3 frame
Payload FCS
3 1 1
2
2
3 1 1
3
2
1 1
2 3 1 1
2 3 1 1
PSC
Residential
subscriber
MDU
subscribers
LAN/PAN
OLT
ONU1
ONU2
ONU3
180 Chapter 7
the central OLT controller at any and every moment of time is aware of the scheduled
transmissions from individual ONUs. The only exception to this centrally managed
upstream channel access scheme is the so-called discovery process [7], where new and
not initialized ONUs are allowed to register in the EPON system.
A multiple access protocol is required in the upstream direction, since the EPON
operates as a multipoint-to-point network and every single ONU talks directly to the
OLT. A contention-based media access mechanism (similar to CSMA/CD [8, 9]) is difficult
to implement; in the typical network deployment, ONUs cannot detect a collision at
the OLT, and providing the architecture with a feedback loop leading to every single
ONU is not feasible. Contention-based schemes also have the drawback of providing a
nondeterministic service, i.e., node throughput and channel utilization may be described
as statistical averages, and hence, there is no guarantee of an ONU getting access to
the media in any small interval of time, which means this type of access protocol is
ill-suited for delay-sensitive transmissions, such as video conferencing or VoIP.
Pages:
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458