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					      Computer Networking

        Yishay Mansour (mansour at cs.tau.ac.il)


                Teaching Assistant: Hillel Avni


Oct 18, 2009                                      1
                                                   Lecture 1: Oct 18, 2009


      Course Information
 Lectures: Sunday     4–7                     Orenstein 111
 Exercises: Wednesday 11 –12, 12 – 1        Super Center 315 ‫מרכזי על‬


 Web site:
 http://www.cs.tau.ac.il/research/hillel.avni/courses/comnet10.html


  Books:
 •A Top-down Approach to Computer Networking / Kurouse-Ross



1. An Engineering Approach to Computer Networking / Keshav
2. Computer Networks / Tanenbaum
3. Data Networks / Bertsekas and Gallager
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                                   Lecture 1: Oct 18, 2009


   Practical Information


Homework assignment:
Mandatory
Both theoretical and programming




Grades:
Final Exam:                60%
theory exercises:          20%
Programming exercises:     20%
                                                         3/71
                                       Lecture 1: Oct 18, 2009


                       Motivation

   Today‟s economy
       manufacturing, distributing, and retailing goods
       but also creating and disseminating information
            publishing
            banking
            film making….
    part of the ‘information economy’
   Future economy is likely to be dominated by
    information!

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                                             Lecture 1: Oct 18, 2009




                     Information?
   A representation of knowledge
   Examples:
       books
       bills
       CDs & DVDs
   Can be represented in two ways
       analog (atoms)
       digital (bits)
   the Digital Revolution
       convert information as atoms to information as bits
       use networks to move bits around instead of atoms
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The Challenges                     Lecture 1: Oct 18, 2009




   represent all types of information as bits.
   move the bits
       In large quantities,
       everywhere,
       cheaply,
       Securely,
       with quality of service,
       ….


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                             Lecture 1: Oct 18, 2009




Today’s Networks are complex!

   hosts
   routers
   links of various media
   applications
   protocols
   hardware, software


Tomorrow‟s will be even more!
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            Internet Physical Infrastructure
   Residential access
          Cable
          Fiber
          DSL          ISP         Backbone ISP     ISP
          Wireless




                               The Internet is a network
                                of networks
Campus access,                 Each individually
  e.g.,                         administrated network is
      Ethernet
       Wireless
                                called an Autonomous
                                System (AS)
   
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                                   Lecture 1: Oct 18, 2009




        This course’s Challenge
   To discuss this complexity in an
    organized way, that will make today‟s
    computer networks (and their
    limitations) more comprehensive.
   identification, and understanding relationship
    of complex system‟s pieces.
   Problems that are beyond a specific
    technology

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                                      Lecture 1: Oct 18, 2009




Early communications systems
   I.e. telephone
   point-to-point links
   directly connect together the users wishing to
    communicate
   use dedicated communication circuit
   if distance between users increases beyond the
    length of the cable, the connection is formed by a
    number of sections connected end-to-end in series.




                                                          10/71
                                        Lecture 1: Oct 18, 2009

                Data Networks

   set of interconnected nodes exchange information
   sharing of the transmission circuits= "switching".
   many links allow more than one path between every
    2 nodes.
   network must select an appropriate path for each
    required connection.




                                                            11/71
           Qwest backbone                                     Lecture 1: Oct 18, 2009




http://www.qwest.com/largebusiness/enterprisesolutions/networkMaps/preloader.swf 12/71
         Networking Issues - Telephone

Addressing - identify the end user

phone number 1-201-222-2673 = country code + city code +
exchange + number

   Routing - How to get from source to destination.
Telephone circuit switching: Based on the phone number.

   Information Units - How is information sent
telephone Samples @ Fixed sampling rate. not self
descriptive! have to know where and when a sample came
    Oct 18, 2009                                       13/71
                                          Lecture 1: Oct 18, 2009




           Networking Issues - Internet
   Addressing - identify the end user
IP addresses 132.66.48.37, Refer to a host interface =
   network number + host number

   Routing- How to get from source to destination
Packet switching: move packets (chunks) of data among
  routers from source to destination independently.

