1. Bayero University , Kano CSC7307 Computer Communication & Networks
Computer Communication and Networks
(CSC7303)
DEPARTMENT OF COMPUTER SCIENCE,
Bayero University, Kano.
1
A. A. Datti
Lecture 2: Network Models

2. Bayero University , Kano CSC7307 Computer Communication & Networks
Network Models
 For successful communication , two systems must follow a common set of
rules for generating and interpreting messages
 The set of rules to be followed is very complex
 Layered approach provides a viable approach to deal with a complex problem
 The communication functions are partitioned into a hierarchical set of layers
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3. Bayero University , Kano CSC7307 Computer Communication & Networks
Why Layered Approach ?
 A complex problem is divided into a number of
pieces of manageable and comprehensible size.
 It provides structured modular approach
 Each module can be developed and tested
independently
 Allows easy enhancement and implementation of
the functions of a particular layer without affecting
layers 3

4. Bayero University , Kano CSC7307 Computer Communication & Networks
Why Layered Approach ?
 A protocol is required when two entities need to
communicate.
 When communication is not simple, we may divide
the complex task of communication into several
layers.
 In this case, we may need several protocols, one for
each layer.
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5. Example of Three Layered Communication
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6. Bayero University , Kano CSC7307 Computer Communication & Networks
Some Basic Principles Followed In Layering
 Use optimum number of layers
 Put Similar function at the same layer
 Create layer where there is need for different
abstraction
 Allow changes of function to be made within a layer
without affecting others
 Create layer boundaries for each layer with its upper
and lower layers
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7. Bayero University , Kano CSC7307 Computer Communication & Networks
Using Layers to Describe Data Communication

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ISO’s Open System Interconnection
Architecture
 The most popular layered architecture that dominated data
communication and networking before 1990 was the
International Standards Organization’s (ISO’s) open
system interconnection architecture known as OSI
reference model.
 Everyone believed that the OSI model would become the ultimate
standard for data communications—but this did not happened.
 The TCP/IP protocol suite became the dominant architecture
because it was used and tested extensively in the Internet; the OSI
model was never fully implemented.
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9. Bayero University , Kano CSC7307 Computer Communication & Networks
THE OSI MODEL
 Established in 1947, the International Standards Organization
(ISO) is a multi- national body dedicated to worldwide
agreement on international standards.
 An ISO standard that covers all aspects of network
communications is the Open Systems Interconnection (OSI)
model.
 It was first introduced in the late 1970s.
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10. Bayero University , Kano CSC7307 Computer Communication & Networks
ISO is the organization; OSI is the
model.
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11. Bayero University , Kano CSC7307 Computer Communication & Networks
OSI Model Cont.
 An open system is a set of protocols that allows any two
different systems to communicate regardless of their
underlying architecture.
 The purpose of the OSI model is to show how to facilitate
communication between different systems without requiring
changes to the logic of the underlying hardware and
software.
 The OSI model is not a protocol; it is a model for
understanding and designing a network architecture that is
flexible, robust, and interoperable.
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12. Bayero University , Kano CSC7307 Computer Communication & Networks
OSI Model Cont.
 The OSI model is a layered framework for the
design of network systems that allows
communication between all types of computer
systems.
 It consists of seven separate but related layers,
each of which defines a part of the process of
moving information across a network.
 Understanding the fundamentals of the OSI model
provides a solid basis for exploring data 12

13. Bayero University , Kano CSC7307 Computer Communication & Networks
The OSI Model
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14. Bayero University , Kano CSC7307 Computer Communication & Networks
Layers in the OSI Model
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15. Bayero University , Kano CSC7307 Computer Communication & Networks
Summary of OSI layer
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16. Bayero University , Kano CSC7307 Computer Communication & Networks
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17. Bayero University , Kano CSC7307 Computer Communication & Networks
OSI Layers

18. Bayero University , Kano CSC7307 Computer Communication & Networks
Physical Layer 1/3
 Line configuration. The physical layer is concerned
with the connection of devices to the media. It can be
either point-to-point configuration or multipoint
configuration (a link is shared between several
devices).
 Physical topology. The physical topology defines how
devices are connected to make a network. Devices can
be connected using a mesh topology (every device
connected to every other device), a star topology
(devices are connected through a central device), a
ring topology (each device is connected to the next,
forming a ring), or a bus topology (every device on a
common link). 18

19. Bayero University , Kano CSC7307 Computer Communication & Networks
Physical Layer 2/3
 Concerned with transmission of raw bits over a communication
channel
 Data rate . The transmission rate- number of bits sent each
second
 Syncronization of bits . The sender and receiver must not only
use the same bit rate but must also be synchronized at the bit
level. In other words, the sender and the receiver clocks must be
synchronized.
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20. Bayero University , Kano CSC7307 Computer Communication & Networks
Physical Layer 3/3
 Transmission mode. The physical layer also defines
the direction of transmission between two devices:
simplex, half-duplex, or full-duplex. In the simplex
mode, only one device can send; the other can only
receive. The simplex mode is a one- way
communication. In the half-duplex mode, two
devices can send and receive, but not at the same
time. In a full-duplex (or simply duplex) mode, two
devices can send and receive at the same time.
 Deal with Physical Transmission media. It can be
guided or unguided media.
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21. Bayero University , Kano CSC7307 Computer Communication & Networks
Physical Layer
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22. Bayero University , Kano CSC7307 Computer Communication & Networks
OSI Layers
•Provides connectivity and path selection between
two host
•Provides Logical address
•No error correction, best effort delivery.

