ITU-T approves XGS-PON, amends NG-PON2 specifications

ITU-T Study Group 15 members have given first-stage approval (or, as the ITU-T calls it, "consent") to a pair of next-generation PON specifications. Recommendation ITU-T G.9807.1 "10-Gigabit-capable symmetric passive optical network," more commonly known as XGS-PON, will offer symmetrical 10-Gbps optical transmission capacity. ITU-T G.989.2 Amendment 1 is an update to the NG-PON2 physical layer specification that uses time and wavelength division multiplexing (TWDM) to support four or eight symmetrical 10-Gbps PONs on the same fiber.

The physical layer of XGS-PON follows XG-PON (ITU-T G.987.2) and 10GE-PON (IEEE Standard 802.3), which means systems can be designed using existing 10-Gbps symmetrical optical transceiver components. The XGS-PON protocol layer is based on NG-PON2 (ITU-T G.989.3) and XG-PON (ITU-T G.987.3), and its ONU management and control mechanism is specified in ITU-T G.988.

Use of these various specifications has significant implications. XGS-PON likely will be used as an intermediate step between GPON and NG-PON2, enabling operators to support symmetrical 10-Gbps applications that may not require the multi-wavelength future-proofing of NG-PON2 or where immediate competitive situations demand a lower-cost, more immediate approach. The lower expected cost versus the use of a single-wavelength NG-PON2 implementation is due to the use of fixed-wavelength optical transceivers, rather than the tunable optics the NG-PON2 specifications will codify.

The ITU-T says a typical distance between an XGS-PON optical line terminal (OLT) and an optical network unit (ONU) will be 20 km, with one OLT capable of supporting up to 128 ONUs.

The availability of XGS-PON systems likely will limit, if not eliminate, demand for XG-PON1 systems, which offer 10-Gbps only in the downstream direction. The availability of 10 Gbps upstream via XGS-PON is seen as more appropriate for the business services and mobile backhaul applications for which such systems are most likely to be used.

Meanwhile, the NG-PON2 amendment affects ITU-T G.989.2, which specifies characteristics of the NG-PON2 physical media dependent (PMD) layer. ITU-T G.989.1 describes the general requirements of NG-PON2 systems, while ITU-T G.989.3 specifies the NG-PON2 frame formats, messages, and protocols for data transmission.

The ITU-T isn't providing details of what's new in Amendment 1, other than it "continues the maintenance and evolution of physical media dependent (PMD) layer specification as defined in Recommendation ITU-T G.989.2 (2014)."

Several vendors already have announced NG-PON2 systems (see, for example, "ADTRAN unveils NG-PON2 platform for 10G PON, TWDM PON,""Calix offers NG-PON2 TWDM PON support,""Huawei intros next-generation PON offering," and "Alcatel-Lucent lowers entry cost of TWDM-PON with Universal TWDM").

The ITU notes that Study Group 15 also has begun to look at capacities greater than 10 Gbps, particularly 25 Gbps. The effort matches a similar initiative by the IEEE to boost EPON speeds beyond 10 Gbps through the work of the IEEE P802.3ca 100G-EPON Task Force.

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