   Information Units - How is information sent.
Self-descriptive data: packet = data + metadata (header).
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                                   Lecture 1: Oct 18, 2009




Telephone networks support a single, end-to-
  end quality of service but is expensive to boot

Internet supports no quality of service but is
flexible and cheap


Future networks will have to support a wide
range of service qualities at a reasonable cost
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                                        Lecture 1: Oct 18, 2009


                           History
       1961-1972: Early packet-switching principles

1961: Kleinrock - queuing theory shows effectiveness of
  packet-switching
1964: Baran - packet-switching in military networks
1967: ARPAnet – conceived by Advanced Research Projects
  Agency
1969: first ARPAnet node operational
1972: ARPAnet demonstrated publicly
    NCP (Network Control Protocol) first host-host

     protocol
    first e-mail program

    ARPAnet has 15 nodes
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                           History        Lecture 1: Oct 18, 2009


          1972-1980: Internetworking, new and
                    proprietary nets


1970: ALOHAnet satellite network in Hawaii
1973: Metcalfe‟s PhD thesis proposes Ethernet
1974: Cerf and Kahn - architecture for interconnecting
   networks
late70‟s: proprietary architectures: DECnet, SNA, XNA
late 70‟s: switching fixed length packets (ATM precursor)
1979: ARPAnet has 200 nodes




                                                              17/71
                               Lecture 1: Oct 18, 2009




Cerf and Kahn‟s internetworking principles:


   minimalism, autonomy - no internal
    changes required to interconnect networks
   best effort service model
   stateless routers
   decentralized control


Defines today‟s Internet architecture
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                                        Lecture 1: Oct 18, 2009
                        History
              1980-1990: new protocols,
               proliferation of networks


1983:   deployment of TCP/IP
1982:   SMTP e-mail protocol defined
1983:   DNS defined for name-to-IP-address translation
1985:   FTP protocol defined
1988:   TCP congestion control


new national networks: CSnet, BITnet, NSFnet, Minitel
100,000 hosts connected to confederation of networks

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                                          Lecture 1: Oct 18, 2009


                          History
         1990 - : commercialization and WWW


early 1990‟s: ARPAnet decommissioned
1991: NSF lifts restrictions on commercial use of NSFnet
  (decommissioned, 1995)
early 1990s: WWW
   hypertext [Bush 1945, Nelson 1960‟s]
   HTML, http: Berners-Lee
   1994: Mosaic, later Netscape
   late 1990‟s: commercialization of WWW


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                                      Lecture 1: Oct 18, 2009


    Demand and Supply

   Huge growth in users
       The introduction of the web
   Faster home access
       Better user experience.
   Infrastructure
       Significant portion of telecommunication.
   New evolving industries
       Although, sometimes temporary setbacks
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                                              Lecture 1: Oct 18, 2009


                  Internet: Users

                1400
                1200
Million users




                1000
                 800
                 600
                 400
                 200
                  0
                   1995 1997 1999 2001 2003 2005 2007 2009
                                   year                           22/71
                                                                                Lecture 1: Oct 18, 2009


        Penetration around the Globe (2009)
80                           %Population     %Penetration               USA+Canada

70
                                                                                                          Australia
60                  Asia/Pacific
                                            Europe
50
40
                                                                                      Latin America
30                                                        Middle East
                         Asia/Pacific
20   Africa
                                        Europe
                                                                                 Latin America
10         Africa
                                                     Middle East
                                                                   USA+Canada
                                                                                                  Australia
 0

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       http://www.internetworldstats.com/stats.htm                                                    23/71
                                                          U
                                                                       Lecture 1: Oct 18, 2009


  Users around the Globe (2002/5/9)
800
               Asia/Pacific
700
                                                                                            2009
600
500                                                                                         2005
                              Europe
400
                                                                                            2002
300                                                  USA+Canada

                                                                Latin America
200
100   Africa                           Middle East
                                                                                Australia
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                                                                                            24/71
                                             Lecture 1: Oct 18, 2009


Technology: Modem speed

       100000

        80000
                                                56000
        60000
 bps




        40000                               33600
                                        28800
        20000                       14400
                                9600
                300 1200 2400
            0
             79

             80

             84

             87

             91

             93

             95

             97

             08
           19

           19

           19

           19

           19

           19

           19

           19

           20
                                   year
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                                  Lecture 1: Oct 18, 2009


Today‟s options

   Modem: 56 K
   ISDN: 64K – 128K       OBSOLETE

   Frame Relay: 56K ++
   Today High Speed Connections
       Cable, ADSL, Satellite.
       All are available at
            5Mb (2005)
            30 Mb (2009)
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                 Lecture 1: Oct 18, 2009


Coming soon   (1999)




