n301/Computer Networks n301.tplt Basic Components Nodes Links Communication Links Over Telephone Over dicated wires Figure 12.1a Figure12.1b Two forms of information representation Conversion Digital -------> Analog Analog ------> Digital This is where modems come into play Figure 12.2 Modulation of a carrier to encode binary information Transmission Media (Links) Twisted-pair copper wire (ex: tel)

• cheap, low rate, low range, noise

Coaxial Cable (ex: TV Cable)

• more exp., higher rate, higher range, noise tolerance

Fiver-optic

• expensive, very fast, no noise, long distance

Figure 12.3 Transmission time of an image at different transmission speeds Ubiguitous Computing "Computers everywhere"! Local Area Networks (LAN) Connects several nodes within a small geographical area. => lab, office Polular ones: Ethernet, Token Ring Ethernet 1970 by Xerox Pane 10Mbs Fast Ethernet 100Mbs Figure 12.4a Figure 12.4b An Ethernet LAN implemented using cables Communication Protocol How does machines communicate? CSMA/CD each machine has an Ethernet Address each message contains the address of the receiver everybody recieves every message discards, if not theirs Figure 12.6a Figure 12.6b Figure 12.6c Sending messages across an ethernet Wide Area Networks (WAN) General structure of a Wide area network Protocol Uses Store - and - forward, packet switching protocol Packets: representation of messages. (unit) Protocol 2 Routing: Process selecting specific packets Shortest path: Complexity? Topological change Network failures Figure 12.8 Store-and-Forward Packet Switching Network configuration Internet is a collection of LANS & WANS connected by routers. Typical LAN/WAN Network Configuration Network configuration 2

1. Node A on LAN1 broadcasts a message containing the destaination address "Node B, LAN2"
2. Every node on LAN1 recieves this message, but only router R1 accepts it, becuae R1 has the ecplicit responsibility to accept and handle all messages destined for nodes outside LAN1.

Network configuration 3

3. Router R1 reconfigures the message so that it is consistent whit the protocols and formats of messages sent through the WAN. R1 then passes the message into the WAN via node C.

Network configuration 4

4. The message is routed from node C to node D using the store-and-forward packet-switching protocols just described. When the message arrives at node D, it is passed onto router R2.

Network configuration 5

5. R2 again reformats the message, this time to be consistent with the protocols and formats of LAN2, which need not be the same protocols used by LAN1. Then it broadcasts the message on LAN2, where is it recieved by all nodes.

Network configuration 6

6. Node B, the node whose address is contained in the destination address field, accepts the message. All other nodes on LAN2 disregard it. The message has correctly arrived at its destination.

Internet & WWW Gateways: Provides connectivity between different WANs. How different networks communicate? "Networks of Networks" TCP Transport Content Protocol Technology borrowed from telephone co. country code, city code, phone # etc. 01 317 274 9727 Cerf and Kahn Developed two conventions in the networking work: Addressing - hierarchical addressing scheme that uniquely identifies a computer user located anywhere in the world. Protocols (TCP/IP) - describes the rules and procedures that networks would use for sddressing, message formats, routing, and error control Figure 12.12 State of Networking in the late 1980s The World Wide Web Hypertext: Collection of coduments connected by pointers] Operations of the web Figure 12.15a Figure 12.15b Figure 14.6 Figure 20.1