts the optical signals in the optical fiber to electrical signals for delivery to the tenants. The ONU is not powered locally, but retrieves power from a remote power station in a remote terminal (RT) that can be located far away. The powering is provided via already existing twisted pair telephone copper from the RT to the ONU. The twisted pair telephone copper was once being used for Plain Old Telephone Service (POTS) and later for Asymmetric Digital Subscriber Line (ADSL) containing POTS. Due to the small wire gauge of the twisted pair telephone copper the power supply in the RT must deliver the power using high DC voltage since the use of low DC or AC voltage would result in high losses. The RT may therefore up-convert −48V (which is the typical operating voltage of the RT) to +/−190V which is necessary to deliver enough power (˜95 W per power channel, 380V, 250 mA max) to the ONU. Depending on power consumption and distance, one or two power pairs are typically utilized. Accordingly, the ONU down-converts +/−190V to −48V to power internal electronics. The RT may be placed in the central office or in another location where access to the AC power grid is available.
To protect the RT and ONU equipment from surges during lightning, primary (gas discharge tubes) and secondary (sidactors) protection mechanisms are deployed in both the RT and the ONU, effectively shortening power leads to ground, bleeding surge energy off. This causes power outages in the area of tens of milliseconds that leads to service unavailability for users. Therefore, hold-over capability in form of an inexpensive hold-over circuit containing low-lost and low-value capacitors may be installed at the ONU to power the device during the short time period of power loss from the remote up-converter.
When a ground fault is detected on the power leads, the up-converter performs a safety power shutdown of the RT for between 60 milliseconds to several seconds according to safety standards. This results in loss of power from the up-converter of the RT to the ONU. For economic reasons an ONU should preferably not be provided with any expensive hold-over capability of several seconds such as ultra-capacitors, batteries, etc. Consequently, a ground fault leads to a reboot of the ONU leading to service outage to all costumers of the ONU for some minutes.
Ground faults do not occur frequently since they are introduced by safety breaches caused by humans or animals touching the leads or power crossover. However, the poorly shielded and aged telephone copper that feeds from RT to the ONU can be very sensitive to external disturbances such as interferences from close-by power distribution and transmission lines causing common mode transients which result in current imbalance. Sometimes the up-converter misinterprets these disturbances as ground fault events causing unnecessary safety power shut downs.
There is thus a need for a method and an apparatus that is resilient to interferences but still complies with existing safety standards.