Copyright © 2003 by Prentice Hall 2
Communication Model

3
Communication Model
•Source
Generates data to be transmitted
• Transmitter
Converts data into transmittable signals
• Transmission System
Carries data
• Receiver
Converts received signal into data
• Destination
Takes incoming data

4
Communication over a point-to-point link:
A simplified model
Generate
Data
Data to
Signals
Signals to
Data
Receive
Data

5
Simplified Communications Model
 Source (e.g. PC)
 Generates data to be transmitted
 Transmitter
 Converts data into transmittable signals (modulation, encoding)
 Transmission System (medium + equipment)
 Carries signals, but introduces attenuation, noise, interference, etc.
 Receiver
 Converts received signals into data (demodulation, decoding)
 Destination
 Takes and uses incoming data
SignalData
1101...
Data
1101…
Noise, Distortion
Interference
Attenuation

6
Interfacing Addressing
Signal generation Routing
Synchronization Recovery
Exchange management: Message formatting
Error detection and correction Security
Error control Network management
Flow control Transmission system utilization
= Tasks covered in some detail in this course
SignalData
1101...
Data
1101…
Noise, Distortion
Interference
Attenuation
Tasks covered in some detail in this course

7
Simplified Communications Model
Speech, Speech,

1- Data communications
 Data communication : Is the exchange of
data between devices via some form of
transmission medium such as a wire cable .
 For data communication to occur , the
communicating devices must be part of
communication system made up of a
combination of
1- Hardware ( physical equipment )
2- Software (programs)

 Block diagram of data communication system
Sender Receiver
Massage
Protocol
Rule 1
Rule 2
Rule 3
Rule 1
Rule 2
Rule 3
Protocol
Medium

Component of Data
communications system
 Data communications system has five components:
1- Message : is the information (data) to be
communicated such as ( text – picture – audio –
video) .
2- Sender : the device that sends the data massage
such as ( computer – workstation – telephone –
video camera )
3-Receiver : is the device that receives the massage
such as ( computer – workstation – telephone –
video camera )

4- Transmission medium : is the physical path
by which a massage travels from sender to
receiver such as ( Twisted pair wire – Coaxial
cable – fiber optic cable - Radio waves)
5- Protocol : is a set of rules that govern data
communications. It represents an agreement
between the communicating devices . Without a
Protocol , Tow devices may be connected but
not communicating . Just as a person speaking
French can not be understood by a person who
speaks only Japanese .

 The effectiveness of data communications
system depend on four fundamental
characteristics :
1- Delivery 2- Accuracy
3- Timelines 4- Jitter
Fundamental characteristics
of Data communications

1- Delivery : the system must deliver data to the
correct destination
2- Accuracy : the system must deliver data accurately .
3- Timelines : the system must deliver data in a timely
manner . Data delivered late are useless .
4- Jitter : refers to the variation of the packet arrival
time it is the uneven delay in the delivery of data .
Fundamental characteristics
of Data communications

Data Representation
 Information has a different forms such as :
1- Text
2- Numbers
3- Images
4- Audio
5- Video

Data Flow
 Communication between two devices can be :
1- Simplex : in simplex mode the communication is
unidirectional , as on a one way street .only one of the
two devices on a link can transmit ; the other can only
receive .
• Example of simplex devices . The keyboard can only
introduce input & the Monitor can only accept output .
• * the simplex mode can only the entire capacity of the
channel to send data in one direction .

Data Flow
2- Half - Duplex : 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
 In half-Duplex transmission , the entire of
channel is taken over by whichever of the two
devices is transmitting at the time .
 Example on Half Duplex is walkie-talkies.

Data Flow
3- Full-Duplex :
In the full-Duplex mode , both station can transmit and
receive simultaneously.
- Signal going in one direction share the capacity of the link with
signals going in the other direction . This share can occur in two
ways :
- Either the link must contain two physically separate
transmission paths one for sending and the other for
receiving
or The capacity of the channel is divided between signals
traveling in both direction
• The full -Duplex mode is used when communication in both
directions is required all the time. The capacity of the channel
must be divided between the tow directions.
• EX: The Telephone Network.

