The first multi wavelength PON system, NG-PON2, has been specified by ITU-T (Q2/15). The tutorial presents a comprehensive review of the NG-PON2 trans... Passive optical networks - Optical network units - Wavelength division multiplexing - Protocols - Tutorials - Calibration - Synchronization - passive optical networks - NG-PON2 transmission convergence layer - multiwavelength PON system - transmission convergence layer specification
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NG-PON2 Transmission Convergence Layer: A Tutorial. Denis A. Khotimsky, Verizon Communications, [email protected] Abstract The first multi wavelength PON system, NG-PON2, has been specified by ITU-T (Q2/15). The tutorial presents a comprehensive review of the NG-PON2 trasnmission converegence layer specification and is targeted at development and verification engineers, system engineers, as well as product line management experts. The paper addresses selected key aspect within the tutorial scope. Introduction NG-PON2 is the latest of the ITU-T standardized PON systems. In 2010, the Full Service Access Network (FSAN) industry group started investigation of the 40 Gbit/s capable optical access technologies, and by April of 2012, arrived at the decision to start developing a set of specifications for an NG-PON2 system based on the TWDM-PON approach with optional point-to-point WDM overlay. The General Requirements document ITU-T G.989.1 was approved in March 2013, and the Physical Media Dependent (PMD) layer specification, in December 2014. The Transmission Convergence (TC) layer specification is expected to be consented in July 2015. The present tutorial provides insights into the NG-PON2 TC layer, placing it in the context of the earlier PON standards and outlining the principal differences and advances. NG-PON2 is historically the first PON system that supports multiple wavelength channels, including wavelength channels of two types. The first type is represented by the wavelength channels providing point-to-multipoint connectivity using conventional TDM/TDMA PON mechanisms. These are known as time and wavelength division multiplexing (TWDM) channels. The second type is represented by the wavelength channels that provide point-topoint connectivity using some well-defined synchronous or asynchronous mechanism, such as Gigabit or 10 Gigabit Ethernet, CIPRI or OBSAI. Each TWDM or PtP WDM channel is understood to include a pair of a downstream wavelength channel and an upstream wavelength channel. With respect to individual PtP WDM channel, the NG-PON2 TC layer specification relies on framing, encapsulation, and management specified for the underlying point-to-point client and is only concerned with managing the multi-wavelength aspect of the PtP WDM subsystem. With respect to the TWDM channels, the NG-PON2 TC layer specification includes full protocol stack (TWDM TC PHY adaptation sublayer, TWDM TC framing sublayer, TWDM TC service adaptation
sublayer), as well as descriptions of the TWDM PON management channels and the individual protocol-based functions: security key management, ONU power management, wavelength channel management, protection. Within this scope, the TWDM TC specification draws extensively upon the ITU-T G.987.3, the XG-PON1 TC layer specification. TWDM PON protocol stack The TC layer protocol stack for an individual TWDM PON channel is composed of three sublayers: service adaptation sublayer, framing sublayer, and PHY adaptation sublayer.
Figure 1: Downstream PHY frame
The OLT downstream (DS) transmitter operates in the continuous mode at one of two possible line rates: 9.95328 Gbit/s or 2.48832 Gbit/s. At the PHY layer the transmission is slotted with a Physical Synchronization Block (PSBd) inserted at the start of each 125s frame (see Figure 1). Forward Error Correction (FEC) in the downstream direction can be turned on or off, depending on the operational considerations.
Figure 2: Downstream FS frame
The payload of the downstream PHY frame carries the Framing Sublayer (FS) frame (see Figure 2). The FS frame header contains two variable length partitions: BWmap and PLOAMd, of which the latter transports the downstream Physical layer operation, administration and maintenance (PLOAM) messaging channel, and the former provide sthe bandwidth map which controls the upstream transmission by the associated ONUs. The ONU upstream (US) transmitter operates in the burst mode at the nominal line rates of 9.95328 Gbit/s or 2.48832 Gbit/s. The specification allows combining the ONUs operating at both line rates on the same ODN (see Figure 3).
