Passive Optical Networks

To maintain low cost, traditionally all Ethernet transmitters are running

asynchronously on their own local clock domains. There is no global synchron-

ization. A receiver derives the clock signal for gating the received data from its

received digital symbols. Mismatches between clock sources are accounted for by

adjusting the interframe gap (IFG) between Ethernet frames.

In an EPON system, the downstream physical link maintains continuous

signal stream and clock synchronization. In the upstream direction, in order to

maintain a common timing reference with the OLT, ONUs use loop timing for

the upstream burst mode transmission, i.e. the clock for upstream signal trans-

mission is derived from the downstream received signal.

2.7.4 PCS Layer and Forward ...