Data Communications Introduction and Review

Transmission Media Copper Wires Copper Wires Low resistance. Electrical signal produces miniature radio station – interference. Low resistance. Electrical signal produces miniature radio station – interference. When one wire encounters another – electromagnetic wave occurs. When one wire encounters another – electromagnetic wave occurs. Interference has to be minimized in order for communications to occur. Interference has to be minimized in order for communications to occur. Twisted Pair Twisted Pair Coaxial Cable Coaxial Cable

Transmission Media Glass Fibres Glass Fibres Transmitter at one end uses a light emitting diode (LED) Transmitter at one end uses a light emitting diode (LED) Receiver at other end uses light sensitive transistor to detect the pulse. Receiver at other end uses light sensitive transistor to detect the pulse. Advantages No interference Reflect Inward Light encodes more Travel over a single fibre Disadvantages Special equipment to polish ends Finding breaks is difficult

Transmission Media Radio Waves Radio Waves Radio frequency or RF Radio frequency or RF No direct connection No direct connection Cannot bend around the earth Cannot bend around the earth Can be combined with satellites Can be combined with satellites Microwaves Microwaves Electromagnetic radio wave beyond the frequency used by Radio and Television Electromagnetic radio wave beyond the frequency used by Radio and Television Do not broadcast in all directions but can be directed. Do not broadcast in all directions but can be directed. Carry more information than RF transmission Carry more information than RF transmission Cannot penetrate metal structures Cannot penetrate metal structures

Transmission Media Infrared Infrared Limited to small area Limited to small area Transmitter pointed toward receiver Transmitter pointed toward receiver Inexpensive – no antenna Inexpensive – no antenna TV Remote TV Remote Laser Laser Similar to Microwave ie. line of sight Similar to Microwave ie. line of sight

Asynchronous Communication Sender and receiver not synchronized Sender and receiver not synchronized   Parity Bit Stop Bit Start Bit ASCII “G” Start Bit ASCII “J” Electrical signal transmitted does not contain information the receiver can use to determine where individual bits begin and end.

Asynchronous Communication Using electric current to send bits Using electric current to send bits Negative voltage could be used to represent ‘1’ while a positive voltage could be used to represent ‘0’ Negative voltage could be used to represent ‘1’ while a positive voltage could be used to represent ‘0’

Asynchronous Communication time Waveform Diagram Problem: there is no set interval that a bit is transmitted

Asynchronous Communication Standards for communication Standards for communication How long should the sender hold the voltage on the wire for a single bit? How long should the sender hold the voltage on the wire for a single bit? What is the maximum rate at which hardware can change the voltage? What is the maximum rate at which hardware can change the voltage? How can users be sure that different vendor’s hardware will be compatible? How can users be sure that different vendor’s hardware will be compatible?

Asynchronous Communication Standards for communication cont’d Standards for communication cont’d International Telecommunications Union – ITU International Telecommunications Union – ITU Electronic Industries Association – EIA Electronic Industries Association – EIA Institute for Electrical & Electronics Engineers - IEEE Institute for Electrical & Electronics Engineers - IEEE

Asynchronous Character Transmission with RS-232 Standard by Electronic Industries Association (EIA) Standard by Electronic Industries Association (EIA) Connection less than 50 feet long Connection less than 50 feet long Voltage range = -15 to +15 volts Voltage range = -15 to +15 volts Specifies transmission of characters Specifies transmission of characters Each character uses 7 bits Each character uses 7 bits Defines serial asynchronous communication Defines serial asynchronous communication Never leaves 0 volts on the wire Never leaves 0 volts on the wire

Asynchronous Character Transmission with RS-232 time stop idleIdle start Sending and receiving hardware agree on the length of time for transmission of a bit

Asynchronous Character Transmission with RS-232 Baud rate = number of changes in the cycle per second Baud rate = number of changes in the cycle per second =bps =bps Manufacturers make hardware to operate at various rates Manufacturers make hardware to operate at various rates Framing errors can occur Framing errors can occur If voltages do not agree or the stop bit does not occur at the time expected. If voltages do not agree or the stop bit does not occur at the time expected.

Asynchronous Character Transmission with RS-232 Communication is full duplex. Communication is full duplex. TRGTRG TRGTRG A modem transmits on pin 2 and receives on pin 3, while a computer transmits on pin 3 and receives on pin 2. The ground is pin 7. The cable that connects a computer to a modem has a wire from pin 2 to pin 2, pin 3 to pin 3, and pin 7 to pin 7 for the ground.

Asynchronous Character Transmission with RS-232 Real hardware is limited. Real hardware is limited. Real Ideal Bandwidth and bit transmission. Bandwidth and bit transmission. Measured in cycles per second or Hertz (Hz). Measured in cycles per second or Hertz (Hz).

The End.