3 Basic Principles of Passive Optical Network that define its Constitution



3 Basic Principles of Passive Optical Network that define its Constitution

Passive Optical Network is the concept clearly seen all around and its main implication is reduction in the number of cables travelling from source to the end user(s). A Passive Optical Network has only passive components and is capable of handling the data-centric demands arising from both the home as well as enterprise networks. In bringing fiber to the home, Passive Optical Network has played a major role. So, how a PON is constituted and what principles govern its constitution? Let’s understand it.

A typical PON shows the origin of the service network at an optical line terminal (OLT) that is normally a head office or central unit of the network. The services are then carried along an optical feeder extending to 10-12 km of distance, eventually splitting the optical power into multiple distribution fibers using a splitter that resides on a remote node. Each distribution fiber carries the service to the destined Optical Network Unit which is the termination point of optical signal and this signal is then distributed to all the end users connected with this ONU.

PON has made the data-centric services available 24 by 7 by 365. This omnipresence of the data networks requires lot of power as well as no flaw in connectivity. It can be made possible only if the networks are completely pre-planned and have features integrated for easier fault checking and access, as well as quicker resolution of issues. Moreover, the demand of the enhanced level of protection cannot be denied.

So, the three main principles that govern the constitution of a PON are:

  1. Network sustenance and protection

There are two common approaches that are followed for developing fully protected and long sustainable PON networks. These approaches are – pre-planned protection schedule and on-site dynamic restoration. In pre-planned protection, a protective optical path is introduced at the designing stage that sends alerts in case of any kind of network failure; whereas in dynamic restoration, the expert search for the extra optical paths only when the traffic is disrupted as a result of an instance of network failure. The traffic restoration takes longer in the second approach as the action is taken only on the basis of possibilities available.

  1. Easily comprehensive topology

The PONs is designed on the principles of tree and ring topologies. In the former, the optical signal emanating from the OLT is split on the nodal point and is delivered to the targeted ONUs with individual distribution fibers. While following the ring pattern of network topology, OLT is linked to multiple nodal points through single or double fiber rings. The access node features an optical power splitter that is further connected to multiple ONUs in concentric circles, sometimes in star like structure too. Topology determines the ease of management of the networks. While designing the network architecture, one needs to pre-plan how the connections between the OLT and ONU would be formed – whether the protection light trajectory would be duplicated or just integrated.

  1. Unmatched flexibility 

Flexibility is the most important principle to follow in PON. Looking at the flexibility requirements, WDM-PONs have been developed that include protection feature in the form of alternate lightpaths that provide extra routes to the optical signals in case of the fiber failure. Experts have come up with the idea of star-ring topology where multiple fiber feeders are connected separately to individual RNs. And, the individual RNs are interconnected through a ring network. This set-up is flexible enough to isolate the failed feeder without disrupting the connection between CO (central office) and the adjacent ONUs. Thus, the system does not come to a stand-still in case of individual fiber failure and the network services are not affected at the user’s end.

Understanding the constitution of a Passive Optical Network

So, broadly speaking, a Passive Optical Network comprises of following components. Whether it is FTTH, FTTB, FTTC or any other PON type, the optical access network (OAN) is common to all types.

  1. Central Office: The central point of the network, or starting point from where the optical signals are initiated and made to travel till nodal points.
  2. OLT (Optical Line Transmission): The Optical Line Termination (OLT) interface stretches from the SNI (Service Node Interface) to service nodes, leading up to the PON. The OLT manages the PON related issues of an ATM transport system. The ONU and OLT extend a visible transport service of ATM type between UNI and SNI across a PON. The OLT comprises of:
  3. SDH/SONET interfaces
  4. ATM UNI at 155-622Mbps
  5. IP traffic over Fast Ethernet, Gigabit Ethernet, 10Gigabit Ethernet protocols
  6. Optical Network Unit (ONU): ONU is the termination point of the PON and is presented to the end user as a service interface where cables can be inserted or wireless connections can be established, reaching up to the end device. The service interface can be an access point of either or all of Data connection (Ethernet, for e.g.), Voice over IP (VoIP point), or telemetry. A typical ONU normally comprises of:
  7. Native service converged interface such as a coaxial cable, DSL or multiservice Ethernet point
  8. Equipment that marks the network termination such as plain old telephone service or/and Ethernet

OLT sends ONU specific packets of data; the targeted ONU recognizes the signal meant for it and the end-to-end encryption helps in transferring data to the end user without any loss or leakage. PON has, thus, brought in the convenience as well as security to the data communication services and has eased the lives of service providers who can now deliver quality as well as comfort in a package.


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