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					Computer Networking


Chapter 1 Introduction
           Chapter 1: Introduction

Our goal:                   Overview:
   get “feel” and             Why use network
    terminology                what’s the Internet
   more depth, detail         what’s a protocol?
    later in course            network edge
   approach:                  network core
       use Internet as        access net, physical media
        example                Internet/ISP structure
                               performance: loss, delay
                               protocol layers, service models
                               network modeling


                     Computer Networking @ UTC/CS             2
     Chapter 1: roadmap

1.1 Why use Network?
1.2 What is the Internet?
1.3 Network edge
1.4 Network core
1.5 Network access and physical media
1.6 Internet structure and ISPs
1.7 Delay & loss in packet-switched networks
1.8 Protocol layers, service models
1.9 History
             Computer Networking @ UTC/CS   3
          Why Use Networks?

   Network: group of computers and other
    devices that are connected by some type of
    transmission media
   Advantages of using networks over standalone
    computers:
       Networks enable multiple users to share the
        network’s resources (devices and data)
       Networks allow you to manage, or administer,
        resources on multiple computers from a central
        location

                     Computer Networking @ UTC/CS        4
     Chapter 1: roadmap

1.1 Why use Network?
1.2 What is the Internet? (Nuts and Bolts,
  Service, Protocols)
1.3 Network edge
1.4 Network core
1.5 Network access and physical media
1.6 Internet structure and ISPs
1.7 Delay & loss in packet-switched networks
1.8 Protocol layers, service models
1.9 History
             Computer Networking @ UTC/CS   5
         What’s the Internet: “nuts and
                  bolts” view
                                                     router        workstation
   millions of connected                                 server
    computing devices: hosts = end                                     mobile
    systems                                     local ISP
   running network apps
   communication links
        fiber, copper, radio, satellite
    
                                                                   regional ISP
       transmission rate = bandwidth
   routers: forward packets
    (chunks of data)
    path, route, packet switching

                                               company
                                               network
                           Computer Networking @ UTC/CS                         6
           What’s the Internet: “nuts
                and bolts” view
                                                         router    workstation
   protocols control sending,
    receiving of msgs                                     server
                                                                      mobile
       e.g., TCP, IP, HTTP, FTP, PPP              local ISP
   Internet: “network of
    networks”
       loosely hierarchical                                       regional ISP
       public Internet versus
        private intranet
   Internet standards
       RFC: Request for comments
       IETF: Internet Engineering                company
        Task Force                                network
                          Computer Networking @ UTC/CS                      7
        What’s the Internet: a service view

   communication
    infrastructure enables
    distributed applications:
        Web, email, games, e-
         commerce, file sharing
   communication services
    provided to apps:
        Connectionless unreliable
        connection-oriented
         reliable



                         Computer Networking @ UTC/CS   8
        What’s the Internet: a service view


   File services
       Capability of a server to share data files,
        applications, and disk storage space
       File server
   Print services: used to share printers across a
    network




                       Computer Networking @ UTC/CS   9
    What’s the Internet: a service view


   Communications services
       Allow remote users to connect to the network
       Remote access server (also known as
        communications server or access server)
   Mail services
       Coordinate the storage and transfer of e-mail
        between users on a network
       Mail server



                    Computer Networking @ UTC/CS        10
What’s the Internet: a service view

   Internet services
       Supplying Web pages, file transfer capabilities,
        Internet addressing schemes, security filters,
        and a means for directly logging on to other
        computers on the Internet
       Web server
   Management services: centrally administer
    management tasks on the network
       Traffic monitoring and control
       Load balancing


                  Computer Networking @ UTC/CS       11
What’s the Internet: a service view


   Management services (continued)
       Hardware diagnosis and failure alert
       Asset management
       License tracking
       Security auditing
       Software distribution
       Address management
       Backup and restoration of data



                   Computer Networking @ UTC/CS   12
           What’s a protocol?

