Transcript ICOM 6115
ICOM 6115 – Computer Networks and the WWW Manuel Rodriguez-Martinez, Ph.D. Lecture 9 ICOM 6115 ©Manuel Rodriguez-Martinez Another view of bandwidth • Each signal will be made up of components of various frequencies • If a signal has a range of frequencies f1 through f2 – Bandwidth is f2 – f1 – Ex. If a signal has frequencies 10Hz through 60Hz, then the bandwidth will be 50Hz – Telephone has 3kHz of bandwidth • In this view (communications guys) our “bandwidth” is called data rate. ICOM 6115 ©Manuel Rodriguez-Martinez Bandwidth and Data Rate • Henry Nyquist proved that – A signal with bandwidth H can be reconstructed if the medium is sampled with a frequency 2H. • Noiseless channel • If the signal has V discrete levels then – Data rate = 2H log_2 V bits/sec – Data rate is proportional to the bandwidth • So, we will call data rate the bandwidth ICOM 6115 ©Manuel Rodriguez-Martinez A few numbers • Bandwidth and data rates for a binary signaling scheme – Telephone - 3kHz – gives 6000 bps • Need to user more tones to encode more bits – Go faster – Cooper • Cat 3 - 16 MHz • Cat 5 – 100 Mhz – Fiber • 25,000 GHz ICOM 6115 ©Manuel Rodriguez-Martinez Signal-to-Noise ratio • Data Channels are not noiseless • Signal-to-noise ratio – How much of the signal is noise – Decibel (dB) – 10 log_10 S/N, where S/N is the signal-to-noise ratio • Claude Shannon proved the following – Maximum Data rate = H log_10 (1 + S/N) bits/sec • Telephone has 3kHz bandwidth and 30dB S/N – Maximum data rate = 30,000 bps ICOM 6115 ©Manuel Rodriguez-Martinez Guided Media • • • • Tape or a DVD Twisted Pair Cable Coaxial Cable Fiber Optics ICOM 6115 ©Manuel Rodriguez-Martinez Tape • Store data in tapes, pack your tapes, and carry them around – This is a silly example, but illustrates difference between bandwidth and latency • One tape can hold hundreds of gigabytes – UItrium – 200 gigabytes – 1 box of 1000 tapes can hold 200 Terabytes, or 1600 terabits (1.6 petabits) – Ship the box by 24hours FedEx – 86,400 sec – Bandwidth of this link: 19Gbps • From San Juan to Mayaguez (2 hr drive) – 200 Gbps ICOM 6115 ©Manuel Rodriguez-Martinez The Problem with Tape • Latency . . . – It takes 1 day to see any data by 24 hr FedEx • But it is a 19Gbps link – It takes 2 hours to see data any by driving from SJ to Mayaguez • But is a 200Gbps link – It takes .5ms to see data on a 100Mbps link with a RTT of 1ms • Many times bandwidth is not the issue, is latency! ICOM 6115 ©Manuel Rodriguez-Martinez Unshielded Twisted Pair Cooper CAT 3 – 16MHz CAT 5 – 100MHz • Cables are twisted to like DNA – Make cable radiate less • Use by telephone, and in many LANs (Ethernet) • Most common cables are Cat. 3 and now Cat. 5 (most popular currently) – New comers (Cat 5e, Cat 6 and Cat 7) ICOM 6115 ©Manuel Rodriguez-Martinez Coaxial Cable (old timer) • Can span longer distances (shielding) – 75 ohm – Cable TV – 50 ohm – Digital Transmission (old days Ethernet) • Bandwidth – 1GHz (modern day) ICOM 6115 ©Manuel Rodriguez-Martinez Cat 5, Coaxial and Ethernet ICOM 6115 ©Manuel Rodriguez-Martinez Fiber Optics • Way of the future – 1Gbps (“cheap fiber”) – 10Gbps (“expensive fiber”) • Limited by the ability to convert between light and electrical signals. • Fiber gives the possibility of infinite bandwidth – Old days: avoid moving data over network – The Future: Spread data around network • It is cheap! ICOM 6115 ©Manuel Rodriguez-Martinez Basics of Fiber Light Source Fiber glass Detector Fiber glass Detector Light Source • Bits are converted into light pulses by light source (1 is a light pulse, 0 is lack of light) • Then moved by fiber of glass. • Detector maps light to electrical signal • Put two lines: one to send, one to receive – Full duplex fiber ICOM 6115 ©Manuel Rodriguez-Martinez Physics of Fiber • Prepare the glass so it can make light reflect completely in the wire. ICOM 6115 ©Manuel Rodriguez-Martinez Types of Fiber • Multi-mode fiber – Light pulses hit the glass at various angles • Need repeaters to amplify signal • Single-mode fiber – Very thin glass – light goes almost on a straight line – Can go longer distances (100km) without repeaters ICOM 6115 ©Manuel Rodriguez-Martinez Fiber Networks: Active Repeaters - Failure of 1 link breaks the network + Links can be kilometers in length (campus backbone) ICOM 6115 ©Manuel Rodriguez-Martinez Fiber Networks: Passive Star ICOM 6115 ©Manuel Rodriguez-Martinez Passive Star: Tradeoff • Benefit – Failure in one interface won’t break network • Disadvantage – Light is broadcasted, so need good photodiodes • Limits the number of nodes you can have on the network ICOM 6115 ©Manuel Rodriguez-Martinez Radio Waves • Use air as the medium for data transmission – Unguided – Natural Broadcast network – Security issue here • How to protect your data? • Need good encryption mechanism • Network card has radio transmitter and receiver ICOM 6115 ©Manuel Rodriguez-Martinez Example: 801.11 Family Base Station ICOM 6115 Ad-hoc Network ©Manuel Rodriguez-Martinez Some Issues • Lack of coverage • Overlapping coverage ICOM 6115 ©Manuel Rodriguez-Martinez The Telephone System • Why should we care? • Phone lines run through vast regions of the Earth – They reach homes, schools, offices • Big Idea! – Move data over phone lines • Build wide-area networks on top of leased lines from phone companies – Sprint, AT&T, Verizon ICOM 6115 ©Manuel Rodriguez-Martinez ICOM 6115 ©Manuel Rodriguez-Martinez