Download Conversions 4-Data Communication-Lecture Notes and more Study notes Data Communication Systems and Computer Networks in PDF only on Docsity!
LECTURE
Conversion
Digital To Analog Conversion o Process of changing one of the characteristics of an analog signal based on the info in a digital signal o When you Tx data from one computer to the other using a public telephone line o Original data is digital but because telephone wires carry analog signal, original data must be converted o Digital data must be modulated on an analog signal that has been manipulated to look like two distinct values corresponding to binary 1 to binary 0
o Figure shows the relationship b/w digital info the digital to analog conversion hardware & resultant analog signal Variation in Characteristics of Sine Wave
o A sine wave is defined by 3 characteristics:
Amplitude Frequency Phase
o By changing one aspect of a simple electrical signal back & forth,we can use it to represent digital data o When we vary any one of these characteristics ,we create a second version of that wave o If we than say that the original wave represents binary 1,the variation can represent binary 0 or vice versa o So by changing one aspect of a simple electrical signal back & forth,we can use it to represent digital data
Mechanisms for Modulating Digital Data to Analog Signals
o Any of the three characteristics listed above can be altered in this way, giving us at least 3 mechanisms for modulating digital data into analog signals
Amplitude shift keying(ASK) Frequency shift keying(FSK) Phase shift keying (PSK)
docsity.com
Fourth Mechanism
o In addition, there is a fourth and better mechanism that combines changes in both amplitude and phase called Quadratue Amplitude Modulation(QAM) o QAM is the most efficient of these options and is the mechanism used in all modern modems
Types of digital to analog modulation
Aspects of Digital to Analog Conversion
o Before we discuss specific methods of digital to analog modulation,two basic issues must be defined: Bit/Baud rate Carrier signal
Bit Rate & Baud Rate o Two terms used frequently in data communication
- Bit rate
- Baud rate Bit rate : no of bits transmitted during one second Baud rate : no of signal units per second that are required to represent that bit Bit Rate & Baud Rate o In discussion of computer efficiency, bit rate is more important –we want to know how long it takes to process each piece of info o In data transmission, however ,we are more concerned with how efficiently we can more that data from place to place, whether in pieces or blocks o The fewer signal units required, the most efficient the system and less bandwidth required to transmit more bits ,so we are more concerned with baud rate o The baud rate determines the B.W required to send the signal
Relationship b/w bit rate &band rate
docsity.com
o Speed of transmission during ASK is limited by the physical characteristics of Tx. Medium
Effect Of Noise on ASK
o ASK is highly susceptible to noise interference o NOISE : Unintentional voltages introduced onto a line by various sources such as Heat or Electromagnetic Radiation from other sources o These unintentional voltages combine with signal to change the amplitude o A 1 can be changed to 0 and a 0 to a 1 o ASK relies solely on Amplitude for recognition o Noise usually affects the amplitude, therefore ASK is the modulating method, that is most affected by Noise
On-Off Keying (OOK) o A popular ASK Technique o In OOK, one of the bit values is represented by no voltage o The advantage is the reduction in the amount of energy required to transmit Information
Bandwidth for ASK o Bandwidth of a signal is total range of frequencies occupied by that signal o When we decompose an ASK modulated signal, we get a spectrum of many simple frequencies o The most significant ones are those b/w, fc-Nbaud/2 and fc+Nbaud / with carrier frequency fc at the middle
o Bandwidth requirements for ASK are calculated using the formula:
docsity.com
o BW=(1+d) * Nbaud o BW = Bandwidth o Nbaud= Baud Rate ondition of line (min.value = 0) Example 5 bandwidth required for an ASK signal TX at 2000 bps. TX. Mode is half
: ASK, Baud Rate= Bit Rate
bandwidth equal to its Baud Rate
Summ tal-to Analog Conversion
Keying (ASK)
eading Sections “Data Communications and Networking” 4th Edition by Behrouz A.
o d= factor related to c . Find minimum duplex Solution
- In
- Therefore, Baud Rate = 2000
- Also ASK requires a minimum
- Therefore Minimum BW = 2000 Hz ary Digi Bit Rate and Baud Rate Carrier Signals Amplitude Shift
R Section 5.3, Forouzan
docsity.com
