Lec 2 OSI and TCP IP model by Dr. Shilpy.pptx

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Communication Architecture

Strategy for connecting host computers and other communicating equipment.

Defines necessary elements for data communication between devices.

A communication architecture, therefore, defines a standard for the communicating hosts.

A programmer formats data in a manner defined by the communication architecture and passes it on to the communication software.

Separating communication functions adds flexibility, for example, we do not need to modify the entire host software to include more communication devices.

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2.3

2-1 LAYERED TASKS

We use the concept of

layers

in our daily life.

• _{As an example, let us consider two friends who}

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2.4

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5

PROTOCOL LAYERS

In Chapter 1, we discussed that a

protocol is required when two entities

need

to

communicate.

When

communication is not simple, we may

divide

the

complex

task

of

communication into several layers. In

this

case,

we

may

need

several

protocols, one for each layer.

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Assume Maria and Ann are neighbors with a lot of common ideas. However, Maria speaks only Spanish, and Ann speaks only English. Since both have learned the sign language in their childhood, they enjoy meeting in a cafe a couple of days per week and exchange their ideas using signs. Occasionally, they also use a bilingual dictionary. Communication is face to face and Happens in one layer as shown in Figure 2.1.

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Now assume that Ann has to move to another town because of her job. Before she moves, the two meet for the last time in the same cafe. Although both are sad, Maria surprises Ann when she opens a packet that contains two small machines. The first machine can scan and transform a letter in English to a secret code or vice versa. The other machine can scan and translate a letter in Spanish to the same secret code or vice versa. Ann takes the first machine; Maria keeps the second one. The two friends can still communicate using the secret code, as shown in Figure 2.2.

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Layer Architecture

Layer architecture simplifies the network design.

A Layered architecture allows us to discuss a well-defined, specific part of the layer and complex system.

It is easy to debug network applications in a layered architecture network.

The network management is easier due to the layered architecture.

Network layers follow a set of rules, called protocol.

The protocol defines the format of the data being exchanged, and the control and timing for the handshake between layers.

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Open Systems

Interconnection (OSI) Model

International standard organization (ISO) established a committee in 1977 to develop an architecture for computer communication.

Open Systems Interconnection (OSI) reference model is the result of this effort.

In 1984, the Open Systems Interconnection (OSI) reference model was approved as an international standard for communications architecture.

Term “open” denotes the ability to connect any two systems which conform to the reference model and associated standards.

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OSI Reference Model

The OSI model is now considered the primary Architectural model for inter-computer communications.

The OSI model describes how information or data makes its way from application programmes (such as spreadsheets) through a network medium (such as wire) to another application programme located on another network.

The OSI reference model divides the problem of moving information between computers over a network medium into SEVEN smaller and more manageable problems .

This separation into smaller more manageable functions is known as layering.

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2.13

ISO is the organization. OSI is the model.

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2.14

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OSI Reference Model: 7

Layers

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OSI: A Layered Network

Model

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

Each layer provides a service to the layer above 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 three 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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Physical Layer

Provides physical interface for transmission of information.

Defines rules by which bits are passed from one system to another on a physical communication medium.

Covers all - mechanical, electrical, functional and procedural - aspects for physical communication.

Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other similar attributes are defined by physical layer specifications.

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2.19

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2.20

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

Note

The physical layer is also concerned with the following:

1. Physical characteristics of interfaces and medium 2. Representation of bits.

3. Data rate.

4. Synchronization of bits.

5. Line configuration: this layer is concerned with the connection of devices to the media i.e. point-to-point or multipoint

6. Physical topology.

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Data Link Layer

Data link layer attempts to provide reliable communication over the physical layer interface.

Breaks the outgoing data into frames and reassemble the received frames. Create and detect frame boundaries.

Implement Error Control by implementing an acknowledgement and retransmission scheme of damaged or lost frames.

Implement flow control.

Supports point-to-point as well as broadcast communication. Supports simplex, half-duplex or full-duplex communication.

Examples of Link layer protocols are Ethernet, WiFi and PPP (point-to-point)

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2.22

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Functions of Data Link Layer

• _Framing:_{Frames are the streams of bits received from the network layer into}

manageable data units. This division of stream of bits is done by Data Link Layer.

• _{Physical Addressing:}_{The Data Link layer adds a header to the frame in order to define}

physical address of the sender or receiver of the frame, if the frames are to be distributed to different systems on the network.

• _{Flow Control:}_{A flow control mechanism to avoid a fast transmitter from running a slow}

receiver by buffering the extra bit is provided by flow control. This prevents traffic jam at the receiver side.

• _{Error Control:}_{Error control is achieved by adding a trailer at the end of the frame.}

Duplication of frames are also prevented by using this mechanism. Data Link Layers adds mechanism to prevent duplication of frames.

• _{Access Control:}_{Protocols of this layer determine which of the devices has control over}

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2.25

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

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Network Layer

Implements routing of frames (packets) through the network.

Defines the most optimum path the packet should take from the

source to the destination

Defines logical addressing so that any endpoint can be identified.

Handles congestion in the network.

Facilitates interconnection between heterogeneous networks

(Internetworking).

The network layer also defines how to fragment a packet into smaller

packets to accommodate different media.

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2.27

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2.28

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

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Transport Layer

Purpose of this layer is to provide a reliable mechanism for the

exchange of data between two processes in different computers.

Ensures that the data units are

delivered error free

.

Ensures that data units are

delivered in sequence

.

Ensures that there is

no loss or duplication

of data units.

Provides

connectionless or connection oriented

service.

Provides for the connection management.

Multiplex multiple connection over a single channel.

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Session Layer

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.

Synchronization : This layer allows a process to add checkpoints which are considered as synchronization points into stream of data.

Example: If a system is sending a file of 800 pages, adding checkpoints after every 50 pages is recommended. This ensures that 50 page unit is successfully received and acknowledged. This is beneficial at the time of crash as if a crash happens at page number 110; there is no need to retransmit 1 to100 pages.

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Presentation Layer

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.

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Application Layer

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 (FTP), electronic mail (SMTP), remote login (TELNET),

HTTP, DNS etc.

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Functions of Application Layer

• _{Mail Services:}

_{This layer provides the basis for E-mail forwarding and storage.}

• _{Network Virtual Terminal:}

_{It allows a user to log on to a remote host.}

• _{Directory Services:}

_{This layer provides access for global information about}

various services.

• _{File Transfer, Access and Management (FTAM):}

_{It is a standard mechanism to}

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OSI In Action

A message begins at the top application layer and moves down the OSI

layers to the bottom physical layer.

As the message descends, each successive OSI model layer adds a header

to it.

A

header

is layer-specific information that basically explains what

functions the layer carried out.

Conversely, at the receiving end, headers are striped from the message

as it travels up the corresponding layers.

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TCP/IP Model

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TCP/IP Model

TCP/IP Model

Application Layer

Application programs using the network

Transport Layer (TCP/UDP)

Management of end-to-end message transmission, error detection and error correction

Network Layer (IP)

Handling of datagrams : routing and congestion

Data Link Layer

Management of cost effective and reliable data delivery, access to physical networks

Physical Layer

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