human protocols:                   network protocols:
 “what’s the time?”                machines rather than
                                     humans
 “I have a question”
                                    all communication
 introductions
                                     activity in Internet
                                     governed by protocols
… specific msgs sent
… specific actions taken         protocols define format, order of
   when msgs received, or              msgs sent and received
   other events                     among network entities, and
                                        actions taken on msg
                                        transmission, receipt

                        Computer Networking @ UTC/CS             13
          What’s a protocol?
     a human protocol and a computer network protocol:



          Hi                               TCP connection
                                           req
          Hi
                                           TCP connection
       Got the                             response
        time?
                                        Get http://www.awl.com/kurose-ross
        2:00
                                                    <file>
                            time
Q: Other human protocols?
                     Computer Networking @ UTC/CS                            14
  Chapter 1: roadmap
1.1 Why use Network?
1.2 What is the Internet?
1.3 Network edge
1.4 Network core
1.5 Network access and physical media
1.6 Internet structure and ISPs
1.7 Delay & loss in packet-switched
  networks
1.8 Protocol layers, service models
1.9 History
          Computer Networking @ UTC/CS   15
        A closer look at network
               structure:
   network edge:
    applications and hosts
    (client, server)
   network core:
       routers
       network of networks
   access networks,
    physical media:
    communication links



                 Computer Networking @ UTC/CS   16
                The network edge:
   end systems (hosts):
       run application programs
       e.g. Web, email
       at “edge of network”
   peer-peer model:
       minimal (or no) use of
        dedicated servers
   client/server model
       client host requests, receives
        service from always-on server
       e.g. Web browser/server;
        email client/server


                           Computer Networking @ UTC/CS   17
          Types of networks:
        Peer-to-peer networks

   Every computer can communicate directly with
    every other computer
   By default, no computer has more authority
    than another
   Every computer is capable of sending and
    receiving information to and from every other
    computer




                  Computer Networking @ UTC/CS   18
      Peer-to-peer networks
           (continued)




Resource sharing on a simple peer-to-peer network
                Computer Networking @ UTC/CS        19
        Peer-to-peer Networks
              (continued)
   Peer-to-Peer Networks
       Advantage
            Simple to configure
            Less expensive to set up
       Disadvantage
            Not flexible
            Not necessarily secure
            Not practical for large number of computers




                     Computer Networking @ UTC/CS          20
        Client/Server networks

   Server: central computer used to facilitate
    communication and resource sharing between
    other computers on the network (called clients)
   Client/server network: uses a server to enable
    clients to share data, data storage space, and
    devices
   To function as a server, a computer must be
    running a network operating system (NOS)




                 Computer Networking @ UTC/CS         21
Client/Server networks
      (continued)




  Resource sharing on a client/server network
          Computer Networking @ UTC/CS          22
         Client/Server networks
               (continued)
   Advantages of using client/server networks
    over peer-to-peer networks:
       User logon accounts and passwords can be
        assigned in one place
       Access to multiple shared resources can be
        centrally granted to a single user or groups of
        users
       Problems can be tracked, diagnosed, and often
        fixed from one location



                   Computer Networking @ UTC/CS       23
         Client/Server networks
               (continued)

   Advantages of using client/server networks
    over peer-to-peer networks (continued):
       Servers are optimized to handle heavy
        processing loads and dedicated to handling
        requests from clients, enabling faster response
        time
       Because of their efficient processing and
        larger disk storage, servers can connect more
        than a handful of computers on a network


                   Computer Networking @ UTC/CS      24
    LANs, MANs, and WANs

   Local area network (LAN): confined to a
    relatively small space, such as a building or an
    office
   Metropolitan area network (MAN): larger than
    a LAN; connects clients and servers from
    multiple buildings
   Wide area network (WAN): connects two or
    more geographically distinct LANs or MANs



                 Computer Networking @ UTC/CS     25
LANs, MANs, and WANs
      (continued)




       A simple WAN
      Computer Networking @ UTC/CS   26
    Network edge: connection-oriented
                service

