The document discusses communication architectures and protocol layers. It introduces the OSI reference model, which divides communication tasks into 7 layers - physical, data link, network, transport, session, presentation, and application layer. Each layer provides services to the layer above it and communicates with corresponding layers on other systems. The layers simplify complex communication tasks and allow standardized protocols for each layer.

1. Protocol Layers and Their Service Models

2. Communication Architecture
Strategy for connecting host computers and other
communicating equipment.
Defines necessary elements for data communication
between devices.
A communication architecture, therefore, defines a
standard for the communicating hosts.
A programmer formats data in a manner defined by the
communication architecture and passes it on to the
communication software.
Separating communication functions adds flexibility, for
example, we do not need to modify the entire host software
to include more communication devices.
OSI Model

3. 2.3
2-1 LAYERED TASKS
We use the concept of layers in our daily life.
• As an example, let us consider two friends who
communicate through postal mail. The process of
sending a letter to a friend would be complex if there
were no services available from the post office.

4. 2.4
Figure Tasks involved in sending a letter

5. 5
PROTOCOL LAYERS
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.

6. 6
Assume Maria and Ann are neighbors with a lot of common
ideas. However, Maria speaks only Spanish, and Ann speaks
only English. Since both have learned the sign language in their
childhood, they enjoy meeting in a cafe a couple of days per
week and exchange their ideas using signs. Occasionally, they
also use a bilingual dictionary. Communication is face to face
and Happens in one layer as shown in Figure 2.1.
Example 2.1

7. 7
Figure Example 2.1

8. 8
Now assume that Ann has to move to another town because of
her job. Before she moves, the two meet for the last time in the
same cafe. Although both are sad, Maria surprises Ann when
she opens a packet that contains two small machines. The first
machine can scan and transform a letter in English to a secret
code or vice versa. The other machine can scan and translate a
letter in Spanish to the same secret code or vice versa. Ann
takes the first machine; Maria keeps the second one. The two
friends can still communicate using the secret code, as shown
in Figure 2.2.
Example 2.2

9. 9
Figure 2.2 Example 2.2

10. Layer Architecture
Layer architecture simplifies the network design.
A Layered architecture allows us to discuss a well-defined,
specific part of the layer and complex system.
It is easy to debug network applications in a layered
architecture network.
The network management is easier due to the layered
architecture.
Network layers follow a set of rules, called protocol.
The protocol defines the format of the data being
exchanged, and the control and timing for the handshake
between layers.
OSI Model

11. Open Systems Interconnection (OSI) Model
International standard organization (ISO) established a
committee in 1977 to develop an architecture for computer
communication.
Open Systems Interconnection (OSI) reference model is the
result of this effort.
In 1984, the Open Systems Interconnection (OSI) reference
model was approved as an international standard for
communications architecture.
Term “open” denotes the ability to connect any two
systems which conform to the reference model and
associated standards.
OSI Model

12. OSI Reference Model
The OSI model is now considered the primary Architectural model for inter-
computer communications.
The OSI model describes how information or data makes its way from application
programmes (such as spreadsheets) through a network medium (such as wire) to
another application programme located on another network.
The OSI reference model divides the problem of moving information between
computers over a network medium into SEVEN smaller and more manageable
problems .
This separation into smaller more manageable functions is known as layering.
OSI Model

13. 2.13
ISO is the organization.
OSI is the model.
Note

14. 2.14
Figure Seven layers of the OSI model

15. OSI Reference Model: 7 Layers
OSI Model

16. OSI: A Layered Network Model
The process of breaking up the functions or tasks of networking into layers
reduces complexity.
Each layer provides a service to the layer above it in the protocol specification.
Each layer communicates with the same layer’s software or hardware on other
computers.
The lower 4 layers (transport, network, data link and physical —Layers 4, 3, 2, and
1) are concerned with the flow of data from end to end through the network.
The upper three layers of the OSI model (application, presentation and session—
Layers 7, 6 and 5) are orientated more toward services to the applications.
Data is Encapsulated with the necessary protocol information as it moves down
the layers before network transit.
OSI Model

17. FIG. OSI Model

18. Physical Layer
Provides physical interface for transmission of information.
Defines rules by which bits are passed from one system to another on a physical
communication medium.
Covers all - mechanical, electrical, functional and procedural - aspects for
physical communication.
Such characteristics as voltage levels, timing of voltage changes, physical data
rates, maximum transmission distances, physical connectors, and other similar
attributes are defined by physical layer specifications.
OSI Model

19. 2.19
Figure. Physical layer

20. 2.20
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.
Note
The physical layer is also concerned with the following:
1. Physical characteristics of interfaces and medium
2. Representation of bits.
3. Data rate.
4. Synchronization of bits.
5. Line configuration: this layer is concerned with the connection of devices to the media i.e. point-to-point
or multipoint
6. Physical topology.
7. Transmission mode : The physical layer also defines the direction of transmission between two devices:
simplex, half-duplex, or full-duplex

