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Short note: WDM

Marks: 5M

Year: Dec2014, Dec2013, May2015, May2014

• WDM (Wavelength Division Multiplexing) corresponds to the scheme in which multiple optical carriers at different wavelengths (produced by LASER) are modulated by using independent electrical bit streams (which may themselves use TDM and FDM techniques in the electrical domain) and are then transmitted over the same fiber.

• The optical signal at the receiver is demultiplexed into separate channels by using an optical technique.

• The main system feature of WDM are as follows:

1. Capacity

2. Transparency

3. Wavelength routing

4. Wavelength switching

• There are two types of WDM

  i. Unidirectional

  ii. Bidirectional

• In unidirectional WDM system Single Carrier wavelengths are fed into single fibers at one end and then separate them into their corresponding detectors at other end.

• The insertion loss, channel width and cross talk are the three basic parameters which are used to decide the performance of a WDM system.

• The bidirectional WDM technique enables bidirectional communications over one strand of fibre, as well as multiplication of capacity.

• WDM systems are divided in different wavelength patterns, conventional or coarse and dense WDM.

• Conventional WDM systems provide up to 8 channels in the 3rd transmission window (C-band) of silica fibres around 1550nm. Dense WDM uses the same transmission window but with denser channel spacing.

Advantages of WDM

Wavelength division multiplexing has several advantages over the other presented approaches to increase the capacity of a link:

• Works with existing single mode communication fibre

• Works with low speed equipment

• Is transparent: Doesn’t depend on the protocol that has to be transmitted.

• Is scalable: Instead of switching to a new technology, a new channel can easily be added to existing channels. Companies only have to pay for the bandwidth they actually need.

• It is easy for network providers to add additional capacity in a few days if customers need it.

This gives companies using WDM an economical advantage. Parts of a fibre can be leased to a customer who then gets fast network access without having to share the connection with others. The telecommunication company on the other hand still has an independent part of the fibre available for other customers