23. Bayero University , Kano CSC7307 Computer Communication & Networks
Hop-to-Hop Delivery
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24. Bayero University , Kano CSC7307 Computer Communication & Networks
Data Link Layer 1/3
 The data link layer transforms the physical layer, a
raw transmission facility, to a reliable link.
 It makes the physical layer appear error-free to the
upper layer (network layer). Other responsibilities of
the data link layer include the following:
 Framing. The data link layer divides the stream of
bits received from the network layer into
manageable data units called frames.
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25. Bayero University , Kano CSC7307 Computer Communication & Networks
Data Link Layer 2/3
 Physical addressing. If frames are to be distributed
to different systems on the network, the data link
layer adds a header to the frame to define the
sender and/or receiver of the frame.
 Flow control. If the rate at which the data is
absorbed by the receiver is less than the rate
produced at the sender, the data link layer imposes
a flow control mechanism to prevent overwhelming
the receiver.
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26. Bayero University , Kano CSC7307 Computer Communication & Networks
Data Link Layer 3/3
 Error control. The data link layer adds reliability to
the physical layer by adding mechanisms to detect
and retransmit damaged or lost frames. It also uses
a mechanism to recognize duplicate frames
 Access control. When two or more devices are
connected to the same link, data link layer protocols
are necessary to determine which device has
control over the link at any given time.
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27. Bayero University , Kano CSC7307 Computer Communication & Networks
Data Link Layer
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28. Bayero University , Kano CSC7307 Computer Communication & Networks
OSI Layers

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30. Bayero University , Kano CSC7307 Computer Communication & Networks
Network Layer
 The network layer is responsible for the source-to-destination delivery of a
packet, between networks (links).
 Whereas the data link layer oversees the delivery of the packet between
two systems on the same network (link), the network layer ensures that
each packet gets from its point of origin to its final destination.
 If two systems are connected to the same link, there is usually no need for a
network layer.
 However, if the two systems are attached to different networks (links) with
connecting devices between the networks (links), there is often a need for
the network layer to accomplish source-to-destination delivery.
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31. Bayero University , Kano CSC7307 Computer Communication & Networks
Network Layer
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32. Bayero University , Kano CSC7307 Computer Communication & Networks
Functions of Network Layer
 Logical addressing. The physical addressing
implemented by the data link layer handles the
addressing problem locally. If a packet passes the
network boundary, we need another addressing system
to help distinguish the source and destination systems.
This address is called IP address
 Routing. When independent networks or links are
connected together to create internetworks (network of
networks) or a large network, the connecting devices
(called routers or switches) route or switch the packets
to their final destination. One of the functions of the
network layer is to provide this mechanism
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33. Bayero University , Kano CSC7307 Computer Communication & Networks
OSI Layers

34. Bayero University , Kano CSC7307 Computer Communication & Networks
Transport Layer
 Responsible for True end-to-end Communication. Whereas, network layer
oversees source-to-destination delivery of individual packets.
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35. Bayero University , Kano CSC7307 Computer Communication & Networks
Transport Layer
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36. Bayero University , Kano CSC7307 Computer Communication & Networks
Function of Transport Layer
 Error control. Like the data link layer, the transport
layer is responsible for error control. However, error
control at this layer is performed process-to-process
rather than across a single link.
 Flow control. Like the data link layer, the transport
layer is responsible for flow control. However, flow
control at this layer is performed end to end rather
than across a single link.
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37. Bayero University , Kano CSC7307 Computer Communication & Networks
Function of Transport Layer
 Connection control. The transport layer can be
either connectionless or connection- oriented .
 Segmentation and reassembly. A message is
divided into transmittable segments, with each
segment containing a sequence number. These
numbers enable the transport layer to reassemble
the message correctly upon arriving at the
destination and to identify and replace packets that
were lost in transmission.
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38. Bayero University , Kano CSC7307 Computer Communication & Networks
Function of Transport Layer
 Port Addressing : Computers often run several
programs at the same time. For this reason, source-
to-destination delivery means delivery not only from
one computer to the next but also from a specific
process (running program) on one computer to a
specific process (running program) on the other.
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39. Bayero University , Kano CSC7307 Computer Communication & Networks
OSI Layers

