1. Lecturer: Mrs. Rohani bt Hassan
Chapter 2
Network Models
Lecturer: Mrs. Rohani bt Hassan
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2. Network Model
A network is a combination of hardware and software that sends data from one location to another.
The hardware consists of the physical equipment that carries signals from one point of the network to another.
The software consists of instruction set that make possible the services that we expect from a network.
For example, the task of sending an from one point in the world to another can be broken into several tasks, each performed by a separate software package. Each software package uses the services of another software package. At the lowest layer, a signal is sent from the source computer to the destination computer.

3. Topics discussed in this section:
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.
Figure 2.1 shows the steps in this task.
Topics discussed in this section:
Sender, Receiver, and Carrier Hierarchy

4. Figure 2.1 Tasks involved in sending a letter

5. Sender, Receiver and Carrier
In figure 2.1, we have a sender, a receiver and a carrier that transports the letter. There is a hierarchy of tasks.
At the sender site – the activities is write a letter, picked up by a letter carrier and a carrier transports the letter.
On the way – the letter is then on the way to the recipient.
At the receiver site – the carrier transports the letter, delivered to the recipient’s mailbox and receiver picks up the letter.
Hierarchy
There are three different activities at the sender site and another three activities at the receiver site.
The task of transporting the letter between the sender and the receiver is done by the carrier.
The task must be done in the order given in the hierarchy.

6. 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.
An open system is a set of protocols that allows any two different systems to communicate regardless of their underlying architecture.
The purpose of the OSI model is to show how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software.
The OSI model is not a protocol. It is s model for understanding and designing a network architecture that is flexible, robust and interoperable.

7. Topics discussed in this section:
THE OSI MODEL FRAMEWORK
The OSI model is a layered framework for the design of network systems that allows communication between all types of computer system.
It consists of seven separate but related layer, each of which defines a part of the process of moving information across a network ( see figure 2.2 ).
An understanding of the fundamentals of the OSI model provides a solid basis for exploring data communications.
Topics discussed in this section:
Layered Architecture Peer-to-Peer Processes
Encapsulation

8. ISO is the organization. OSI is the model.
Note
ISO is the organization. OSI is the model.

9. Figure 2.2 Seven layers of the OSI model

10. Layered Architecture
The OSI model is composed of seven (7) order layers.
Figure 2.3 shows the layered involve when a message is sent from device A to device B. As the message travels from A to B, it may pass through many intermediate nodes. The intermediate nodes usually involve only the first three layers of the OSI model.
In developing the model, the designers distilled the process of transmitting data to its most fundamental elements. They identified which networking functions had related uses and collected those functions into discrete groups that became the layers. Each layers defines a family of functions distinct from those of the other layers. The designers created an architecture that is both comprehensive and flexible. Most importantly, the OSI model allows complete interoperability between otherwise incompatible systems.

11. Peer-to-Peer Processes
At the physical layer, communication is direct.
In Figure 2.3 device A sends a stream of bits to device B through intermediate nodes. At the higher layer, however, communication must move down through the layers on device A, over to device B, and then back u through the layers. Each layer in the sending device adds its own information to the message it receives from the layer just above its and passes the whole package to the layer just below it.
At layer 1 entire package is converted to a form that can be transmitted to the receiving device. At the receiving machine, the message is unwrapped layer by layer, with each process receiving and removing the data meant for it. For example, layer 2 remove the data meant for it , and then passes the rest to layer 3. Layer 3 then removes the data meant for it and passes the rest to layer 4, and so on.

12. Encapsulation
Figure 2.3 reveals another aspect of data communications in the OSI model: encapsulation. A packet (header and data) at level 7 is encapsulated in a packet at level 6. The whole packet at level 6 is encapsulated in a packet at level 5, and so on.
In other words, the data portion of a packet at level N – 1 carries the whole packet (data, header and trailer) from level N. The concept is called encapsulation: level N – 1 is not aware of which part of the encapsulated packet is data and which part is the header or trailer. For level N – 1, the whole packet coming from level N is treated as one integral unit.

