1. WXET1143 Lecture5: Internet: How it Works? (Continuation)

2. Data movement  Even the simplest communication over the Net is broken up into scores of discrete packages of data.  Those packages are sent over via telephone lines, satellite signals, microwaves…  Packages in a single communication might not be sent over on a single path.  They also run through systems that has different OS such as Windows, Unix, AppleTalk, NetWare and so on.

3.  Messages are coded and decoded compressed and decompressed Translated and retranslated Bungled and corrected Lost and repeated Shredded and stitched back together  The amazing thing about all this is that…

4.  NO ONE IS IN CHARGE!! No one owns the Internet People and companies own bits and pieces Some organization rule over things such as domain names  The Internet is built on cooperation

5. How networks communicate  For a message to travel through a network, it must pass several layers.  These layers are designed to make sure the data gets through intact and accurate.  These layers are called the OSI layer.  There are 7 layers in all.

6. 1. Application layer  It is the first layer.  The only part of the process the user sees.  Even so, user doesn’t see most of the work the application does to prepare a message for sending over a network.  This layer converts a message’s data into bits.  It then attaches a header identifying the sending and receiving computers.

7. 2. Presentation layer  The second layer.  Translates the message into a language that the receiving computer understands. Usually in ASCII  this layer also compresses and perhaps encrypts the data.  It then adds another header specifying the language as well as the compression and encryption schemes.

8. 3. Session layer  The third layer.  Opens communication.  Sets brackets (boundaries) for the beginning and end of the message.  Establishes whether the message will be sent half-duplex – each computer taking turns sending and receiving Full-duplex – both computers sending and receiving at the same time.  These details are placed into a session header.

9. 4. Transport layer  Protects the data being sent.  It subdivides the data into segments and creates checksum tests Mathematical sums based on the contents of data. Can be used later to determine whether the data was scrambled  It also makes backup copies of the data.  The transport header identifies each segment’s checksum and its position in the message.

10. 5. Network layer  Selects a route for the message.  Forms segments into packets and counts them.  Adds a header containing The sequence of packets The address of the receiving computer.

11. 6. Data-link layer  Supervises the transmission.  Confirms the checksum and addresses.  Duplicates the packets.  This layer keeps a copy of each packet until it receives confirmation from the next point along the route that the packet has arrived undamaged.

12. 7. Physical layer  Encodes the packets into the medium that will carry them. Such as an analog signal, if the message is going across a telephone line  Send the packets along that medium.

13. Finally,  At the receiving node, the layered process that sent the message on its way is reversed. Physical – converts message into bits Data-link – recalculates the checksum, confirms arrival, logs in packets Network – recounts incoming packets Transport – recalculates the checksum, reassembles the message segments. Session – holds parts of the message until it is complete and sends it to the next layer. Presentation – decrypts, expands, & translates it. Application – identifies recipient, converts bits into readable characters & directs data to the correct application.

14. Information travels the Internet  Typically in a office, A PC jacks in to the Internet by being a part of a LAN. The network, wires directly to the Internet through a port called a T-connection.  In a SOHO, a PC is more likely to use a modem to connect to a network  Either way, information is asked and received through a browser.

15.  Your local host network makes a connection on another line to another network.  If the second network is some distance away, your host LAN might have to go through a router.  Router inspects your request to determine what other part of the Internet it’s addressed to.  Then, based on available connection and traffic, the router determines the best path to set the request back on its track to the proper destination.

16.  If the destination for your request isn’t on the same mid-level network, the router send the request to a network access point (NAP).  The pathway can take any route along the Internet backbone.  Backbone: A collection of networks that link powerful supercomputers associated with the National Science Foundation.  Along the way, your request might pass through repeaters, hubs, bridges, and gateways.

17.  Repeaters Used to amplify or refresh the stream of data which deteriorates the farther it travels. Repeaters let the data signals reach more remote PCs.  Hubs Link groups of networks so that the PC and terminals attached to each of those networks can talk to any of the other networks.

18.  Bridge Links LANs so that data from one network can pass through another network on its way to a third LAN.  Gateways Similar to bridges They also translate data between one type of network and another.

19.  When the request reaches its destination, the packets of data, address and error- correction are read.  The remote computer then takes the appropriate action, such as: Running a program Sending data back to your PC Posting a message on the Internet Etc..