Ethernet passive optical networks are known, as described for example in U.S. Patent Application No. 20020196801 to Haran et al. In general, as shown in FIG. 1, an EPON 100 comprises an optical link terminal (OLT) 102 connected through a splitter 104 to a plurality (in this case 3) of optical network units (ONUs) 106, 108 and 110. The OLT transmits downlink transmissions to each ONU. Each ONU transmits uplink transmissions to the OLT. In a “unified” EPON, all devices transmit at the same rate, for example currently at 1 Gigabit per second (1 G).
The new IEEE 802.3 EPON standard specifies a 1 G access network shared between multiple users. The current definition of the uplink/downlink channels is symmetric. Looking forward, the existing bandwidth (BW) might not be sufficient when customer BW requirements will increase, for example when Internet protocol (IP) high-definition television (HDTV) services become popular.
A time-dependent channel is a communication channel in which the data arrival time plays as important a role as the data content itself. Examples for such channels are:                Control messages with embedded timestamps, as in IEEE802.3 EPON        A channel with encryption based on packet transmission time, as commonly implemented in EPON solutions        
Doubling of a downlink channel (path) BW can be performed simply by time division multiplexing (TDM), as well known in the art. In this case, two independent channels are used, placing on the transmission line information from one channel followed by information from the other channel, based on a selected scheme. The problem with this type of solution is that the line code appears as two normal rate (1 G) links, not one double rate (2 G) link.
It would therefore be advantageous to have mechanisms required for higher rate downlink transmission support in EPON. In particular, it would be advantageous to have a 2 G downlink BW in EPON. It would be further advantageous that these mechanisms retain maximal similarity to the existing 1 G solution, thereby allowing fast time-to-market.