21. Data Link Layer
Data link layer attempts to provide reliable communication over the physical layer
interface.
Breaks the outgoing data into frames and reassemble the received frames.
Create and detect frame boundaries.
Implement Error Control by implementing an acknowledgement and retransmission
scheme of damaged or lost frames.
Implement flow control.
Supports point-to-point as well as broadcast communication.
Supports simplex, half-duplex or full-duplex communication.
Examples of Link layer protocols are Ethernet, WiFi and PPP
(point-to-point)
OSI Model

22. 2.22
Figure 2.6 Data link layer

23. Functions of Data Link Layer
• Framing: Frames are the streams of bits received from the network layer into
manageable data units. This division of stream of bits is done by Data Link Layer.
• Physical Addressing: The Data Link layer adds a header to the frame in order to
define physical address of the sender or receiver of the frame, if the frames are to
be distributed to different systems on the network.
• Flow Control: A flow control mechanism to avoid a fast transmitter from running a
slow receiver by buffering the extra bit is provided by flow control. This prevents
traffic jam at the receiver side.
• Error Control: Error control is achieved by adding a trailer at the end of the frame.
Duplication of frames are also prevented by using this mechanism. Data Link
Layers adds mechanism to prevent duplication of frames.
• Access Control: Protocols of this layer determine which of the devices has control
over the link at any given time, when two or more devices are connected to the
same link.

25. 2.25
The data link layer is responsible for moving
frames from one hop (node) to the next.
Note

26. Network Layer
Implements routing of frames (packets) through the network.
Defines the most optimum path the packet should take from the
source to the destination
Defines logical addressing so that any endpoint can be identified.
Handles congestion in the network.
Facilitates interconnection between heterogeneous networks
(Internetworking).
The network layer also defines how to fragment a packet into smaller
packets to accommodate different media.
OSI Model

27. 2.27
Figure. Network layer

28. 2.28
The network layer is responsible for the
delivery of individual packets from
the source host to the destination host.
Note

29. Transport Layer
Purpose of this layer is to provide a reliable mechanism for the
exchange of data between two processes in different computers.
Ensures that the data units are delivered error free.
Ensures that data units are delivered in sequence.
Ensures that there is no loss or duplication of data units.
Provides connectionless or connection oriented service.
Provides for the connection management.
Multiplex multiple connection over a single channel.
OSI Model

30. Session Layer
Session layer provides mechanism for controlling the dialogue between the two end
systems. It defines how to start, control and end conversations (called sessions)
between applications.
This layer requests for a logical connection to be established on an end-user’s request.
Any necessary log-on or password validation is also handled by this layer.
Session layer is also responsible for terminating the connection.
Synchronization : This layer allows a process to add checkpoints which are considered
as synchronization points into stream of data.
Example: If a system is sending a file of 800 pages, adding checkpoints after every 50 pages is
recommended. This ensures that 50 page unit is successfully received and acknowledged. This is
beneficial at the time of crash as if a crash happens at page number 110; there is no need to
retransmit 1 to100 pages.
OSI Model

31. Presentation Layer
Presentation layer defines the format in which the data is to be
exchanged between the two communicating entities.
Also handles data compression and data encryption.
OSI Model

32. Application Layer
Application layer interacts with application programs and is the
highest level of OSI model.
Application layer contains management functions to support
distributed applications.
Examples of application layer are applications such as file
transfer (FTP), electronic mail (SMTP), remote login (TELNET),
HTTP, DNS etc.
OSI Model

33. Functions of Application Layer
• Mail Services: This layer provides the basis for E-mail forwarding and
storage.
• Network Virtual Terminal: It allows a user to log on to a remote host.
• Directory Services: This layer provides access for global information about
various services.
• File Transfer, Access and Management (FTAM): It is a standard mechanism
to access files and manages it. Users can access files in a remote computer
and manage it. They can also retrieve files from a remote computer.

34. OSI In Action
A message begins at the top application layer and moves down the OSI
layers to the bottom physical layer.
As the message descends, each successive OSI model layer adds a header
to it.
A header is layer-specific information that basically explains what
functions the layer carried out.
Conversely, at the receiving end, headers are striped from the message
as it travels up the corresponding layers.
OSI Model

35. TCP/IP Model
TCP/IP MODEL

36. OSI & TCP/IP Models
TCP/IP Model

37. TCP/IP Model
TCP/IP Model
Application Layer
Application programs using the network
Transport Layer (TCP/UDP)
Management of end-to-end message transmission,
error detection and error correction
Network Layer (IP)
Handling of datagrams : routing and congestion
Data Link Layer
Management of cost effective and reliable data delivery,
access to physical networks
Physical Layer
Physical Media