Analog data, digital signal

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Transcript Analog data, digital signal

Chap. 3 Signalling and topology
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Encoding and modulation
• Digital data, digital signal
 Encoding
• Analog data, digital signal
 sampling to digital data, and then encoding to digital signal
• Digital or analog data, analog signal
 Using carrier signal
digital signal
digital or
analog data
encoder
decoder
digital or
analog data
demodulator
digital or
analog data
analog signal
digital or
analog data
modulator
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Parameter for encoding
• Signal detection of receiver
 Level detection
 Edge detection
data
0
0
1
1
data
(a)
0
0
1
1
(b)
• Timing
 Start time and end time of a bit
 Transmitter and receiver have same information for bit timing and character (or frame)
timing
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• Synchronization
 If many 1s are transmitted, receiver does not detect exact number of 1, because of
propagation delay, jitter, delay distortion, etc.
 Hence, synchronization of transmitter and receiver is necessary
1
1
1
1
1
transmitted
signal
received
signal
propagation time
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1
1
1
1
0
 Synchronization method
 Using start bit
 voltage varies 1 to 0 (0 -> 5V transition), when digital signal is started
 Receiver detects transition of start bit, and then receive several bits
 ex. asynchronous transmission
 Using transition per each bit
 voltage varies high to low, or low to high at mid of bit
 Receiver detects transition of each bit, then judges 1 or 0
 ex. Manchester code for IEEE 802.3
 Using separate clock line
 Transmitter sends clock signal via clock line, and then sends signal via data line
 Receiver detects clock signal via clock line, and then receives data signal
 ex. SPI communication for connecting microcontroller
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• dc component
 dc component is avoided for encoding of digital signal
 dc component : long period of high signal (+5V or -5V)
 dc component causes loss of synchronization
0
100 200 300 400 500
600 700 800 900 1000 1100
transmitted
signal
0
95 190 285 380 475 570 665 760 855 950 1045 1140
received
signal
propagation time
bit reception error
due to timing
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• Error detection
 Any signal can detects error
 Differential encoding
 transition at mid of bit
 If transition is not, this bit is error
 Robust to noise
• Noise immunity
 Robust to noise
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Encoding digital data to digital signal
• NRZ-L (Nonreturn to Zero)
 Two different voltages for 0 and 1 bits
 Voltage constant during bit interval
 no transition
 ex. Absence of voltage for zero, constant positive voltage for one
 More often, negative voltage for one value and positive for the other
data
0
0
1
1
0
5V
NRZ-L
0V
8
0
1
0
1
0
• NRZI (Nonreturn to Zero, invert on ones)






Nonreturn to zero inverted on ones
Constant voltage pulse for duration of bit
Data encoded as presence or absence of signal transition at beginning of bit time
Transition (low to high or high to low) denotes a binary 1
No transition denotes binary 0
An example of differential encoding
data
0
0
1
1
0
5V
NRZI
0V
9
0
1
0
1
0
• Manchester





Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
data
0
0
1
1
5V
Manchester
0V
10
0
0
1
0
1
0
• CAN (controller area network)





