ZyXEL Communications P-3202HN-Ba User Manual page 243

Absorption happens as the light's energy is converted into heat; scattering occurs
when the light hits stray particles of other matter inside the cable and some its
photons are redirected in other directions. As a result, the further the light in the
pipe travels, the less coherent it becomes and eventually it disintegrates. In
current PON implementations, there are two standard distances that a service
provider can choose: 1-10 kilometers and 1-20 kilometers. Light does not
attentuate differently over the greater of the two distances; rather, the service
provider simply uses much more powerful equipment to transmit the light signals
into the network, thus boosting the relative signal strength to such a point that
attentuation does not set in as rapidly.
The other major limitation is the "splitting." To make the most of available
bandwidth, service providers must split a backbone line into many smaller lines
which are then extended to multiple customers.
For example, a backbone line leaving the service provider's central office may split
twice, sending subsidiary lines to branch office ONUs or secondary OLTs. These, in
turn, can be split again and again until a certain number of customers have been
served. However, each time a light signal is split each subsequent subsidiary beam
is at a markedly lower intensity than the original. If a service provider's maximum
bandwith allocation is approximately 60 Mbps/sec (the physical limitation of the
fiber), this bandwidth must be shared among all customers connected to the
backbone. If only one customer is connected, they reap the benefits of full 60
Mbps/sec bandwidth; on the other hand, if the service provider splits the signal so
that three customers in three disparate locations can benefit, each one only
receives ~20 Mbps/sec of bandwidth because of the tripartite split. The maximum
number of splits that a service provider can make is 64, at which point the data
flood from the signal source becomes but a trickle by the time it reaches the end
of its journey.
Bit Rate Requirements
The kind of transmission speeds a PON provides depends primarily on the kind of
network your service provider maintains and any bandwidth limits it enforces (if it
does so at all). Various programs and applications can take advantage of the
network's bandwidth as long as it meets their requirements.
Below is a table listing the minimum bit rates various types of applications require
in order to operate at their full potential over a PON. If you are not sure about
IAD User's Guide
Appendix A Passive Optical Networks
243