40. Bayero University , Kano CSC7307 Computer Communication & Networks
Session Layer
 Establishes connection and termination : The
session layer establishes connection and when the
data transfer is complete it does the termination.
 Perform dialing management
◦ Who speaks , when , how long
◦ Simplex
◦ Half-duplex
◦ Full-duplex
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41. Bayero University , Kano CSC7307 Computer Communication & Networks
Session Layer
 Recovery using check-point(synchronization):
The session layer allows a process to add
checkpoints (synchronization points) into a stream
of data.
◦ For example, if a system is sending a file of 2,000 pages,
it is advisable to insert checkpoints after every 100 pages
to ensure that each 100-page unit is received and
acknowledged independently. In this case, if a crash
happens during the transmission of page 523, the only
pages that need to be resent after system recovery are
pages 501 to 523
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42. Bayero University , Kano CSC7307 Computer Communication & Networks
OSI Layers

43. Bayero University , Kano CSC7307 Computer Communication & Networks
Presentation Layer
 Translation : Because different computers use different encoding
systems (e.g. ASCII , UTF ) , the presentation layer is responsible
for interoperability between these different encoding methods. The
presentation layer at the sender changes the information from its
sender-dependent format into a common format. The presentation
layer at the receiving machine changes the common format into its
receiver-dependent format.
 Data compression and Decompression : Data compression
reduces the number of bits contained in the information. Data
compression becomes particularly important in the transmission of
multimedia such as text, audio, and video.
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44. Bayero University , Kano CSC7307 Computer Communication & Networks
Presentation Layer
 Encryption and Decryption: Sometimes for
secured communication encryption and decryption
has to be done and this is the function of this
presentation layer.
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45. Bayero University , Kano CSC7307 Computer Communication & Networks
OSI Layers

46. Bayero University , Kano CSC7307 Computer Communication & Networks
Application Layer
 The application layer concern with user application
 Specific application provided include
 File Transfer
 Email
 File Management
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47. Bayero University , Kano CSC7307 Computer Communication & Networks
Summary of OSI layer
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48. Bayero University , Kano CSC7307 Computer Communication & Networks
Encapsulation
Data
Segments
Packet
Frame
Bits
Data
Data

49. Bayero University , Kano CSC7307 Computer Communication & Networks
Lecture 1 Continued... TCP/IP Protocol Suite
 The designers of OSI assumed that this model and the protocols
developed within this model would come to dominate computer
communications and replace the rival models such as TCP/IP .
 This has not happened and instead, the TCP/IP architecture has
dominated.
 The TCP/IP protocol suite was developed prior to the OSI model and it is
a four layer model
 Perhaps the most important reason why domination occur is that the
key TCP/IP protocols were mature and well tested at a time when similar
OSI protocols were in the development stage .
 Another reason is that the OSI model is unnecessarily complex, with
seven layers to accomplish what TCP/IP does with fewer layers.
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50. Bayero University , Kano CSC7307 Computer Communication & Networks
The TCP/IP Model
The Department of Defense (DoD) developed the
TCP/IP reference model to provide a communication
network that could continue to function in wartime.
•ARPANET (Advanced Research Projects
Agency Network) was a research network
sponsored by the DoD (U.S. Department
of Defense), 21/11/1969 UCLA-Stanford.
•It became TCP / IP (Transmission Control
Protocol / Internet Protocol) Reference
Model in 1974.

51. Bayero University , Kano CSC7307 Computer Communication & Networks
TCP/IP Applications

52. Bayero University , Kano CSC7307 Computer Communication & Networks
Transport Layer Protocols

53. Bayero University , Kano CSC7307 Computer Communication & Networks
Transport Layer Protocols
TCP is responsible for:
• end-to-end communication
• flow control
• reliability of data delivery
TCP supports a logical connection between the sending and receiving hosts

54. Bayero University , Kano CSC7307 Computer Communication & Networks
Internet Layer Protocols
The IP Protocol is
responsible for:
• defining packet format
and addressing scheme
• routing packets to remote
hosts
• transferring data between
the internet layer and the
network access layer

55. Bayero University , Kano CSC7307 Computer Communication & Networks
Internet Layer Protocols
 IP - connectionless, best-effort delivery routing of
packets.
 ICMP - control and messaging capabilities.
 ARP - determines the data link layer address for known
IP addresses.
 RARP - determines the IP address for a known MAC
address.

56. Bayero University , Kano CSC7307 Computer Communication & Networks
Internet Path Determination

57. Bayero University , Kano CSC7307 Computer Communication & Networks
Network Access Protocols
Main service : transfer data on physical
medium
• Encapsulation of IP packets into frames
• Interface to the physical medium

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59. Bayero University , Kano CSC7307 Computer Communication & Networks
Comparing TCP/IP with the OSI Model