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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Stephen Hardy | Editorial Director and Associate Publisher
Stephen Hardy has covered fiber optics for more than 15 years, and communications and technology for more than 30 years. He is responsible for establishing and executing Lightwave's editorial strategy across its digital magazine, website, newsletters, research and other information products. He has won multiple awards for his writing.
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