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371-1-0291: An Introduction to Computer Networks

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Title: 371-1-0291: An Introduction to Computer Networks


1
371-1-0291 An Introduction to Computer Networks
Homepage http//www.cse.bgu.ac.il/Courses/course.a
sp?ID63
  • Handout 1 Introduction

Additional Reading Text book Chaps. 1, 9.2.1
2
Outline
  • An Introduction to the mail system
  • An Introduction to the Internet
  • Important concepts in Networking
  • Key Enablers of Large Networks

3
An Introduction to the mail system
Technion
Beer Sheva
Letter
Shimon
Reuven
Envelop
4
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

5
An Introduction to the mail system
Technion
Beer Sheva
Application Layer
Shimon
Reuven
Transport Layer
6
An Introduction to the Internet
mail.technion.ac.il
bgumail.bgu.ac.il
Shimon
Reuven
7
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?

8
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 ?

9
An Introduction to the mail system
Application Layer
Shimon
Reuven
Transport Layer
10
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.

11
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?

12
Outline
  • An Introduction to the mail system
  • An Introduction to the Internet
  • Important concepts in Networking
  • Key Enablers of Large Networks

13
Important Concepts in Networking
  • Multiplexing
  • Buffering
  • Encapsulation
  • Socket API
  • Common Applications

14
Multiplexing
  • A method of sharing resources among users
  • Provides a cost-effective resource sharing
  • Packetizing messages enables Time Mux
  • Routers enable Space Mux

15
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

16
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

17
An Introduction to the mail system
Application Layer
Shimon
Reuven
Transport Layer
18
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

19
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)

20
Outline
  • An Introduction to the mail system
  • An Introduction to the Internet
  • Important concepts in Networking
  • Key Enablers of Large Networks

21
Three Key Enablers of Networks
  • Digitization of Signals
  • Economies of Scale
  • Network Externalities

22
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.

23
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
24
Network Externalities
  • Value of the network increases as the number of
    users increases, which in turn attracts more
    users.

Benefit
Cost
Users
25
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
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