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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 1 Terminal-Host Platform Processing done at a central host computer – Mainframe (very large) – Minicomputer (mid-size) Programs Execute on the Host Host 2 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 3 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 Service File Server Database Server Internet Access Gateway Server File Server Program Access Programs executed on the client PC – Program and data files downloaded over the network Program Executed On Client PC Downloaded Over Network Client PC Network Server 6 Client Workstation 7 Workstations are more powerful than PCs Come in client and server versions 21” display Powerful RISC microprocessor Large, fast RAM, hard disk Usually UNIX OS, can be Windows NT Client Workstation Networked PC The Dominant 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 8 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 9 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 10 Network Computers (NCs) 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 11 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 12 Networks Mix Traffic of Multiple Platforms 13 Network (LAN, WAN, internet) PC network traffic Terminal-host traffic F2-7: Program Interactions on a StandAlone PC Application Program Request Response Operating System Commands related to files Hardware Files or error messages 14 Client/Server Processing Client Program 15 Server Program Platform independent TWO programs on TWO machines Client Machine (PC) Server Machine (UNIX Minicomputer) 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 16 17 Internet Access Webserver Internet Service Provider (ISP) To connect to the Internet, you must have an ISP The Internet Backbone PC with browser Internet Service Provider (ISP) 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) 18 19 Internet Access with PPP Modem, TCP/IP software PPP software PPP link over telephone line ISP 20 Internet Access with PPP: routers Router Router Router The Internet: 1. a network of routers connected by transmission lines 2. Subnets connected ISP by routers ISP Router 21 Layered Interactions in Internet Access User PC Router Webserver Browser Webserver Software TCP Software TCP Software IP Software CSLIP/PPP Software Subnet Layer (PPP) IP Software IP Software PPP Software Subnet Point-to-Point Connection 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 22 23 Layered Interactions in Internet Access User PC Browser TCP Software Routers Webserver Internet Layer Governs Routing Across Multiple Routers On the Internet IP Webserver Software TCP Software IP IP Software IP Software IP Software PPP Software Subnet Software Subnet 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 24 25 Layered Interactions in Internet Access User PC Browser Routers Transport Layer (TCP) TCP Software Webserver Webserver Software TCP Software IP Software IP Software IP Software PPP Software Subnet Software Subnet 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 26 27 Layered Interactions in Internet Access User PC Browser Routers Application Layer (HTTP, HTML) TCP Software Webserver Webserver Software TCP Software IP Software IP Software IP Software PPP Software Subnet Subnet Protocol Data Units (PDUs) At each layer, messages are strings of 1s and 0s These Called Each messages are organized PDU protocol data units (PDUs) layer (except Physical) has a PDU – E.g., Application PDU or APDU – E.g., Computer (Transport) PDU or CPDU 28 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 29 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 CSLIP/PPP Or Other Subnet Software CPDU SnT IPDU IH Internet PDU (IPDU) Subnet Layer SnH Subnet PDU (SnPDU) CSLIP or PPP At Each Router 2. Subnet process removes IP PDU from SnPDU, delivers it to the IP process. IP PDU 3. IP process considers Router best route IP Software Subnet Process Subnet A 31 SnPDU for Subnet A 1. Router subnet SnA SnB process accepts the SnPDU 4. IP process delivers the IP PDU IP PDU to the subnet process SnPDU on Subnet B Subnet B At the Webserver 32 Webserver 4. TCP Software delivers APDU to Webserver Program 3. IP Software delivers CPDU to Transport Process 2. delivers IPDU to IP Software Process 1. accepts SnPDU Subnet APDU CPDU IPDU SnPDU Webserver Software TCP Software IP Software Subnet Software 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 33 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 Hacker No 34 35 Extranets Give Buyers or Suppliers Access – Limited to certain resources, such as inventory and pricing databases – Uses the Internet for transmission Customer OK Extranet Resource Internal No Any Other Resource 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 36