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Optical network refers to the use of fibre optic cable to provide data and connectivity usually across large distances. Fibre carries more data over longer distances than any other medium and is used to provide, for instance, undersea connectivity between continents as well as intra-city high capacity circuits.
Whereas Time Division Multiplexing (TDM) technology effectively gave each signal a “timeslot” in which to travel down an electrically attenuated cable, Wave Division Multiplexing (WDM) splits white light into its constituent colours similar to a prism splitting daylight into a rainbow. Each colour, or wavelength, carries a signal which does not interfere with the other wavelengths within the fibre. This allows multiple signals to travel down the fibre at the same time. In this way large quantities of data can be carried per wavelength.
When a provider wishes to run WDM technology they either run fibre themselves or use a “dark fibre” service from another provider. Dark fibre is a fibre that is not currently carrying any wavelengths. The provider may then use WDM technology to multiplex signals onto the fibre as they wish.
A provider may also request a wavelength from a fibre provider and will therefore require a “coloured” optic which is tuned to the specific wavelength that the fibre provider is able to offer.
As data volumes continue to grow and WDM technology becomes available on Small Form Factor Pluggable Optics (SFPs), providers have started to provision wavelengths directly within their routers.
WDM technology is split into two different areas:
Course Wave Division Multiplexing (CWDM)
This technology supports up to 18 wavelengths transmitted down a fibre at the same time with each wavelength being 20nm (nanometres) apart. As wavelength spacing is quite wide CWDM interfaces tend to be cost effective and reliable up to approximately 70km of travel, although the number of channels it is able to handle diminishes as range increases. Each wavelength can transmit 10Gbps of data.
Dense Wave Division Multiplexing (DWDM)
DWDM can handle 80 wavelengths per fibre at any one time and the wavelengths are only 0.8nms apart. DWDM signals can also be regenerated and amplified, meaning that they can travel much further distances. Although the wavelengths are closer together each one is capable of transmitting up to 400Gbps of data.
Passive vs. Active
As routers become more proficient with xWDM technologies there is a need to provide lower cost optical equipment that combines wavelengths but leaves other functions to the router, such a system is called a Passive Optical System.
If you wish to convert a short range optical signal to a longer one then an Active Optical System is required which will be powered.
Reconfigurable Optical Add-Drop Multiplexer (ROADM)
As optical networks become more complex and bandwidth requirements change constantly there is a need to provide a flexible way of “routing” wavelengths across an optical network and essentially adding them to and dropping them off the network at the required site. A ROADM achieves this functionality remotely, saving costly site visits and allowing for optical networks to grow and adapt as required.