Downstream Transmission in EPON Systems
In the downstream direction, Ethernet packets broadcast by the OLT pass through a
1 ?— N PSC or a PSC cascade to reach the ONUs. Each ONU receives a copy of every
downstream data packet. The number of connected ONUs can typically vary between
4 and 64, limited by the available optical power budget.
The downstream channel properties in this PON system make it a shared-medium
network: packets broadcast by the OLT are selectively extracted by the destination
ONU, which applies simple packet-filtering rules based on MAC and LLID addresses
(see IEEE 802.3ah, Clause 64 [7] for details). The downstream channel operation is best
depicted in Figure 7.3, where packets destined to different end subscribers are filtered
out by the ONUs from the broadcast downstream data flow.
Figure 7.1 Standard EPON deployment with various scenarios of FTTx solution: FTTB, FTTC, FTTH,
and mixed FTTH
FTTH mixed solution
FTTH solution
FTTC solution
for residental areas
Optical link
FTTB solution
Servers/data servers
disk arrays
Feeder section
PSC
Drop
section
ONU
OLT
ONU
LAN switch
ONU
CO of ISP
Copper link
Passive Optical Networks (PONs) 179
Upstream Transmission
In the upstream direction, from the ONU toward the OLT (see Figure 7.
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