2.pptx

Data Communication
Chapter Two

Contents
• Data communication
• Computer network

 Communication is the process of sending and receiving
messages to share meanings.
 Data communications are the exchange of data between
two devices via some form of transmission medium
(channels).
 It needs communicating devices as hardware (physical
equipment) and software (programs) for the occurrence
of data communications.
What is data communication?

Cont’d…
For Data Communications to occur, the communicating
devices must be part of a communication system made
up of combining hardware and software.
The effectiveness of data communication system
depends on four fundamental characteristics:
-Delivery,
-Accuracy,
-Timeliness, and
- Jitter.

Delivery:
 The system must deliver data to the correct destination.
 Data must be received by the intended device or user
and only by that device or user.
Accuracy:
 The system must deliver data accurately without
modification of the original data.
 Data that have been altered in transmission and left
uncorrected are unusable.

Timeliness: Is information delivered in timely
fashion?
 The system must deliver data in timely manner .
 Data that delivered late/delayed are useless.
Jitter: Is information delivered in sufficiently smooth
fashion?
 Refers to the variation in the packet arrival time.
 It is uneven delay in the delivery of audio or video
packets.
For example: Let us assume that video packets sent every
30ms. If some of packets arrive with 30ms delay and
others 40ms delay, uneven quality in the video is the
result.

Communication Types
Unicasting (one-to-one)
Multicasting (one-to-many)
Broadcasting (one-to-all)

Components of Data Communications
A data communications system has five components
1. Message: is the information (data) to be
communicated. Popular forms of information include
text, numbers, pictures, audio, and video.
2. Sender: is the device that sends the data message. It
can be a computer, workstation, telephone handset,
video camera, and so on.
3. Receiver: is the device that receives the message.

4. Transmission medium:
It is physical path in which a message travels from
sender to receiver.
Can be both guided and unguided.
5. Protocol: is a set of rules that govern data
communications.
It represents an agreement between the
communicating devices.
Without a protocol, two devices may be connected
but not communicating, just as a person speaking
English cannot be understood by a person who speaks
only Amharic.

Data Transmission
 It is sending of binary data from source to
destination as bit by bit or grouping bits together.
The transmission of binary data across a link
(communication channel) can be either parallel or
serial mode.
 In parallel mode multiple bits are sent together each
clock, where as
 In serial mode, 1 bit is sent with each clock tick.

Parallel Transmission
Multiple signals are sent in parallel
Cannot travel long distances
Example: printer connections

Serial Data transmission
 Signals are sent one bit at a time
 Travels long distances
Example: telephone wires

 Receiver waits ready for sender’s message and
responds in real time (e.g. phone call, video-
conferencing, Chats).
1. Synchronous Data Transmission
The transmission is full-duplex type and is more reliable
and efficient than asynchronous to transfer large amount
of data.
So, useful for high-speed applications and does not use
stop/start bits – instead devices agree on timing.

2. Asynchronous Data Transmission
Both sender and receiver require no clock signals.
It is half-duplex type of transmission.
Have added parity bits (which called start and stop bit)
for synchronize clock signals b/n sender and receiver.
There is gap b/n frames or group of bits (bytes) by start
and stop bits
(e.g. Email, Letters).

Analog and Digital Data Transmission

I. Digital Data Transmission
1. Digital data
Produced by computers, in binary form, represented
as a series of ones and zeros
Can take only 0 and 1
2. Digital transmissions
Made of square waves with a clear beginning and
ending
Computer networks send digital data using digital
transmissions.

II. Analog Data Transmission
1. Analog data
Produced by telephones
Sound waves, which vary continuously over time
Can take on any value in a wide range of possibilities
2. Analog transmissions
Analog data transmitted in analog form (vary
continuously)
Examples of analog data being sent using analog
transmissions are broadcast TV and radio.

a) Simplex mode:
In simplex mode, communication is unidirectional.
Only one of the devices on a link can transmit, the other
can only receive.
Example: Radio and TV.
b) Half-duplex mode:
In half-duplex mode, each station can both transmit and
receive, but not at the same time.
When one device is sending, the other can only receive
and vice versa. Example: walkie-talkies for soldiers
It used when no need for communication in both
direction at the same time.

c) Full-duplex mode:
In full-duplex mode, both stations can transmit
and receive simultaneously.
Example: telephone network
When people are communicating by a
telephone line, both can talk and listen at the
same time.

Transmission Impairments
 Signals travel through transmission media, which are
some time 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.
With any communications system, the signal that is
received may differ from the signal that is transmitted,
due to transmission impairments.

