371-1-0291: An Introduction to Computer Networks

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371-1-0291 An Introduction to Computer Networks
Homepage http//www.cse.bgu.ac.il/Courses/course.a
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• Handout 1 Introduction

Additional Reading Text book Chaps. 1, 9.2.1
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Outline

• An Introduction to the mail system
• An Introduction to the Internet
• Important concepts in Networking
• Key Enablers of Large Networks

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An Introduction to the mail system
Technion
Beer Sheva
Letter
Shimon
Reuven
Envelop
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Characteristics of the mail system

• Each envelope is individually routed.
• No time guarantee for delivery.
• No guarantee of delivery in sequence.
• No guarantee of delivery at all !!!
• Things get lost
• How can we acknowledge delivery?
• Retransmission
• How to determine when to retransmit? Timeout?
• Need local copies of contents of each envelope.
• How long to keep each copy.
• What if an acknowledgement is lost?

• Each mailing functionality is carried out by a
  specific network architecture layer

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An Introduction to the mail system
Technion
Beer Sheva
Application Layer
Shimon
Reuven
Transport Layer
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An Introduction to the Internet
mail.technion.ac.il
bgumail.bgu.ac.il
Shimon
Reuven
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Characteristics of the Internet Protocol (IP)

• Each packet is individually routed.
• No time guarantee for delivery.
• No guarantee of delivery in sequence.
• No guarantee of delivery at all !!!
• Packets get lost
• Packet acknowledgements
• Packet retransmission
• How to determine when to retransmit? Timeout?
• Need local copies of contents of each packet.
• How long to keep each copy?
• End-to-end or hop-by-hop?
• What if an acknowledgement is lost?

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Further Characteristics of the Internet Protocol
(IP)

• No guarantee of integrity of data.
• Packets can be fragmented.
• Packets may be duplicated.

• These need to be handled by the network
• So which layer does what ?

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An Introduction to the mail system
Application Layer
Shimon
Reuven
Transport Layer
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Layering in the Internet
Three out of seven

• Transport Layer
• Provides reliable, in-sequence delivery of data
  from end-to-end on behalf of the application.
• Network Layer
• Provides best-effort, but unreliable, delivery
  of datagrams.
• Link Layer
• Carries data over (usually) point-to-point links
  between hosts and routers or between routers and
  routers.

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Some questions about the mail system

• How many sorting offices are needed and where
  should they be located?
• How much sorting capacity is needed?
• Should we allocate more for Rosh Hashana?
• How can we guarantee timely delivery?
• What prevents delay guarantees?
• Or delay variation guarantees?
• How do we protect against fraudulent mail
  deliverers, or fraudulent senders?

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Outline

• An Introduction to the mail system
• An Introduction to the Internet
• Important concepts in Networking
• Key Enablers of Large Networks

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Important Concepts in Networking

• Multiplexing
• Buffering
• Encapsulation
• Socket API
• Common Applications

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Multiplexing

• A method of sharing resources among users
• Provides a cost-effective resource sharing
• Packetizing messages enables Time Mux
• Routers enable Space Mux

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Buffering

• The process of temporarily storing data until a
  resource becomes available
• Enables to overcome random delays and async
  transmission
• Handles temporarily overflow and reduces loss of
  data

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Encapsulation

• The process of embedding data in a new message
  format
• The new message will consist of headerdata
• Enables hierarchical layering where one layer
  places its data into another layer format
• The latter is responsible to provide its
  end-to-end service to the encapsulated data

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An Introduction to the mail system
Application Layer
Shimon
Reuven
Transport Layer
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Socket API

• API Application Programming Interface
• Socket API is a common interface by which remote
  application can exchange messages
• Enables distributed development of applications
  that can talk to each other

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Common Applications

• A key factor for the Internet success
• Part of Internet suite
• Notable ones are
• Domain Name Server (DNS)
• Mail
• Network File System (NFS)
• HTTP (one of the Web protocols)
• RTSP (Streaming another Web protocol)

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Outline

• An Introduction to the mail system
• An Introduction to the Internet
• Important concepts in Networking
• Key Enablers of Large Networks

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Three Key Enablers of Networks

• Digitization of Signals
• Economies of Scale
• Network Externalities

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Digitization of Signals

• Any information bearing signal can be represented
  by a binary string with arbitrarily high
  accuracy.
• Sample quantize a Digital stream.
• Digital data is cheaper to store, manipulate and,
  most importantly, reliably communicate.

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Economies of Scale

• Cost of capacity grows slowly with increased
  capacity.
• A network has fixed costs only (almost) and zero
  marginal costs.

Cost
Capacity or users
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Network Externalities

• Value of the network increases as the number of
  users increases, which in turn attracts more
  users.

Benefit
Cost
Users
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Next Lecture

• A Detailed FTP Example
• Layering
• Packet Switching and Circuit Switching
• Some terms
• Data rate, Bandwidth and throughput
• Propagation delay
• Packet, header, address
• Bandwidth-delay product, RTT

Additional Reading Text book Chaps. 1.2, 3.1