Modems, Signals and Modulations Flashcards
What is a digital signal?
A digital signal is a set of numbers that represent the exact strength of the signal at regular time intervals
A digital signal is discontinuous (gaps) and varies instantaneously
The jumps in size between each bar are what is meant by varying instantaneously
What is an analog signal?
Analog data is in a continuous form and varies smoothly
A voltage that is continuously going up and down
Difficult and expensive to get a computer to work with analog data
Advantages of digital over analogue
Less error because of only two distinct values (0, 1)
More efficient transmission (switching, multiplexing)
Higher maximum transmission rate
More secure (easier to encrypt)
Integration (of voice, data and video) easier with digital transmission
Define Frequency
Frequency is the number of waves in an analogue signal that occur per second
Measured in hertz (Hz) (i.e. 1Hz is one cycle or wave per second)
Think of frequency as being like counting waves at the beach. If you are out swimming, but stop in the water and count the number of waves that pass you in a second, then you have counted the frequency of the waves.
Define Band
A band is a range of frequencies (i.e. the FM radio band covers the range of frequencies from 80MHz to 108 MHz).
What is bandwidth?
Bandwidth is the width of the band
The difference between the highest and lowest frequencies in a band (i.e. each FM station has a bandwidth of 25kHz, so an FM station broadcasting on 99MHz (99,000kHz) can broadcast a signal that varies between 98,987.5kHz and 99,012.5kHz)
In networks, bandwidth is used to refer to the amount of data that can be sent per second
What is Baud Rate?
Baud rate is the number of times that a signal changes in one second (i.e. 1,200 baud means that the voltage in an electrical cable changes 1,200 times every second. A baud rate of 2,400 baud means that the voltage changes 2,400 times every second)
Measured in baud
What is a Bitrate?
Bit rate is the number of bits of data sent every second
Measured in bits per second (bps)
A bit rate of 33,600 bps means that 33,600 bits are sent every second. A bit rate of 9.6kbps (9,600bps) means that 9,600 bits are sent every second.
Bit rate = Baud rate * number of bits per signal element/symbol/interval
What does a modem do?
Modems are used by computers to exchange data, such as this web page, over a telephone line.
Modern modems also allow your computer to send and receive faxes.
Modems take the digital data from a computer and turn it into analogue data - specifically sounds that can be transmitted down a telephone line.
How does a modem work?
The namemodemcomes frommodulation/demodulation.
It takes a digital signal from the computer and turns it into an analogue signal. This analogue signal is sound; the sound is “played” into the telephone line. This process is calledmodulation.
It can also take an analogue signal - the sound from the telephone line - and turn it into a digital signal. This process is calleddemodulation.
Finally, it performs some control functions, such as dialling a telephone number.
What are Modulation and Demodulation?
Modulationmeans that a electrical signal, such as a digital signal in a computer, is converted to a sound signal that can be transmitted along a normal analogue telephone line.
Demodulationis the opposite process. When the receiver gets the signal, it demodulates the received signal to get the original data back. Demodulation is simply the reverse of modulation.
What are the three modulation techniques?
Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)
How does AM work?
Modulates/changes the amplitude/size/voltage/volume
Frequency and phase remain constant
How does FM work?
Modulates/changes the frequency/pitch/tone/period
Amplitude and phase remain constant
In FM, the frequency of the signal (that is the pitch or tone of the sound) is modulated. The result is that the frequency changes slightly (or “wobbles”) depending on the bit value being represented. The amplitude and phase of the signal do not change.
How does PM work?
Instantaneously alters phase position of signal
Amplitude and frequency remain constant
Digital only technique
1 to 1 and 0 to 0 no change, 0 to 1 and 1 to 0 phase change
