- A radical reconfiguration: wireless endpoints and an optical backbone.
- Wireless includes radio and infrared.
- Radio bandwidth is 25 GHz vs. 25 THz for fiber
- Optical pipes are fast 50 Tbits/sec and almost errorless.
- All the usual suspects are providing the motivation.
- Web browsers.
- Graphics and visualization.
- Medical images.
- Multimedia conferencing.
- Wired video (broadband).
- The rise of uniform memory access (UMA) distributed systems.
- A light source, an optical path, and a light detector; also include
repeaters (a.k.a amplifiers).
- Lightpaths are optical end-to-end.
- Otherwise, optical-electrical conversions occur (either oeo or eoe).
- Fiber transmission properties.
- Low loss and (electrical) noise resistant; non-radiating (difficult to
eavesdrop).
- Higher bandwidths due to greater information densities (electrons vs
photons).
- Comparable costs to copper, but less price volatility.
- Light physics.
- Attenuation diminishes the signal via scattering and absorption.
- Also external factors, such as manufacturing, the environment, and
physical layout (bending).
- Scattering is due to Rayleigh scattering, a result of non-uniform
cooling; effects short wavelengths.
- Absorption results from impurities (water included) in the glass and
effects longer wavelengths.
- Dispersion spreads and distorts the shape of the light pulses.
- Chromatic dispersion (the red shift) results from differing
propagation speeds.
- Light
- Light purity is a measure of wavelength population; the smaller the
population, the purer the light.
- Infrared (0.01 cm - 7*10-5 cm), visible (7*10-5 - 4*10-5 cm), and ultraviolet (4*10-5 - 10-7 cm).
- Infrared (3*1012 - 4.3*1014 Hz), visible (4.3*1014 - 7.5*1014 Hz), and ultraviolet (7.5*1014 -
3*1017 Hz).
- An optical fiber is an ultra-pure glass wire.
- Fiber structure.
- The core, high density.
- The cladding, low density.
- The sheath, optical isolation.
- The jacket, protection.
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- Regions of low optical attenuation at 850, 1310, 1550, and 1625 nm.
- Single-mode vs. multi-mode fiber: how many colors in your rainbow?
Fiber is harder to handle than is copper wire.
- Fiber is easy to lay, but hard to exploit.
- Fiber is more fragile in terms of layout and shock resistance.
- Splicing and connecting fiber is much harder than for copper.
Fiber does suffer from optical dispersion and absorption.
- Optical absorption reduces signal strength, dispersion distorts signal
shape (the pulse).
Light sources need to be fast, bright, and pure.
- Sub-microsecond or picosecond switching times.
- Light emitting diodes and injection laser diodes.
Advances in fiber transmission rates result from multiplexing and
amplification.
Wavelength division multiplexing (WDM).
- Rather than use a pure color (wavelength), use multiple colors.
- Multiplex several beams of pure but different wavelength into the same
pipe.
- The higher carrier frequency leads to greater carrying capacity.
- Between 1 to five different colors can be mixed.
- Dense WDM uses shorter wavelengths (nanometers) and shorter pulses
(femtoseconds).
Optical time division multiplexing.
- Electronic TDM is a serial operation; scaling with the bit-rate and
number of ports.
- Optical TDM is a parallel operation, weaving multiple bit streams into
a single, mixed color pulse.
All-optical networks.
- WDM point to point links.
- Broadcast and select star networks.
- Similar to radio.
- Uses a passive star coupler to broadcast on outgoing links.
- Wavelength limited, and attenuates the signal by splitting it.
- Wavelength routing networks.
Latency vs. bandwidth.
- Going faster does not necessarily mean going quicker.
- Adding more wavelengths does not make this faster.
Network architectures and services.
- Move from the electrical to the optical domain.
- Support circuits, datagrams, and virtual circuits.
- Analog and digital circuits; analog and digital packet and cells.
Optical memories.
- Buffer lookup and IP routing.
- Use delay-line techniques to implement non-random access data.
Three dimensions for improving bandwidth: space, time, and wavelength
multiplexing.
This page last modified on 18 November 2004.