Consequences over two types of signals :
For analog signals: degradation of signal quality
For digital signals: bit errors
The most significant impairments include
Attenuation
Distortion
Noise

1. Attenuation
 Attenuation means a loss of energy.
 When a signal travels through a medium, it loses some
of its energy in overcoming the resistance of the
medium.
 Some of the electrical energy in the signal is converted
to heat.
 This conversion of electrical energy causes loos of
signal strength (attenuation).

Measurement of Attenuation
 To show the loss or gain of energy the unit
“decibel” is used.
dB = 10log10P2/P1
Where as
P1 is input signal and P2 is output signal
 These problems are dealt with by the use of
amplifiers or repeaters by amplifying the signal.

 Suppose a signal travels through a transmission
medium and its power is reduced to one-half. This
means that P2 is (1/2)P1.
 In this case, the attenuation (loss of power) can be
calculated as
A loss of 3 dB (–3 dB) is equivalent to losing one-half
the power.
Example 1 of Attenuation…

A signal travels through an amplifier, and its power is
increased 10 times. This means that P2 = 10P1 .
Then compute the amplification or gain of power by
amplifier.
Solution:
Example 2 of Attenuation…

2. Distortion
 Distortion means that the signal changes its form or
shape.
 Each frequency component has its own propagation
speed traveling through a medium.
 The different components therefore arrive with
different delays at the receiver.
 That means that the signals have different phases at
the receiver than they did at the source.
Reading Assignment: what is the solution for Distortion?

Additional signals inserted between transmitter and
receiver.
Noise is any electrical energy on the transmission
cable that makes it difficult for a receiver to interpret
the data sent from the transmitter.
Noise is a major limiting factor in communications
system performance.
3. Noise

Noise - solutions
 Twisting cables – effect of one signal cancels the
other.
Shielding – reduce interference from outside
source.

Network Devices and Their Role

1. Hub
Hubs are Layer-1(physical layer) devices that physically
connect network devices together for communication.
It is a simple network device and sometimes it is called
Repeater.
if two devices transmit a frame simultaneously, a collision
will occur.

As an active hubs regenerate signals, it increases the
distance that can be spanned by the LAN (up to 100
meters per segment).
Hubs can also be connected locally to a maximum of
two other hubs, thereby increasing the number of
devices
Active hubs are usually used against attenuation, which
is a decrease in the strength of the signal over distance.
Advantages of Hub

Limitation of Hub
Hubs cannot filter data: i.e. data packets are sent to
all connected devices/computers and do not have
intelligence to find out best path for data packets.
This leads to inefficiencies and wastage.
Any data packet coming from one port is sent to all
other ports. It is then up to the receiving computer
to decide if the packet is for it.

2. Switch
Switches, like hubs, provide a centralized connection.
In networks, it is a device that filters and
forwards packets between LAN segments.
Switches can support both layer 2 (based on MAC
Address) and layer 3 (based on IP address) depending on
the type of switch(example: core switch or multi-layer
switch).
A network Bridge connects two local area networks while
Switch connects multiple clients to a network.

 Rather than forwarding data to all the connected ports, a
switch forwards data only to the port on which the
destination system is connected.
 It looks at the Media Access Control (MAC) addresses of
the devices connected to it to determine the correct
port.
 A MAC address is a unique number that is stamped into
every NIC.
Switch…

3. Repeaters
 It operates on physical layer (i.e. layer 1) of the OSI
Reference Model.
 Two, or more, such LANs could be interconnected to
extend the length limitation by means of a device
known as a repeater.
 It is primarily concerned with the transmission of
electrical signals at bit level.

4. Bridge
 Bridges are used to connect separate LANs together.
 It performs ‘filtering’ function.
 All other intra-network frames are rejected by the
bridge.

Bridge…
The function of the bridge is to make intelligent
decisions about whether or not to pass signals to
the next segment of a network.
When a bridge sees a frame on the network, it
looks at the destination MAC address and
compares it to the forwarding table to determine
whether to filter, flood, or copy the frame onto
another segment.
A bridge only has one incoming and one outgoing
port.
Operate at data link layer of network

5. Router
Routers operate at the Network layer of the OSI model.
They are slower than bridges and switches but make
"smart" decisions on how to route packets received on one
port to a network on another port.
A router, like a switch forwards packets based on address.
Usually, routers use the IP address to forward packets,
which allows the network to go across different protocols.

Routers support different WAN technologies but
switches do not.
Networks served by a router are not required to use
the same protocol.
Routers are frequently used to place additional
security on sensitive networked resources.
Routers are located at gateways, the places where
two or more networks connect.

6. Gateway
Gateways serve as an entry and exit point for a
network as all data must pass through or
communicate with the gateway prior to being
routed.

Cont…
It serves as a single access point and a converter to
connect dissimilar networks using different protocols
and different models(i.e OSI and TCP/IP models).
A gateway is a computer that operates in all seven
layers of the OSI model.
A gateway takes an application message, reads it, and
interprets it.

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