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               Lecture 1: Oct 18, 2009


Today (2005)




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                                  Lecture 1: Oct 18, 2009


      Why do we need Standards

   Networks (and other media) support
    communication between different entities



   Need agreement to ensure correct, efficient
    and meaningful communication


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                                          Lecture 1: Oct 18, 2009



      Various Organizations Issue Standards


   IEEE (Institute for Electrical and Electronic Engineers)

   IETF (Internet Engineering Task Force)

   ITU (International Telecommunications Union)

   ISO (International Organization for Standardization)

   W3C (World Wide Web Consortium)

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                                    Lecture 1: Oct 18, 2009


    Protocol Layers


   A way for organizing structure of network


 … Or at least our discussion of networks


   The idea: a series of steps

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                                                Lecture 1: Oct 18, 2009


        Protocol Layering
   Necessary because communication is complex
   Intended primarily for protocol designers
   Divides the problem into intellectually manageable pieces
   Provides a conceptual framework that can help us understand
    protocols
   Think of layering as a guideline, not a rigid specification
   Understand that optimizations may violate strict layering
   Should be invisible to users

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                                      Lecture 1: Oct 18, 2009


 Mail system functionality



          QuickTime™ and a
TIFF (Un compressed) decompressor
   are neede d to see this picture.




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                                                Lecture 1: Oct 18, 2009


            How do we Communicate?

   Send a mail from Alice to Bob                  Bob
        Alice in Champaign, Bob in Hollywood
   Example:
        US Postal Service


    Alice




                                                    Hollywood, California

            Champaign, Illinois

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                                                         Lecture 1: Oct 18, 2009


    What does Alice do?

             Alice
             200 Cornfield Rd.
             Champaign, IL 61820



                                Bob
                                100 Santa Monica Blvd.
                                Hollywood, CA 90028

   Bob‟s address (to a mailbox)
   Bob‟s name – in case people share mailbox
   Postage – have to pay!
   Alice‟s own name and address
         in case Bob wants to return a message
        In case the mail has to be returned.

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                                                   Lecture 1: Oct 18, 2009


    What does Bob do?

          Alice
          200 Cornfield Rd.
          Champaign, IL 61820



                          Bob
                          100 Santa Monica Blvd.
                          Hollywood, CA 90028


   Install a mailbox
   Receive the mail
   Get rid of envelope
   Read the message

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        Layers:

       Person delivery of parcel

       Post office counter handling

       Ground transfer: loading on trucks            Peer entities

        Airport transfer: loading on airplane

       Airplane routing from source to destination


each layer implements a service
   via its own internal-layer actions
   relying on services provided by layer below
  Oct 18, 2009                                              37/71
                                    Lecture 1: Oct 18, 2009


      Advantages of Layering

   explicit structure allows identification &
    relationship of complex system‟s pieces
      layered reference model for discussion

   modularization eases maintenance &
    updating of system
      change of implementation of layer‟s

       service transparent to rest of system


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                                   Lecture 1: Oct 18, 2009


    Protocols

   A protocol is a set of rules and formats
    that govern the communication
    between communicating peer
       set of valid messages - syntax
       meaning of each message - semantics


   Necessary for any function that requires
    cooperation between peers
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                                        Lecture 1: Oct 18, 2009

    Protocols

   A protocol provides a service
       For example: the post office protocol for reliable
        parcel transfer service


   Peer entities use a protocol to provide a
    service to a higher-level peer entity
       for example, truck drivers use a protocol to
        present post offices with the abstraction of an
        unreliable parcel transfer service

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                                              Lecture 1: Oct 18, 2009


        Protocol Layers

   A network that provides many services needs
    many protocols
   Some services are independent, But others
    depend on each other
   A Protocol may use another protocol as a step in
    its execution
       for example, ground transfer is one step in the
        execution of the example reliable parcel transfer
        protocol
   This form of dependency is called layering
       Post office handling is layered above parcel ground
        transfer protocol.                                41/71
                                           Lecture 1: Oct 18, 2009


        Open protocols and systems

   A set of protocols is open if
       protocol details are publicly available
       changes are managed by an organization whose
        membership and transactions are open to the public
   A system that implements open protocols is
    called an open system
   International Organization for Standards (ISO)
    prescribes a standard to connect open systems
       open system interconnect (OSI)
   Has greatly influenced thinking on protocol
    stacks                                                     42/71
                                           Lecture 1: Oct 18, 2009


    ISO OSI reference model

   Reference model
       formally defines what is meant by a layer, a service
        etc.
   Service architecture
       describes the services provided by each layer and the
        service access point
   Protocol architecture
       set of protocols that implement the service
        architecture
       compliant service architectures may still use non-
        compliant protocol architectures
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                                       Lecture 1: Oct 18, 2009


     The seven Layers
               There are only 5 !!

