Course Overview
Summer 2010
CS 372 Introduction to Computer Networks*
Monday, June 21, 2010
School of Electrical Engineering and Computer Science
Oregon State University
* Based in part on slides by Bechir Hamdaoui and Paul D. Paulson.
Chapter 1, slide:
1
Lecture/Office/Lab Hours
 Course website


http://classes.engr.oregonstate.edu/eecs/summer2010/cs372/index.html
Please write down this URL—all course material and information will
be provided thru this site
 Lectures

M-F 1-1:50, WNGR 287
 Instructor

Majid Alkaee Taleghan (alkaee@eecs.oregonstate.edu)

Office hours: MW 2-3 @ 3130 KEC
Chapter 1, slide:
2
Lecture/Office/Lab Hours
 Teaching Assistant
 Lab
 Location: Dearborn 205
 Access code
 Hours
Chapter 1, slide:
3
Prerequisite/Textbook
Prerequisite:
 CS 261 or an equivalent data structure course
 Programming skills in Java
 Basic Linux familiarity
 Textbook is Required
Textbook
Computer Networking: A Top-Down Approach
Featuring the Internet, 5th Edition, Games F.
Kurose, Keith W. Ross
Chapter 1, slide:
4
Grading Policy
 Assignments: 20%
 Each student must hand in one copy
 5 assignments: approx. 1 every week
 Labs: 15%
 Groups of 2 are required; each group must hand in
one and only one copy
 5 labs: approx. 1 every week
 Quizzes: 5%
 5 quizzes: approx.1 every week
 One midterm exam: 25%
 Final exam: 35%
Chapter 1, slide:
5
Course Goals
 Learn the basics of computer networks: concepts,
principles, and architecture
Goals
 Understand how various network protocols work:
the Internet will be our underlying example
 Observe and analyze network protocols in actions
 Write network programs
Methods
Lecture
Measurement
“principles & concepts”
“Real networks”
Assignments
Labs
Chapter 1, slide:
6
Topics To Be Covered
 Architecture of the Internet: network of networks
 Network protocols: HTTP, FTP, IEEE
 Congestion and flow control: TCP
 Routing algorithms: IP and datagram
 Packet-switching and circuit-switching
 Data link layers and Ethernet: ARP, CSMA/CD
 Medium access control and local area networks
Chapter 1, slide:
7
Labs: Objective & Approach
Objective
 Understand how Internet protocols work
 Force network protocols to perform certain actions
 Observe and analyze protocols’ behavior
Approach
 Real measurement of Internet traffic
 Sniff and analyze traffic sent/received from/by
your end system
 Software tool: Wireshark (also known as Ethereal)


already installed in Lab DEAR 205
To run, type: sudo wireshark then enter your eecs psswd
Chapter 1, slide:
8
Assignments: Objective & Approach
Objective
 Practice problems to deepen your understanding of
basic networking concepts
Approach
 You are expected to do your assignments individually
 Some problems will be solved in class: this gives you
the opportunity to clarify things further
Chapter 1, slide:
9
Keep in mind …
 My mission is to help you learn and understand
the material—please do not hesitate to ask for
help
 I will be available for you to help you with
everything: material/assignments/labs
 We’ll start after a 5-mn break
 please team up for labs…
 find your partner …
 sign up sheet …
Chapter 1, slide: 10
Chapter 1: Introduction
Our goal:
 learn basic network
terminologies
 more depth, detail
later in course
 approach:
 use Internet as
example
Acknowledgement: slides drawn heavily from Kurose & Ross
Chapter 1, slide: 11
Chapter 1: roadmap
1 What is the Internet?
2 Network edge
3 Network core
4 Network access and physical media
5 Internet structure and ISPs
6 Protocol layers, service models
7 Delay & loss in packet-switched networks
Chapter 1, slide: 12
What’s the Internet: a “service” view
 communication
infrastructure enables
distributed applications:

Web, email, games, ecommerce, file sharing
 communication services
provided to apps:


Connectionless:
unreliable
connection-oriented:
reliable
Chapter 1, slide: 13
What’s the Internet: “nuts and bolts” view
 millions of connected
computing devices: called
hosts or end systems


e.g., Laptops, workstations
running network apps
router
server
mobile
local ISP
 routers & switches:
 forward packets (chunks of
data)
 communication links
 e.g., fiber, copper, radio,
satellite
workstation
regional ISP
company
network
Chapter 1, slide: 14
What’s the Internet: “nuts and bolts” view
 network protocols


control sending/receiving of
msgs
e.g., TCP, IP, HTTP, FTP, PPP
router
workstation
server
mobile
local ISP
 Internet standards

