• A light-emitting diode (LED) is a semiconductor light source.• LEDs are used as indicator lamps in many devices and are increasingly used for other lighting.• Introduced as a practical electronic component in 1962.• early LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness.• LEDs are often small in area (less than 1 mm2), and integrated optical components may be used to shape its radiation pattern.• LEDs present many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved robustness, smaller size, and faster switching.• The LED consists of a chip of semiconducting material doped with impurities to create a p-n junction.• As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction.
LASER Principal components: 1. Gain medium 2. Laser pumping energy 3. High reflector 4. Output coupler 5. Laser beam
• A laser is a device that emits light (electromagnetic radiation) through a process of optical amplification based on the stimulated emission of photons.• The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation.• The emitted laser light is notable for its high degree of spatial and temporal coherence, unattainable using other technologies.• The gain medium is a material with properties that allow it to amplify light by stimulated emission.• In its simplest form, a cavity consists of two mirrors arranged such that light bounces back and forth, each time passing through the gain medium.• Typically one of the two mirrors, the output coupler, is partially transparent. The output laser beam is emitted through this mirror.
(Q). Explain fiber optic cable mode?.(Q). What are single and multimode form fiber optics cables?.
• Single-mode fibers – used to transmit one signal per fiber (used in telephone and cable TV).• They have small cores(9 microns in diameter) and transmit infra-red light from laser.• Single mode fibers (SMF) are characterized by a small core diameter (from 5 to 10 microns.• The cladding diameter in both cases is 125m.• One can say that because of small core diameter, in the single mode optical fibers light propagates along one path that is nearly parallel to the fiber axis Single-mode fibers are used in applications in which low signal loss and high data rates are required, such as in long spans where repeater/amplifier spacing must be maximized.
• Multi-mode fibers – used to transmit many signals per fiber (used in computer networks).• They have larger cores(62.5 microns in diameter) and transmit infra-red light from LED. Step-index multimode fiber has an index of refraction profile that “steps” from low to high to low as measured from cladding to core to cladding.• Relatively large core diameter and numerical aperture characterize this fiber.• The core/cladding diameter of a typical multimode fiber used for telecommunication is 62.5/125 μm (about the size of a human hair).• The term “multimode” refers to the fact that multiple modes or paths through the fiber are possible.• multimode fiber is used in applications that require high bandwidth (< 1 GHz) over relatively short distances (< 3 km) such as a local area network or a campus network backbone.
• Discuss the losses in fiber optic transmission?.• State and Explain different types of losses of fiber cable?.
1. Absorption Loss• Light travels best in clear substances. Impurities such as metal particles or moisture in the fiber can block some of the light energy,• It absorb the light and dissipate it in the form of heat energy, which caused absorption loss.• The solution is to use ultra-pure glass and dopant chemicals to minimize impurities.• To eliminate loss at the water peak wavelength during the process of fiber manufacturing.
2. Bending Loss• Bending losses occurs in two forms - macrobending and microbending. When a cable is bent and it disrupts• The path of the light signal. The tighter the bends of a cable, the greater it is of the light loss.
3. Insertion Loss (IL)• Insertion loss is the most important performance indicator of a fiber optic interconnection.• This is the loss of light signal, measured in decibels (dB), during the insertion of a fiber optic connector.• Insertion loss can be minimized by proper selection of interconnect materials, good polishing and termination process of fiber connectors.
FDDI (Fiber Distributed Data Interface)(Q). What is FDDI ?. OR(Q). Explain the Terms…(1). FDDI specification.(2). FDDI MAC framing.(3). FDDI physical layer.
• Fiber Distributed Data Interface (FDDI) provides a 100 Mbit/s optical standard for data transmission in a local area network that can extend in range up to 200 kilometers.• FDDI local area networks can support thousands of users.• As a standard underlying medium it uses optical fiber, although it can use copper cable, in which case it may be referred to as CDDI (Copper Distributed Data Interface).• FDDI was considered an attractive campus backbone technology in the early to mid 1990s since existing Ethernet networks only offered 10 Mbit/s transfer speeds and Token Ring networks only offered 4 Mbit/s or 16 Mbit/s speeds.• Thus it was the preferred choice of that era for a high- speed backbone.
