Download Lecture 26

How the Internet Works
The Internet and the Web
• The Web is actually just one of many computer
applications that run on the Internet
• Among others are email and file transfer programs
for text, sound, and video
• The Internet provides the physical basis for
sending and receiving information of any kind
• Some similarities to the telephone system, but not
totally
Structure and Behavior
• The Internet has a structure or physical
organization of its parts
• The Internet has a set of protocols or rules
for transmitting information
Structure:
Point to Point Connections
I connect my laptop to my desktop via
an ethernet cable or wireless card
I connect my iPod or cell phone to my
laptop or desktop via a USB line or
Bluetooth card
Wide Area Network (WAN)
Each computer
or resource is
called a node
All connections are point to point
Not practical as the number of nodes
gets large
A Central Network Server
Easy to add or remove nodes
Not robust: if the server goes down, so
does the network
A Better Structure:
Distributed Servers
The central unit is actually a collection
of many machines through which
messages are routed (one link for many
machines, but many of these links are
available)
The network is faulttolerant: if a server goes
down, the net stays alive
Internet Service Providers (ISPs)
Local ISP
Local ISP
Regional ISP
National ISP
International ISP
A hierarchy of connections
Behavior: Internet Protocols
• With so many different types of machines,
operating systems, and applications, we need a set
of universal rules to exchange information
• These rules are called protocols
• Established and maintained by the Internet Society
Protocol Hierarchy
5
Application
HTTP, FTP
4
Transport
TCP
3
Network
IP
2a
Logical link control
PPP, ethernet
2b Medium access control ethernet
1
Physical
Modem, DSL
Application
Transport
Network
The Physical Layer
Logical link control
Medium access
control
Physical
• Sets up a “pipe” through which bits are
transmitted
• Concerned with voltage levels, radio frequencies,
converting between analog and digital signals, etc.
• Mechanics of phone line (simple modem) or
broadband (DSL or cable modem) transmission
Transmission Speeds and Times
Application
Transport
Network
Medium Access Control
Logical link control
Medium access
control
Physical
• Need to establish a connection between sender and
receiver across a physical link
• Could have two or more senders competing for the
same link
• Poll until the link becomes idle, then transmit
• If two senders collide, both stop sending, wait
random times, and then resend
Application
Transport
Network
Logical Link Control
Logical link control
Medium access
control
Physical
• Medium access sets up a channel, but
logical link ensures that a message arrives
correctly
• Uses ARQ (automatic repeat request)
algorithm to check information for damage
Packets
• Before information is sent, it’s bundled up into
one or more packets
• A packet contains
– Beginning and ending marks of the information
– The addresses of the sender and receiver
– A number indicating the position of the packet’s
information in the original source information
– A damage indicator
Receiving the Message
• When the receiver gets a packet, it
– Checks for damage
– Sends an acknowledgement message (ACK) to
the sender if the packet is not damaged
ARQ Algorithm
• The sender waits a brief time after sending a
packet
• If an acknowledgement message is not received
after that time, the sender resends a copy of the
packet
• If the receiver gets the copy, it’s thrown away
Application
Transport
Network
Network Layer
Logical link control
Medium access
control
Physical
• Maintains a universal addressing scheme for
all nodes in the network
• Delivers messages between any two nodes
Addressing a Machine
• Use the Internet Protocol (IP) system
• An IP address is a 32-bit number that
contains four 8-bit quantities, each quantity
ranging from 0..255
• A domain name is a symbol associated with
an IP address
Resolving a Domain Name to an
IP Address
• Domain name: www.wlu.edu
• IP Address: 137.113.100.113
• A domain name service (DNS) keeps a table
of domain names and their IP addresses
Routing a Message
Ideally, the network layer would find the shortest path through the
network from one machine to the other
As the network gets very large, this method becomes impractically
slow, so we have to approximate the shortest path
Transport Layer
• One machine might be running several
applications (email server, Web server, FTP
server, etc.)
• Need to locate the port that the application uses to
receive information
• A port number identifies an application’s port
• Example: a Web server uses port 80
Transport Control Protocol
(TCP)
• Establishes a “logical” connection between two
applications running on different machines
• Example: A Web browser on my home computer
connects to W&L’s Web server
• Uses the same ARQ algorithm as the network
layer
Application
Transport
Network
Application Layer
Logical link control
Medium access
control
Physical
• Web browser/server (HTTP)
• File client/server (FTP)
• Email client/server