TransmissionMedia 1.pdf

Introduction to Transmission Systems Overview Guided - wire Unguided - wireless Characteristics and quality determined by medium and signal For guided, the medium is more important For unguided, the bandwidth produced by the antenna is more important Key concerns are data rate and d...

Introduction to Transmission Systems Overview Guided - wire Unguided - wireless Characteristics and quality determined by medium and signal For guided, the medium is more important For unguided, the bandwidth produced by the antenna is more important Key concerns are data rate and distance Example of Communication System Design Factors Bandwidth —Higher bandwidth gives higher data rate Transmission impairments —Attenuation Interference Number of receivers —In guided media —More receivers (multi-point) introduce more attenuation Electromagnetic Spectrum Classes of transmission media Guided Transmission Media Twisted Pair Coaxial cable Optical fiber  Transmission Characteristics of Guided Media Frequency Typical Typical Repeater Range Attenuation Delay Spacing Twisted pair 0 to 3.5 kHz 0.2 dB/km @ 50 µs/km 2 km (with loading) 1 kHz Twisted pairs 0 to 1 MHz 0.7 dB/km @ 5 µs/km 2 km (multi-pair 1 kHz cables) Coaxial cable 0 to 500 MHz 7 dB/km @ 10 4 µs/km 1 to 9 km MHz Optical fiber 186 to 370 0.2 to 0.5 5 µs/km 40 km THz dB/km Attenuation: Loss of signal strength as it travels down the wire. Sample problem A signal is transmitted through a 10-kilometer cable with an attenuation of 3 dB/km. If the input signal power at the beginning of the cable is 10 mW, what is the output signal power after the 10-kilometer transmission? Step 1: Calculate the total attenuation: Step 2: Apply the attenuation to the input power: Electromagnetic Interference and Crosstalk EMI introduces noise into the signal and thereby reduces the signal-to-noise ratio (SNR). The effect of EMI on communication lines is observed as reduced distance and transmission rates. Crosstalk refers to any undesired influence on the signal due to the communication lines running adjacent to it. Near End Crosstalk Measurement of signal coupling or crosstalk near the ends of the cable. NEXT is more likely to occur at the ends due to stronger signal levels. Weaker receive-signal levels make the cable more vulnerable to crosstalk. Desirable NEXT Value : A higher NEXT (dB) value indicates better performance. Attenuation-to-Crosstalk Ratio (ACR) ACR : A combined measurement of attenuation and crosstalk. Importance : Ensures signal integrity by balancing signal loss with reduced crosstalk. A higher ACR value indicates better performance. A good Attenuation to Crosstalk Ratio (ACR) is typically considered to be 20 dB or higher ACR = Attenuation – Crosstalk dB Sample Problems NEXT A signal of 10 mW is transmitted at the near end of a twisted pair cable. If the NEXT loss at 100 MHz is 45 dB, calculate the crosstalk power received on the neighboring pair. ACR You are testing a network cable to ensure it meets the required performance standards. During the test, you measure the following: Attenuation: 45 dB and Crosstalk: 20 dB Determine the cable performance in terms of ACR. The Predecessors Two-wire open line - consists of two wires that are generally spaced from 2 to 6 inches apart by insulating spacers. - most often used for power lines, rural telephone lines, and telegraph lines. It is sometimes used as a transmission line between a transmitter and an antenna or between an antenna and a receiver. Advantage: simple construction. Disadvantages: high radiation losses and electrical noise pickup because of the lack of shielding. Twin lead cable a two-conductor flat cable used as a balanced transmission line to carry radio frequency (RF) signals. It is constructed of two, stranded copper wires, or solid copper-clad steel wires. The wires are held a fixed distance apart by a plastic ribbon that is a good insulator at radio frequencies (usually polyethylene). It is also called (two- wire) ribbon cable. Twisted Pair The size of wire, type of insulation, and tightness of the twist (twists per inch) determine its characteristics. Pair twists are what help to cancel out NEXT--different twist rates on each pair prevent the pairs from picking up signals from adjacent pairs. It's important to maintain pair twists as close to termination as possible UTP and STP cables Categories of unshielded twisted- pair cables Categories of unshielded twisted-pair cables Twisted Pair - Applications Most common medium Telephone network —Between house and local exchange (subscriber loop) Within buildings —To private branch exchange (PBX) For local area networks (LAN) —10Mbps or 100Mbps Higher category cables are used for data center and professional applications - in Gbps transfer rates Twisted Pair - Transmission Characteristics Analog —Amplifiers every 5km to 6km Digital —Use either analog or digital signals —repeater every 2km or 3km Limited distance Unshielded and Shielded TP Unshielded Twisted Pair (UTP) —Ordinary telephone wire —Cheapest —Easiest to install —Suffers from external EM interference Shielded Twisted Pair (STP) —Metal braid or sheathing that reduces interference —More expensive —Harder to handle (thick, heavy) Coaxial Cable Categories of coaxial cables Coaxial Cable Applications Most versatile medium Television distribution —Arial to TV —Cable TV Long distance telephone transmission —Can carry 10,000 voice calls simultaneously —Being replaced by fiber optic Short distance computer systems links Local area networks Coaxial Cable - Transmission Characteristics Analog —Amplifiers every few km —Closer if higher frequency —Up to 500MHz Digital —Repeater every 1km —Closer for higher data rates