Analog and Digital Data Transmission

 Data

¨ Entities that convey meaning

 Signals

¨ Electric or electromagnetic representations of data

 Signaling

¨ Physical propagation of signal along a suitable medium

 Transmission

¨ Communication of data by propagation and processing of signals

 

Analog and Digital Data

 Analog

¨ Continuous values within some interval

¨ e.g. sound, video

 Digital

¨ Discrete values

¨ e.g. text, integers

Components of Speech

 Frequency range (of hearing) 20Hz-20kHz

¨ Speech 100Hz-7kHz

 Easily converted into electromagnetic signal for transmission

 Sound frequencies with varying volume converted into electromagnetic frequencies with varying voltage

 Limit frequency range for voice channel

¨ 300-3400Hz 

Binary Digital Data

 From computer terminals etc.

 Two dc components, one for binary 1 and other for binary 0.

 Bandwidth depends on data rate

  

Data and Signals

 Usually use digital signals for digital data and analog signals for analog data

 Can use analog signal to carry digital data

¨ Modem

 Can use digital signal to carry analog data

¨ Compact Disc audio

Digital Signals Carrying Analog and Digital Data

Analog Transmission

 Analog signal transmitted without regard to content

 May be analog or digital data

Analog signal become weaker (attenuates) after a certain distance. Use amplifiers to boost signal

Amplifiers also amplifies noise

 

Digital Transmission

 Concerned with content of the signal.

 Integrity endangered by noise, attenuation etc.

 To achieve greater distance repeaters are used.

¨ Repeater receives signal

¨ Extracts bit pattern

¨ Retransmits

¨ Thus attenuation is overcome

 Noise is not amplified

 Analog and Digital Transmission

 Important distinction is in the manner signal attenuation is handled at repeater / amplifiers

 Analog - Attenuated signal is amplified and retransmitted

 Digital - Data encoded in attenuated signal is recovered, a new signal is generated encoding that data, and retransmitted

 Digital signals always digitally transmitted, but analog signals can be transmitted either way (assuming the signal carries digital data