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Packet-Optical

The Promise of Next Generation Metro WDM Access

Some tier 1 operators including China Unicom and Telefonica are championing a low-cost version of WDM for access network applications, including mobile fronthaul and backhaul and for fixed broadband access. The next generation of metro WDM technology, or NGM-WDM, has been discussed within the ITU since 2011, but after more than two years of discussions, the ITU-T began the official standardization process in April 2014.

The standardization is being led by ITU-T Study Group 15 (SG15), which is responsible for optical transport, and in particular, falls within the scope of Question 6 under the topic "multichannel bi-directional DWDM applications with port agnostic single-channel optical interfaces." Also called G.metro, the NGM-WDM standard is currently under study with a proposed completion timeframe of 2016.

Many details are yet to be worked out, but the recommendation describes an architecture in which a head-end connects to tail end equipment (TEE) using a single-fiber bidirectional "black link" connection. The black link connection creates a passive WDM access network that eliminates transponders from the network and thus saves cost compared to traditional WDM architectures.

The WDM elements include an optical multiplexier/demultiplexer, one or more optical OADMs, and an optical amplifier (which is optional). The initial recommendation calls for up to 10 Gbit/s of upstream/downstream bandwidth running on 40 or 80 channels per band and with reach spanning 20 km to 80 km.

There are different technology options being proposed for achieving the requirements of NGM-WDM, including:

  • Tunable lasers in the tail end equipment to achieve the port-agnostic requirements.

  • Ultra dense channel spacing combined with coherent detection to achieve large system capacity.

  • Wavelength reuse, meaning that the laser from the head end to tail end equipment is reused for the tail end to head end transmission. This technique eliminates the need for a TEE-located laser.

    In June, NGM-WDM proponents China Unicom and Telefonica released a white paper highlighting the benefits and applications of such a technology. The white paper identified increasing mobile access bandwidth requirements as the primary driver for the new WDM-based access architecture. In particular, authors observed that the mobile access bandwidth provided by LTE is approaching fixed line network speeds (i.e., up to hundreds of megabits per second). In the LTE era, NGM-WDM can be applied to the mobile backhaul network as well as to mobile fronthaul, or the transmission between remote radio units and the larger baseband units.

    While mobility requirements will drive the evolution, report authors also cite additional applications for NGM-WDM, including residential broadband, high-capacity leased lines, and fixed and mobile network convergence architectures. There are, of course, other competing and somewhat overlapping technologies in play targeting these applications, including TWDM-based NGPON2, so NGM-Metro success is not a given.

    However, tier 1 operator interest combined with the official ITU-T standards track sanction indicate that this is a new access option worth paying attention to in the coming years.

    — Sterling Perrin, Senior Analyst, Heavy Reading

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