CableLabs backhaul spec gets speed boost

Faster fibre will help networks cope with the Netflix effect

Late last week, CableLabs launched another bandwidth-boosting project, this time designed to sweat more gigabits-per-second out of the optical leg of cable networks.

The US cable industry's research arm launched its Full Duplex Coherent Optics project, designed to double the capacity of the upstream channels from HFC nodes to the exchange.

The new work is a development on CableLabs' existing point-to-point Coherent Optics specification project, distinguished technologist Steve Jia wrote.

Coherent optics replaces simple on-off keying (OOK, as used in PON deployments) with modulation on both amplitude (amplitude shift keying, ASK) and phase (phase shift keying, PSK) to expand the capacity of the fibre.

CableLabs last year decided to adopt coherent optics to solve both the capacity crunch and a fibre shortage in cable networks. As Alberto Campos (also a distinguished technologist) wrote at the time: “In some cases, only the two primary fibres that are feeding the fibre node remain available for access transport”, because other fibres in the infrastructure had been repurposed to provide business services and cell backhaul.

The latest project, Full Duplex Coherent Optics, was designed both to add capacity and to make coherent technologies more deployable in cable network access fibres, Jia wrote.

The capacity boost is simple: with full duplex operation, each fibre would get double its current capacity.

Where there are two fibres available, the approach is simple: upstream traffic uses one fibre, downstream traffic uses the other, and both directions use the same wavelength.

However, Jia added that a survey of US cable operators found 20 per cent of them only had a single fibre available, and predicted that number to grow to 60 per cent within five years.

The simplest way to handle full-duplex optical communications is to use two fibres at different wavelengths, but that needs two expensive and power-hungry lasers, so CableLabs is taking a different approach, using “two optical circulators on each end in a special configuration”.

The circulators isolate upstream from downstream signals at the same wavelength, meaning the transceivers only need a single laser.

This “direction-division multiplexing”, Jia wrote, is not sensitive to either bandwidth or wavelength, and CableLabs said it's been tested to distances up to 100 km.

Since the circulator is a common off-the-shelf component, products implementing the Full-Duplex Coherent Optics spec would probably integrate it into existing transceivers, giving carriers a pluggable upgrade.

The Register notes that this project is not related to the full-duplex DOCSIS 3.1 project, which offers a speed boost on the copper leg of the network. ®

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