Ch02

2.1

Chapter 2

Network Models

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2-1 LAYERED TASKS2-1 LAYERED TASKS

We use the concept of We use the concept of layerslayers in our daily life. As an in our daily life. As an example, let us consider two friends who communicate example, let us consider two friends who communicate through postal mail. The process of sending a letter to a through postal mail. The process of sending a letter to a friend would be complex if there were no services friend would be complex if there were no services available from the post office. available from the post office.

Sender, Receiver, and CarrierHierarchy

Topics discussed in this section:Topics discussed in this section:

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Figure 2.1 Tasks involved in sending a letter

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2-2 THE OSI MODEL2-2 THE OSI MODEL

Established in 1947, the International Standards Established in 1947, the International Standards Organization (Organization (ISOISO) is a multinational body dedicated to ) is a multinational body dedicated to worldwide agreement on international standards. An ISO worldwide agreement on international standards. An ISO standard that covers all aspects of network standard that covers all aspects of network communications is the Open Systems Interconnection communications is the Open Systems Interconnection ((OSIOSI) model. It was first introduced in the late 1970s. ) model. It was first introduced in the late 1970s.

Layered ArchitecturePeer-to-Peer ProcessesEncapsulation


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ISO is the organization.OSI is the model.

Note

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Figure 2.2 Seven layers of the OSI model

Main benefits of layered network

The process of breaking up the functions or tasks of networking into layers reduces complexity.

Each layer provides a service to the layer below it in the protocol specification.

Each layer communicates with the same layer’s software or hardware on other computers.

The lower 4 layers (transport, network, data link and physical —Layers 4, 3, 2, and 1) are concerned with the flow of data from end to end through the network.

The upper four layers of the OSI model (application, presentation and session—Layers 7, 6 and 5) are orientated more toward services to the applications.

Data is Encapsulated with the necessary protocol information as it moves down the layers before network transit.

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Figure 2.3 The interaction between layers in the OSI model

This interface defines what information and services a layer must provide for the layer above it. Between machines,

layer x on one machine communicates with layer x on another machine, by using a protocol (this is Peer-to-Peer Process).

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Figure 2.4 An exchange using the OSI model

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2-3 LAYERS IN THE OSI MODEL2-3 LAYERS IN THE OSI MODEL

In this section we briefly describe the functions of each In this section we briefly describe the functions of each layer in the OSI model.layer in the OSI model.

Physical LayerData Link LayerNetwork LayerTransport LayerSession LayerPresentation LayerApplication Layer


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Figure 2.5 Physical layer

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The physical layer is responsible for movements ofindividual bits from one hop (node) to the next.

Note

Physical Layer

The physical layer coordinates the functions required to transmit a bit stream over a physical medium.

Physical characteristics of interfaces and media: The physical layer defines the characteristics of the interface between devices and the transmission media, including its type.

Representation of the bits: Transmitted, bits must be encoded into signals –electrical or optical-. The physical layer defines the type of encoding.

Data rate: The physical layer defines the transmission rate, the number of bits sent each second.

Line configuration: the physical layer is concerned with the connection of devices to the medium.

Synchronization of bits Physical topology Transmission Mode

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Figure 2.6 Data link layer

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

Note

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Figure 2.7 Hop-to-hop delivery

Datalink Layer Fuctions

Framing. The data link layer divides the stream of bits received from the network layer into data units called frames.

Physical addressing. The data link layer adds a header to the frame to define the physical address of the sender (source address) and/or receiver (destination address) of the frame.

Flow Control Error Control :- Retransmit damaged or lost frame, recognize

duplicate frmae Access control :- In multi-point connection, which device has

control over medium If the frame is intended for a system outside the sender’s

network, the receiver address is the address of the device that connects one network to the next.

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Figure 2.8 Network layer

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The network layer is responsible for the delivery of individual packets from

the source host to the destination host.

Note

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Figure 2.9 Source-to-destination delivery

Network Layer Functions

Routing:- Defines the most optimum path the packet should take from the source to the destinationLogical addressing so that any endpoint can be identified. Facilitates interconnection between heterogeneous networks (Internetworking).

