The Power of Reverse Power

Telcos are gearing up for the hot new ultra broadband access, G.fast, which delivers up to 1 Gbps of ultra high-speed broadband access over existing copper wiring. The operators who are quickly deploying G.fast services are leveraging 3 central technologies: G.fast communications, fiber-to-the-distribution-point architectures, and reverse power feeding.

G.fast is the International Telecommunications Union (ITU) standard for ultra broadband Internet access that was approved in December 2014. Fiber-to-the-distribution-point (FTTdp) is the system architecture established by the Broadband Forum, which is designed to move the centralized equipment closer to the home to the distribution point (from the DSLAM). Reverse Power Feed (abbreviated RPF) is a capability that makes the FTTdp easy to deploy away from existing electronic infrastructure while ensuring that G.fast stays the lowest-cost-per-megabit ever delivered. The RPF handshake protocol is being standardized by the European Telecommunication Standards Institute (ETSI) and the RPF deployment profiles and requirements are formalized by the Broadband Forum.

G.fast Benefits

This ultra broadband standard offers plenty for everyone to get excited about:

High performance. G.fast delivers up to 1 Gbps over existing copper wires through a combination of 5x more spectrum, shorter distances (400 meters or closer), and built-in vectoring (which eliminate crosstalk). It also supplies flexible upstream and downstream ratios to better adapt to new consumer requirements.

Consumer self-installation. G.fast is very robust and doesn’t require pair bonding to achieve great performance. The infrastructure is designed for the operator to set up the FTTdp unit and then simply ship a CPE device to the consumer for self-installation (similar to the installation of ADSL). The distribution point unit (DPU) is installed without the need to add new power sources/infrastructure at the distribution point.

Zero touch maintenance. G.fast is very robust and is designed to better handle interferers that could harm the signal or cause the system to lose its link. From the beginning, G.fast was designed to avoid the need for telcos to roll trucks if there is any downtime or provisioning. Everything will be possible to manage and provision remotely.

In the FTTdp architecture, G. fast gets these results because the DPU is located close to the home — at least 50 percent closer than most DSLAMs — in many cases as close as 50-100 meters from the subscriber. This allows for the achievement of high performance, but at the challenge of how to power the device affordably in this new location. That’s where reverse power comes in.

The Power of Reverse Power

To power the box at this distribution point, current must be supplied. There are 3 options:

Option 1: Have the local utility to install new electrical services.

Option 2: Forward power the DPU from the cabinets. As the distance between the Cabinets and DPUs is very long, this option requires additional power supplies and batteries in the already busy cabinets. In addition, the aging copper cables between the cabinets and the DPU need to be maintained
alongside the new fiber infrastructure.

Option 3: Use reverse power feed: power that is delivered over the phone line from the CPE device to the DPU. That’s where RPF comes into play. With RPF, the CPE device injects current onto the telephone wires, which are connected to the DPU. Each subscriber shares in the load of power.

Safe Operation

Reverse power feed is applied by the RPF power sourcing equipment (PSE) located at the CPE. Power would be sourced only upon a successful detection of a valid RPF-powered device (PD) located at the DPU. This capability is guaranteed by the RPF startup protocol, which performs detection and
classification of the DPU as part of the power-up sequence.

The RPF protocol would ensure immediate safe power removal at any of the following events:
• a valid DPU does no longer exist on the other end.
• the cable connecting the CPE and the DPU is removed or shorted.
• a phone connected to the line without an adaptor goes off hook.
• any line error condition defined by the ETSI standard.

G.fast is very power efficient and green. By using time division duplexing (TDD) instead of frequency division duplexing (FDD) used by VDSL, G.fast modems consume power only when they transmit and receive. Inactive ports are powered down to reduce power. This allows for G.fast DPUs to be deployed with a small power footprint — in under 15 watts when only 1 line is active.

Will this cost the consumer extra? The average consumer will pay between 2-8 cents per month in electricity for powering the DPU — which is actually less than they pay for the power consumption of VDSL today. Plus, with reverse power feeding systems, DPU power consumption is evenly distributed over all subscribers so the load is shared equally.