2. Digital Transmission
A computer network is designed to send information from one point to another .
This information needs to be converted to either a digital signal (digital transmission) or an analog
signal (analog transmission).
In Digital transmission:(1) methods which convert digital data to digital signals (digital-to-digital
conversion) and(2) methods which convert analog signals to digital signals (analog-to-digital
conversion)
4. Analog-to Digital Conversion
The techniques described earlier convert digital data to digital signals. Sometimes,
however, we have an analog signal such as one created by a microphone or camera .
Since digital signal is superior to analog signal in processing, the tendency today is to
change analog signal to digital signal .
The most common technique to change an analog signal to digital signal is called pulse
code modulation (PCM).
5. PCM
A PCM encoder has three processes:
1. Sampling: The analog signal is sampled
2. Quantization: The sampled signal is quantized
3. Encoding: The quantized values are encoded as stream of bits.
7. sampling
Line coding and block coding can be used to convert binary data to a digital signal .
If we want to store voice recording in the computer or send it digitally, we need to change it through a process called
sampling.
8. Analog Transmission
To send the digital data over an analog media, it needs to be converted into analog signal . There
can be two cases according to data formatting.
Bandpass: The filters are used to filter and pass frequencies of interest. A bandpass is a band of
frequencies which can pass the filter.
Low-pass: Low-pass is a filter that passes low frequencies signals.
When digital data is converted into a bandpass analog signal, it is called digital-to-analog
conversion. When low-pass analog signal is converted into bandpass analog signal, it is called
analog-to-analog conversion.
9. Digital-to-Analog Conversion
When data from one computer is sent to another via some analog carrier, it is first converted into
analog signals. Analog signals are modified to reflect digital data.
An analog signal is characterized by its amplitude, frequency, and phase. There are three kinds of
digital-to-analog conversions:
10. Amplitude Shift Keying
1)In this conversion technique, the amplitude of analog carrier signal is modified to reflect binary data.
2)When binary data represents digit 1, the amplitude is held; otherwise it is set to 0. Both frequency
and phase remain same as in the original carrier signal.
11. Frequency Shift Keying
In this conversion technique, the frequency of the analog carrier signal is modified to reflect
binary data.
This technique uses two frequencies, f1 and f2. One of them, for example f1, is chosen to
represent binary digit 1 and the other one is used to represent binary digit 0. Both amplitude and
phase of the carrier wave are kept intact.
12. Phase Shift Keying
In this conversion scheme, the phase of the original carrier signal is altered to reflect the binary
data.
When a new binary symbol is encountered, the phase of the signal is altered. Amplitude and
frequency of the original carrier signal is kept intact.
13. Analog-to-Analog Conversion
Analog signals are modified to represent analog data. This conversion is also known as Analog
Modulation. Analog modulation is required when bandpass is used. Analog to analog conversion
can be done in three ways:
14. Amplitude Modulation
In this modulation, the amplitude of the carrier signal is modified to reflect the analog data.
Amplitude modulation is implemented by means of a multiplier. The amplitude of modulating
signal (analog data) is multiplied by the amplitude of carrier frequency, which then reflects analog
data.
The frequency and phase of carrier signal remain unchanged.
16. Frequency Modulation
In this modulation technique, the frequency of the carrier signal is modified to reflect the change
in the voltage levels of the modulating signal (analog data).
17. Phase Modulation
In the modulation technique, the phase of carrier signal is modulated in order to reflect the
change in voltage (amplitude) of analog data signal
Phase modulation is practically similar to Frequency Modulation, but in Phase modulation
frequency of the carrier signal is not increased. Frequency of carrier is signal is changed (made
dense and sparse) to reflect voltage change in the amplitude of modulating signal.
20. Meaning of Topologies
Topology is a way of connecting devices with each other either physically or
logical. Two or more devices make link and two or more links form a topology.
It is basically a geometrical representation of how network is laid out.
Topology is way of connecting several devices with each other on a network.
21. Types of Topology
BUS TOPOLOGY
MESH TOPOLOGY
STAR TOPOLOGY
RING TOLPOLOGY
TREE TOPOLOGY
23. Bus Topology
Bus topology consists of a single cable called a backbone—
connecting all nodes on a network without intervening connectivity
devices
24. Bus
Advantages
Works well for small networks
Easy to install
Relatively inexpensive to implement
Disadvantage
Management costs can be high
Network disruption when computers are
added or removed
A break in the cable will prevent all
systems from accessing the network.
Difficult to troubleshoot
26. Ring Topology
Each node is connected to the two nearest nodes so the entire
network forms a circle
One method for passing data on ring networks is token passing
Data travels around the network
Traffic flows in one direction
Slow performance
One workstation goes down; whole network goes down
Network is highly dependent
27. Ring
Advantages
Cable faults are easily located, making
troubleshooting easier
Ring networks are moderately easy to
install
Disadvantage
Expensive
Requires more cable and network
equipment at the start
Expansion to the network can cause
network disruption
A single break in the cable can disrupt the
entire network
28. Star topology
Every node on the network is connected through
a central device called hub or switch.
29. Star (continued)
Star topology send and receive data indirectly , the data passes to and from the
hub.
If the data is required to be sent from one computer and received by another
computer, then this can be accomplished only by the central controller as a]each
data is first sent to the hub, which then relays that data to the destination
computer.
31. Star
Advantages
Good option for modern networks
Low startup costs
Easy to manage
Offers opportunities for expansion
Disadvantage
Hub is a single point of failure
Requires more cable than the bus
33. Mesh Topology
Mesh Topology: Each computer connects to every other.
High level of redundancy.
Rarely used.
Wiring is very complicated
Cabling cost is high
Troubleshooting a failed cable is tricky
34. Mesh
Advantages
Robust
There is the advantage of privacy or
security
The network can be expanded
without disruption to current uses
Disadvantage
Requires more cable than the other
topologies
Complicated implementation
Installation and reconnection are
difficult.
Sheer bulk of wiring can be greater
than the available space can
accommodate
Expensive
35. Tree Topology
• Hybrid Topology is Tree topology
• Tree topology is a combination of Bus and Star topology.
• It consists of groups of star-configured workstations connected
to a linear bus backbone cable.
• If the backbone line breaks, the entire segment goes down
• An example of this network could be cable TV technology
36. Tree
Advantages
• Point to point wiring for individual
segment
• Error detection and correction is
easy
• Expansion of the network is
possible and easy
• If one segment is damaged other
segment are not be affected
Disadvantage
• It requires more cabling
• If the backbone link break, the entire
segment totally goes down.
• As more and more nodes and segment
are needed, the maintenance becomes
difficult
• Heavily traffics cause the network and
make it too slow
