7.0 Modern Communication Media
7.0 Modern Communication Media
7.1 Optical Fiber Communications
An optical fiber is a dielectric waveguide that transmits information as light pulses. It operates on the principle of Total Internal Reflection, where light entering the fiber core is continuously reflected at the boundary between the core and the cladding, guiding it along the fiber.
- Structure: Composed of a central Core with a refractive index n1 and an outer Cladding with a lower refractive index n2. The cladding adds mechanical strength and reduces scattering losses.
- Types:
- Step-index fiber: Has a uniform core refractive index.
- Graded-index fiber: The core’s refractive index varies with distance from the center.
- Single-mode fiber: Used for long-distance, high-bandwidth applications (excited by laser).
- Multi-mode fiber: Used for shorter distances (excited by LED).
- Advantages: High transmission bandwidth, low power loss, immunity to electromagnetic interference, and enhanced security.
- Applications: Telephone systems, submarine cable networks, computer data links (CATV), CCTV, and emergency services.
7.2 Satellite Communications
A communication satellite is a microwave repeater station in space that revolves around the Earth in a predictable path called an orbit. It receives signals from Earth on an uplink frequency, changes the frequency band using a transponder, and retransmits the signal back to Earth on a downlink frequency.
Kepler’s Laws of Orbital Motion
The motion of satellites is governed by Kepler’s three laws:
- First Law: A satellite revolves around the Earth in an elliptical orbit, with the Earth at one of the two foci.
- Second Law: The line joining the satellite to the Earth’s center sweeps out equal areas in equal intervals of time.
- Third Law: The square of the orbital period is proportional to the cube of the mean distance between the satellite and the Earth.
Types of Earth Orbits
- Geosynchronous Earth Orbit (GEO): An orbit at an altitude of 35,900 km (22,300 miles) with a period matching the Earth’s rotation (23 hours, 56 minutes). A satellite in this orbit appears stationary from Earth if it is also circular and in the equatorial plane (Geostationary). Used for broadcasting and weather forecasting.
- Medium Earth Orbit (MEO): Orbits at altitudes of about 8,000 miles. They offer lower transmission delay and require smaller receiving terminals than GEO satellites.
- Low Earth Orbit (LEO): Orbits at altitudes of 500 to 1000 miles. They provide very low transmission delay (0.05 seconds) and are used for applications requiring real-time communication, including video.