Computer Networks

1
Computer Networks

• Chapter 4 - Digital Transmission

2
Bits vs. Electric Pulses

• Binary digits, 1s and 0s, generated by a computer
  are translated into a sequence of voltage pulses
  that can be propagated through the medium.

0 1 1 0 0 0 1 0
Line encoder
01100010
time
Electric pulses Two signal levels, V1 and V0
bits
3
Signal levels vs. Data levels

• A pulse can carry more then one bit
• For example with four different voltage levels,
  each can carry 2 bits

00
00
01
10
11
01
11
3V
2V
1V
0V
time
4
Pulse Rate vs. Bit Rate

• Pulse rate pulses per second
• Bit rate bits per second
• Bit rate Pulse rate x log2L
• L number of levels
• Example A signal has eight levels with pulse
  duration of 1ms.
• Pulse rate 1/10-3 1000 pulses/sec.
• Bit rate 1000xlog281000x33000 bits/sec or bps

5
Data Pulses at the Receiver

• Data pulses are attenuated and distorted during
  the transmission
• The receiver parses the voltage at the same pulse
  rate at which the sender is sending

0 1 1 0 0 0 1 0
V
Pulses sent at the sender
t
0
Time
Signal received at the receiver
0 1 ? 0 0 0 1 0
V
threshold
t
0
Time
6
Synchronization

• The sender and the receiver clock cannot be
  exactly synchronized
• Errors are made because of bad synchronization
• Solution encode the signal in such a way that
  the timing information is included in the signal
  (self-synchronizing signal)

7
Types of Digital Encoding
Digital Encoding
Polar
Unipolar
Bipolar
8
Manchester Encoding

• Binary 1 is encoded as low-to-high signal and a
  binary 0 is encoded as high-to-low signal
• The transition occurs at the center of the bit.
• This transition is used by the clock extraction
  circuit to produce a clock pulse

1
0
1
1
1
0
0
1
Bit stream and binary encoding
Help 2f wave
Manchester encoded signal
9
Differential Manchester Encoding

• Binary 1 is encoded as absence of transition at
  the begining of the bit period
• Binary 0 is encoded as presence of transition at
  the begining of the bit period
• There is always transition in the second-half of
  the period

1
0
1
1
1
0
0
1
Bit stream and binary encoding
Help 2f wave
Differential Manchester encoded signal
10
Analog-to-digital Conversion (A/D)

• Transformation of analog (continuous) signals
  into digital signals
• Required by the telephone companies for long
  distance voice transmission
• Reasons for doing this
• When amplifying analog signals, the noise built
  upon the signal is amplified, too
• Digital signals can be regenerated and the
  regenerated signals are equivalent to their
  originals

11
CODEC

• Infinite number of values of the amplitude need
  to be represented (coded) as a digital stream
  with a minimum loss of information
• The device that codes the analog signal into
  digital signal is called a coder. The device that
  performs the inverse operation is called a
  decoder. Both are usually assembled in one box
  called a codec.

V
V
Codec
t
t
CODer-DECoder
12
Sampling First step in A/D Conversion
Amplitude
Amplitude
Time
Time
Analog signal
Samples from the analog signal
13
Quantization

• A method of assigning sign and magnitude values
  to quantized samples. Each value is translated
  into its seven-bit binary equivalent

01100100
102
00011000
024
01101110
110
00100110
038
125
125 100 75 50 25 0
01111101
124
00110000
048
110
105
102
88
01101001
105
00100111
39
48
01011000
85
00011010
26
38
39
26
24
10001111
-015
1101000
-80
-25 -50 -75 -100 -125
-15
10110010
-50
-50
Sign bit is 0 , - is 1
-80
14
Sampling Rate

• Nyquist theorem
• The sampling rate must be at least twice the
  highest frequency of the analog signal
• In that case only the analog signal can be
  reconstructed back from the digital signal
• The theorem is discovered in the early 20th
  century, but it was used after more than half a
  century

15
Pulse Code Modulation (PCM)

• The process of sampling, quantization, binary
  encoding and digita-to digital encoding is known
  as pulse code modulation
• PCM is used to transmit voice signals
• Voice data (phone conversation) is limited to
  below 4000Hz
• Require 8000 sample per second
• Each analog sample is assigned a digital value

16
Transmission Modes
Transmission Modes
Used in Data Communication
Parallel
Serial
Used for communication with computer peripherals
Synchronous
Asynchronous
17
Parallel Transmission
Sender
Reciever
0 1 1 0 0 0 1 0
Eight lines are needed
18
Serial Transmission
The eight bits are sent one after another
Sender
Receiver
0 1 1 0 0 0 1 0
0 1 1 0 0 0 1 0
0 1 1 0 0 0 1 0
Only one line is needed
Parallel/serial conventor
Serial/parallel conventor
19
Synchronous vs. Asynchronous Transmission

• Asynchronous transmission (computer
  communication)
• The start bit is inserted before each byte and
  stop bit is inserted at the end
• Asynchronous here means asynchronous at the
  byte level, but there is still a clock for the
  bits (the bits are still synchronous)
• Synchronous transmission (digitized voice
  transmission)
• Bits are sent one after the other without the
  start and the stop bit

20
Simplex, Half-duplex, Duplex

• Simplex channel
• The transmission is only in one direction
• Half-duplex channel
• The transmission is in both directions, but only
  one at a time (both directions cannot be used at
  the same time)
• Duplex
• The transmission is in both directions without
  limitation