THE OSI MODEL Computer Networks - part V
LAYERED TASKS • 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. Figure 1- Tasks involved in sending a letter
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. • ISO is the organization. OSI is the model. • Topics covered: • Layered Architecture • Peer-to-Peer Processes • Encapsulation
THE OSI MODEL • 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 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 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 MODEL • 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 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 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
User support layers Network support layers Figure 3- The interaction between layers in the OSI model
Data Encapsulation • 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.
PDU’s and the OSI Model Encapsulation Decapsulation
LAYERS 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).
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 6- Data link layer
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 7- Hop-to-hop delivery
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 8- Network layer
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 9- Source-to-destination delivery
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 10- Transport 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 12- Session 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 13- Presentation layer
The application layer is responsible for providing services to the user. Figure 14- Application layer