Passive optical network PON
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12-11-2009, 09:34 AM


Abstract:
A passive optical network (PON) is a point-to-multipoint, fiber to the premises network architecture in which unpowered optical splitters utilizing Brewster's angle principles are used to enable a single optical fiber to serve multiple premises, typically 32-128. A PON consists of an optical line terminal (OLT) at the service provider's central office and a number of optical network units (ONUs) near end users. A PON configuration reduces the amount of fiber and central office equipment required compared with point to point architectures.Downstream signals are broadcast to each premises sharing a single fiber. Encryption is used to prevent eavesdropping.Upstream signals are combined using a multiple access protocol, usually time division multiple access (TDMA). The OLTs "range" the ONUs in order to provide time slot assignments for upstream communication
see en.wikipediawiki/Passive_optical_network for more
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18-06-2012, 02:46 PM

Passive Optical Networks


.ppt   Passive Optical Networks.ppt (Size: 4.04 MB / Downloads: 85)
Aside – why is fiber better ?

attenuation per unit length
reasons for energy loss
copper: resistance, skin effect, radiation, coupling
fiber: internal scattering, imperfect total internal reflection
so fiber beats coax by about 2 orders of magnitude
e.g. 10 dB/km for thin coax at 50MHz, 0.15 dB/km l =1550nm fiber
noise ingress and cross-talk
copper couples to all nearby conductors
no similar ingress mechanism for fiber
ground-potential, galvanic isolation, lightning protection
copper can be hard to handle and dangerous
no concerns for fiber

Why not fiber ?

but fiber has its own problems
harder to splice, repair, and need to handle carefully
regenerators and even amplifiers are problematic
more expensive to deploy than for copper
digital processing requires electronics
so need to convert back to electronics
we will call the converter an optical transceiver
optical transceivers are expensive
switching easier with electronics (but possible with photonics)
so pure fiber networks are topologically limited:
point-to-point
rings

More physical layer problems

Near-far problem
OLT needs to know signal strength to set decision threshold
If large distance between near/far ONUs, then very different attenuations
If radically different received signal strength can't use a single threshold
EPON: measure received power of ONU at beginning of burst
GPON: OLT feedback to ONUs to properly set transmit power
Burst laser problem
Spontaneous emission noise from nearby ONU lasers causes interference
Electrically shut ONU laser off when not transmitting
But lasers have long warm-up time
and ONU lasers must stabilize quickly after being turned on

How does it work?

ONU stores client data in large buffers (ingress queues)
ONU sends a high-speed burst upon receiving a grant/allocation
Ranging must be performed for ONU to transmit at the right time
DBA - OLT allocates BW according to ONU queue levels
OLT identifies ONU traffic by label
OLT extracts traffic units and passes to network
OLT receives traffic from network and encapsulates into PON frames
OLT prefixes with ONU label and broadcasts
ONU receives all packets and filters according to label
ONU extracts traffic units and passes to client
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