Computer Networks - part V THE OSI MODEL. LAYERED TASKS Figure 1- Tasks involved in sending a letter There are 3 different activities at the sender site.

Computer Networks - part V

THE OSI MODELTHE OSI MODEL

LAYERED TASKSLAYERED TASKS

Figure 1- Tasks involved in sending a letter

•There are 3 different activities at the sender site and 3 at the receiver site. •Must be done in the order of the layers.•Each layer at the sending site uses the services of the layer right below it.

THE OSI MODELTHE OSI MODEL

1.1. Established in 1947, the International Standards Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards. worldwide agreement on international standards.

2.2. An ISO standard that covers all aspects of network An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) communications is the Open Systems Interconnection (OSI) model. It was first introduced in the late 1970s. model. It was first introduced in the late 1970s.

3.3. ISO is the organization. OSI is the model.ISO is the organization. OSI is the model.4.4. Topics covered:Topics covered:

1.1. Layered ArchitectureLayered Architecture2.2. Peer-to-Peer ProcessesPeer-to-Peer Processes3.3. EncapsulationEncapsulation

• It is now considered the primary Architectural model for inter-computer communications.

• The Open Systems Interconnection (OSI) reference model is a descriptive network scheme. It ensures greater compatibility and interoperability between various types of network technologies.

• It describes how information or data makes its way from application programmes through a network medium 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.

THE OSI MODELTHE OSI MODEL

Figure 2- Seven layers of the OSI model

THE OSI MODELTHE OSI MODEL

Why use a layered approach ? Data communications requires complex procedures

Sender identifies data path/receiver Systems negotiate preparedness Applications negotiate preparedness Translation of file formats

For all tasks to occur, a high level of cooperation is required

Provide framework to implement multiple specific protocols per layer

THE OSI MODELTHE OSI MODEL

Advantages of Layering Easier application development Network can change without all programs being

modified Breaks complex tasks into subtasks Each layer handles a specific subset of tasks

Communication occurs between different layers on the same node or stack

(INTERFACES) – vertical communications between similar layers on different nodes or stacks

(PEER-TO-PEER PROCESSES) – horizontal communications

• The OSI Reference Model is composed of seven layers, each specifying particular network functions.

• 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.

• The lower 4 layers (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 (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.

THE OSI MODELTHE OSI MODEL

• The seven layers of the OSI model ca be divided into two categories:

• Upper layers (Application set – 3 layers)

• Lower layers (Transport set – 4 layers)

THE OSI MODELTHE OSI MODEL

• Network support layers : Layers 1, 2, 3• Concerned with flow of data from end to end through

Network– Combination of HW & SW– Physical layer always implemented in Hardware

• User support layer : Layer 5, 6, 7 – Always implemented in Software.– It allows interoperability among unrelated software systems

• Transport layer (Layer 4) : links the two subgroups• Layers glued together by interfaces• Each interface defines what info & services it must provide

for the above layer

THE OSI MODELTHE OSI MODEL

(A)ll 7. (A)pplication (A)way

(P)eople 6. (P)resentation (P)izza

(S)eem 5. (S)ession (S)ausage

(T)o 4. (T)ransport (T)hrow

(N)eed 3. (N)etwork (N)ot

(D)ata 2. (D)ata Link (D)o

(P)rocessing 1. (P)hysical (P)lease

MnemonicsMnemonics

Figure 3- The interaction between layers in the OSI model

Network support layers

User support layers

• Data exists at each layer contained within a unit called a Protocol Data Unit (PDU).

• Data Encapsulation is the process of adding a header to wrap the data that flows down the OSI model.

• The 5 Steps of Data Encapsulation are: 1. The Application, Presentation and Session layers create

DATA from users' input. 2. The Transport layer converts the DATA to SEGMENTS 3. The NW layer converts the Segments to Packets

(datagram) 4. The Data Link layer converts the PACKETS to FRAMES 5. The Physical layer converts the FRAMES to BITS.

Data EncapsulationData Encapsulation

Figure 4- An exchange using the OSI model

Layer PDU Name

7. Application Data

6. Presentation Data

5. Session Data

4. Transport Segment

3. Network Packet

2. Data Link Frame

1. Physical Bits

PDU’s and the OSI ModelPDU’s and the OSI Model

LAYERS IN THE OSI MODELLAYERS IN THE OSI MODEL

Figure 5- Physical layer

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

•The interface and the type of the physical transmission medium•Raw bits -> signals•Bit duration•How the devices are connected to the media (point-to-point, or multipoint)•How devices are connected to each other(mesh, star, ring, bus, or hybrid)•The direction of transmission( simplex, half-duplex, or full-duplex).

Figure 6- Data link layer

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

•Makes the raw transmission facility (physical layer), reliable. Error-free to the upper layer (network).•Divides the stream of bits into frames (data units)•Adds a header to define the send and/or receiver of the frame•Flow control mechanism to avoid overwhelming the receiver (receiver slower than sender). Detect and retransmit damaged or lost frames. Recognize duplicate frames.

Figure 7- Hop-to-hop delivery

•Communication at the data link layer occurs between two adjacent nodes.

•For a to f, 3 partial deliveries are made. a to b, b to e, and e to f. Different headers.

Figure 8- Network layer

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

•Ensures that each packet gets from origin to final destination.•If two systems are connected to the same link, there is usually no need for a network layer. If different links, need. •Network layer adds logical addresses of the sender and receiver•Routing: routers/switchers route or switch the packets to their final destination.

Figure 9- Source-to-destination delivery

When packet gets B, B makes a decision based on the final F. B is a router, it uses its routing table to find that the next hop is router E, so send to E.

Figure 10- Transport layer

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

•Ensures the whole message arrives intact and in order, overseeing both error control and flow control at the source-to-destination level. •Service-point addressing: specific process (like email, msn, etc)•A message is divided into segments, containing a sequence number

Figure 11- Reliable process-to-process delivery of a message

Figure 12- Session layer

•Establishes, maintains, and synchronized the interaction among communicating systems.•Synchronization: allows a process to add checkpoints, or synchronization points, to a stream of data. •Responsible for enforcing the rules of dialog (e.g., Does a connection permit half-duplex or full-duplex communication?), synchronizing the flow of data, and reestablishing a connection in the event a failure occurs.

Figure 13- Presentation layer

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

• Provides for data formats, translations, and code conversions.• Concerned with syntax and semantics of data being transmitted.• Encodes messages in a format that is suitable for electronic transmission.• Data compression and encryption done at this layer.• Receives message from application layer, formats it, and passes it to the session layer.

Figure 14- Application layer

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

Figure 15- Summary of layers