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June 2018

PON NETWORKS


Part-1
By Devchand Haria
“Passive Optical Networks” (PON) Offers Carriage Of CATV, Telephony & Internet, All Distributed Simultaneously On The Same Passive Fibre Distribution Network.
Part-1 Provides an overview, while
Part-2 will provide details on practical implementation.


Today cable TV networks are beginning to face stiff competition from DTH services & telecom companies. It is not enough to provide only digital video. Consumers also want other services like voice Internet data on the same network. Triple play solutions are required, for all future Cable TV networks. High definition TV & interactive games & high speed internet delivery all require huge bandwidth. Future CATV networks will have to deliver not only video but also these high bandwidth intensive services.



EXISTING TECHNOLOGY

Currently, most Indian CATV networks use ‘HFC’ or Hybrid Fiber Coaxial systems, ie both Optical Fibre & Coaxial cables are used in the network.

However, copper based coaxial cables are much more expensive; require high maintenance and expensive amplifiers at close intervals. These networks require active components in the field. Hence it is necessary to maintain battery power and UPS power supplies throughout the network.

For internet delivery using Cable Modems, it is necessary to run reverse path amplifiers. Reverse path noise and ingress also are concerns, and difficult to maintain at low levels, particularly for large networks.

As a result networks are beginning to shift to total fibre solutions, with no coaxial cables. Signals are delivered by Fibre-To-The-Building (FTTB) or Fibre-To-The-Home (FTTH).

Figures 1, 2 & 3 indicate various FTTB and FTTH solutions that are possible.


POINT-TO-POINT ETHERNET

This is the most basic system but also the most expensive and complicated to implement. As shown in Fig. 1, separate fibres are installed from the control room to each building / user. If the optical transreceivers are bi-directional then 1 fibre is required per building. If the transreceivers are not bi-directional, 2 fibres – 1 to send & 1 to receive are required between the control room and each building !



As a result an impractically large number of fibres and transreceivers would be required if a Point-To-Point Ethernet system was used to distribute Internet and other digital signals, to a large number of homes.



CURB SWITCHED ETHERNET

This system offers fewer fibres but the number of optical transreceivers remains inordinately large.

As shown in Fig.2 a single fibre is run from the control room to a central point in the area to be served (Fibre to the Served Area)

From that central point, each building is serviced by a separate fibre and a separate transreceivers. This distribution scheme is also impractical and often difficult to find a suitable location in the served area where a large number of optical transreceivers can be located and powered. As all cable operators know, obtaining power in the field is difficult, expensive and often unreliable.



ETHERNET PON

The Ethernet Passive Optical Network (PON) provides an ideal solution in the field. As shown in Figure-3, a single fibre runs from the Control Room to a central point. From that point, only passive optical splitters divide the signal and distribute it to each building. This article will discuss, in detail, PON Networks.



They are rapidly gaining popularity due to their immense advantages and ease of operation.


PON DISTRIBUTION

Signal from the Optical Line Termination (OLT) transmitter located in the Control Room is carried on a single fibre, and then distributed to each building, through a series of cascaded, PON Optical splitters.

An Optical Network Unit (ONU) at each building, provides separate CATV, Data/internet and telephony to each home.

A PON network can deliver multiple services throughout the network. Services such as voice video and data can be intermixed using IP format & traditional cable TV services can be added on the network, at a separate optical wavelength.

The PON is a high bandwidth, Point-To-Multipoint access technology that enables data & video communication using passive optical components.

Since it is Point (The Control Room) to multipoint (each building) distribution, the OLT transmitter transmits the same signal to all the Nodes / ONUs.

Each node then receives only those signals it is authorised to receive. Each data packet is embedded with its user’s Machine Address Code (MAC) address, which is the unique identity of the receiving computer. When this matches with the ID of the computer that is connected to that particular ONU, the ONU receives and decodes the information. This is similar to the system used for cable modems, for data distribution.



The ONU is a sophisticated unit & implements the standard PON transport protocol, stringent QoS for a multi services environment and support of IP protocols such as IGMP for efficient distribution of IP video.


ENCRYPTION & SECURITY

PON technology also uses a very high level of security with 128 – bit Advanced Encryption Standard (AES) encryption of traffic between the OLT and ONU. A different encryption key is automatically generated and used for every ONU.

Part-2 in the July Issue will deal with the practical implementation of a PON network. n



ABOUT THE AUTHOR

Devchand Haria - B.E. (Elect. & Telecom) is Director at Optilink Networks Pvt. Ltd. a leading solution provider for digital headend solutions & distributors for RF, Networking and Fiber Optic Products.

Over the last 26 years, Optilink Networks Pvt. Ltd. has developed a leading market position as a thematic system provider & distributor of high quality satellite receiving equipments, fiber optic equipments & digital headend solutions.