Transcript Why networks?

Introduction to Networks
and the Internet
CMPE 150
Fall 2005
Lecture 2
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Announcements
• Permission codes.
• Labs.
• Homework 1.
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Last class…
• What’s a computer network?
• Why networks?
• Examples of networks:
– Postal system.
– Telephone network.
• Telephone network:
– Voice.
– Real-time.
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Last class (cont’d)
• The evolution of the telephone system.
• Addressing.
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Today
• Data networks.
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Data Communication Model
Network
Source
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Destination
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Simplified Communication Model
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Simplified Data Communications
Model
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Components
• End systems (or hosts),
• Routers/switches/bridges, and
• Links (twisted pair, coaxial cable, fiber, radio,
etc.).
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Components (cont’d)
• Source
– generates data to be transmitted
• Transmitter
– Converts data into transmittable signals
• Transmission System
– Carries data
• Receiver
– Converts received signal into data
• Destination
– Takes incoming data
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Key Tasks
• Transmission.
• Signal Generation.
• Synchronization.
• Error detection and correction.
• Addressing and routing
• End-to-end Recovery.
• Security.
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Key Tasks
• Transmission.
• Signal Generation.
• Synchronization.
Physical Layer
• Error detection and correction.
• Addressing and routing
• End-to-end Recovery.
• Security.
Data Link Layer
Network Layer
Transport Layer
Application Layer
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Networking
• Point to point communication not usually
practical
– Devices are too far apart.
– Large set of devices would need impractical
number of connections.
• Solution is a communications network.
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Simplified Network Model
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Connecting End Systems
Dedicated link
Multiple access / shared medium
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Connecting End Systems (cont’d)
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Shared Communication
Infrastructure
A stream of packets from sender to receiver.
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Types of Data Networks
• Several ways to classify data networks.
• For example, according to “coverage”.
– Local Area Networks (LANs) typically
provide networking capabilities within a
building, campus.
• Typically within 5-mile radius.
– Wide-Area Networks (WANs) span greater
geographic distances (e.g., world-wide).
– Metropolitan Area Networks (MANs) span
more restricted distances, e.g., geographic
regions (e.g., Los Nettos network in Southern
California, etc.)
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The Internet
Backbone
Regional
Stub
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Types of Networks (cont’d)
• Classification according to type of connection.
– Dedicated link.
– Shared medium (multiple access).
– Switched point-to-point.
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Types of Networks (cont’d)
• Classification according to topology…
• What is network topology?
– The way network elements are
interconnected.
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Network Topologies: Examples
Star
Ring
Tree
Irregular
Bus
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More Concepts…
• Network protocols.
• Layering.
• Network/protocol architecture.
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Network Protocols
• Diplomats use rules, called protocols, as
guides for formal interactions.
• A communication protocol is a set of rules
that specify the format and meaning of
messages exchanged between computers
across a network.
• A set of related protocols that are designed
for compatibility are called protocol suite.
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Human and Computer Protocols
Human Protocol
Hi
Hi
Got the
time?
Computer Protocol
Web client
open
connection
OK
Web server
send me data
2:00
<data>
time
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Layering
• What is it?
• Building complex systems is hard!
– Approach: “Divide and conquer”.
– Split job into smaller jobs, or layers.
• Analogy to other fields.
– Building a house: digging, foundation, framing,
etc.
– Car assembly line…
• Basic idea: each step dependent on the
previous step but does not need to be aware
of how the previous step was done.
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Analogy: Air Travel
• The problem: air travel.
• Decomposed into series of steps:
Arrival at airport
Departure from airport
Check-in
Baggage claim
Boarding
Deplane
Takeoff
Landing
Traveling
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Arrival
Departure
Check-in
Baggage claim
Boarding
Deplane
Takeoff
Landing
intermediate air traffic sites
Airplane routing
Arriving airport
Departing airport
More on the air travel analogy…
Airplane routing
Traveling
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Protocol Architecture
• Task of communication broken up into modules
• For example file transfer could use three
modules
– File transfer application
– Communication service module
– Network access module
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Simplified File Transfer
Architecture
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A Three Layer Model
•
•
•
Application Layer
Transport Layer
Network Access Layer
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Network Access Layer
• Exchange of data between the computer and
the network
• Sending computer provides address of
destination
• May invoke levels of service
• Dependent on type of network used (LAN,
packet switched etc.)
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Transport Layer
• Reliable data exchange
• Independent of network being used
• Independent of application
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Application Layer
• Support for different user applications
• e.g. e-mail, file transfer
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Layered Protocol Design
• Layering model is a solution to the problem of
complexity in network protocols
• The model divides the network protocols into
layers, each of which solves part of the
network communication problem
– Each layer has its own protocol!
• Each layer implements a service to the layer
above
– Relying on services provided by the layers
below.
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Layers
• Layers are the different components that
need to be designed/implemented when
designing/implementing networks.
• Each layer responsible for a set of functions.
• Top layer relies on services provided by
bottom layer.
• Layer makes it service available to higher
layer through an interface.
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Network/Protocol Architecture
• Set of layers, what their functions are, the
services each of them provide, and the
interfaces between them.
• A.k.a, protocol architecture or protocol stack.
• Examples:
– ISO-OSI 7 layer architecture.
– TCP-IP architecture (Internet).
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Protocol Data Units (PDU)
• At each layer, protocols are used to
communicate.
• At the source, control information is added to
user data at each layer, a.k.a., encapsulation.
• At the receiver, control information is stripped off
at each layer going up the stack, a.k.a.,
decapsulation.
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Operation of a Protocol
Architecture
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Example 1: ISO OSI
Architecture
• ISO: International Standards Organization
• OSI: Open Systems Interconnection.
Application
Presentation
Session
Transport
Network
Data link
Physical
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Layers of Interest in ISO Model
• Layer 7: Application
– Application-specific protocols (e.g. ftp, http, smtp)
• Layer 4: Transport
– Delivery of data between computers (end-to-end).
• Layer 3: Network
– Data routing across a network.
• Layer 2: Data Link
– Reliable transmission over physical medium.
• Layer 1: Physical
- Transmission of bits between two nodes.
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Example 2: TCP/IP Architecture
• Model employed by the Internet.
TCP/IP
Application
Application
Presentation
Transport
Session
Transport
Internet
Network
Access
Physical
CMPE 150- Introduction to Computer Networks
ISO OSI
Network
Data link
Physical
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TCP/IP Protocol Architecture
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