DATA COMMUNICATIONS

1
DATA COMMUNICATIONS

• Telecommunication communication at a distance.
• Data information presented in whatever form is
  agreed upon by the parties creating and using the
  data.
• Data communications the exchange of data between
  two devices via some form of transmission medium
  such as a wire cable.

2
1-1 DATA COMMUNICATIONS

• Four fundamental characteristics
• Delivery correct destination
• Accuracy correct data
• Timeliness fast enough
• Jitter uneven delay

3
Components
Figure 1.1 Five components of data communication
4
Data Representation

• Text
• Email, articles, etc
• Coding (Unicode, ASCII)
• Numbers
• Direct conversion
• Images
• Pixels, resolution, gray scale, RGB, YCM
• Audio
• Continuous, signal conversion
• Video
• Movie, continuous/discrete

5
Data Flow (Transmission Modes)
Figure 1.2 Data flow (simplex, half-duplex, and
full-duplex)
6
NETWORKS
A network is a set of devices (often referred to
as nodes) connected by communication links.
7
Type of Connections (line configurations)
Figure 1.3 Types of connections point-to-point
and multipoint
8
Physical topology
Figure 1.4 Categories of physical topology
9
Mesh
1
2
4
5
3
Figure 1.5 A fully connected mesh topology (five
devices)
10
Star
Figure 1.6 A star topology connecting four
stations
11
Bus
Figure 1.7 A bus topology connecting three
stations
12
Ring
Figure 1.8 A ring topology connecting six
stations
13
Network models

• Local area network (LAN)
• Wide area network (WAN)
• Metropolitan area networks

14
Figure 1.12 A heterogeneous network made of four
WANs and two LANs
15
THE INTERNET

• History of the Internet
• ARPA
• ARPANET
• Transmission control Protocol (TCP)
• Internetworking Protocol (IP)
• Internet today
• Internet service providers (ISPs)

16
1-4 PROTOCOLS AND STANDARDS

• Protocols (rules)
• Why do we need protocols?
• Key elements of protocols
• Syntax
• Semantics
• Timing
• Standards
• De facto vs. De jure
• Organizations
• Internet standards (Internet draft RFC)

17
Chapter 2 Network Models
18
Figure 2.2 Seven layers of the OSI model
19
Figure 2.3 The interaction between layers in the
OSI model
User support layers
Network support layers
20
Figure 2.4 An exchange using the OSI model
21
LAYERS IN THE OSI MODEL
Figure 2.5 Physical layer
The physical layer is responsible for movements
of individual bits from one hop (node) to the
next.
22
Figure 2.6 Data link layer
The data link layer is responsible for moving
frames from one hop (node) to the next.
23
Figure 2.8 Network layer
The network layer is responsible for the delivery
of individual packets from the source host to the
destination host.
24
Figure 2.10 Transport layer
The transport layer is responsible for the
delivery of a message from one process to
another.
25
Figure 2.12 Session layer
The session layer is responsible for dialog
control and synchronization.
26
Figure 2.13 Presentation layer
The presentation layer is responsible for
translation, compression, and encryption.
27
Figure 2.14 Application layer
The application layer is responsible for
providing services to the user.
28
Figure 2.15 Summary of layers
29
TCP/IP PROTOCOL SUITE

1. The layers in the TCP/IP protocol suite do not
   exactly match those in the OSI model. The
   original TCP/IP protocol suite was defined as
   having four layers host-to-network, internet,
   transport, and application.
2. However, when TCP/IP is compared to OSI, we can
   say that the TCP/IP protocol suite is made of
   five layers physical, data link, network,
   transport, and application.
3. Topics covered
4. Physical and Data Link Layers
5. Network Layer
6. Transport Layer
7. Application Layer

30
Figure 2.16 TCP/IP and OSI model
31
Figure 2.18 Relationship of layers and addresses
in TCP/IP
32
Chapter 3 Data and Signals
33
ANALOG AND DIGITAL
To be transmitted, data must be transformed to
electromagnetic signals. Data can be analog or
digital. Analog data refers to information that
is continuous digital data refers to information
that has discrete states. Analog data take on
continuous values. Digital data take on discrete
values. Signals can be analog or digital. Analog
signals can have an infinite number of values in
a range digital signals can have only a limited
number of values.
Topics discussed in this section
Analog and Digital DataAnalog and Digital
SignalsPeriodic and Nonperiodic Signals
34
Figure 3.1 Comparison of analog and digital
signals
35
In data communications, we commonly use periodic
analog signals and nonperiodic digital signals.
Periodic signals repeat patterns
Nonperiodic signals no patterns
36
PERIODIC ANALOG SIGNALS

• Periodic analog signals can be classified as
  simple or composite.
• A simple periodic analog signal, a sine wave,
  cannot be decomposed into simpler signals.
• A composite periodic analog signal is composed of
  multiple sine waves.

