Next-Gen PON: An interview with BT

An interview with Peter Bell, Access Platform Director, BT Innovate & Design on next generation passive optical networks (PONs). These include 10 Gigabit Ethernet PON (EPON) and Gigabit PON (GPON) and wavelength division multiplexing PON (WDM-PON).

Peter Bell, Access Platform Director, BT Innovate & Design

Q: The status of 10 Gigabit PON – 10G EPON and 10G GPON (XG-PON): Applications, where it will be likely be used, and why is it needed?

PB: IEEE 10G EPON: BT not directly involved but we have been tracking it and believe the standard is close to completion  (gazettabyte: The standard was ratified in September 2009.)

ITU-T 10Gbps PON:  This has been worked on in the Full Service Access Network group (FSAN) where it became known as XG-PON.   The first version XG-PON1 is 10Gbps downstream and 2.5Gbps upstream and work has started on this in ITU-T with a view to completion in the 2010 timeframe.  The second version XG-PON2 is 10Gbps symmetrical and would follow later.

Not specific to BT’s plans but an operator may use 10Gbps PON where its higher capacity justified the extra cost.  For example: business customers, feeding multi-dwelling units (MDUs) or VDSL street cabinets

Q: BT's interest in WDM-PON and how would it use it?

PB: BT is actively researching WDM-PON. In a paper presented at ECOC '09 conference in Vienna (24th September 2009) we reported the operation of a compact DWDM comb source on an integrated platform in a 32-channel, 50km WDM-PON system using 1.25Gbps reflective modulation.

 We see WDM-PON as a longer term solution providing significantly higher capacity than GPON.   As such we are interested in the 1Gbps per wavelength variants of WDM-PON and not the 100Mbps per wavelength variants.

Q: FSAN has two areas of research regarding NG PON: What is the status of this work?

PB: NG-PON1 work is focussed on 10 Gbps PON (known as XG-PON) and has advanced quite quickly into standardisation in ITU-T.

NG-PON2 work is longer term and progressing in parallel to NG-PON1

Q: BT's activities in next gen PON – 10G PON and WDM-PON?

PB: It is fair to say BT has led research on 10Gbps PONs.  For example an early 10Gbps PON paper by Nesset et al from ECOC 2005 we documented the first, error-free physical layer transmission at 10Gbps, over a 100km reach PON architecture for up and downstream.

 We then partnered with vendors to achieve early proof-of-concepts via two EU funded collaborations. 

Firstly in MUSE we collaborated with NSN et al to essentially do first proof-of-concept of what has become known as XG-PON1 (see attached long reach PON paper). 

Secondly, our work with NSN, Alcatel-Lucent et al on 10Gbps symmetrical hybrid WDM/TDMA PONs in EU project PIEMAN has very recently been completed. 

Q: What are the technical challenges associated with 10G PON and especially WDM-PON?

For 10Gbps PONs in general the technical challenges are:

• Achieving the same loss budgets - reach - as GPON despite operating at higher bitrate and without pushing up the cost.

• Coexistence on same fibres as GPON to aid migration.

• For the specific case of 10Gbps symmetrical (XG-PON2) the 10 Gbps burst mode receiver to use in the headend is especially challenging. This has been a major achievement of our work in PIEMAN.

For WDM-PONs the technical challenges are:

• Reducing the cost and footprint of the headend equipment (requires optical component innovation)

• Standardisation to increase volumes of WDM-PON specific optical components thereby reducing costs.

• Upgrade from live GPON/EPON network to WDM-PON (e.g. changing splitter technology)

Q: There are several ways in which WDM-PON can be implemented, does BT favour one and why, or is it less fussed about the implementation and more meeting its cost points?

PB: We are only interested in WDM-PONs giving 1Gbps per wavelength or more and not the 100Mbps per wavelength variants. In terms of detailed implementation we would support the variant giving lowest cost, footprint and power consumption.

Q: What has been happening with BT's Long Reach PON work

PB: We have done lots of work on the long reach PON concept  which is summarised in a review published paper from IEEE JLT and includes details of our work to prototype a next-generation PON capable of 10Gbps, 100km reach and 512-way split.  This includes EU collaborations MUSE and PIEMAN 

From a technical perspective, Class B+ and C+ GPON (G.984.2) could reach a high percentage of UK customers from a significantly reduced number of BT exchanges.   Longer reach PONs would then increase the coverage further. 

Following our widely published work in amplified GPON, extended reach GPON has now been standardised (G.984.6) to have 60 km reach and 128-way split, and some vendors have early products.   And 10Gbps PON standards are expected to have same reach as GPON.