Layers

1. Lecture#03
Computer Communications
& Networks CS-576

2.  Need of a Layered Architecture
 OSI Model
 7 Layers of OSI Model
 Functional Details of OSI Layers
• Physical Layer
• Data Link Layer
• Network Layer
• Transport Layer
• Session Layer
• Presentation Layer
• Application Layer

3. Layered Architecture
 Design Philosophy of Layered Architecture
 The complex task of communication is broken into
simpler sub- tasks or modules
 Each layer performs a subset of the required
communication functions
 Each layer relies on the next lower layer to perform
more primitive functions
 Each layer provides services to the next higher layer
Changes in one layer should not require changes in other
layers
 Helps in troubleshooting and identifying the problem

4. An example Sending a letter

5. OSI Model
 To standardize the design of communication system, the
ISO created the OSI model.
 Open Systems Interconnection
 Open System: A model that allows two different systems
to communicate regardless of their underlying network
 Purpose of the OSI MODEL
 Open Communication between different systems without
requiring changes to the hardware and software.

6. OSI MODEL
 Definition of OSI Model
 A layered framework for the design of network systems
that allows communication across all types of computer
systems regardless of their underlying architecture.
 Each layer performs a subset of the required
communication functions
 Each layer relies on the next lower layer to perform more
primitive functions
 Each layer provides services to the next higher layer
 Changes in one layer should not require changes in other
layers

7. OSI MODEL
 Layers of the OSI Model
1. Physical (Bits)
2. Data Link (Frames)
3. Network (Packets)
4. Transport (Segment)
5. Session (Dialog units)
6. Presentation (Raw Data)
7. Application (Text, Numbers)

8. Interfaces between Layers
 The passing of the data and network information down
through the layers of the sending device and back up
through the layers of the receiving device is made possible
by an interface between each pair of adjacent layers.

9. Organization of layers
 The layers are divided into two groups
 Network support layers:
 Deals with physical aspects of moving from one device to
another.
 Layers 1,2,3
 User Support layers
 Allow interoperability among unrelated software systems.
 Layers 5,6,7
 Layer 4 links the two subgroups what the lower layers have
transmitted in form that upper layer can use.
 The upper OSI are always implemented in software lower layers
are combination of hardware and software. Except for the
physical layer which is mostly hardware.

10. An exchange using the OSI
model

11. Physical layer

12. Physical layer responsibilities
 The physical layer is responsible for transmitting individual
bits from one node to the next.
 Characterises of Media: The physical layer defines the
charactertics and type of transmission medium.
 Representation of bits: The physical layer data consists of a
stream of bits(sequence of 0,s and 1,s) with no interpretation. To
be transmitted, bits must be encoded into signals-electrical or
optical. The physical layer defines the type of encoding.
 Data Rate: The transmission rate- the number if bits sent each
second is also defined by the physical layer.
 Line configuration: The physical layer defines the attachment
of devices with medium
 Transmission Mode: The physical layer also defines the
direction of transmission between two devices: simplex, half
duplex,full duplex

13. Data link layer

14. Data link layer responsibilities
 The data link layer is responsible for moving frames from one hop
(node) to the next.

15. Example 1

16. Network Layer

17. Network Layer responsibilities
 The network layer is responsible for the delivery of individual packets
from the source host to the destination host
.

19. Transport Layer responsibilities

20. Transport Layer responsibilities
 The transport layer is responsible for the delivery of a message from
one process to another.

22. Session Layer

23. Session Layer responsibilities
 The session layer is responsible for dialog control and synchronization.

24. Presentation Layer

25. Presentation Layer responsibilities
 The presentation layer is responsible for translation, compression, and
encryption.

26. Application Layer

27. Application Layer responsibilities
 The application layer is responsible for providing services to the user.
 Network virtual terminal (Remote log-in)
 File transfer and access
 Mail services
 Accessing the World Wide Web