Need for multiplexing and WDM

Why multiplexing is needed

Multiplexing lets many signals share one transmission medium. It is needed because:

• A transmission medium (coax, fibre, microwave) has far more capacity than one voice channel needs; multiplexing uses that capacity fully.
• It greatly reduces the cost per channel: one cable, fibre or radio link replaces hundreds of separate lines.
• Fewer physical lines mean less right-of-way, duct space and maintenance.
• Radio spectrum is limited; multiplexing lets many users share it.
• It makes it possible to build trunk networks and hierarchies (T1/E1, SDH) between exchanges.

Wavelength Division Multiplexing

WDM carries several optical signals over a single fibre, each on a different wavelength of light. It is the optical equivalent of FDM.

 l1 -->|\                    /|--> l1
 l2 -->| MUX ==== fibre ==== DEMUX|--> l2
 l3 -->|/   (l1+l2+...+lN)   \|--> l3
 lN -->                         --> lN
 l = wavelength (lambda)

Working:

1. Each input signal modulates its own laser at a different wavelength $\lambda_1, \lambda_2, \ldots, \lambda_N$ (in the 1310/1550 nm windows).
2. A passive optical multiplexer (diffraction grating, prism, arrayed waveguide grating or thin-film filter) combines all wavelengths into one fibre.
3. Optical amplifiers (EDFA) can boost all wavelengths together on long routes.
4. At the far end a demultiplexer separates the wavelengths and photodetectors convert each to an electrical signal.

Types:

• CWDM (coarse): 20 nm spacing, up to 18 channels, cheap uncooled lasers, metro distances.
• DWDM (dense): 0.8 nm (100 GHz) or less, 40–160 channels, long-haul backbones, terabits per second per fibre.

Advantages: huge increase in capacity of existing fibre; each wavelength is independent and can carry any format and bit rate; easy upgrades; bidirectional use of one fibre is possible. Limitations: costly, stable lasers and filters; crosstalk between adjacent wavelengths; dispersion and fibre non-linearities at high power.