Operating System Part 2

Operating System Part 2

1. Multiplexing

   • The process of combining several signals down a single communication channel and splitting them up at the receiver.
   • Time Division Multiplexing
     • First Sender A and Receiver A are joined.
     • Then Sender B and receiver B are joined.
     • The switching between senders and receivers takes place every few milliseconds.
   • Frequency Division Multiplexing
     • Each signal is assigned a different frequency along the channel.

2. ADSL

   • Asymmetric Digital Subscriber Line makes use of frequency division multiplexing.
   • 256 channels are used each of which can send a bit at the same time as the others.
   • Each channel makes use of QAM to allow 60000bps.
     • ADSL standard allows speeds of 8 Mbps downstream and 1Mbps upstream.
   • Must also allow the connection between the exchange and house to carry conventional analogue signals.
   • A NID (network interface device) on a customers premises marks the end of phone companies property and the start of the customers property.
   • Digital subscriber line access multiplexer (DSLAM)

3. Data Packets

   • When a file is sent across a network it is broken into blocks of data and placed into packets.
   • Each packet will have...
     • packet number.
     • the address of the machine it came from.
     • address of machine its going to.
     • The data.
     • A checksum to make sure data is not corrupted..

4. Communication Networks

   • Provide town to town and country to country communication over digital signals.
   • May be...
     • Copper cables.
     • Fibre optics.
     • Line of sight microwaves.
     • Microwave via satellite.
   • Data is broken down into packets and sent across the network from node to node until it reaches its destination.
     • Two ways that this may happen.
       • Circuit switching
         • A route is negotiated and space is made available.
         • The data is transferred and the route is closed.
         • Route is guaranteed.
         • Also known as a collection based system.
       • Packet Switching
         • Each packet is individually routed through a network.
         • Different packets may follow different paths so packets may arrive out of order.
         • Each packet is placed onto a network and will be queued if necessary when it arrives to a router.
         • Each packet will be individually routed so different packets in the same message may go by different routes.
         • As packets may arrive out of order, they are sorted into packet number order when arriving at the receiver.
   • Local Area networks
     • Bus
     • Star
     • Ring
     • Mesh

5. CSMA/CD

   • Carrier sense multiple access with collision detection.
     • Method used by ethernet and similar to systems used on wireless networks.
   • When a network card needs to send a data packet.
     • Network card listens to see if packet is on the network.
     • If no packet is present then the data packet is sent.
     • If the network card detects the network packet is being corrupted a collision has occured.
       • The card stops sending the packet.
       • The card waits a random amount of time.
       • The data packet is sent again.
   • Works well except that:
     • On a busy network collisions occur more often causing more repeated packets increasing traffic.
     • The greater the physical size of the network the greater the chance that a data packet has been put on but goes undetected when a card looks to see if the network is clear.

6. Token Networks

   • A token, permission to send is transferred from machine to machine.
   • Only when data is received can data be sent, by adding it to the token.
   • When the token reaches the receiving machine the data is removed.
   • More efficient but are now little used because...
     • Need specialist hardware making them expensive.
     • Tend to have slower basic data rate rather than CSMA/CD.

7. Joining Networks

   • A bridge is a piece of hardware that joins two or more networks together.
     • The networks must use the same protocol.
     • A bridge will only pass through data packets that need to travel from one side of the bridge to the other.
   • Gateway - Joins a network to another network that may have a different protocol.
   • On a packet switched network, a router is responsible for passing a data packet on to a node nearer to its destination.
   • Routers build up a list of where destination addresses are.
     • May take into account the levels of traffic flow.
     • Data packets may be routed over the fastest, not the shortest route.