Application                                   Application
Presentation     Application                  Presentation
Session                                       Session
Transport                                     Transport
Network                Network                Network
Data Link              Data Link              Data Link
Physical               Physical               Physical

End system              Intermediate            End system
                         system
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                                                               Lecture 1: Oct 18, 2009

       The seven Layers - protocol stack

                                                            data


  Application                                      AH       data      Application
  Presentation                           PH             data          Presentation
  Session                           SH             data               Session
  Transport                    TH             data                    Transport
  Network               Network               NH        data          Network
  Data Link             Data Link             DH+data+DT              Data Link
  Physical              Physical                     bits             Physical

Session   and presentation layers are not so important, and are often ignored
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                                          Lecture 1: Oct 18, 2009


                     ‫עיקרון השכבות‬
Source            ‫ מתקבלת הודעה‬X ‫בשכבה‬       Destination
                  ‫זהה להודעה ששכבה‬
    Application   ‫ מסרה בצד המקור‬X   Application
                      Identical message

    Transport                               Transport
                      Identical message

     Network                                 Network
                      Identical message

    Data-Link                               Data-Link


                     Network
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                                      Lecture 1: Oct 18, 2009


Postal network

   Application: people using the postal system
   Session and presentation: chief clerk sends some
    priority mail, and some by regular mail ;
    translator translates letters going abroad.
   Transport layer: mail clerk sends a message,
    retransmits if not acked
   Network layer: postal system computes a route and
    forwards the letters
   datalink layer: letters carried by planes, trains,
    automobiles
   physical layer: the letter itself
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                                             Lecture 1: Oct 18, 2009

        Internet protocol stack

   application: supporting network applications       application
       ftp, smtp, http

   transport: host-host data transfer                   transport
       tcp, udp

   network: routing of datagrams from source             network
    to destination
       ip, routing protocols                                 link
   link: data transfer between neighboring
    network elements                                      physical
       ppp, ethernet

   physical: bits “on the wire”                                 48/71
                                            Lecture 1: Oct 18, 2009


             Protocol layering and data


                source      destination
        M     application   application              M       message
     Ht M      transport     transport         Ht M          segment
   Hn Ht M      network       network        Hn Ht M          datagram
Hl Hn Ht M        Link          Link      Hl Hn Ht M           frame
                physical      physical




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                                               Lecture 1: Oct 18, 2009


             Physical layer                  L1

   Moves bits between physically connected
    end-systems
   Standard prescribes
       coding scheme to represent a bit
       shapes and sizes of connectors
       bit-level synchronization
   Internet
       technology to move bits on a wire, wireless link, satellite
        channel etc.

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                                                Lecture 1: Oct 18, 2009


        Datalink layer                     L2

   (Reliable) communication over a single link.
   Introduces the notion of a frame
       set of bits that belong together
   Idle markers tell us that a link is not carrying a
    frame
   Begin and end markers delimit a frame
   Internet
       a variety of datalink layer protocols
       most common is Ethernet
       others are FDDI, SONET, HDLC
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                                                           Lecture 1: Oct 18, 2009


              Datalink layer (contd.)

       Ethernet (broadcast link)
             end-system must receive only bits meant for it
             need datalink-layer address
             also need to decide who gets to speak next
             these functions are provided by Medium ACcess sublayer (MAC)


       Datalink layer protocols are the first layer of software
       Very dependent on underlying physical link properties
       Usually bundle both physical and datalink in hardware.

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                                         Lecture 1: Oct 18, 2009


     Network layer                      L3

   Carries data from source to destination.
   Logically concatenates a set of links to form the
    abstraction of an end-to-end link
   Allows an end-system to communicate with any other
    end-system by computing a route between them
   Hides idiosyncrasies of datalink layer
   Provides unique network-wide addresses
   Found both in end-systems and in intermediate systems



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                                            Lecture 1: Oct 18, 2009


        Network layer types

   In datagram networks
       provides both routing and data forwarding
   In connection-oriented network
       separate data plane and control plane
       data plane only forwards and schedules data
        (touches every byte)
       control plane responsible for routing, call-
        establishment, call-teardown (doesn‟t touch data
        bytes)


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                                            Lecture 1: Oct 18, 2009


        Network layer (contd.)