FDDI SpecificationFDDI is defined by four separate specifications.(1). Media Access Control (MAC)—Defines how the medium is accessed, including frame format, token handling, addressing, algorithm for calculating a cyclic redundancy check value, and error recovery mechanisms.(2). Physical Layer Protocol (PHY)—Defines data encoding/decoding procedures, clocking requirements, framing, and other functions.(3). Physical Layer Medium (PMD)—Defines the characteristics of the transmission medium, including the fiber-optic link, power levels, bit error rates, optical components, and connectors.(4). Station Management (SMT)—Defines the FDDI station configuration, ring configuration, and ring control features, including station insertion and removal, initialization, fault isolation and recovery, scheduling, and collection of statistics.
• Fax (short for facsimile), sometimes called telecopying, is the telephonic transmission of scanned printed material (both text and images), normally to a telephone number connected to a printer or other output device.• The original document is scanned with a fax machine (or a telecopier), which processes the contents (text or images) as a single fixed graphic image, converting it into a bitmap, the information is then transmitted as electrical signals through the telephone system.• The receiving fax machine reconverts the coded image, printing a paper copy.• Fax machines still retain some advantages, particularly in the transmission of sensitive material which, if sent over the Internet unencrypted, may be vulnerable to interception, without the need for telephone tapping.• In some countries, because electronic signatures on contracts are not recognized by law while faxed contracts with copies of signatures are, fax machines enjoy continuing support in business.
(Q). What is satellite communication?. Explain Wi-Fi network with their advantages and disadvantages.(Q). What is satellite communication?. State and Explain two types satellite communication with their merits and demerits.
• What is Satellite Communication? In satellite communication, signal transferring between the sender and receiver is done with the help of satellite. In this process, the signal which is basically a beam of modulated microwaves is sent towards the satellite. Then the satellite amplifies the signal and sent it back to the receiver’s antenna present on the earth’s surface.
Advantages of Satellites Communication• The advantages of satellite communication over terrestrial communication are: The coverage area of a satellite greatly exceeds that of a terrestrial system. Transmission cost of a satellite is independent of the distance from the center of the coverage area. Satellite to Satellite communication is very precise. Higher Bandwidths are available for use.
Disadvantages of Satellites Communication• The disadvantages of satellite communication: Launching satellites into orbit is costly. Satellite bandwidth is gradually becoming used up. There is a larger propagation delay in satellite communication than in terrestrial communication.
• There are following two methods of satellite communication. (1). FDMA (Frequency Division Multiplexing Access). (2). TDMA (Time Division Multiplexing Access).
FDMA (Frequency Division Multiplexing Access).• Frequency Division Multiple Access (FDMA), different signals are assigned frequency channels.• FDMA is a basic technology in the analog Advanced Mobile Phone System (AMPS).• With FDMA, each channel can be assigned to only one user at a time.• FDMA is also used in the Total Access Communication System (TACS).• FDMA technique is both used in analog as well as in digital systems.• FDMA is a simplest form of multiplexing technique.• There is no problem regarding transmission traffic.
TDMA (Time Division Multiplexing Access)• Time Division Multiple Access (TDMA), it makes use of the same frequency spectrum but allows more users on the same band of frequencies by dividing the time into “slots” and shares the channel between users by assigning them different time slots.• TDMA is utilized by Digital-Advanced Mobile Phone System (D- AMPS) and Global System for Mobile communications (GSM).• However, each of these systems implements TDMA in a somewhat different and incompatible way.• There is increase in transmission efficiency.• TDMA technique is used in voice communication system.• Cost of installation is very less.
• What is Digital Telephony?. OR• Explain Digital Telephony?.
• Digital telephony is the technique behind voice communication (i.e. Telephone Networks).• Telephony generally involves analog electrical signaling over copper wire.• In this we handles large quantity of high quality voice connections of higher distances.• Because analog signals have problems of longer distances.• Digital Telephony also avoids the problem of noise by encoding original audio signal into digital form this process Is also called as digitization• There is a better efficiency level.• Transmission is also good.