ENGS Lecture 3 ENGS 4 - Lecture 3 Technology of Cyberspace Winter 2004 Thayer School of Engineering Dartmouth College Instructor: George Cybenko, x Assistant: Sharon Cooper (“Shay”), x6-3546

ENGS Lecture 3 Basic Terminology - bits bit – basic unit of information, having two possible values, 0 or 1. –example: if b is a bit, then either b=0 or b=1 –b 1 b 2 are two bits, then the possible values are 00, 01, 10, 11. –b 1 b 2 b 3 could be 000,001,010,011,100,101,110,111 –b 1 b 2 b 3 b 4 could have how many values? –k bits could have 2 k values –information transfer rates are measures in bits per second

ENGS Lecture 3 Basic Terminology - bytes byte – 8 bits, so 256 possible values. – 26 characters, lower and upper case = 52 – 10 digits, 52+10=62 – add punctuation, control characters, etc – 128 “traditional” characters (ASCII characters) – typically, one typed character is stored in 8 bits = 1 byte –storage and memory sizes are expressed in units of bytes

ENGS Lecture 3 kilo, mega, tera, peta kilo is Greek prefix for bits can have 2 10 values (= 1024) a kilobit is 1024 bits, kilobyte is 1024 bytes mega is Greek prefix for 1,000, bits = 1 megabit, 2 20 bytes = 1 megabyte 2 30 bits = 1 terabit, 2 30 bytes = 1 terabyte

ENGS Lecture 3 Some simple examples Google home page has about 4000 bytes 4000 bytes = 32,000 bits Internet has to “move” 32,000 bits for a user to see the Google home page. Internet has to move 3,200,000 bits for 100 users to see Google, IE, about 3 megabits Dartmouth has 10,000 people on campus

ENGS Lecture 3 Basic Web Browsing User enters a URL or clicks on a link. eg Browser sends request for web page into the network Domain Name Server is really “The web” web page at

ENGS Lecture 3 Network fabric: hubs, switches, routers User enters a URL or clicks on a link. eg Hub User enters a URL or clicks on a link. eg User enters a URL or clicks on a link. eg Switch Router Printer Hub

ENGS Lecture 3 Network fabric: hubs, switches, routers MAC address – hardware address assigned by manufacturer IP address – 32 byte address used for all devices attached to the Internet (4 billion addresses) hub – broadcasts all data to all ports switch – directs data to ports according to the IP to hardware/MAC address correspondence router – directs data to other routers for deliver based on IP addresses aggregated data transfer requirements are huge

ENGS Lecture 3 Bandwidth and latency bandwidth – measured in bits per second example: dialup modem has bandwidth of 56kbps = 56 kilobits per second = 56 x 1024 bits per second wireless ethernet has nominal bandwidth of 11 megabits per second wired ethernet has 10 or 100 megabits per second bandwidth latency is the time required to move to move the first bit from one point to another

ENGS Lecture 3 Bandwidth versus latency Truck load of DVD’s driven from Hanover to Boston – 2 hour drive 10,000 DVD’s in truck, each DVD stores 4.7 gigabytes of data = 4.7x 8 Gb = 37.6 gigabits = about 3.8 x bits 7,200 seconds in 2 hours bandwidth is 3.8 x bits/ 7.2 x x / 7.2 x 10 3 = 5 x bps = 50 gbps latency is 2 hours

ENGS Lecture 3 Bandwidth versus latency 400,000 bits per web page 10,000 users 4,000,000,000 bits = 4 gigabits if the 10,000 users get their web pages in 1 second, then the network has to offer at least 4 gigabits per second bandwidth observed latency is only a second or so Quality of Service (QOS) – guaranteed latency and bandwidth

ENGS Lecture 3 Cisco Systems Leading manufacturer of high performance routers and switches for the Internet Founded in late 1980’s Stanford spinoff, based on software developed there Huge growth in 1990’s Internet tech collapse in 2000

ENGS Lecture 3 Cisco Stock now

ENGS Lecture 3 Engineering Challenges of Different Content Types ContentBandwidth Bursty QoS? Web, data Medium Yes No Voice Low Yes Yes TV, video High No Yes Monitoring Variable No No

ENGS Lecture 3 Homework 1 – Due Jan 20 1.Estimate the number of bytes in the ORC ( edition, printed) 2.How much time would downloading it require on a 56 kbps modem line? 3.How much time would downloading it require on a 10 mbps ethernet? 4.How much time would downloading it require on a 100 mbps ethernet? 5.What is the bandwidth and latency of the NASA Mars Rover to earth channel? 6.Create a web page with the answers to these questions on the webpage.

ENGS Lecture 3 Homework 1 – Due Jan 20 Create a web page with the answers to these questions on the webpage.