Computing CG3 - Hardware

A simple set on notes about Hardware, section CG3.1

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Data transmission

There are two types of transmission, Parallel and Serial transmission

Parallel Transmission

  • A number of bits can be sent at the same time along a number of paths. 
  • Used internally when data is sent along the buses.                              
  • Faster than Serial Transmission.                                                              
  • Can only be used for short distance.

Serial Transmission 

  • Bits are sent one after another on a single wire.
  • Used for greater distances.                             
  • Can be fast on fibre optic cables.                   
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Multiplexing and Switches

Transmission speeds can be increased using multiplexing, a method of combining and sending more than one signal at the same time, Several data streams can be combined and transmitted over the same wire.

A switch connects segments of a network together. Packets of data received by the switch from one network are transmitted into the other connecting network.

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Circuit Switching

A number of switched are set to create a path along which data can be sent.

E.g.

The old telephone system. Switches are set in the exchange between the caller   and receiver and this path is maintained while the call is made. Each switch is a   possible cause of interference and is unsuitable for transmitting data.                   

All data follows the same path in order and the path cannot be used by any other data. Packet switching is preferred over this method.

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Packet Switching

In a packet-switching system, data messages are split into packets of a fixed size.

A packet will contain: 

  • The data being sent.                                                     
  • The address of the source and destination of the data.   
  • A checksum for the purpose of checking for corrupt files.
  • Data which allows the original data to be re-assembled.

The packet data is re-directed by each computer on the network to the next until it arrives at it intended destination.

A single set of data can be split up into a number of packets, each packet may take a different route and may also arrive out of order but are re-assembled at the end.This method is preferred  because there is no waiting during gaps and is less likely to be affected by network failure. It also has better security then Circuit Switching.

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Bus Network

A Bus Network is a number of computers that use a single channel for sending data. Only one computer can send data at a time. The packet passes along the network from one computer to the next until it arrives at the receiving computer.

Before a computer can send data down a bus, it must check if the channel is free, if it is not then the computer will wait. Once the data is sent, the computer will see if a collision occurs (two pieces of data sent at once). If one does occur a computer will wait a random short amount of time and try sending again.

(http://i56.tinypic.com/2j2dpp5.jpg)

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Token Ring Network

A small packet called a token pass continuously round the ring network, always in the same direction. A computer can attach data to a token and the address of the computer it wishes to send it to.  Each station receives the package and forwards the message on if necessary.

(http://i51.tinypic.com/29upx92.jpg)

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Protocols and Gateways

Protocol

A set of rules for transmitting data around a network. Computers would not be able to communicate if there was no protocol as they would not be able to interpret the other's data.

There are a number of standards set down for protocols to follow. The most commonly used protocol on networks and the Internet is TCP/IP.

Gateway

If two different networks with different protocols need to be connected together, a gateway needs to be used. The gateway intercepts data passing through and converts it to the proper format.

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More on Protocols

A typical protocol may state:

  • What data should be placed in the header of a data packet.
  • How to format the data in a packet.
  • Handshaking procedures
  • What to do if errors are detected.

Some common protocols include:

  • TCP/IP (Transmission Control Protocol/Internet Protocol) - The basic protocol used for transmitting data over any network or the Internet.
  • HTTP (HyperText transfer Protocol)-Used for transmitting web pages over the Internet.
  • FTP (File Transfer Protocol) -A protocol for transmitting files over the Internet.
  • POP3 (Post Officer Protocol 3) - A protocol for retrieving emails.
  • SMTP (Simple Mail Transfer Protocol) - A protocol for sending emails.
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