Transcript Lecture 3 - Thayer School of Engineering

ENGS 4 - Lecture 3
Technology of Cyberspace
Winter 2004
Thayer School of Engineering
Dartmouth College
Instructor: George Cybenko, x6-3843
[email protected]
Assistant: Sharon Cooper (“Shay”), x6-3546
ENGS4 2004 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
– b1b2 are two bits, then the possible values
are 00, 01, 10, 11.
– b1b2b3 could be
000,001,010,011,100,101,110,111
– b1b2b3b4 could have how many values?
– k bits could have 2k values
– information transfer rates are measures in bits
per second
ENGS4 2004 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
ENGS4 2004 Lecture 3
kilo, mega, tera, peta
•
•
•
•
•
kilo is Greek prefix for 1000
10 bits can have 210 values (= 1024)
a kilobit is 1024 bits, kilobyte is 1024 bytes
mega is Greek prefix for 1,000,000
220 bits = 1 megabit, 220 bytes = 1
megabyte
• 230 bits = 1 terabit, 230 bytes = 1 terabyte
ENGS4 2004 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
ENGS4 2004 Lecture 3
Basic Web Browsing
Domain
Name
Server
www.cnn.com
Browser sends request
for web page www.cnn.com
into the network
User enters
a URL or clicks
on a link.
eg www.cnn.com
207.25.71.20
www.cnn.com
is really
207.25.71.20
207.25.71.20
web page at
www.cnn.com
ENGS4 2004 Lecture 3
“The web”
Network fabric: hubs, switches, routers
Hub
Router
Hub
User enters
a URL or clicks
on a link.
eg www.cnn.com
Switch
Printer
User enters
a URL or clicks
on a link.
eg www.cnn.com
User enters
a URL or clicks
on a link.
eg www.cnn.com
ENGS4 2004 Lecture 3
Router
Router
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
ENGS4 2004 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
ENGS4 2004 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 1014 bits
• 7,200 seconds in 2 hours
• bandwidth is 3.8 x 1014 bits/ 7.2 x 103
• 38 x 1013 / 7.2 x 103 = 5 x 1010 bps = 50 gbps
• latency is 2 hours
ENGS4 2004 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
ENGS4 2004 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
ENGS4 2004 Lecture 3
Cisco Stock 1999- now
ENGS4 2004 Lecture 3
Engineering Challenges of Different Content Types
Content
Bandwidth
Web, data
Medium
Bursty
QoS?
Yes
No
Voice
Low
Yes
Yes
TV, video
High
No
Yes
Monitoring
Variable
No
No
ENGS4 2004 Lecture 3
Homework 1 – Due Jan 20
1.
2.
3.
4.
5.
6.
Estimate the number of bytes in the ORC (2003-2004
edition, printed)
How much time would downloading it require on a 56
kbps modem line?
How much time would downloading it require on a 10
mbps ethernet?
How much time would downloading it require on a 100
mbps ethernet?
What is the bandwidth and latency of the NASA Mars
Rover to earth channel?
Create a web page with the answers to these
questions on the webpage.
ENGS4 2004 Lecture 3
Homework 1 – Due Jan 20
Create a web page with the answers to these
questions on the webpage.
ENGS4 2004 Lecture 3