An Introduction to
Computer Network
1-1 DATA COMMUNICATIONS
Data communications are the exchange of data
between two devices via some form of transmission
medium such as a wire cable.
A network is a set of devices (often referred to as
nodes) connected by communication links.
A node can be a computer, printer, or any other
device capable of sending and/or receiving data
generated by other nodes on the network.
A link can be a cable, air, optical fiber, or any
medium which can transport a signal carrying
information.
1.1.1 Components
A data communications system has five components:
1. Message
2. Sender
3. Receiver
4. Transmission Medium
5. protocols
Figure 1.1: Five components of data communication
Data Representation
Information today comes in different forms such as:
• Text
• Numbers
• Images
• Audio
• Video
Three Categories of Connecting
Devices
Hubs
• A hub is a device that operates only in the physical
layer.
• Signals that carry information within a network can
travel a fixed distance before attenuation endangers
the integrity of the data.
• A repeater receives a signal and, before it becomes
too weak or corrupted, regenerates and retimes the
original bit pattern.
Link-Layer Switches
• A link-layer switch (or switch) operates in both the
physical and the data-link layers.
• As a physical-layer device, it regenerates the signal
it receives.
• As a link-layer device, the link-layer switch can
check the MAC addresses (source and destination)
contained in the frame.
Learning switch
Routers
• We will discuss routers in Unit-3 when we discuss
the network layer. In this section, we mention routers
to compare them with a two-layer switch and a hub.
• A router is a three-layer device; it operates in the
physical, data-link, and network layers.
NETWORK MODELS
Tasks involved in sending a letter
OSI Reference Model
An exchange using the OSI model
Functions of Physical Layer
Physical characteristics
Representation of bits
Data rate
Synchronization of bits
Line Configuration
Physical topology
Transmission mode
Functions of Data Link Layer
Framing
Physical addressing
Flow control
Error control
Access control
Functions of Network Layer
Logical addressing
Routing/Switching
Source to destination connectivity
Congestion control
Fragmentation
Responsibilities of Transport Layer
End to End connectivity (Process to process
delivery of entire message) [Service Point
Addressing-SPA]
Multiplexing/De-multiplexing
Congestion control
Flow control
Error control
Segmentation and reassembly
Session, Presentation, and Application layers
The Session layer is responsible for dialog
control and synchronization.
The Presentation layer is responsible for
translation, compression, and encryption.
The Application layer is responsible for
providing services to the user.
Network virtual terminal
File transfer, access & management
Mail services
Directory services
Summary of OSI Model
TCP/IP Model
CTP [Stream Control Transmission Protocol] is a connection oriented protocol &
rovides full duplex transmission of stream.
FTP: File Transfer Protocol
SMTP: Simple Mail Transfer Protocol
DNS: Domain Name System
SNMP: Simple Network Management protocol
Signal Levels Vs Data Levels
S=N/r
Where S= signal rate, N=bit rate, r=signal
element
Information rate/bit rate/data rate
Signal rate/modulation rate/baud rate/pulse
rate
Capacity/frequency/bandwidth
Line Coding Schemes
Note
In unipolar encoding, we use only
one voltage level.
12
2
Note
In polar encoding, we use two
voltage levels: positive &
negative
12
4
• Return to Zero (RZ) is a binary bit stream encoding scheme in
which the signal returns to zero voltage in between the data bits.
• A bit 0 is represented by negative voltage and bit 1 is represented
by positive voltage.
Manchester Encoding
There are two types of conventions in Manchester encoding:
1. Dr. Thomas: In this manchester encoding 0 is
represented as low-to-high and 1 is represented as high-
to-low.
2. IEEE802.3: In this manchester encoding, 0 is
represented as high-to-low and 1 is represented as low-
to-high.
Application of Manchester Encoding:
• It is used for IR protocols, RFID, and NFC system.
• Manchester Encoding is transparent.
• There is no signal-dropping issue.
Differential Manchester
Encoding
• It is also known as the Biphase mark code, etc.
• The presence and absence of the transition indicate the
value.
• In Differential Manchester Encoding 0 should contain an
edge but 1 should not contain any edge it should be
continuous.
Manchester Encoding
13
2
Differential Manchester Encoding
13
3
Note
In Manchester and differential
Manchester encoding, the
transition
at the middle of the bit is used
for synchronization.
13
4
Note
In bipolar encoding, we use three
levels: positive, zero, and
negative.
13
7
Alternate Mark Inversion :A binary '0' is represented by no signal and a 'binary
1' as a positive or negative pulse. The pulses corresponding to the binary 1's
must have an alternating polarity.
Pseudoternary:
A binary '1' is represented by no signal and a binary '0' as a positive or negative
pulse. The pulses corresponding to the binary 0's must have an alternating
polarity.
ADVANTAGES
• There is no synchronization problem (binary 1 string in
AMI and binary 0 string in Pseudoternaries ).
• There is no continuous component (string of 1 in AMI and
string of 0 in Pseudoternaries).
• Bandwidth is lower compared to NRZ.
• Error detection is easy due to the alternation between
the pulses.
tical Fibre Transmission Mode
Source-to-destination data transfer in a circuit-
switch network
Delays in a circuit-switched network
Data transfer
Setup request in a virtual-circuit network
Setup acknowledgment in a virtual-circuit network
Delay in a virtual-circuit network
Integrated Digital Network
Benefits to Subscribers
Single access line for all services
Ability to tailor service purchased to suit
needs
Competition among equipment vendors
due to standards
Availability of competitive service providers
