4 and Figure 7.5),
EPON operates in the multipoint-to-point mode (MP2P), where numerous ONUs transmit
their data packets to a single receiver module located in the OLT. Moreover, since individual
ONUs are not aware of other ONUs??™ transmissions (as the PSC is a directional
device, an ONU cannot see the signal transmitted upstream by any other ONU), the
resulting connectivity appears similar to the P2P architecture, where centrally managed
access to the upstream channel allows for only a single ONU at a time to deliver pending
packets. However, because all ONUs belong to a single collision domain, centrally managed
channel access is required (typically via a dynamic bandwidth allocation algorithm or
DBA for short), and ONUs in their default state are not allowed to transmit any data
unless granted specifically by the OLT. In this way, data collisions are avoided because
Figure 7.2 Standard PON deployment topologies supported by EPONs
ONU
ONU
ONU
ONU
ONU
ONU
ONU
ONU ONU
ONU ONU
OLT OLT
OLT
(a) Tree topology (b) Bus topology
(c) Ring topology
ONU
Figure 7.3 Downstream channel transmission in an EPON (P2M operation??”broadcast??”and LLID
packet filtering)
Header
Ethernet 802.
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