1 Computer (Transport) Layer F Manages communication between two computers F Platform independence: one computer does not know even what platform the other is (mainframe, PC, etc.) F TCP/IP’s most popular transport standard is TCP

2 Terminal-Host Platform F Processing done at a central host computer –Mainframe (very large) –Minicomputer (mid-size) Host Programs Execute on the Host

3 PC Network F 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 F Servers –Provide services to client PCs –Usually PCs but not always Server Service

5 PC Network F Servers are Specialized –Optimized for their services for cost, performance, and reliability File Server Database Server Internet Access Gateway Server File Service

6 File Server Program Access F Programs executed on the client PC –Program and data files downloaded over the network Client PC Server Program Executed On Client PC Downloaded Over Network Network

7 Client Workstation F Workstations are more powerful than PCs F Come in client and server versions Client Workstation 21” display Powerful RISC microprocessor Large, fast RAM, hard disk Usually UNIX OS, can be Windows NT

8 Networked PC F The Dominant Computer Platform Today is the Networked PC –High penetration of PCs on desktops –Most are already networked F “Legacy” Systems Must be Maintained –Mainframe terminal-host systems –Minicomputer terminal-host systems –Many are being “downsized” to PC networking

9 Managed PCs F 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 F 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

11 Network Computers (NCs) F 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 The Internet

12 Low-Cost PCs for the Home F 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 F Sub $1,000 PCs

13 Networks Mix Traffic of Multiple Platforms Network (LAN, WAN, internet) Terminal-host traffic PC network traffic

14 F2-7: Program Interactions on a Stand- Alone PC Application Program Operating System Hardware Commands related to filesFiles or error messages Response Request

15 Client/Server Processing Client ProgramServer Program Client Machine (PC) Server Machine (UNIX Minicomputer) TWO programs on TWO machines Platform independent

16 Client/Server Processing F Application Layer Protocols –Control the requests and responses –Application independence: any browser can work with any webserver program F Transport Layer Protocols –Allow two computers with different architectures to work together –Brings platform independence

17 Internet Access Webserver PC with browser Internet Service Provider (ISP) Internet Service Provider (ISP) Internet Service Provider (ISP) Internet Service Provider (ISP) The Internet Backbone The Internet Backbone To connect to the Internet, you must have an ISP

18 Internet Service Provider (ISP) F Basic Service: Links You to the Internet –Provides a connection –Pays for your use. (The Internet is NOT Free.) F Most Provide Additional Services – –World Wide Web Homepage Hosting –Premium Providers have Specialized Services (America Online, CompuServ, MSN)

19 Internet Access with PPP Modem, TCP/IP software PPP software PPP link over telephone line ISP

20 Internet Access with PPP: routers ISP The Internet: 1. a network of routers connected by transmission lines Router ISP Router 2. Subnets connected by routers

21 Layered Interactions in Internet Access Browser TCP Software TCP Software IP Software IP Software CSLIP/PPP Software CSLIP/PPP Software User PC IP Software IP Software PPP Software PPP Software Router Webserver Software Webserver Software TCP Software TCP Software IP Software IP Software Subnet Webserver Subnet Layer (PPP) Point-to-Point Connection

22 Point-to-Point Connection F Physical Layer –Serial Port –Modem –Telephone Transmission F Data Link Layer –Packaging of data for delivery over line –Error detection and correction –CSLIP or PPP standard

23 Layered Interactions in Internet Access Browser TCP Software TCP Software IP Software IP Software PPP Software PPP Software User PC Webserver Software Webserver Software TCP Software TCP Software IP Software IP Software Subnet Webserver Internet Layer Governs Routing Across Multiple Routers On the Internet IP Software IP Software Subnet Software Subnet Software Routers IP

24 Routing Across the Internet F 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 Browser TCP Software TCP Software IP Software IP Software PPP Software PPP Software User PC Webserver Software Webserver Software TCP Software TCP Software IP Software IP Software Subnet Webserver Transport Layer (TCP) IP Software IP Software Subnet Software Subnet Software Routers

26 Transport Layer F 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 Browser TCP Software TCP Software IP Software IP Software PPP Software PPP Software User PC IP Software IP Software Subnet Routers Webserver Software Webserver Software TCP Software TCP Software IP Software IP Software Subnet Webserver Application Layer (HTTP, HTML)

28 Protocol Data Units (PDUs) F At each layer, messages are strings of 1s and 0s F These messages are organized F Called protocol data units (PDUs) F Each layer (except Physical) has a PDU –E.g., Application PDU or APDU –E.g., Computer (Transport) PDU or CPDU PDU

29 Organization of PDU F Header (delivery information) F Trailer F Trailer (sometimes present for error handling) F Data Field –Often PDU of next higher layer Trailer Data Field Header PDU at Layer N PDU at Layer N+1

30 PDU Transmissions from the PC Browser IP Software IP Software CSLIP/PPP Or Other Subnet Software CSLIP/PPP Or Other Subnet Software User PC APDU CH Computer (Transport) Layer (CPDU) TCP PDU Application Layer CPDUIH Internet Layer TCP Software TCP Software Internet PDU (IPDU) SnTIPDUSnH Subnet PDU (SnPDU) CSLIP or PPP Subnet Layer Application PDU

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

32 At the Webserver Webserver Software Webserver Software TCP Software TCP Software IP Software IP Software Subnet Software Subnet Software Webserver Subnet SnPDU 1. accepts SnPDU IPDU 2. delivers IPDU to IP Software Process CPDU 3. IP Software delivers CPDU to Transport Process APDU 4. TCP Software delivers APDU to Webserver Program

33 Intranets F 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 F Must keep out hackers F Yet users need access to Internet services outside the Internet F Solution: create a firewall to limit access from the outside Intranet Hacker OK No Firewall

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

36 Extranets F 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