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     Computer (Transport) Layer
   Manages communication between two computers
   Platform independence: one computer does not
    know even what platform the other is (mainframe,
    PC, etc.)
   TCP/IP’s most popular transport standard is TCP
                                                       2
         Terminal-Host Platform
   Processing done at a central host computer
    – Mainframe (very large)
    – Minicomputer (mid-size)

                                          Programs
                                          Execute on
                                          the Host




                        Host
                                                     3
                  PC Network
   Client PC sits on desks of ordinary users
    – Receives services from servers
    – Stand-alone PC with hardware, software added

Client
   PC




Client
   PC
                                                                  4
                   PC Network
   Servers
    – Provide services to client PCs
    – Usually PCs but not always

                                                         Server
                           Service

                               Service

                Service

                                       Server
                                                Server
                                                                 5
                   PC Network
   Servers are Specialized
    – Optimized for their services for cost, performance, and
      reliability

                                                        File
                           File Service                 Server




               Database
                 Server

                                           Internet Access
                                           Gateway Server
                                                           6
        File Server Program Access
   Programs executed on the client PC
    – Program and data files downloaded over the network

Program Executed         Downloaded
  On Client PC           Over Network




       Client              Network                Server
        PC
                                                     7
              Client Workstation
  Workstations are more powerful than PCs
  Come in client and server versions

21” display
                                             Client
                                             Workstation
Powerful RISC
microprocessor

Large, fast RAM,
hard disk

Usually UNIX OS, can be
Windows NT
                                                  8
             Networked PC
 TheDominant Computer Platform Today is
 the Networked PC
  – High penetration of PCs on desktops
  – Most are already networked

 “Legacy”   Systems Must be Maintained
  – Mainframe terminal-host systems
  – Minicomputer terminal-host systems
  – Many are being “downsized” to PC networking
                                                    9
              Managed PCs
 Designed   to be Managed Remotely
  – Slashes maintenance costs
  – Slashes software upgrade costs
  – Users have less control over their machines
  – Company can search for illegal games
  – Management standards are still immature
  – Eventually, all networked PCs will be managed
                                            10
                    NetPC
 Standard   Pushed by Microsoft, Intel
  – Closed box with limited upgradability
  – Should ease maintenance by reducing
    incompatibilities and add-in problems
  – Will users be content?
  – Remote management built in
    Network Computers (NCs)                               11

 Designed     for Use on the Internet (Java)
  – Little or no local disk drive storage
  – Programs downloaded over Internet when needed (or
    from corporate servers using Internet standards)
  – User always gets the latest version of the software
  – Downloading can take time; small applications would
    load faster but might lack functionality
  – When the network is down, so are you

                                              The
                                            Internet
                                                            12
   Low-Cost PCs for the Home
 WebTV     (Microsoft)
  – Uses TV for its display
  – User only needs $500 box
  – Problem: TV display resolution is far lower than that
    of a TV screen; webpages look bad
  – Problem: Need special Internet service provider who
    cannot work with your regular PC
  – Problem: No printing
  – Problem: Limited to WWW and email

 Sub   $1,000 PCs
      Networks Mix Traffic of Multiple                          13

                Platforms

                           Network
                     (LAN, WAN, internet)



PC network traffic                          Terminal-host traffic
F2-7: Program Interactions on a Stand-                     14

             Alone PC

                Application Program

   Request                        Response

                  Operating System

  Commands related to files      Files or error messages


                      Hardware
                                                     15
     Client/Server Processing

Client Program             Server Program
                                        Platform independent
             TWO programs on
              TWO machines




Client Machine            Server Machine
     (PC)               (UNIX Minicomputer)
                                                16
     Client/Server Processing
 Application   Layer Protocols
  – Control the requests and responses
  – Application independence: any browser can
    work with any webserver program

 Transport   Layer Protocols
  – Allow two computers with different
    architectures to work together
  – Brings platform independence
                                              17
            Internet Access

                              To connect to
Webserver                     the Internet,
                Internet      you must have
                Service       an ISP
                Provider
                 (ISP)
                              The Internet
                               Backbone
                Internet
 PC with        Service
 browser        Provider
                 (ISP)
                                                     18
 Internet Service Provider (ISP)
 Basic   Service: Links You to the Internet
  – Provides a connection
  – Pays for your use. (The Internet is NOT Free.)

 Most    Provide Additional Services
  – E-mail
  – World Wide Web Homepage Hosting
  – Premium Providers have Specialized Services
    (America Online, CompuServ, MSN)
                                        19
     Internet Access with PPP




   Modem,                         ISP
TCP/IP software
 PPP software

   PPP link over telephone line
                                         20
Internet Access with PPP: routers

    Router                 Router


                  Router



 The Internet:
 1. a network of
 routers connected               ISP
 by transmission lines          Router

 2. Subnets connected
                  ISP
       by routers
                                                  21
  Layered Interactions in Internet Access
User PC                    Router     Webserver

                                      Webserver
 Browser
                                      Software

   TCP                                  TCP
 Software                             Software

             Subnet
    IP                        IP         IP
 Software    Layer         Software   Software
             (PPP)
CSLIP/PPP                    PPP
                                       Subnet
 Software                  Software