Goal: data transfer                  TCP service [RFC 793]
    between end systems
                                        reliable, in-order byte-
   handshaking: setup                   stream data transfer
    (prepare for) data                       loss: acknowledgements
    transfer ahead of time                    and retransmissions
        Hello, hello back human        flow control:
         protocol
                                             sender won’t overwhelm
        set up “state” in two                receiver
         communicating hosts
                                        congestion control:
   TCP - Transmission
                                              senders “slow down sending
    Control Protocol
                                          

                                              rate” when network
        Internet’s connection-               congested
         oriented service
                       Computer Networking @ UTC/CS                    27
              Network edge:
           connectionless service

Goal: data transfer between            App’s using TCP:
    end systems                           HTTP (Web), FTP (file
       same as before!                    transfer), Telnet (remote
   UDP - User Datagram                    login), SMTP (email)
    Protocol [RFC 768]:
       connectionless                 App’s using UDP:
       unreliable data                   streaming media,
        transfer                           teleconferencing, DNS,
                                           Internet telephony
       no flow control
       no congestion control



                    Computer Networking @ UTC/CS               28
Chapter 1: roadmap

1.1 Why use Network?
1.2 What is the Internet?
1.3 Network edge
1.4 Network core
1.5 Network access and physical media
1.6 Internet structure and ISPs
1.7 Delay & loss in packet-switched
  networks
1.8 Protocol layers, service models
1.9 History
          Computer Networking @ UTC/CS   29
           The Network Core
   mesh of interconnected
    routers
   the fundamental question:
    how is data transferred
    through net?
       circuit switching:
        dedicated circuit per
        call: telephone net
       packet-switching: data
        sent thru net in
        discrete “chunks”


                     Computer Networking @ UTC/CS   30
         Network Core: Circuit
              Switching

End-end resources
  reserved for “call”
   link bandwidth, switch
    capacity
   dedicated resources: no
    sharing
   circuit-like (guaranteed)
    performance
   call setup required



                    Computer Networking @ UTC/CS   31
          Network Core: Circuit Switching


network resources (e.g.,               dividing link bandwidth into
  bandwidth) divided                    “pieces”
  into “pieces”                             frequency division
   pieces allocated to calls               time division
   resource piece idle if not
    used by owning call (no
    sharing)




                        Computer Networking @ UTC/CS                   32
              Circuit Switching:
                FDM and TDM

                                           Example:
FDM
                                           4 users

      frequency

TDM
                                  time


      frequency

                                 time
                   Computer Networking @ UTC/CS       33
         Numerical example
   How long does it take to send a file of
    640,000 bits from host A to host B over a
    circuit-switched network?
       All links are 1.536 Mbps
       Each link uses TDM with 24 slots
       500 msec to establish end-to-end circuit

Work it out!




                  Computer Networking @ UTC/CS     34
         Network Core: Packet Switching

each end-end data stream                  resource contention:
  divided into packets                     aggregate resource

 user A, B packets share                    demand can exceed
  network resources                          amount available
 each packet uses full link               congestion: packets

  bandwidth                                  queue, wait for link use
 resources used as needed                 store and forward: packets
                                             move one hop at a time
                                                  Node receives complete
 Bandwidth division into “pieces”                  packet before forwarding
     Dedicated allocation
     Resource reservation

                         Computer Networking @ UTC/CS                         35
            Packet Switching:
          Statistical Multiplexing

         10 Mb/s
 A       Ethernet       statistical multiplexing     C

                             1.5 Mb/s
     B
            queue of packets
            waiting for output
                   link


                            D                        E
Sequence of A & B packets does not have fixed pattern 
   statistical multiplexing.
In TDM each host gets same slot in revolving TDM frame.
                      Computer Networking @ UTC/CS        36
        Packet switching versus circuit switching

     Packet switching allows more users to use network!

   1 Mb/s link
   each user:
       100 kb/s when “active”
       active 10% of time
                                  N users
   circuit-switching:                                    1 Mbps link
       10 users
   packet switching:
       with 35 users,
        probability > 10 active
        less than .0004
                           Computer Networking @ UTC/CS           37
     Packet switching versus circuit switching

    Is packet switching a “slam dunk winner?”