   Internet
       network layer is provided by Internet Protocol (IP)
       found in all end-systems and intermediate systems
       provides abstraction of end-to-end link
       segmentation and reassembly
       packet-forwarding, routing, scheduling
       unique IP addresses
       can be layered over anything, but only best-effort
        service

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                                                Lecture 1: Oct 18, 2009


         Network layer (contd.)

   At end-systems
        primarily hides details of datalink layer
        segments and reassemble
        detects errors
   At intermediate systems
      participates in routing protocol to create routing

       tables
      responsible for forwarding packets

      schedules the transmission order of packets

      chooses which packets to drop
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                                                      Lecture 1: Oct 18, 2009


         Transport layer                                 L4

   Reliable end-to-end communication.
   creates the abstraction of an error-controlled,
    flow-controlled and multiplexed end-to-end link
    (Network layer provides only a „raw‟ end-to-end service)
   Some transport layers provide fewer services
        e.g. simple error detection, no flow control, and no retransmission

   Internet
        TCP provides error control, flow control, multiplexing
        UDP provides only multiplexing
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                                                    Lecture 1: Oct 18, 2009


        Transport layer (contd.)

   Error control
       GOAL: message will reach destination despite packet loss,
        corruption and duplication
       ACTIONS: retransmit lost packets; detect, discard, and
        retransmit corrupted packets; detect and discard duplicated
        packets
   Flow control
       match transmission rate to rate currently sustainable on the path
        to destination, and at the destination itself
   Multiplexes multiple applications to the same
    end-to-end connection
       adds an application-specific identifier (port number) so that
        receiving end-system can hand in incoming packet to the correct
        application                                                 58/71
                                       Lecture 1: Oct 18, 2009


        Session layer


   Not common
   Provides full-duplex service, expedited data
    delivery, and session synchronization
   Internet
       doesn‟t have a standard session layer



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                                                    Lecture 1: Oct 18, 2009


        Session layer (cont.)

   Duplex
       if transport layer is simplex, concatenates two transport
        endpoints together
   Expedited data delivery
       allows some messages to skip ahead in end-system queues,
        by using a separate low-delay transport layer endpoint
   Synchronization
       allows users to place marks in data stream and to roll back
        to a prespecified mark



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                                            Lecture 1: Oct 18, 2009


        Presentation layer

   Usually ad hoc
   Touches the application data
(Unlike other layers which deal with headers)
   Hides data representation differences between
    applications
       characters (ASCII, unicode, EBCDIC.)
   Can also encrypt data
   Internet
       no standard presentation layer
       only defines network byte order for 2- and 4-byte
        integers                                         61/71
                                      Lecture 1: Oct 18, 2009


      Application layer

   The set of applications that use the network
   Doesn‟t provide services to any other layer




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                                   Lecture 1: Oct 18, 2009


                      ‫עיקרון השכבות‬
Source                                          Destination

  VoIP     Email(smtp)       ftp   Application

  UDP               TCP              Transport

  Network (IPv4)                      Network

Modem    Ethernet     WiFi           Data-Link


                      Network
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                                             Lecture 1: Oct 18, 2009


                          ‫עיקרון השכבות‬
 Source                                                   Destination

  app1     app2          app3        app1         app2             app3


  UDP         TCP                      UDP                     TCP

 Network (IPv4)                        Network (IPv4)

Modem Ethernet    WiFi              Modem     Ethernet                 WiFi


                          Network
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                                           Lecture 1: Oct 18, 2009


        Discussion

   Layers break a complex problem into
    smaller, simpler pieces.
   Why seven layers?
       Need a top and a bottom  2
       Need to hide physical link; so need datalink  3
       Need both end-to-end and hop-by-hop actions; so
        need at least the network and transport layers  5



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                                 Lecture 1: Oct 18, 2009

        Course outline
1    Introduction and Layering
2    Data Link: Multi Access
3    Hubs, Bridges and Routers
4    Scheduling and Buffer Management
5    Switching Fabrics
6    Routing
7    Reliable Data Transfer
8    End to End Window Based Protocols
9    Flow Control
10   Multimedia and QoS
11   Network Security
12   Distributed Algorithms                                66

				
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posted:9/26/2011
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