The document discusses the ISO/OSI reference model, which is a standard for how applications can communicate over a network. It describes the seven layers of the model from physical to application layer. The physical layer deals with transmitting raw bits and hardware connections. The data link layer handles coding, addressing, and error control. The network layer establishes and maintains paths across networks by routing packets. The transport layer provides reliable data delivery. The session layer manages communication sessions and dialogs. The presentation layer transforms data formats. The application layer specifies the user interface and common network applications.

1. Chapter 2: The
ISO/OSI
REFERENCE
MODEL
By: IVY B. BUNCARAS

2. ISO
“International Organization for
Standardization”, an independent, non-
governmental international organization with a
membership of 162 national standards bodies.
OSI
“Open System Interconnection”,
a reference model for how applications can
communicate over a network

3. THE
ISO/OSI
REFERENCE
MODEL

4. Physical Layer
concerned with transmitting raw bits over a
communication channel.
concerned mainly with the hardware connection.
defines the mechanical and electrical
specifications of a network medium and network
interface hardware.
responsible for the physical media connections
and specifications, data transmission, and reception
from media.
connects adjacent nodes physically and electrically

5. IBM Compatible IBM Compatible
Cable
volts
time
The Physical Layer

6. Data Link Layer
concerned with coding, addressing and transmitting information.
responsible for the sending and receiving of bits to and from the physical
layer.
responsible for data framing, error control, flow control and physical
addressing.
adds its own control information to the front of the data packet including
the following;
1. a source address
2. destination address
3. length of frame
Physical Layer
Data Link
Layer
11001010
10
trailerheader
11001010
10

7. The Network Layer
Network B
Token Ring
Network A: Ethernet
router

8. The network layer is one layer above data link
later, which ensures that information arrive at its
intended destination at the desired format.
Its function is to establish, maintain and
provide an open path for information to travel across
the network and thus make the actual route
transparent and insignificant of any other layer.
The primary objective of network layer is to
move or route information across a network made up
of multiple network segments.

9. Functions;
transports, routes, message handling and
transfer of data.
Responsible for routing data across the link,
message fragmentation
Routes data in “packets” between source
destination.
Controls traffic and congestion in the network

10. The Transport Layer
Network A Network B
MAIL
Workstation IBM Compatible Workstation IBM Compatible

11. The Transport Layer acts as the interface
between the higher layers and the lower three layers.
This layer ensures reliable data delivery often
compensating for the lack of reliability in the lower
layers.
places data exchange to match end-point
processing capacity and enciphers user data if needed
for security purposes.
Responsible for reliable end-to-end
communication

12. Implementations can usually confirm or deny data
delivery
The application software may take corrective action or
provide the user with options.
Makes use of a variety of mechanisms such us;
1. acknowledgement messages
2. sequence numbers
3. flow control

13. The Session Layer
Mabuhay! Hey, Myline may I speak with you? Over
Do I know you? Over
Yes, this is Aly. Over
Oh, Hi Aly. What do you need? Over.
Send me the source code of the program. Over.
Sure thing. Here it comes! Over
Network Server Remote Server

14. The session layer adds control mechanism to
the data that establish, maintain, synchronize, and
manage dialog between communicating applications.
The session layer is also generally thought to
be the home of Remote Procedure Calls (RPCs). RPCs
send information across the network in a manner that is
almost transparent to the user.

15. Handles the details that must be agreed upon and
followed by the communicating parties that
exchange the information.
Maintains the session between the devices for as
long as it is needed.
Coordinates and regulates the transfer of
information between them.
Responsible for establishing, maintaining and
coordinating communication.

16. Responsible for session management, session
error control, dialog control, (coordination and
synchronization) and remote procedure calls.
Synchronizes flow between end-points,
correlates exchanges and groups related data into
indivisible units

17. THE PRESENTATION LAYER
Session Presentation Application
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010101010001010
010101010010000
110101010010100
Send me the
source code
and all the
associated
files.

18. The presentation layer transforms data into a
mutually agreed-upon format (transfer syntax) that
can be understood by each application and the
computers they run on.
responsible for the text formatting, code
conversion and display.
responsible for data formats, data format
translation, data compression and data encryption
and decryption.
formats data for different presentation media
and coordinates resource sharing.
the primary function is to present the data into
a form that can be understood by the user

19. the presentation layer’s tasks generally
include interpreting codes such as tabs ad other
codes related to presentation.
it may also govern converting encryption and
other codes into a well-defined format.
responsible for translating and interpreting
different character sets so that they may be
presented in a manner that can be understood by
the end-users

20. The Application Layer

21. The Application Layer specifies the communication
interface with the user and manages communication
between computer applications.
The application layer contains variety of protocols that
are commonly needed for various applications.
Functions:
concerned with program to program transfer of
information
Provides application services such as distributed
database access and document interchange to network
users
Responsible for user interface and applications

22. Example of network application are as
follows:
1. File access / transfer
2. Virtual terminal transfer
3. Network management
4. Directory services
5. Mail transfer service

23. Physical Path
User Data
Communication Path
Logical Path
Sending Process Receiving Process
Bits
DataDH DT
DataNH
Data
Data
Data
Data
TH
SH
PH
AH

24. End-User Application Programs
File transfer, access and management,
document and message interchange, job
transfer and manipulation.
Transfer syntax negotiation data
representation transformation
Dialog and synchronization control for
application entities
Distributed information
services
Syntax-independent
message interchange
service
APPLICATION LAYER
PRESENTATION LAYER
SESSION LAYER

25. End-to-end message transfer (connection
management, error control, fragmentation
and flow control
Network routing, addressing, call setup and
clearing
Data Link Control (framing, data
transparency, error control
Mechanical and electrical network interface
definitions
Data Communications Network
Network-independent message
interchange service
Physical connection to network
termination equipment
TRANSPORT LAYER
NETWORK LAYER
DATA LINK LAYER
PHYSICAL LAYER
Protocol Layer Summary of the ISO/OSI Reference Model

27. OSI Services
Two types of services to the layer of OSI
Reference Model:
 Connection-Oriented Service
- the service user first establishes a connection,
uses this connection and then terminates the
connection when it is no longer needed.
Reliable connection-oriented service has
two minor variations:
• Message sequences
• Byte streams

28.  Connectionless Service
- adapts the postal system model. Each of the message (or letter
in postal system) carries the full destination address and is routed
independently through the system.
Service Example
Connection-oriented Reliable message
stream
Sequence of pages
Reliable byte stream Remote login
Unreliable connection Digitized voice
Connectionless Unreliable datagram Electronic junk mail
Acknowledged
datagram
Registered mail
Request-reply Database query
Examples of Different Types of Service

29. END