IETF: Internet Engineering
Task Force
regional ISP
• RFC: Request for comments

IEEE: for links, e.g., Ethernet
company
network
Chapter 1, slide: 15
What’s a protocol?
a human protocol and a computer network protocol:
Hi
TCP connection
request
Hi
TCP connection
response
Got the
time?
Get http://www.awl.com/kurose-ross
2:00
<file>
time
Chapter 1, slide: 16
What’s a protocol?
human protocols:
 “what’s the time?”
 “I have a question”
 introductions
… specific msgs sent
… specific actions taken
when msgs received,
or other events
network protocols:
 machines rather than
humans
 all communication
activity in Internet
governed by protocols
protocols define (1) format,
order of msgs sent and
received among network
entities, and (2) actions
taken on msg
transmission, receipt
Chapter 1, slide: 17
Chapter 1: roadmap
1 What is the Internet?
2 Network edge
3 Network core
4 Network access and physical media
5 Internet structure and ISPs
6 Protocol layers, service models
7 Delay & loss in packet-switched networks
Chapter 1, slide: 18
A closer look at network structure:
 network edge:
applications and
hosts
 network core:
routers
 network of
networks

 access networks,
physical media:
communication links
Chapter 1, slide: 19
The network edge: service models
 end systems (hosts):



run application programs
e.g. Web, email
at “edge of network”
 client/server model


client host requests, receives
service from always-on server
e.g. Web browser/server;
email client/server
 peer-to-peer model:


minimal (or no) use of
dedicated servers
e.g. Skype, BitTorrent, KaZaA
Chapter 1, slide: 20
Chapter 1: roadmap
1 What is the Internet?
2 Network edge
3 Network core
4 Network access and physical media
5 Internet structure and ISPs
6 Protocol layers, service models
7 Delay & loss in packet-switched networks
Chapter 1, slide: 21
The Network Core
 mesh of interconnected
routers
 the fundamental
question: how is data
transferred through net?
 circuit switching:
dedicated circuit per
call: telephone net
 packet-switching: data
sent thru net in
discrete “chunks”
Chapter 1, slide: 22
Network Core: Circuit Switching
End-end resources
reserved for “call”
 dedicated resources: no
sharing
 call setup required
 circuit-like (guaranteed)
performance
 same path for all chunks
Chapter 1, slide: 23
Network Core: Circuit Switching
network resources
(e.g., bandwidth)
divided into “pieces”
 allocated pieces per call
 no sharing
resource piece idle if
not used by owning call
Chapter 1, slide: 24
Network Core: Circuit Switching
 Two ways of dividing bandwidth into “pieces”
frequency division
 time division

Chapter 1, slide: 25
Circuit Switching: FDM and TDM
Example:
Freq. Division Multiplx. (FDM)
4 users
frequency
time
Time Division Multiplx. (TDM)
frequency
time
Chapter 1, slide: 26
Numerical example
 How long does it take to send a file of
640,000 bits from host A to host B over a
circuit-switched network?
The link’s transmission rate = 1.536 Mbps
 Each link uses TDM with 24 slots/sec
 500 msec to establish end-to-end circuit
Let’s work it out! You have 5 minutes!

 Solution:
 Bandwidth of circuit = 1.536/24 = 64 kbps
 Time to send: 640000 bits/64 kbps + 0.5s =
10.5s
Chapter 1, slide: 27
Network Core: Packet Switching
100 Mb/s
Ethernet
A
B
C
1.5 Mb/s
each end-to-end data stream is divided into packets
 no dedication/reservation: all streams share resources
 no setup is required
 resources used as needed
 each packet uses full link bandwidth
 aggregate resource demand can exceed capacity
 no guarantee
Chapter 1, slide: 28
Network Core: statistical multiplexing
100 Mb/s
Ethernet
A
B
statistical multiplexing
C
1.5 Mb/s
queue of packets
waiting for output
link
D
E
Sequence of A & B packets does not have fixed pattern,
shared on demand  statistical multiplexing.
Whereas in TDM, each host gets same slot (periodically)
Chapter 1, slide: 29
Packet-switching: store-and-forward
L
R
R
R
Entire packet must arrive at router before it can be
transmitted on next link: store and forward
 Takes L/R seconds to transmit (push out) packet of
L bits on to link of R bps
 delay = 3L/R (assuming zero propagation delay)
more on this later …
Example:
 L = 10,000,000 bits and R = 1 Mbps
 delay = 30 sec
Chapter 1, slide: 30
Packet switching versus circuit switching
Packet-switching
 Resources
sharing
 Congestion
may lead to it
 Overhead
less overhead;
no connection setup
 Guarantee
Best-effort
no guarantee
Circuit-switching
dedicated
admission control
more overhead;
reserve resources 1st
provide guarantee
good for multimedia
Chapter 1, slide: 31
Packet switching versus circuit switching
A
Circuit switching
B
B: has no
packets to send
A
2 Mb/s
• 2 circuits (use TDM)
• A reserves 1 circuit
• B reserves 1 circuit
Utilization = 50% only = 1 Mb/s
Packet switching
B
2 Mb/s
• statistical multiplex.
• A uses full link since
B is not using it
Utilization = 100% = 2 Mb/s
Chapter 1, slide: 32
Packet switching versus circuit switching
Utilization (average % of bandwidth used)
Packet switching allows more users to use network (better
utilization)!
 3 Mb/s link
 each user:


1 Mb/s when “active”
active 1/3 of time
 circuit-switching:
N users
3 users
 packet switching:
 with 4 users, prob that more
than 3 are active is 0.0123
 with 5 users, prob. that more
than 3 active is ???
(see board notes)

3 Mbps link
More of this in assignment 1
Chapter 1, slide: 33