4/8/10 1 MS in Telecommunications TCOM 500: Modern Telecommunications Dr. Bernd-Peter Paris George Mason University Spring 2009 MS in Telecommunications Outline • Commonly used transmission media and their properties. • Noise: the fundamental limiting factor in all communication systems. • Path loss models for different transmission media. • Link budgets and transmission range. • Distortion in transmission media. Paris 2 TCOM 500: Modern Telecommunications4/8/10 2 MS in Telecommunications Context • Purpose of a digital communication system: • Replicate digital information available at the transmitter’s location at the receiver’s location. • Generally this implies connecting spatially separate locations. • Same principle applies for communicating over time – storage of information. • The communications channel models the link connecting transmitter and receiver. • Channel models the degradation that the transmitted signal experiences. Paris 3 TCOM 500: Modern Telecommunications Transmitter Channel Receiver bits signal signal bits MS in Telecommunications TRANSMISSION MEDIA Paris 4 TCOM 500: Modern Telecommunications4/8/10 3 MS in Telecommunications Classification of Transmission Media Paris 5 TCOM 500: Modern Telecommunications Transmission Media Guided Media (wired) Twisted-Pair Coaxial Fiber-Optic Unguided Media (wireless) Free-space MS in Telecommunications Twisted Pair Cable • Two electrical conductors – usually copper – each covered with insulating plastic. • Used for baseband signaling – i.e., line codes. • Signal is represented as the voltage between the conductors. • Usually, one conductor is grounded; • The other carries the signal. Paris 6 TCOM 500: Modern Telecommunications4/8/10 4 MS in Telecommunications Purpose of Twisting • On a pair of parallel wires, the currents flowing through the conductors interact with each other. • This can significantly reduce the flow of current, • Especially at high frequencies. • By twisting the wires, the direction of the induced current changes with every twist. • The induced currents cancel each other. Paris 7 TCOM 500: Modern Telecommunications 1. Current flows through conductors in opposite directions. 2. Current creates a magnetic field surrounding conductor. 3. Magnetic field induces a current in the other conductor. MS in Telecommunications Shielded and Unshielded Twisted Pair Paris 8 TCOM 500: Modern Telecommunications • UTP is most widely used. • STP provides additional protection against external electromagnetic interference. • Expensive and rarely used.4/8/10 5 MS in Telecommunications Example: Category-5 UTP • Standardized by ANSI/TIA/EIA-568-A. • Includes four twisted pairs in a single cable jacket. • Typically, three twists per inch. • Wire thickness: 24 gauge (0.205 mm²) • Main use: 100Mb/s Ethernet. Paris 9 TCOM 500: Modern Telecommunications MS in Telecommunications UTP Categories Paris 10 TCOM 500: Modern Telecommunications4/8/10 6 MS in Telecommunications Connector • UTP cables are terminated by 8P8C (8 positions, 8 contacts) modular connectors. • Often (incorrectly) referred to as RJ-45. • Assignment of wires to pins is specified by TIA/EIA-568-B. Paris 11 TCOM 500: Modern Telecommunications MS in Telecommunications Coaxial Cable • Coax cable is an electrical cable • with an inner conductor • surrounded by a flexible, tubular insulating layer, • surrounded by a tubular conducting shield. • Main benefit: the electromagnetic field carrying the signal exists only between the inner and outer conductors. • Creates no interference. • Very resilient to interference. Paris 12 TCOM 500: Modern Telecommunications • Coax cables are relatively expensive. • Replaced largely by UTP for data communications.4/8/10 7 MS in Telecommunications Fiber Optic Cables • An optical fiber is made up of • the core (carrying the light pulses), • the cladding (reflecting the light pulses back into the core) and • the buffer coating (protecting the core and cladding from moisture, damage etc.). • Main benefits: • supports very high data rates up to 100Gb/s. • With Wavelength division multiplexing, single fiber can carry up to 1600Gb/s. • Low losses for long range transmission. Paris 13 TCOM 500: Modern Telecommunications MS in Telecommunications Propagation Modes • Propagation of light relies on reflection of light towards the core of the fiber. • At boundary between cladding and fiber light is reflected as long as incident angle is not too steep. • Snell’s law. Paris 14 TCOM 500: Modern Telecommunications Mode Multimode Step Index Graded Index Single Mode4/8/10 8 MS in Telecommunications Propagation Modes Paris 15 TCOM 500: Modern Telecommunications Requires a highly focused beam at the source. MS in Telecommunications Optical Transmitter and Receiver • Optical communications channels must interface with electrical equipment. • Optical transmitter are LEDs or lasers. • They convert an electrical signal into light pulses. • A photodiode converts optical signals to electrical signals. Paris 16 TCOM 500: Modern Telecommunications Optical Interface Electrical Interface4/8/10 9 MS in Telecommunications NOISE IN COMMUNICATION SYSTEMS Paris 17 TCOM 500: Modern Telecommunications MS in Telecommunications Noise • Noise is the term used to describe the random signal that is added to the transmitted signal on its way to the receiver. • We distinguish noise from: • Interference – either intentional or unintentional perturbation from another transmitted signal. • Distortion – the alteration the signal experiences generally due to multiple-propagation paths. • Noise is the fundamental limiting factor in any communication system. Paris 18 TCOM 500: Modern Telecommunications4/8/10 10 MS in Telecommunications Thermal Noise • In most communication systems, thermal noise is the most important form of noise. • Thermal noise is present in any conducting material and is due to the random movement of electrons in the conductor. • Thermal noise is random and is well modeled as following a Gaussian distribution. • Thermal noise is well modeled as covering the entire spectrum uniformly. •