13. Figure 2.3 The interaction between layers in the OSI model

14. Figure 2.4 An exchange using the OSI model

15. An exchange using OSI model
In Figure 2.4, which gives an overall view of the OSI layers. D7 means the data unit at layer 7, D6 means the data unit at layer 6, and so on. The process starts at layer 7 (the application layer), then moves from layer to layer in descending , sequentially order. At each layer, a header or trailer can be added to the data unit. Commonly, the trailer is added only at layer 2. When the formatted data unit passes through the physical layer (layer 1), it is changed into an electromagnetic signal and transported along a physical link.
Upon reaching its destination, the signal passes into layer 1 and is transformed back into digital form. The data units then move back up through the OSI layers. As each block of data reaches the next layer, the header and trailer attached to it at the corresponding sending layer are removed, and actions appropriate to that layer are taken. By the time it reaches layer 7, the message is again in a form appropriate to the application and is made available to the recipient.

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

17. Physical Layer
The physical layer coordinates the functions required to carry a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium. It also defines the procedure and functions that physical devices and interfaces have to perform transmission to occur.
Figure 2.5 shows the position of the physical layer with respect to the transmission medium and the data link layer.
The physical layer is also concerned with the following:
Physical characteristics of interfaces and medium
Representation of bits
Data rate
Synchronization of bits
Line Configuration
Physical topology
Transmission mode

18. Figure 2.5 Physical layer

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

20. Data Link Layer
The data link layer transforms the physical layer, a raw transmission facility, to a reliable link. It makes the physical layer appear error-free to the upper layer (network layer)
Figure 2.6 shows the relationship of the data link layer to the network and physical layer.
Other responsibilities of the data link layer include the following:
Framing
Physical Addressing
Flow Control
Error Control
Access Control

21. Figure 2.6 Data link layer

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

23. Figure 2.7 Hop-to-hop delivery

24. Network Layer
The network layer is responsible for the source-to-destination delivery of a packet, possibly across multiple networks (links). Whereas the data link layer oversees the delivery of the packet between two systems on the same packet (links), the network layer ensures that each packet gets from its point of origin to its final destination.
If two systems are connected to the same link, there is usually no need for a network layer. However, if the two systems are attached to different network (links) with connecting devices between the networks (links), there is often a need for a network layer to accomplish source to destination delivery.
Figure 2.8 shows the relationship of the network layer to the data link and transport layer.
Other responsibilities of the network layer include the following:
Logical Addressing
Routing

25. Figure 2.8 Network layer

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

27. Figure 2.9 Source-to-destination delivery

28. Transport Layer
The transport layer is responsible for process to process delivery of the entire message. A process is an application program running on a host. The transport layer ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-to-destination level.
Figure 2.10 shows the relationship of the transport layer to the network and session layer.
Other responsibilities of the transport layer include the following:
Service-point addressing
Segmentation and reassembly
Connection control
Flow control
Error Control

29. Figure 2.10 Transport layer

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

31. Figure 2.11 Reliable process-to-process delivery of a message

32. Session Layer
The service provided by the first three layer (physical, data link and network) are not sufficient for some processes.
The session layer is the network dialog controller. It establishes, maintains and synchronizes the interaction among communicating systems.
Figure 2.12 illustrates the relationship of the session layer to the transport and presentation layer.
Specific responsibilities of the session layer include the following:
Dialog control
Synchronization

33. Figure Session layer

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

35. Presentation Layer
The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems.
Figure 2.13 shows the relationship between the presentation layer and the application and session layers.
Specific responsibilities of the presentation layer include the following:
Translation
Encryption
Compression

36. Figure 2.13 Presentation layer

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

38. Application Layer
The application layer enables the user, whether human or software, to access the network. It provides user interfaces and support for services such as electronic mail, remote file access and transfer, share database management and other types of distributed information services.
Figure 2.14 shows the relationship of the application layer to the user and the presentation layers. The user in this example employs X.400 to send an message.
Specific services provided by the application layer include the following:
Network virtual terminal
File transfer, access and management
Mail services
Directory services

39. Figure 2.14 Application layer

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

41. Figure 2.15 Summary of layers

42. Topics discussed in this section:
2-4 TCP/IP PROTOCOL SUITE
The TCP/IP protocol suite was developed prior to the OSI model. 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.
Topics discussed in this section:
Physical and Data Link Layers Network Layer Transport Layer
Application Layer

