Networks

Networks

1. LAN (Local Area Network)

   • Network confined to one location or organisation
   • Only accessible by computers on that LAN

2. WAN (Wide Area Network)

   • Network of LANs spanning a wide area
   • e.g. The Internet

3. Features

   • Advantages
     • Share files, so the same files are available whichever computer you use
     • Share resources such as printers and internet connection
     • Provide a single username and password that can be used on every computer on the network
   • Disadvantages
     • Expensive - extra hardware needed
     • More room for error - if something goes wrong it can affect every computer
     • Difficult - it requires a lot of technical expertise

4. Topology

   • Bus
     • A single cable connects all the computers
     • All the clients and servers connect to this cable
     • Terminators are attached to each end of the cable
     • Cheap
     • Easy to install
     • No extra equipment required
     • High chance of collisions
     • More computers mean more collisions
     • Damage to the wire results in catastrophic failure
     • Only suitable for small networks
   • Ring
     • Each computer is connected to two other computers in a ring formation
     • Each computer receives signals from one of its connected computers
     • If the data is for itself, it reads it, if not it transmits it to the next computer
     • The data travels round the ring in one direction until it reaches its intended recipient
     • No collisions, as each cable only has one computer able to transmit on it
     • Number of computers does not affect speed
     • One faulty cable or faulty computer will result in catastrophic failure
     • More difficult to add new computers
   • Star
     • All computers are connected to a central hub or switch
     • A hub sends each message to all the other computers
     • A switch checks the address and sends it only to the correct one
     • No collisions
     • Failure of a cable or computer is not catastrophic
     • Easy to add new comptuers
     • Expensive (requires hub/switch and lots of cabling)
     • Failure of hub/switch is catastrophic and difficult/expensive to fix
   • Mesh
     • Each computer is connected to one or more other computers
     • A full mesh has every computer connected to every other computer
     • A partial mesh has only some of the computers connected
     • Zero catastrophic failure – there are always alternative paths for data
     • Multiple transmissions can happen simultaneously
     • Redundancy - Lots of unnecessary connections
     • Expensive
     • Difficult to maintain and administer

5. Protocols

   • Examples
     • HTTP (Hyper Text Transfer Protocol) is the standard protocol used for requesting an HTML file from a webserver
     • FTP (File Transfer Protocol) is a protocol for transferring files between two computers
     • POP3 (Post Office Protocol 3) is a protocol for downloading email from a mail server.
     • IMAP (Internet Message Access Protocol) replaced POP3 when web-based email took off. It is for reading email from a mail server, but doesn’t require the mail to be downloaded. IMAP allows email to be left on the server, and read from different mail readers.

6. TCP/IP Stack

   • Routing, checking, re-assembling and forwarding of packets is a complex task.
   • Rather than write one single protocol to do it all, the TCP/IP protocol stack is used.
   • Application - Provides applications services to users and programs
   • Transport - Handles data-consistency functions (Error Checking)
   • Network - Provides network addressing and routing (ip addresses)
   • Data Link - Prepares and corrects data to be passed to the physical layer.
   • Physical - Transmits and receives bits