Bit rate Baud rate Goal in data communication
• Bit rate • Baud rate • Goal in data communication is to increase the bit rate while decreasing the baud rate. • Increasing the data rate, increases the speed of transmission. • Decreasing the baud rate decreases the bandwidth requirement. • Bit Rate= Baud rate * Number of bits per second
Figure 5 -1 Different Conversion Schemes
Figure 5 -2 Digital to Digital Encoding
Figure 5 -3 Types of Digital to Digital Encoding
Figure 5 -4 Unipolar Encoding
Figure 5 -5 Types of Polar Encoding
Polar schemes • The voltages are on both side of the time axis. • NRZ (non return to zero) • NRZ-L : The level of the voltage determines the value of bit. • NRZ-I : the change in the level of the voltage determines the level of the bit. If there is no change, the bit is 0, if there is a change, the bit is 1.
Figure 5 -6 NRZ-L and NRZ-I Encoding
5. 9
• when the voltage level in a digital signal is constant for a while, the spectrum creates very low frequencies. These frequencies around zero, called DC components, present problems for a short system that cannot pass low frequencies.
Return to zero • It uses three values: positive, negative and zero. • The signal changes not between bits but during the bit. The signal goes to zero in the middle of each bit. • The main disadvantage is that it requires two signal changes to encode a bit and therefore occupies greater bandwidth. • Another problem is its complexity.
Figure 5 -7 RZ Encoding
Figure 5 -8 Manchester and Diff. Manchester Encodin
• Manchester encoding : the duration of bits is divided into two halves. The voltage remains at one level during the first half and moves to the other level in the second bit. • A negative to positive transition represents binary 1 and a positive to negative transition represents binary 0.
Digital to Analog Conversion • Digital data needs to be carried on an analog signal. • A carrier signal (frequency fc) performs the function of transporting the digital data in an analog waveform. • The analog carrier signal is manipulated to uniquely identify the digital data being carried. 5. 15
Figure 5. 1 Digital-to-analog conversion 5. 16
Figure 5. 2 Types of digital-to-analog conversion 5. 17
Amplitude Shift Keying (ASK) or ON-OFF Keying (OOK) • ASK is implemented by changing the amplitude of a carrier signal to reflect amplitude levels in the digital signal. • For example: a digital “ 1” could not affect the signal, whereas a digital “ 0” would, by making it zero. 5. 18
Figure 5. 3 Binary amplitude shift keying BW= (fc-Nbaud/2)+(fc-Nbaud/2) BW=(1+d) * Nbaud Note: Minimum value of d=0 (factor related to the modulation process) 5. 19
Figure 5. 4 Implementation of binary ASK 5. 20
Frequency Shift Keying • The digital data stream changes the frequency of the carrier signal, fc. • For example, a “ 1” could be represented by f 1=fc + f, and a “ 0” could be represented by f 2=fc- f. 5. 21
Figure 5. 6 Binary frequency shift keying BW= (fc 1 -fc 0)+Nbaud 5. 22
Phase Shift Keyeing • We vary the phase shift of the carrier signal to represent digital data. • PSK is much more robust than ASK as it is not that vulnerable to noise, which changes amplitude of the signal. 5. 23
Figure 5. 9 Binary phase shift keying BW= (fc-Nbaud/2)+(fc-Nbaud/2) BW=(1+d) * Nbaud Note: Minimum value of d=0 (factor related to the modulation process) 5. 24
Quadrature PSK • To increase the bit rate, we can code 2 or more bits onto one signal element. • In QPSK, we parallelize the bit stream so that every two incoming bits are split up and PSK a carrier frequency. One carrier frequency is phase shifted 90 o from the other - in quadrature. • The two PSKed signals are then added to produce one of 4 signal elements. L = 4 here. 5. 25
Figure 5. 11 QPSK and its implementation 5. 26
Note Quadrature amplitude modulation is a combination of ASK and PSK. 5. 27
Figure 8 -1 WCB/Mc. Graw-Hill Multiplexing vs. No Multiplexing The Mc. Graw-Hill Companies, Inc. , 199
Figure 8 -3 FDM WCB/Mc. Graw-Hill The Mc. Graw-Hill Companies, Inc. , 199
Figure 8 -4 WCB/Mc. Graw-Hill FDM, Time Domain The Mc. Graw-Hill Companies, Inc. , 199
Figure 8 -6 WCB/Mc. Graw-Hill Demultiplexing, Time Domain The Mc. Graw-Hill Companies, Inc. , 199
Five channels, each with a l 00 -k. Hz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 k. Hz between the channels to prevent interference?
WDM • WDM is conceptually the same as FDM, except that the multiplexing and de-multiplexing involve optical signals transmitted through fiber-optic channels. • The idea is the same: We are combining different signals of different frequencies. • Although WDM technology is very complex, the basic idea is very simple. • We want to combine multiple light sources into one single light at the multiplexer and do the reverse at the de-multiplexer. The combining and splitting of light sources are easily handled by a prism.
Figure 8 -8 WCB/Mc. Graw-Hill TDM The Mc. Graw-Hill Companies, Inc. , 199
Figure 8 -9 WCB/Mc. Graw-Hill Synchronous TDM The Mc. Graw-Hill Companies, Inc. , 199
Figure 8 -10 WCB/Mc. Graw-Hill TDM, Multiplexing The Mc. Graw-Hill Companies, Inc. , 199
Figure 8 -11 WCB/Mc. Graw-Hill TDM, Demultiplexing The Mc. Graw-Hill Companies, Inc. , 199
Interleaving • TDM can be visualized as two fast-rotating switches, one on the multiplexing side and the other on the de-multiplexing side. • The switches are synchronized and rotate at the same speed, but in opposite directions. • On the multiplexing side, as the switch opens in front of a connection, that connection has the opportunity to send a unit onto the path. This process is called interleaving. • On the de-multiplexing side, as the switch opens in front of a connection, that connection has the opportunity to receive a unit from the path.
Figure 8 -12 WCB/Mc. Graw-Hill Framing Bits The Mc. Graw-Hill Companies, Inc. , 199
• The addressing in its simplest form can be n bits to define N different output lines with n =10 g 2 N. • For example, for eight different output lines, we need a 3 bit address. • Since a slot carries both data and an address in statistical TDM, the ratio of the data size • to address size must be reasonable to make transmission efficient. For example, it • would be inefficient to send 1 bit per slot as data when the address is 3 bits. This would • mean an overhead of 300 percent. • In statistical TDM, the capacity of the link is normally less than the sum of the capacities of each channel.
Statistical TDM or Asynchronous TDM
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