Signal Theory Rei Sanchez

Principles of Signal Theory In a computer network in order to send data a digital signal needs to be sent to the receiving computer, this signal is created by the hardware using a peer to peer interface so both computers know there is a connection. To maintain the connection, a very small trickle of signal will be sent between the computers to keep the connection active, if this was not sent the connection would die and not work.

Digital Signals Digital signalling is also called discrete state signalling. Digital signals change from one state to another almost instantaneously, without stopping at an in-between state. Digital signals represent discrete states over time.

Representing Data (bits, bytes, packets) A bit is either of two states: on (1) or off (1). Physically, this is usually represented as voltages or magnetic fields. A byte is eight bits, for example: 10100111. A byte can represent 256 distinct values, from 0-255.  Packet structure depends on the protocol being used; usually a header and information about the packet size, and perhaps a checksum, followed by the body of the packet, and then a terminator or tail portion of data is contained in a data packet.

Asynchronous Transmission   Asynchronous connection, this connection does not use clocking so the connections are not in sync, but instead on demand. The devices what typically use this type of connection is: monitors, printers etc

Synchronous Transmission Synchronous connection, this connection uses a clock signal to work the computers in pairs; this is used in computer networks.  This means the computers are synchronised hence the name synchronous connection. Usually a DCE will send the data to keep the computers in sync.

Error detection and Correction Error detection is used in a network so that data can be sent correctly around the network. Interference from other things such as weather and hardware may cause data to be corrupted. Error correction is used within a network so that data can be checked for any errors in order to stop data from being corrupted.

Bandwidth / Limitation Bandwidth limitation means decreasing the amount of information transmitted from sender to receiver per second. This either means the information arrives slower, or the information contains less detail. Noise introduces randomness into a signal. This means that some of the information is lost and replaced with nonsense. In a radio signal, the randomness of noise results in swirling or hissing sounds that come and go randomly. 

Channel Types Radio – the transmission on telephone is by radio signals. This allows people to talk to each other no matter how far they are to each other. Satellite - Satellite transmission requires an unobstructed line of sight. The line of site will be between the orbiting satellite and a station on Earth. Satellite signals must travel in straight lines but do not have the limitations of ground based wireless transmission. Satellite is useful for broadcasting live content around the world. Because satellites are in orbit, the signals can be sent instantaneously into space and then redirected to another satellite or directly to their destination.