OSI - THE MODEL A layer model (Open Systems Interconnection) Each layer performs a subset of the required communication functions Each layer relies on the next lower layer to perform more primitive functions Each layer provides services to the next higher layer Changes in one layer should not require changes in other layers
WHY LAYERS…? A layered architecture allows us to discuss a well- defined ,specific part of a large and complex system. Breaks down the complex operation of networking into simple elements. Helps developers in developing software. It is also much easier to change the implementation of the service provided by the layer. Helps in troubleshooting and identifying the problem. It provides a way to understand how an internet work operates. Task broken into subtasks
REAL TIME EXAMPLE
PHYSICAL LAYER The physical layer is responsible for transmitting individual bits from one node to the next. The physical layer defines Media type ,Connector type and Signaling type . Physical layer defines the following characteristics of interface between data transmission device (e.g. computer) and transmission medium or network. Mechanical Electrical Functional Procedural Representation of bits Data rates
DATA LINK LAYER Data link layer is responsible for activating, maintaining and deactivating a reliable link between two nodes. The data link layer is responsible for transmitting frames from one node to another. 1. Framing 2. Physical Addressing 3. Flow control 4. Error control 5. Access control
DATA LINK LAYER
NETWORK LAYER The network layer is responsible for the delivery of packets from the original source to the final destination. 1. Data is in the form of Packets 2. Logical (IP) Addressing 3. Routing 4. Main protocols at this layer are IP ,ICMP ,ARP
NOTE The network layer is responsible for the source to destination delivery of a packet possibly across multiple networks. Where as the data link layer oversees the delivery of the packet between two systems on the same network.
TRANSPORT LAYERThe transport layer is responsible for delivery of a message from one process to another.It defines: 1. Reliable delivery of data 2. Ordering of delivery 3. Port addressing 4. Segmentation and reassembly 5. Connection control 6. Flow control and Error control 7. Main protocols are TCP and UDP
SESSION LAYER This OSI layer, as the name implies: 1. Establishes 2. Maintains 3. Ends sessions between nodes 4. Manage transmit data Dialog controller i.e. whether connection is half duplex or full duplex.
PRESENTATION LAYER Defines the format of data being sent The presentation layer is designed to handle the syntax and semantics of the information exchanged between two systems Main functions of this layer are 1. Compression 2. Encryption and Decryption Some Examples of services used are listed below: 1. HTML (Hyper Text Markup Language) 2. GIF (Graphics Interchange Format ) 3. JPEG (Joint Photographic Experts Group) 4. ASCII (American Standard Code for Information Interchange)
APPLICATION LAYER The application layer is responsible for providing services to the user. The application layer enables the user to access the network. 1. Mail services 2. File transfer and access 3. Remote log-in 4. Accessing the world wide web 5. Main protocols at this layer are HTTP, SMTP, SNMP, FTP, TELNET