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Figure 2.10 Transport layer

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

Note

Segmentation and reassembly Connection control Flow control Error Control (process to process

rather than across a single link) Service point addressing(port

addressing)

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

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Figure 2.12 Session layer

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The session layer is responsible for dialog control and synchronization.

Note

Session layer provides mechanism for controlling the dialogue between the two end systems. It defines how to start, control and end conversations (called sessions) between applications.

This layer requests for a logical connection to be established on an end-user’s request.Any necessary log-on or password validation is also handled by this layer.Session layer is also responsible for terminating the connection.Session layer can also provide check-pointing mechanism such that if a failure of some sort occurs between checkpoints, all data can be retransmitted from the last checkpoint.

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Figure 2.13 Presentation layer

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

Note

Presentation layer defines the format in which the data is to be exchanged between the two communicating entities.

Also handles data compression and data encryption (cryptography).

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Figure 2.14 Application layer

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

Note

Application layer interacts with application programs and is the highest level of OSI model.

Application layer contains management functions to support distributed applications.

Examples of application layer are applications such as file transfer, electronic mail, remote login etc.

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Figure 2.15 Summary of layers

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2-4 TCP/IP PROTOCOL SUITE2-4 TCP/IP PROTOCOL SUITE

The layers in the The layers in the TCP/IP protocol suiteTCP/IP protocol suite do not exactly do not exactly match those in the OSI model. The original TCP/IP match those in the OSI model. The original TCP/IP protocol suite was defined as having four layers: protocol suite was defined as having four layers: host-to-host-to-networknetwork, , internetinternet, , transporttransport, and , and applicationapplication. However, . However, when TCP/IP is compared to OSI, we can say that the when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five layers: TCP/IP protocol suite is made of five layers: physicalphysical, , data linkdata link, , networknetwork, , transporttransport, and , and applicationapplication..

Physical and Data Link LayersNetwork LayerTransport LayerApplication Layer


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Figure 2.16 TCP/IP and OSI model

Advantages/Disadvantages of TCP/IP

A standard, routable enterprise networking protocol that is the most complete and accepted protocol available. All modern operating systems support TCP/IP, and most large private networks rely on TCP/IP for much of their traffic.

A technology for connecting dissimilar systems. Many TCP/IP application protocols were designed to access and transfer data between dissimilar systems. These protocols include HTTP, FTP, and Telnet.

A robust, scaleable, cross-platform client/server framework. A method of gaining access to the Internet. Disadvantage :- Overhead is higher, so not suitable for

video transmission

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2-5 ADDRESSING2-5 ADDRESSING

Four levels of addresses are used in an internet employing Four levels of addresses are used in an internet employing the TCP/IP protocols: the TCP/IP protocols: physicalphysical, , logicallogical, , portport, and , and specificspecific..

Physical AddressesLogical AddressesPort AddressesSpecific Addresses


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Figure 2.17 Addresses in TCP/IP

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Figure 2.18 Relationship of layers and addresses in TCP/IP

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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

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Figure 2.19 Physical addresses

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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.

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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

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Figure 2.20 IP addresses

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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

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Figure 2.21 Port addresses

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

Note

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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.

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

Note

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Summary1.The OSI model originally distinguishes between service,interface and protocols.1.The TCP/IP model doesnt clearly distinguish between service,interface and protocol.2.The OSI model is a reference model.2.The TCP/IP model is an implementation of the OSI model.3.In OSI model,the protocols came after the model was described.3.In TCP/TP model, the protocols came first and the model was really just a description of theexisting protocols.4.In OSI model,the protocols are better hidden.4.In TCP/IP model ,the protocols are not hidden.5.The OSI model has 7 layers.5.The TCP/IP model has only 4 layers.6.The OSI model supports both connectionless and connection-oriented communication in thenetwork layer,but only connection -oriented communication in transport layer.6.The TCP/IP model supports both connectionless and connection-oriented communication in the transport layer,giving users the choice.

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