Engineering Chemistry

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
Figure 2.1 Tasks involved in sending a letter

4. 2.4
2-2 THE OSI MODEL
Established in 1947, the International Standards
Organization (ISO) is a multinational 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.
Layered Architecture
Peer-to-Peer Processes
Encapsulation
Topics discussed in this section:

5. 2.5
ISO is the organization.
OSI is the model.
Note

6. 2.6
Figure 2.2 Seven layers of the OSI model

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

11. 2.11
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.
Note

12. 2.12
Figure 2.6 Data link layer

13. 2.13
The data link layer is responsible for moving
frames from one hop (node) to the next.
Note

14. 2.14
Figure 2.7 Hop-to-hop delivery

15. 2.15
Figure 2.8 Network layer

16. 2.16
The network layer is responsible for the
delivery of individual packets from
the source host to the destination host.
Note

17. 2.17
Figure 2.9 Source-to-destination delivery

18. 2.18
Figure 2.10 Transport layer

19. 2.19
The transport layer is responsible for the delivery
of a message from one process to another.
Note

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

21. 2.21
Figure 2.12 Session layer

22. 2.22
The session layer is responsible for dialog
control and synchronization.
Note

23. 2.23
Figure 2.13 Presentation layer

24. 2.24
The presentation layer is responsible for translation,
compression, and encryption.
Note

25. 2.25
Figure 2.14 Application layer

26. 2.26
The application layer is responsible for
providing services to the user.
Note

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. Physical Addresses
 The physical address, also known as the link address, is the address
of a node as defined by its LAN or WAN. It is included in the frame
used by the data link layer. It is the lowest-level address.
 The size and format of these addresses vary depending on the
network. For example, Ethernet uses a 6-byte (48-bit) physical
address that is imprinted on the network interface card (NIC).
 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.
2.33

34. 2.34
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

35. 2.35
Figure 2.19 Physical addresses

36. Continue..
 In Figure below a node with physical address 10 sends a frame to a node with
physical address 87. The two nodes are connected by a link (a LAN).
 At the data link layer, this frame contains physical (link) addresses in the header.
These are the only addresses needed. The rest of the header contains other
information needed at this level.
 The trailer usually contains extra bits needed for error detection.
 The data link layer at the sender receives data from an upper layer. It encapsulates
the data in a frame, adding a header and a trailer.
 The header, among other pieces of information, carries the receiver and the sender
physical (link) addresses.
 Note that in most data link protocols, the destination address 87 in this case, comes
before the source address (10 in this case). The frame is propagated through the
LAN. Each station with a physical address other than 87 drops the frame because the
destination address in the frame does not match its own physical address. The
intended destination computer, however, finds a match between the destination
address in the frame and its own physical address. The frame is checked, the header
and trailer are dropped, and the data part is decapsulated and delivered to the upper
layer.
2.36

37. 2.37
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.

38. Unicast, Multicast, and
Broadcast Physical Addresses
 Physical addresses can be either unicast (one single
recipient), multicast (a group of recipients), or broadcast
(to be received by all systems in the network).
 Some networks support all three addresses. A source
address is always a unicast address—the frame comes
from only one station. The destination address, however,
can be unicast, multicast, or broadcast.
 The least significant bit of the first byte defines the type
of address.
2.38

39.  Q: Define the type of the following
destination addresses:
 1. 4A:30:10:21:10:1A
 2. 47:20:1B:2E:08:EE
 3. FF:FF:FF:FF:FF:FF
2.39

40. 2.40
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

41. 2.41
Figure 2.20 IP addresses

42. 2.42
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

43. 2.43
Figure 2.21 Port addresses

44. 2.44
The physical addresses will change from hop to hop,
but the logical addresses usually remain the same.
Note

45. 2.45
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.

46. 2.46
The physical addresses change from hop to hop,
but the logical and port addresses usually remain the same.
Note