Figure 3: Upstream PHY burst
At the PHY layer, each upstream ONU burst contains the Physical Synchronization Block (PSBu) composed of a preamble and delimiter, and an upstream FS frame containing a header used for identification, status indication, and OAM, a trailer, and series of individual allocations, each of which can include a buffer occupancy report for the purpose of Dynamic Bandwidth Assignment (DBA). The OLT exercises on/off control over the upstream FS FEC.
specific timing reference point, which is coordinated with similar reference points for all ONUs on the PON via the equalization delay mechanism (see Figure 4). In both upstream and downstream direction, the FS payload XGEM-encapsulated client traffic, whereby an 8-byte header is prepended for each service data unit or fragment thereof, providing traffic flow association, encryption control and fragmentation control. ONU wavelength channel mobility The NG-PON2 adds an extra degree of freedom to a PON system: controlling a tunable ONU transceiver to attach to one of the available set of TWDM channels. A TWDM channel is understood as a fixed pair of one downstream wavelength channel and one upstream wavelength channel, associated with a single OLT channel termination (OLT CT). The OLT CTs forming a single TWDM PON system are connected to a common trunk fiber via a Wavelength Multiplexor (WM). The specification does not impose any restrictions on the physical location of the OLT CTs of a single TWDM PON system: they can belong to the same line card, to multiple line cards within the same chassis, to multiple chassis within a CO, etc. Various location options open diffrent functional possibilities in front of a TWDM PON system.
Figure 4: Upstream transmission control
For each upstream ONU transmission burst, the BWmap in the downstream FS frame specifies the Start Time of the burst and the Grant Size for individual allocations within the burst. The StartTime is measured with respect to an ONU-
Figure 5: OLT CT arrangement options
Consider the arrangement with OLT CTc across the line cards of a chassis, as shown in bottom part of Figure 5. In addition to simple load
balancing, when the ONUs can be moved between the TWDM channels based on a combination of the static and dynamic load criteria, the across the cards arrangement provides several unique advantages. As one example, of a scheduled software upgrade requires a line card reboot with termination of a data path, the upgrade of different cards could be sequentially staged, and ONUs may be moved temporarily to unaffected line card thus avoiding service interruption. As another example, in case of a single line card failure, the attached ONUs minimize service outage by automatically reattaching to an OLT CT located at a working line card. The TWDM TC layer supports these and other wavelength channel mobility scenarios. TWDM channel management The TWDM channel management makes use of the PLOAM messaging channel to provide the functionality of profile announcement, ONU activation and calibration support, ONU wavelength channel handover, and ONU wavelength channel locking. To begin operations on a TWDM PON system, an ONU scans the downstream operating wavelength band and randomly selects a downstream wavelength channel to synchronize to. It then collects system, channel and burst profile information, which is broadcast by the respective OLT CT. The system and channel profile information allows the ONU to build a view of the entire TWDM system, including the parameter descriptors for each active downstream and upstream wavelength channel. Once the successfully declares its presence on the system with a Serial_Number_ONU PLOAM message transmitted upstream, the OLT CT guides the ONU through upstream wavelength calibration, ranging and authentication process. The TWDM TC layer provides tools to accommodate the ONUs with wide range of precalibration properties. These include precalibrated ONUs with autonomously stabilized transmission wavelength, pre-calibrated ONUs with externally adjustable transmission wavelength, as well as ONUs with no precalibration. To ensure that an ONU that supports fine wavelength tiuning remains locked to a specific upstream wavelength channel, the OLT CT may provide closed loop upstream wavelength control through a dithering mechanism: the OLT CT requests small Tx wavelength adjustments and monitors the quality parameters of the received optical signal. At any point during the ONU’s activation cycle,
the ONU may be instructed to execute a scheduled handover from one TWDM channel (source) to another TWDM channel (target). The OLT CTs that are involved in the handover operation coordinate the effort via the InterChannel-Termination protocol (ICTP). The ICTP is relied upon in several other procedures, including performance monitoring, TWDM PON protection, rogue ONU mitigation. The ICTP is referred via the defined primitive set in G.989.3 and is being specified normatively by Broadband Forum in WT-352. ONU power management NG-PON2 employs the unified method of protocol-based ONU power management based on the Watchful sleep power saving mode. This method combines the advantages of the earlier Sleep and Doze modes, while allowing to emulate either of them through appropriate parameterization.
Figure 6: ONU power in Watchful sleep mode
Point-to-point WDM extension The PtP WDM extension of the NG-PON2 TC layer specification allows to apply the principle, techniques, and formats developed for TWDM TC layer to the transport of the PtP WDM Auxiliary Management and Control Channel (AMCC) traffic. The AMCC TC layer can operate in either transparent or transcoded mode transporting the client data flow intact while being primarily concerned with multi-wavelength management. The downstream and upstream frames have the same format. References  Recommendation ITU-T G.989.1 (2013), 40-Gigabitcapable passive optical networks (NG-PON2): General requirements  Recommendation ITU-T G.989.2 (2014), 40-Gigabitcapable passive optical networks (NG-PON2): Physical Media Dependednt Layer Specification,  Recommendation ITU-T G.989.3 (Work in progress), 40Gigabit-capable passive optical networks (NG-PON2): Transmission Convergence Layer Specification,  Broadband Forum.WT-352. (Work in progress) Multiwavelength PON Inter-Channel-Termination Protocol (ICTP) Specification.