   Great for bursty data
        resource sharing
        simpler, no call setup
   Excessive congestion: packet delay and loss
        protocols needed for reliable data transfer,
         congestion control
   Q: How to provide circuit-like behavior?
        bandwidth guarantees needed for audio/video
         apps
        still an unsolved problem (chapter 6)

                        Computer Networking @ UTC/CS    38
      Packet-switching: store-and-forward

                   L
                       R          R           R

   Takes L/R seconds to                     Example:
    transmit (push out)
                                              L = 7.5 Mbits
    packet of L bits on to link
                                              R = 1.5 Mbps
    or R bps
                                              delay = 15 sec
   Entire packet must
    arrive at router before it
    can be transmitted on
    next link: store and                          Applet
    forward
   delay = 3L/R
                           Computer Networking @ UTC/CS         39
Packet segmenting




Segmentation and Reassembly
       Computer Networking @ UTC/CS   40
             Packet-switched networks:
                    forwarding

   Goal: move packets through routers from source to destination
       we’ll study several path selection (i.e. routing) algorithms
        (chapter 4)
   datagram network:
      destination address in packet determines next hop

      routes may change during session

      analogy: driving, asking directions

   virtual circuit network:
      each packet carries tag (virtual circuit ID), tag determines

        next hop
      fixed path determined at call setup time, remains fixed thru

        call
        routers maintain per-call state

                           Computer Networking @ UTC/CS                41
           Network Taxonomy
                  Telecommunication
                      networks



    Circuit-switched                         Packet-switched
        networks                                networks



FDM                                    Networks         Datagram
                 TDM
                                       with VCs         Networks


• Datagram network is not either connection-oriented
or connectionless.
• Internet provides both connection-oriented (TCP) and
connectionless services (UDP) to apps.
                    Computer Networking @ UTC/CS               42
    Chapter 1: roadmap

1.1 Why use Network?
1.2 What is the Internet?
1.3 Network edge
1.4 Network core
1.5 Network access and physical media
1.6 Internet structure and ISPs
1.7 Delay & loss in packet-switched networks
1.8 Protocol layers, service models
1.9 History

             Computer Networking @ UTC/CS      43
      Access networks and physical media


Q: How to connect end systems
   to edge router?
   residential access nets
   institutional access networks
    (school, company)
   mobile access networks
Keep in mind:
   bandwidth (bits per second)
    of access network?
   shared or dedicated?



                       Computer Networking @ UTC/CS   44
          Residential access: point to point access


        Dialup via modem
           up to 56Kbps direct access to router

             (often less)
           Can’t surf and phone at same time:

             can’t be “always on”

       ADSL: asymmetric digital subscriber line
          up to 1 Mbps upstream (today typically < 256 kbps)

          up to 8 Mbps downstream (today typically < 1 Mbps)

          FDM: 50 kHz - 1 MHz for downstream

                   4 kHz - 50 kHz for upstream
                   0 kHz - 4 kHz for ordinary telephone

                              Computer Networking @ UTC/CS      45
    Residential access: cable modems

   HFC: hybrid fiber coax
       asymmetric: up to 30Mbps downstream, 2
        Mbps upstream
   network of cable and fiber attaches homes to ISP
    router
       homes share access to router
   deployment: available via cable TV companies




                  Computer Networking @ UTC/CS         46
     Cable Network Architecture: Overview




                                Typically 500 to 5,000 homes




cable headend

                                 home
          cable distribution
         network (simplified)

                       Computer Networking @ UTC/CS        47
     Cable Network Architecture: Overview




cable headend

                                 home
          cable distribution
         network (simplified)

                       Computer Networking @ UTC/CS   48
     Cable Network Architecture: Overview
server(s)




cable headend

                                   home
            cable distribution
                network

                         Computer Networking @ UTC/CS   49
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