Used two transmission medium with similar NRZ-L
For example, one medium uses 1.5V or 2.5V, the other uses 3.5V or 2.5V
If voltage between two medium is positive, signal denotes 0
If voltage between two medium is 0, signal denotes 1
CAN signaling has noise immunity
data
0
0
1
1
0
3.5V
CAN 2.5V
1.5V
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0
1
0
1
0
• Bit stuffing
 Solution for dc component in NRZ-L
 ex. CAN’s bit stuffing
 If five bits of 1 is succeed, stuffed bit of 0 is inserted by transmitter
original data
1
1
1
1
1
1
1
1
1
0
stuffed data
1
1
1
1
1
0
1
1
1
1
transmitted
signal
stuffed bit
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0
Encoding analog data to digital signal
• Digitization
 Analog data firstly converts to digital data, and then is encoded to digital signal
 Used digitizer such as ADC (analog to digital converter)
5V
digitizer
analog data
1001001
modem
0V
digital data
analog signal
5V
transceiver
0V
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digital signal
• PCM (pulse code modulation)
 Sampling theory
 If a signal is sampled at regular intervals at a rate higher than twice the highest signal
frequency, the samples contain all the information of the original signal
 Frequency band of voice : 100 Hz – 7KHz
 But voice data limited to below 300 – 3400Hz
 Require 8,000 sample per second
 Procedure
 PAM (pulse amplitude modulation) : real number is sampled per sampling period
 Quantizing : real number value is converted to integer
 Digitizing : integer is converted binary number
 Ex. temperature
 Restoration
 ZOH (zero order holder), FOH (first order holder)
 Restored data is different to original data
 Consideration for PCM
 Sampling frequency
 n value for digitization
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restored data
original data
analog data
100℃
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14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0℃
PAM sampled
0.0
data
quantized data
digitized data
0
7.5
9.2
7.0
8.5
11.9
10.1
7.5
14.3
6.1
7
9
7
8
11
10
7
14
6
0000 0111 1001 0111 1000 1011 1010 0111 1110
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0110
Simplex, half duplex, full duplex
• Simplex
 One direction
 ex. Television, radio
• Half duplex
 Either direction, but only one way at a time
 ex. CAN, RS485, Police radio
• Full duplex
 Both directions at the same time
 ex. RS232, IEEE 802.3, telephone
• Medium access control method is necessary
 If several nodes are transmitted, several signals are mixed.
 Only one node must send at one time for successful transmission
Rx
Tx
Tx & Rx
Rx
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Direct, point-to-point, multi-point link
• Direct link
Tx
 No intermediate devices
• Point-to-point link




Rx
Used direct link
Only 2 devices share link
But one point or nodes are shared
ex. IEEE 802.3 Ethernet
• Multi-point link
 More than two devices share the link using tap
 ex. CAN
 Multi-drop link
 Used direct link between adjacent two nodes
 But, nodes existed between transmitter and receiver are shared
 ex. RS485, CAN without tap
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• Topology
 Physical arrangement of cabling between nodes
• Types of topology
 Mesh, Bus, Tree, Ring, Star
• Mesh





Point-to-point between two nodes
n(n-1)/2 channel is necessary
topology for wireless network
Not used for wired network
Ex.IEEE 802.15.4 WPAN
flow of data
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• Bus and tree
 Transmission propagates throughout medium
 Heard by all stations
 Need to identify target station
 Each station has unique address
 Full duplex connection between station and tap
 Allows for transmission and reception
 Need to regulate transmission
 To avoid collisions
 To avoid hogging
 Data in small blocks - frames
 Terminator absorbs frames at end of medium
terminator
flow of data
headend
tap
flow of data
terminator
tap
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• Star
 Each station connected directly to central node
 Usually via two point to point links
 Hub
 Physical star or logical bus
 Transmitter nodes can send, then hub broadcast to all nodes
 Switch
 Physical star, and logical star
 Transmitter can send specific node by switch
hub
switch
flow of data
flow of data
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• Ring
 Repeaters joined by point to point links in closed loop
 Receive data on one link and retransmit on another
 Links unidirectional
 Stations attach to repeaters
 Data in frames
 Circulate past all stations
 Destination recognizes address and copies frame
 Frame circulates back to source where it is removed
 Media access control determines
when station can insert frame
switch
repeater
flow of data
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Consideration of topology
• Problem for transmission
 Data is broadcasted, then address of each node is necessary
 A node watches whether other nodes are transmitted
 Only one node must be transmitted
• Segmentation
 Avoid data hogging
 One node transmits only one frame, and the waits some interval
 The other nodes can be transmitted during some interval
• Addressing
 Distinguish each nodes
 Insert source address and destination address to frame header
• Medium access control
 Only one node can be transmitted by using MAC
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