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Chapter 1
Introduction
Computer Networking:
A Top Down Approach
Featuring the Internet,
3rd edition.
Jim Kurose, Keith Ross
Addison-Wesley, July
2004.
Adapted from original book supplement slides
All material copyright 1996-2005
J.F Kurose and K.W. Ross, All Rights Reserved
Introduction
1-1
What’s a network
A group of devices that can communicate with
one another - hosts
Physical connections between devices are called links
workstation
workstation
mobile
workstation
server
Introduction
1-2
What’s a network
A group of devices that can communicate with
one another - hosts
Physical connections between devices are called links
workstation
workstation
router
mobile
router
workstation
server
Introduction
1-3
Who can talk on a network?
Desktops
Laptops
Routers
Cell phones
Printers
Scanners
Cameras
Cars
Toasters
Fridges
Microwaves
TVs
TiVos
Phones
Picture
frames
Watches
Satellites
Sensors
and more!
Introduction
1-4
What we couldn’t do without networks?
Introduction
1-5
What’s the Internet
A worldwide, public network connecting millions of
devices
Internet isworkstation
a network of networks!
workstation
workstation
workstation
workstation
workstation
workstation
workstation
workstation
workstation
workstation
router
workstation
workstation
router
router
workstation
workstation
router
router
workstation
router
router
router
router
router
router
mobile
workstation
router
router
mobile
workstation
router
router
mobile
workstation
router
mobile
workstation
server
mobile
workstation
server
mobile
workstation
server
mobile
workstation
server
mobile
workstation
server
server
server
server
Introduction
1-6
What’s the Internet
There is a hierarchy of networks:
At the edge: company or
university networks and end hosts
Middle: local and regional
Internet Service Providers
(ISPs)  TMobile, AOL
Tier 3 and Tier 2 ISPs
Top: national/international
ISPs  AT&T, Sprint
Tier 1 ISPs
router
server
local ISP
company
network
workstation
mobile
regional ISP
Introduction
1-7
How to talk on the Internet?
Must speak the same language!
Accessing the physical media of the link
Physical protocol
Accessing the devices on the same link
Link protocol
Traveling to a host at the other end via routers on the path
Internet Protocol (IP) – just making sure messages travel from
source to destination host, like post office
Delivering message to the right application, handling loss
Transport protocols, some of them are:
 Transport Control Protocol (TCP)
 User Datagram Protocol (UDP)
Making sure the application understands what we want
Application protocols
Introduction
1-8
What’s a protocol?
Human protocols:
“What’s the time?”
“I have a question”
Introductions
… specific messages
sent
… specific actions taken
when messages are
received,
or other events
occur
Network protocols:
Machines rather than
humans
All communication
activity in Internet
governed by protocols
Protocols define format,
order of messages sent
and received among
network entities, and
actions taken on message
transmission, receipt
Pretty much like game rules!
Introduction
1-9
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
Q: Can you think of other human protocols?
Introduction
1-10
Internet standards
Internet standard body
IETF (Internet Engineering Task Force)
Define Internet protocols and best practices
Standard proposals – Internet drafts
Accepted standards – Requests for comments (RFCs)
http://www.ietf.org
Introduction
1-11
Network structure
Network edge: generates data for end users
Applications
Hosts (end systems)
Client/server model
Peer-to-peer model
Network core: makes sure messages arrive to their
destination
Routers
Introduction
1-12
How do end-hosts talk?
Connectionless service
I only care that some data gets to the destination
Best-effort
Connection-oriented service
I would like to have some guarantees
 Delay
 Loss
Introduction
1-13
Connectionless service
Goal: Data transfer between end systems
Simply send data from local application to a remote
one
UDP – User Datagram Protocol [RFC 768]
Doesn’t care if messages are lost or reordered
No speed control
Introduction
1-14
Connection-oriented service
Goal: Reliable data transfer between end systems
Handshaking: connection establishment
Setup (prepare for) data transfer ahead of time
Hello, hello back human protocol
Set up “state” in two communicating hosts
TCP - Transmission Control Protocol [RFC 793]
Transmit and receive messages in order
Detect message loss and retransmit
Control the speed of the transfer so that the receiver
does not get overwhelmed
Reduce the speed if there is network congestion
Introduction
1-15
Connections: yes or no?
Applications using TCP
Web applications
Telnet
File transfer
E-mail
Applications using UDP
Streaming video
Teleconferencing
Internet telephony
Introduction
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The Network Core
Mesh of interconnected routers
Links have limited resource
Bandwidth – how much data can be
transferred in time unit
The fundamental question:
how is data transferred
through a network?
circuit switching:
dedicated circuit per call,
reserved resources
packet-switching:
data sent through network in “chunks”,
may need to wait or get dropped
Introduction
1-17
Network Core: circuit switching
Call setup required
Network resources (bandwidth)
are divided into pieces
Pieces reserved for the
“call” on the path
Dedicated resources:
no sharing, idle if not used
Guaranteed performance
Bandwidth is:
Frequency range
Data rate
Pieces can be divided by:
Time
Frequency
Introduction
1-18
Circuit switching: FDM and TDM
Example:
FDM (Frequency division multiplexing)
4 users
frequency
frequency
band
time
TDM (Time division multiplexing)
frame
frequency
slot
time
Introduction
1-19
Exercise
How long does it take to send a file of 640,000
bits from host A to host B over a circuit-switched
network?
All links are 1.536 Mbps (in the whole freq. range)
Each link uses TDM with 24 slots/sec
500 msec to establish end-to-end circuit
Introduction
1-20
Exercise
How long does it take to send a file of 640,000
bits from host A to host B over a circuit-switched
network?
All links are 1.536 Mbps (in the whole freq. range)
Each link uses FDM with 24 channels/frequency band
500 msec to establish end-to-end circuit
Introduction
1-21