Message passing
CPU
Memory
Storage
CPU
Memory
Storage
Programs send messages to and from each others’ memories

Half duplex: one way at a time
CPU
Memory
Storage
CPU
Memory
Storage

Full duplex: both ways at the
same time
CPU
Memory
Storage
CPU
Memory
Storage

Networks
 A network is a set of devices (nodes ) connected by
communication links , A node can be a computer,
printer , or any other device capable of sending or
receiving data generated by other nodes on the network
 Distributed processing : Task is divided among
multiple computers instead of single large machine.
 Network Criteria : A network must be able to meet
a certain number of criteria , the most important of this
are : 1-performance 2- reliability 3-security .

1- Performance : can be measured in many ways , including
transit time and response time .
- Transit time : is the amount of time required for a message to
travel from one device to another .
- Response time : is the elapsed time between an inquiry and a
response .
- The performance of a network depends on a number of
factors , including:
1- The number of users 2- the type of transmission medium
3- The capabilities of the connected hardware
4- The efficiency of the software
- performance is often evaluated by two network matrices :
throughput and delay.

2- Reliability : in addition to accuracy of delivery ,
network reliability is measured by the frequency of
failure , the time it takes to recover from
a failure , and the network’s robustness in
a catastrophe
3- Security : network security include protecting data
from unauthorized access , protecting data from
damage development , and implementing policies
procedures for recovery from preaches and data
losses

Physical Structures
 We need to define some network attributes :
type of connection : there are two possible type of connections:
1- Point to Point .
2- point to Multipoint.

Physical Topology
 Physical topology refers to the way in which
a network is laid out physical .
 The topology of a network : is the trigonometric
representation of the relationship of all the links
and linking devices.

Categories of Networks
 Computer networks can be classified according
to their size to three categories :
1- Local Area Network (LAN)
2- Metropolitan Area Network (MAN)
3- Wide Area Network (WAN)

1.28
NETWORKS
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.
Distributed Processing
Network Criteria
Physical Structures
Network Models
Categories of Networks
Interconnection of Networks: Internetwork
Topics discussed in this section:

• A LAN is a network that is used for communicating among
computer devices, usually within an office building or home
• LAN’s enable the sharing of resources such as files or
hardware devices that may be needed by multiple users
• Is limited in size.
• Speed from 10 Mbps to 10 G bps
• Has lower cost compared to MAN’s or WAN’s
• Example: The Ethernet (IEEE 802.3 standard
• Data rates are higher (this is made possible by the shorter distances-
small total attenuation  can afford using higher frequencies, e.g.:
Ethernet: 10 Mbps -10 Gbps over 100’s of meters
Originally use a shared broadcast medium, e.g. coaxial cable
1- Local Area Network ( LAN)

30
Metropolitan Area Network (MAN)
 networks that cover a single city
hom
e
cable headend
cable distribution
network (simplified)

2-Metropolitan Area Network (MAN )
 Is a network with a size between a LAN and WAN .
 It normally covers the area inside a town or city .
Requirements: Large capacity (data rate) at low
cost and high efficiency
Can be a private or public network
Middle ground between LAN and WAN:
Stretching of LAN technology
Scaling down of WAN technology
Now also going wireless!:
Example: WiMAX (IEEE 802.16 standard

32
Wide Area Network (WAN)
What is a WAN?
 A WAN (Wide Area Network) : allows you to connect
to other computers over a wider area (i.e. the whole
world).
 Can span the world or link computers across town
 Components
 Communications services
 WAN hardware
 WAN software

3- Wide Area Network (WAN )
 Provides long distance transmission of data.
 It has no geographical limit .
 A WAN is made up of a number of interconnected LANs.
 Ex : Internet
o Large geographical area, e.g. the world
o Usually not owned by one organization
o Alternative technologies
o Circuit switching, e.g. telephone network, ISDN*
o Packet switching, e.g.:
o Frame relay
o Cell relay (Asynchronous Transfer Mode (ATM))

