ICOM 6115©Manuel Rodriguez-Martinez 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 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 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) CAT 3 – 16MHz CAT 5 – 100MHz

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 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 Light Source Detector Light Source Detector Fiber glass

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: Family Base Station Ad-hoc Network

ICOM 6115©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