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1. 2.1
Chapter 2
Network Models
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2. 2.2
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.
Sender, Receiver, and Carrier
Hierarchy
Topics discussed in this section:

3. 2.3
2-1 LAYERED TASKS
 Example: Sending a letter
 Components:
 Sender, Receiver, and Carriers
 Hierarchy:
 The tasks are done in order
 Services: the relations between layers
 Each layer uses the services of the layer below it

4. 2.4
Figure 2.1 Tasks involved in sending a letter

5. 2.5
2-2 THE OSI MODEL
 ISO was Established in 1947
 ISO is a multinational body for worldwide agreement on
international standards
 An ISO standard for network communications is the Open
Systems Interconnection (OSI) model
 The OSI model was first introduced in the late 1970s.
ISO is the organization.
OSI is the model.
Note

6. 2.6
Figure 2.2 Seven layers of the OSI model
 Each layer represents a set
of networking functions
with related uses
 Comprehensive and flexible
modular architecture
 Each layer uses the
services of the layer
immediately below it via an
interface
 Each layer in the sending
side communicates with its
peer in the receiving side
using peer-to-peer
protocols

7. 2.7
Figure 2.3 The interaction between layers in the OSI model

8. 2.8
Figure 2.4 An exchange using the OSI model

9. 2.9
2-3 LAYERS IN THE OSI MODEL
In this section we briefly describe the functions of each
layer in the OSI model.
Physical Layer
Data Link Layer
Network Layer
Transport Layer
Session Layer
Presentation Layer
Application Layer
Topics discussed in this section:

10. 2.10
Figure 2.5 Physical layer
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.
Note

11. 2.11
Functions of the Physical Layer
 The physical characteristics:
 The interface between devices and the transmission media
 Representation of bits:
 The type of encoding into signals
 Transmission data rate:
 The number of bits per second and the bit duration
 Synchronization of bit transmission:
 Clocks between the sender and receiver
 Line configuration:
 The connection of devices to the media
 Physical topology:
 mesh, star, bus, etc.
 Transmission mode:
 duplex, simplex, etc.

12. 2.12
Figure 2.6 Data link layer
The data link layer is responsible for moving
frames from one hop (node) to the next.
Note

13. 2.13
Functions of the Data Link Layer
 Framing: divides the bit stream from network layer into frames
 Physical addressing: specifies the physical addresses (e.g. the
MAC addresses) of the sender and/or the receiver of the frame
 If the receiver is outside the sender’s network, the frame is sent to
the network connecting device (router or gateway)
 Flow control: a mechanism to avoid overwhelming the receiver
buffers by the sender’s data
 Error control: a mechanism to detect and/or correct
transmission errors cause by the physical channel.
 Detection and retransmission of damaged frames
 Prevent frame duplication
 Access control: determines and controls the access of the
communication link among the devices sharing the link

14. 2.14
Figure 2.7 Hop-to-hop delivery

15. 2.15
Figure 2.8 Network layer
The network layer is responsible for the
delivery of individual packets from
the source host to the destination host.
Note

16. 2.16
Functions of the Network Layer
 Packet delivery:
 From source host to destination host across multiple networks.
 Logical addressing:
 For packet delivery outside the local network (or subnet), a
logical address is needed (e.g. IP address)
 Packet routing:
 Defines the routing or switching mechanism for the connecting
devices (i.e.; routers) in order to deliver the packets to the final
destination through an internetwork

17. 2.17
Figure 2.9 Source-to-destination delivery

18. 2.18
Figure 2.10 Transport layer
The transport layer is responsible for the delivery
of a message from one process to another.
Note

19. 2.19
Functions of the Transport Layer
 Message delivery:
 From a specific process in the sending host to a specific process in
the destination host
 Segmenting and reassembling:
 Divide the message from the application layer of the sender side
into transmittable segments containing sequence numbers
 Reassemble the segments into messages at the receiver side
 Connection control:
 The type of connection: connectionless or connection-oriented
 Connectionless: packet are sent independently (e.g. UDP protocol)
 Connection-oriented: a connection is first established with the peer
before delivering the packets (e.g. TCP protocol)
 End-to-end flow control
 End-to-end error control