Topics discussed in this section
Sine WaveWavelengthTime and Frequency
DomainComposite Signals Bandwidth
37
Figure 3.2 A sine wave
Peak amplitude
Frequency
Phase
38
Frequency and period are the inverse of each
other.
39
Table 3.1 Units of period and frequency
40
Frequency is the rate of change with respect to
time. Change in a short span of time means high
frequency. Change over a long span of time means
low frequency.
If a signal does not change at all, its frequency
is zero. If a signal changes instantaneously, its
frequency is infinite.
41
Figure 3.5 Three sine waves with the same
amplitude and frequency,
but different phases
42
Figure 3.6 Wavelength and period
Wavelength is the distance a simple signal can
travel in one period. Wavelength (w) signal
velocity x period Recall period 1 / frequency
43
Figure 3.7 The time-domain and frequency-domain
plots of a sine wave
A complete sine wave in the time domain can be
represented by one single spike in the frequency
domain.
44
Figure 3.8 The time domain and frequency domain
of three sine waves
The frequency domain is more compact and useful
when we are dealing with more than one sine wave.
45
Sine Waves and Composite Signals

• A single-frequency sine wave is not useful in
  data communications we need to send a composite
  signal.
• A composite signal is made of many simple sine
  waves.
• According to Fourier analysis, any composite
  signal is a combination of simple sine waves with
  different frequencies, amplitudes, and phases.
• If the composite signal is periodic, the
  decomposition gives a series of signals with
  discrete frequencies if the composite signal is
  nonperiodic, the decomposition gives a
  combination of sine waves with continuous
  frequencies.

46
Figure 3.12 The bandwidth of periodic and
nonperiodic composite signals
47
DIGITAL SIGNALS
In addition to being represented by an analog
signal, information can also be represented by a
digital signal. For example, a 1 can be encoded
as a positive voltage and a 0 as zero voltage. A
digital signal can have more than two levels. In
this case, we can send more than 1 bit for each
level.
48
Frequency Vs. Bit Rate
Frequency the number of periods in 1s.
Bit rate the number of bits sent in 1s,
expressed in bits per second (bps).
49
Figure 3.16 Two digital signals one with two
signal levels and the other
with four signal levels
50
Wavelength Vs. Bit length
Wavelength is the distance an analog signal can
travel in one period.
Bit length is the distance one bit occupies on
the transmission medium.
51
Figure 3.17 The time and frequency domains of
periodic and nonperiodic
digital signals
A digital signal is a composite analog signal
with an infinite bandwidth.
52
Figure 3.18 Baseband transmission
Baseband transmission sending a digital signal
over a channel without changing the digital
signal to an analog signal.
53
Figure 3.19 Bandwidths of two low-pass channels
54
Figure 3.20 Baseband transmission using a
dedicated medium
Baseband transmission of a digital signal that
preserves the shape of the digital signal is
possible only if we have a low-pass channel with
an infinite or very wide bandwidth.
55
Figure 3.21 Rough approximation of a digital
signal using the first harmonic
for worst case
56
Figure 3.22 Simulating a digital signal with
first three harmonics
57
Table 3.2 Bandwidth requirements
58
Figure 3.23 Bandwidth of a bandpass channel
Broadband transmission changing the digital
signal to an analog signal for transmission. Band
pass channel a channel with a bandwidth that
does not start from zero.
59
TRANSMISSION IMPAIRMENT
Signals travel through transmission media, which
are not perfect. The imperfection causes signal
impairment. This means that the signal at the
beginning of the medium is not the same as the
signal at the end of the medium. What is sent is
not what is received. Three causes of impairment
are attenuation, distortion, and noise.
60
Figure 3.26 Attenuation
Decibel measures the relative strengths of two
signals or one signal at two different points.
Its negative if a signal is
attenuated and positive if a signal is amplified.
61
Example 3.29
Sometimes the decibel is used to measure signal
power in milliwatts. In this case, it is referred
to as dBm and is calculated as dBm 10 log10 Pm
, where Pm is the power in milliwatts.
62
Figure 3.28 Distortion
63
Figure 3.29 Noise

• SNR average signal power/average noise power
• Thermal
• Induced
• Crosstalk
• Impulse

64
Example 3.32
The values of SNR and SNRdB for a noiseless
channel are
We can never achieve this ratio in real life it
is an ideal.
65
Figure 3.30 Two cases of SNR a high SNR and a
low SNR
66
DATA RATE LIMITS
A very important consideration in data
communications is how fast we can send data, in
bits per second, over a channel. Data rate
depends on three factors 1. The bandwidth
available 2. The level of the signals we use
3. The quality of the channel (the level of
noise)
Topics discussed in this section
Noiseless Channel Nyquist Bit RateNoisy
Channel Shannon CapacityUsing Both Limits
67
Nyquist bit rate 2 x bandwidth x log2L
Increasing the levels of a signal may reduce the
reliability of the system.
68
In reality we can not have a noiseless
channel. Shannon capacity bandwidth x
log2(1SNR)
69
Example 3.40
For practical purposes, when the SNR is very
high, we can assume that SNR 1 is almost the
same as SNR. In these cases, the theoretical
channel capacity can be simplified to
For example, we can calculate the theoretical
capacity of the previous example as
70
The Shannon capacity gives us the upper limit
the Nyquist formula tells us how many signal
levels we need.
71
PERFORMANCE
BandwidthThroughputLatency (Delay) Bandwidth-Del
ay Product
72
In networking, we use the term bandwidth in two
contexts. ? The first, bandwidth in hertz, refers
to the range of frequencies in a
composite signal or the range of
frequencies that a channel can pass. ? The
second, bandwidth in bits per second,
refers to the speed of bit transmission in
a channel or link.
73

• Latency (Delay)
• Propagation time
• Transmission time
• Queuing time
• Processing delay