Chapter 4 : Network models. Lecture 9 The OSI model: Layers in the OSI model : 7- Application Layer. 6- Presentation Layer. 5- Session Layer. 4- Transport Layer. 3- Network Layer. 2- Data Link Layer. 1- Physical Layer. 7- Application Layer : - The application layer enables the user, to access the network. - It provides user interfaces and support for services such as electronic mail, remote file access and transfer…. etc. 7- Application Layer : The application layer is responsible for providing services to the user. 6- Presentation Layer : - The presentation layer is responsible for translation, compression, and encryption. - Specific responsibilities of the presentation layer include the following: 1- Translation : At the sender it changes the information from its sender –dependent format into common format. At receiving, changes the common format into its receiver-dependent format. - It can be called network's translators. 6- Presentation Layer : 2- Encryption : To ensure privacy and security. 3- Compression : - Data compression reduces the number of bits contained in the information. - It is important in the transmission of multimedia such as audio or video. 6- Presentation Layer : The presentation layer is responsible for translation, compression, and encryption. 5- Session Layer : - The session layer is responsible for dialog control and synchronization. - Specific responsibilities of the session layer include the following: - Dialog control: - The session layer allows two systems to enter into a dialog. - It allows the communication between two processes to take place in either half-duplex (one way at a time) or full-duplex (two ways at a time) mode. 5- Session Layer : - Synchronization: The Session layer provides synchronization between user tasks by placing checkpoints in the data stream. By this way, if the network fails, only the data after the last checkpoint has to be retransmitted. 5- Session Layer : The session layer is responsible for dialog control and synchronization. 4- Transport Layer : - Layer 4, The transport layer is responsible for processto-process delivery of the entire message and dividing this message into segments. - Specific responsibilities of the transport layer include the following: - Segmentation and reassembly: A message is divided into transmittable segments, with each segment containing a sequence number. Numbers enable the transport layer to reassemble the message correctly upon arriving at the destination. 4- Transport Layer : - Process to process delivery: - Computers often run several programs at the same time. For this reason, source-to-destination delivery means delivery not only from one computer to the next but also from a specific process (running program) on one computer to a specific process (running program) on the other. - The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer. - The transport layer ensures that the messages are delivered error free. 4- Transport Layer : The transport layer is responsible for the delivery of a message from one process to another. Figure : Reliable process-to-process delivery of a message. 3- Network Layer : - Layer 3, the Network layer, Responsible from the delivery of packets from the original source to the final destination across multiple networks, to accomplish this , the network layer defines logical addresses to identify the endpoint of the network. - Network layer adds unique ( IP or logical address) to the packet. 3- Network Layer : - IP addresses 32-bit address that uniquely define a host connected to the Internet . No two host address on the internet can have the same IP address. - This layer also determines the route from the source to the destination computer. 3- Network Layer : The network layer is responsible for the delivery of individual packets from the source host to the destination host. Figure: Source-to-destination delivery 2- Data Link Layer : - Layer 2, the Data Link layer, sends data frames from the Network layer to the Physical layer. - Data Link Layer adds a header to the frame to define the sender and receiver of the frame. - Error control is normally achieved through a trailer added to the end of frame. 2- Data Link Layer : The data link layer is responsible for moving frames from one hop (node) to the next. Figure : Hop-to-hop delivery Example : 1- Physical Layer: - Layer 1, the bottom layer of the OSI model, is the physical layer. - It specifies how data is processed into bits and physically transferred over medium, such as cables. 1- Physical Layer: The physical layer is responsible for the movement of individual bits from one hop (node) to the next. The OSI model: OSI model Layers Data unit 7- Application Layer. 6- Presentation Layer. Data 5- Session Layer. 4- Transport Layer. Segments 3- Network Layer. Packet 2- Data Link Layer. Frame 1- Physical Layer. Bit Encapsulation and de-encapsulation Process: - The process of placing the header before the data and the trailer (not added by all layers) after the data is called Encapsulation of data. - On the receiving computer the process of removing these headers and trailers by the corresponding layers is called de-encapsulation. It’s the Data link layer, which adds the trailer to the data also.