20. 2.20
Figure 2.11 Reliable process-to-process delivery of a message

21. 2.21
Figure 2.12 Session layer
The session layer is responsible for dialog
control and synchronization.
Note

22. 2.22
Functions of the Session Layer
 Dialog Control
 Allows two systems to enter into a dialog
 Allows two processes to communicate
 Synchronization:
 Establish, maintain, and synchronize communicating
system interaction
 Add checkpoints in order to resume a session in case of a
crash

23. 2.23
Figure 2.13 Presentation layer
The presentation layer is responsible for translation,
compression, and encryption.
Note

24. 2.24
Functions of the Presentation Layer
 Handle the syntax and semantics of the information
exchange
 Translation:
 Sender-dependent formats into common formats
 Common formats into receiver-dependant formats
 Encryption and decryption:
 Encrypt sensitive sender information to insure privacy
 Decrypt at the receiver side to retain original message
 Compression:
 Reduce the size of transmitted message to speedup the
transmission

25. 2.25
Figure 2.14 Application layer
The application layer is responsible for
providing services to the user.
Note

26. 2.26
Functions of the Application Layer
 Enables the user, whether human or software, to
access the network
 Network virtual terminal:
 Software emulation of a terminal at a remote host
 File transfer, access, and management:
 Remote access and management of files and contents
 Mail services:
 Transfer and management of email messages
 Directory services:
 Access to distributed database sources

27. 2.27
Figure 2.15 Summary of layers

28. 2.28
2-4 TCP/IP PROTOCOL SUITE
The layers in the TCP/IP protocol suite do not exactly
match those in the OSI model. The original TCP/IP
protocol suite was defined as having four layers: host-
to-network, internet, transport, and application.
However, when TCP/IP is compared to OSI, we can say
that the TCP/IP protocol suite is made of five layers:
physical, data link, network, transport, and
application.
Physical and Data Link Layers
Network Layer
Transport Layer
Application Layer
Topics discussed in this section:

29. 2.29
Figure 2.16 TCP/IP and OSI model

30. 2.30
2-5 ADDRESSING
Four levels of addresses are used in an internet
employing the TCP/IP protocols: physical, logical, port,
and specific.
Physical Addresses
Logical Addresses
Port Addresses
Specific Addresses
Topics discussed in this section:

31. 2.31
Figure 2.17 Addresses in TCP/IP

32. 2.32
Figure 2.18 Relationship of layers and addresses in TCP/IP

33. 2.33
Figure 2.19 Physical addresses
In Figure 2.19 a node with physical address 10 sends a frame to
a node with physical address 87. The two nodes are connected
by a link (bus topology LAN). As the figure shows, the computer
with physical address 10 is the sender, and the computer with
physical address 87 is the receiver.
Example 2.1

34. 2.34
As we will see in Chapter 13, most local-area networks
use a 48-bit (6-byte) physical address written as 12
hexadecimal digits; every byte (2 hexadecimal digits) is
separated by a colon, as shown below:
Example 2.2
07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical address.

35. 2.35
Figure 2.20 IP addresses
Figure 2.20 shows a part of an
internet with two routers
connecting three LANs. Each
device (computer or router)
has a pair of addresses
(logical and physical) for each
connection. In this case, each
computer is connected to only
one link and therefore has
only one pair of addresses.
Each router, however, is
connected to three networks
(only two are shown in the
figure). So each router has
three pairs of addresses, one
for each connection.
Example 2.3

36. 2.36
Figure 2.21 Port addresses
Figure 2.21 shows two
computers communicating via
the Internet. The sending
computer is running three
processes at this time with port
addresses a, b, and c. The
receiving computer is running
two processes at this time with
port addresses j and k. Process
a in the sending computer
needs to communicate with
process j in the receiving
computer. Note that although
physical addresses change
from hop to hop, logical and
port addresses remain the same
from the source to destination.
Example 2.4

37. 2.37
Example 2.5
As we will see in Chapter 23, a port address is a 16-bit
address represented by one decimal number as shown.
753
A 16-bit port address represented
as one single number.
The physical addresses change from hop to hop,
but the logical and port addresses usually remain the same.
Note