Gigabit-capable Passive Optical Networks

4

TRANSMISSION CONVERGENCE LAYER

In this chapter:

• ONU-ID terminology ambiguity
• Payload mapping and framing
• FEC, scrambling and encryption
• ONU discovery, activation, and delay compensation
• Conveying time ofday to the ONU
• Physical layer OAM (PLOAM) messages; an embedded operations channel whose details appear in Appendix II
• Methods by which upstream bandwidth demand is determined and capacity is requested and assigned
• Energy conservation
• Security

The transmission convergence (TC) layer is the heart and soul of a PON and, in particular, of a G-PON or an XG-PON. Here is where we find solutions to the unique problems posed by a PON, as well as to the ordinary issues of any network protocol. These unique aspects are largely related to the tree structure, the point to multipoint nature of the PON:

• Discovering hitherto unknown ONUs in a way that does not disrupt existing traffic on the PON
• Recovering previously known ONUs onto the PON after power failures, power down, or other disruptions
• Coping with the fact that the various ONUs are at different distances from the OLT and, hence, have different propagation delays
• Accommodating drift in the propagation delay that could be caused, for example, by daily or seasonal temperature changes in the ODN
• Defining the upstream burst header for fast and accurate optical receiver calibration so that the header can be quickly delimited and parsed
• Orchestrating upstream burst transmit timing among ONUs, such that
  • (a) No transmissions collide at the ...