OSI Network Model

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OSI Network Model


Layer 1 – Physical

Physical layer defines the cable or physical medium itself, e.g., thinnet, thicknet, unshielded twisted pairs (utp). All media are functionally equivalent. The main difference is in convenience and cost of installation and maintenance. Converters from one media to another operate at this level. Hubs and repeaters operate at this layer. Represents bits onto media as electrical signals or pulses of lights

Layer 2 – Data link

Data link layer defines the format of data on the network. A network data frame, aka packet, includes checksum, source and destination address, and data. The largest packet that can be sent through a data link layer defines the maximum transmission unit (mtu). The data link layer handles the physical and logical connections to the packet’s destination, using a network interface. A host connected to an ethernet would have an ethernet interface to handle connections to the outside world.

Layer 3 – Network

This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing, the internetwork protocol identifies each host with a 32-bit ip address. Ip addresses are written as four dot-separated decimal numbers between 0 and 255, e.g., The leading 1-3 bytes of the ip identify the network and the remaining bytes identify the host on that network. The network portion of the ip is assigned by internic registration services, under the contract to the national science foundation, and the host portion of the ip is assigned by the local network administrators, locally by noc@indiana.edu. For large sites, usually subnetted like ours, the first two bytes represents the network portion of the ip, and the third and fourth bytes identify the subnet and host respectively.

Even though ip packets are addressed using ip addresses, hardware addresses must be used to actually transport data from one host to another. The address resolution protocol (arp) is used to map the ip address to it hardware address.

Layer 4 – Transport

This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer. Transport layer subdivides user-buffer into network-buffer sized datagrams and enforces desired transmission control. Two transport protocols, transmission control protocol (tcp) and user datagram protocol (udp), sits at the transport layer. Reliability and speed are the primary difference between these two protocols. Tcp establishes connections between two hosts on the network through ‘sockets’ which are determined by the ip address and port number. Tcp keeps track of the packet delivery order and the packets that must be resent. Maintaining this information for each connection makes tcp a stateful protocol. Udp on the other hand provides a low overhead transmission service, but with less error checking.

Layer 5 – Session

This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.

Layer 6 – Presentation

This layer provides independence from differences in data representation (e.g., encryptions) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

Layer 7 – Application

This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntaxare identified. Everything at this layer is application-specific. This layer provides application services for fille-transfers, email and other network software services. Telnet and ftp are applications that exist entirely in the application level.

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