LAN Basics
 LAN’s can be either wired or wireless.
 Twisted pair, coaxial or fiber optic cable can be
used in wired LAN’s
 Nodes in a LAN are linked together with a certain
topology. These topologies include:
 Bus
 Ring
 Star
 Mesh
 Branching tree

37
Network Topology
 The physical layout of
a network
 Node - each computer,
printer, or server on network
 Three common topologies
 Bus
 Ring
 Star

Network Topologies
 1- Bus Topology
 Each node is connected one after the other
 Nodes communicate with each other along
the same path called the backbone

1- Bus topology
-Each node is connected one after the other .
-Computers send messages to other computers on network
 If messages collide with other messages, sending
nodes resends their message again.
collision
terminator
terminator

 Advantages of Bus Topology
1- Use of cable is economical
2- media is inexpensive and easy to work with it.
3- Easy to be extended
4- if any computer on the Bus stopped to work,
that does not effect on the entire network.

Disadvantages of bus topology
1- Network can slow in heavy traffic.
2- Cable break effect on the entire network.
3- Number of computers is limited.

2- Ring topology
-The ring network is like a bus network, but the
“end” of the network is connected to the first node.
-Nodes in the network use token to communicate with
each other.
Backbone
Node
Node

2- Ring Topology
*Advantages
- High performance ,despite many users.
*Disadvantages
- Failure of one computer can impact the rest of
the network.
-Failure of one cable impact the entire network

 3- Star Topology
 Each node is connected to a device in the center
of the network called a hub
 The hub simply passes the signal arriving from
any node to the other nodes in the network
 The hub does not route the data
Hub
Node
Node

 Advantages
Adding new computers is easy.
- centralized monitoring and management are
possible .
- failure of one computer does not affect the rest of
the network.
 Disadvantages
1- If the centralized point fails the entire network
will be got down also.
2- Many cables are used for connection.
Star Topology

4- Mesh topology
FULLY CONNECTED
 Advantages
System increased redundancy and reliability.
 Disadvantages
system is expensive to install because it uses a lot of
cabling.
PC

Q (a) For each of the following four networks, discuss the
consequences if a connection fails in :
(1) Five devices arranged in a mesh topology
(2) Five devices arranged in a star topology (not counting the
)hub)
(3) Five devices arranged in a bus topology
(4) Five devices arranged in a ring topology
An.
•If a connection fails in five devices arranged in a mesh topology: if one connection
fails , the other connection will still be working.
• If a connection fails in five devices arranged in a star topology The other devices
will still be able to send data through the hub ; their will be no access to the device
which has the failed connection to the hub.
•If a connection fails in five devices arranged in a bus topology: All transmission
stops if the failure is in the bus . if the drop- line fails , only the corresponding
device cannot operate.
•If a connection fails in five devices arranged in a ring topology: The failed
connection may disable the whole network unless it is a dual ring or there is a by-
pass mechanism.

Protocols and Standards
 Protocol : is a set of rules that govern data
communications. It represents an agreement
between the communicating devices .
Without a Protocol , Tow devices may be connected
but not communicating.
 The key element of a Protocol are:
1- Syntax
2- Semantics
3-Timing

Protocol
 1- Syntax : is the structure or format of data .
- For example : a simple protocol might expect
the first 8 bit of data to be the address of the
sender , the second 8 bits to be the address of
the receiver ,and the rest of the stream to be the
massage itself.
 2- Semantics : refers to the meaning of each
section of bits

Protocol
 3- Timing : refers to two characteristics
1- when data should be sent
2- how fast they can be sent
Ex : if a sender produces data at 100 Mbps but
the receiver can process data at 1 Mbps , the
transmission will overload the receiver and some
data will be lost

Standards
 Standards : are necessary to ensure that
products from different manufacturers can work
together as expected .
 Examples on some organizations involved in
standards creation are :
1- International organization for standardization
(ISO)
2-Institute of Electrical and Electronic Engineers
(IEEE)

Ch 1 network

More Related Content

What's hot (20)

Similar to Ch 1 network (20)

Recently uploaded (20)

Ch 1 network