     Point-to-Point Connection
                                               22
    Point-to-Point Connection
 Physical   Layer
  – Serial Port
  – Modem
  – Telephone Transmission

 Data   Link Layer
  – Packaging of data for delivery over line
  – Error detection and correction
  – CSLIP or PPP standard
                                                         23
 Layered Interactions in Internet Access
User PC                 Routers              Webserver

                                             Webserver
Browser
               Internet Layer Governs        Software
           Routing Across Multiple Routers
  TCP              On the Internet             TCP
Software                                     Software
             IP                     IP
   IP                      IP                   IP
Software                Software             Software

  PPP                    Subnet
                                              Subnet
Software                Software
                                                         24
   Routing Across the Internet
 Routing

  – Message (packet) must travel over a network of
    routers
  – The user PC tells the first router the destination
    address of the webserver (e.g.,
    voyager.cba.hawaii.edu)
  – Working together, routers route the packet to its
    destination host
                                              25
 Layered Interactions in Internet Access
User PC            Routers       Webserver
                                  Webserver
Browser
              Transport Layer     Software
                  (TCP)
  TCP                               TCP
Software                          Software

   IP                 IP             IP
Software           Software       Software

  PPP               Subnet
                                   Subnet
Software           Software
                                                     26
             Transport Layer
 Connects   the Two Computers
  – Establishes a temporary connection
  – Breaks connection when transmission ends
  – Asks for the retransmission of damaged packets
  – Asks for the retransmission of lost packets
  – Flow control: tell the other computer to pause
  – Platform independence: connects computers
    from different vendors
                                              27
 Layered Interactions in Internet Access
User PC            Routers       Webserver
                                  Webserver
Browser
             Application Layer    Software
              (HTTP, HTML)
  TCP                               TCP
Software                          Software

   IP                  IP            IP
Software            Software      Software

  PPP
                     Subnet        Subnet
Software
                                              28
   Protocol Data Units (PDUs)
 Ateach layer, messages are strings of 1s
 and 0s
                                        PDU
 These    messages are organized
 Called   protocol data units (PDUs)
 Each   layer (except Physical) has a PDU
  – E.g., Application PDU or APDU
  – E.g., Computer (Transport) PDU or CPDU
                                                     29
           Organization of PDU
   Header (delivery information)
   Trailer (sometimes present for error handling)
   Data Field
    – Often PDU of next higher layer


                    PDU at Layer N+1
PDU at Layer N

        Trailer         Data Field          Header
                                                       30
       PDU Transmissions from the PC
User PC
                                        Application Layer
 Browser
                  APDU Application PDU
                             Computer (Transport) Layer
   TCP
 Software         APDU CH (CPDU) TCP PDU
                                      Internet Layer
    IP
 Software           CPDU     IH   Internet PDU (IPDU)
CSLIP/PPP                                    Subnet Layer
 Or Other
  Subnet    SnT       IPDU        SnH Subnet PDU (SnPDU)
 Software                                CSLIP or PPP
                          At Each Router                      31

 2. Subnet process
 removes IP PDU                        3. IP process
 from SnPDU,                             considers
                                Router  best route
 delivers it to the IP                               4. IP process
 process.                                            delivers the
                                  IP
                 IP PDU                       IP PDU IP PDU to
                               Software
                                                     the subnet
                                                     process
                               Subnet
           SnPDU for           Process         SnPDU on
            Subnet A                            Subnet B
             1. Router subnet SnA SnB
             process accepts
Subnet A     the SnPDU                                     Subnet B
                     At the Webserver                  32


                                           Webserver
4. TCP Software delivers APDU
     to Webserver Program                  Webserver
                                    APDU
                                           Software

 3. IP Software delivers CPDU                TCP
      to Transport Process         CPDU
                                           Software

        2. delivers IPDU                      IP
                                  IPDU
     to IP Software Process                Software


         1. accepts SnPDU                   Subnet
                                SnPDU
                                           Software
Subnet
                                                33
                   Intranets
 Use   of Internet Technology within Firms
  – Standards are mature
  – Standards for many applications
  – Security is good for internal use
  – Platform independent: No need to create
    separate applications for PCs, Macs, Unix
  – Browser runs software: no need to install
    software on PCs
  – Major concern of corporations today
                                                           34
                     Intranets
   Must keep out hackers
   Yet users need access to Internet services outside
    the Internet
   Solution: create a firewall to limit access from the
    outside
                            Firewall
             OK
                                       Intranet

                       No
Hacker
                                                      35
                  Extranets
 Give   Buyers or Suppliers Access
  – Limited to certain resources, such as inventory
    and pricing databases
  – Uses the Internet for transmission
                                         Extranet
 Customer               OK               Resource

                                 Internal

                       No
                            Any Other Resource
                                                     36
                  Extranets
 Tunneling
  – Extranet transmission uses the unsecure
    Internet
  – Add secure transmission through the Internet
    by means of encryption, other measures
  – Creates a secure “tunnel” through the Internet




        Tunnel

				
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posted:10/10/2011
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