FTTH With Passive Optical Networks

Broadband Internet speeds have been increasing year on year as new technologies are making this possible. The copper local loop has been a limiting factor with Digital Subscriber Line, so moves to provide Optical Fibre runs closer to the home or business can substantially increase the available bandwidth. One technology involves the use of Passive Optical Networks.

Fiber Optic cables allow us to pass huge amounts of data on point to point links by passing light down the fiber, but for multiple broadband connections over fibre we need a way to separate the individual data streams required for each household or business. Two methods have evolved that allow us to do that, Passive Optical Networks and Active Optical Networks.

An AON (Active Optical Network) will use networking equipment such as routers and switches to make the decisions to route individual streams to their respective destinations. They take advantage of the Ethernet standards developed for use over Fiber Optics and have been around for around 15 years or more now. Often an AON will mean subscribers have a dedicated Fibre connection to the home, or FTTH.

A PON (Passive Optical Network) does not rely on routing and switching equipment from the exchange, but instead uses optical splitting techniques. Power is only required at the exchange and at the customer premises. So how does it work?

The Passive Optical Network has 3 main components in the form of:

OLT (Optical Line Termination) equipment normally located at the exchange which converts the data signals present in the provider networking equipment to the optical signals used across the Passive Optical network.

POC (Passive Optical Coupler) is a device used to split the optical signal from a single fiber and distribute those signals from a single fiber to the multiple fibers serving the individual subscribers. In the reverse direction the POC can take multiple data signals from the multiple subscriber fiber connections and multiplex them onto a single fiber for connection to the OLT at the exchange. Inevitably, when the fiber optic signal is split then the original signal is attenuated as the light signal entering the coupler is equally distributed between the subscriber fiber connections. The more subscribers being served by the PON the more attenuation there is. Typically 32 incoming subscriber fibers can be coupled to single run of fibre back to the exchange. The signal on this single fiber is a combination of all the subscriber connection that exist.

An ONU (Optical Network Unit) is used to terminate the fiber signals at the subscriber end with a fiber optic interface pointing to the exchange and single or multiple interfaces connecting to customer equipment depending on the requirements of the customer. For example, a home consumer may just have a simple Ethernet interface, multi-port switch and probably wireless connectivity. With business users the ONU may provide other interfaces depending on the equipment to be connected.

There are two main advantages of a PON over an AON in that 32 subscribers can be served from a single fibre from the exchange and maintenance costs are low due to the fact that there is no electrical switching equipment involved in the PON itself.

A Passive Optical Network is restricted in terms of the range over which it can operate, with 10-12 miles being typical. Bandwidth is shared between the number of subscribers active on the PON, and as such the transmission speed may vary depending on how busy each subscriber is. This slowing down of speed or latency as it is popularly known can have a detrimental effect on voice, video and multimedia services.

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