43. TCP/IP - Introduction
TCP/IP is a hierarchical protocol made up of interactive modules, each of which provides a specific functionality; however the modules are not necessarily interdependent.
The layer of the TCP/IP protocol suite contain relatively independent protocols that can be mixed and matched depending on the need of the system.
The term hierarchical means that each upper-level protocol is supported by one or more lower-level protocols
At the transport layer, TCP/IP define three protocols: Transmission Control Protocol (TCP), User Datagram Protocol (UDP) and Stream Control Protocol (SCTP).
At the network layer, the main protocol defined by TCP/IP is the Internetworking Protocol (IP); there are also some other protocols that support data movement in this layer

44. TCP/IP Model Physical and Data Link Layer Network Layer
At the physical and data link layers, TCP/IP does not define any specific protocol. It support all the standard and proprietary protocols. A network in a TCP/IP internetwork can be LAN or WAN.
Network Layer
At the network layer, TCP/IP supports the Internetworking Protocol (IP). IP uses four supporting protocol: ARP, RARP, ICMP and IGMP.
Transport Layer
Traditionally the transport layer was represented in TCP/IP by two protocols: TCP and UDP. IP is a host-to-host protocol. UDP and TCP are transport level protocols. SCTP is a new transport layer protocol .
Application Layer
The application layer in TCP/IP is equivalent to the combined session, presentation and application layers in the OSI model.

45. Figure 2.16 TCP/IP and OSI model

46. Topics discussed in this section:
2-5 ADDRESSING
Four levels of addresses are used in an internet employing the TCP/IP protocols: Physical (link) Addresses, Logical (IP) Addresses, Port Addresses and Specific Addresses.
See Figure 2.17 for the Addresses in TCP/IP.
Each Address is related to a specific layer in the TCP/IP architecture, as shown on Figure 2.18.
Topics discussed in this section:
Physical Addresses Logical Addresses Port Addresses Specific Addresses

47. Figure 2.17 Addresses in TCP/IP

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

49. Physical Addresses
The physical address (also known as the link address) is the address of a node as defined by its LAN and WAN. It is included in the frame used by the data link layer. It is the lowest-level address.
The physical addresses have authority over the network (LAN or WAN). 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).
LocalTalk (Apple) has a 1-byte dynamic address that changes each time the station comes up.

50. Example 2.1
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.

51. Figure 2.19 Physical addresses

52. A 6-byte (12 hexadecimal digits) physical address.
Example 2.2
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:
07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical address.

53. Logical Addresses
Logical addresses are necessary for universal communications that are independent of underlying physical networks. Physical addresses are not adequate in an internetwork environment where different networks can have different address formats.
A universal addressing system is needed in which each host can be identified uniquely, regardless of the underlying physical network.
The logical addresses are designed for this purpose. A logical address in the Internet is currently a 32-bit address that can uniquely define a host connected to the Internet. No two publicly addressed and visible hosts on the Internet can have the same IP address.

54. Example 2.3
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.

55. Figure IP addresses

56. Port Addresses
The IP address and the physical address are necessary for a quantity of data to travel from a source to the destination host. However, arrival at the destination host is not the final objective of data communication on the Internet.
Today, computers are devices that can run multiple processes at the same time. The end objective of Internet Communication is a process communicating with another process.
For example:
Computer A can communicate with computer C by using TELNET.
At the same time, computer A communicate with computer B by using the File Transfer Protocol (FTP).
For these processes to receive data simultaneously, we need a addresses to label the different processes.
In the TCP/IP architecture, the label assigned to a process is called a port address.
A port address in TCP/IP is 16-bits in length.

57. Example 2.4
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.

58. Figure Port addresses

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

60. A 16-bit port address represented as one single number.
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.

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

62. Specific Addresses
Some application have user-friendly addresses that are designed for that specific address.
Examples include the address and the URL.
The first defines the recipient of an
The second is used to find a document on the World Wide Web.
These addresses get changed to the corresponding port and logical addresses by the sending computer.

63. Thanks You
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