ISO/OSI Network Model

ISO/OSI Network Model

The ISO-OSI model is the International Standard Organizations Open System Interconnect model. It is the standard model for networking protocols and distributed applications. THis model defines seven network layers.

Brief information about each layer of ISO-OSI Model as follows

Physical Layer

Physical layer defines the physical medium like cables e.g. unshielded twisted pairs (UTP) ,thinnet, thicknet etc. All media are functionally nearly equivalent.This includes the layout of pins, voltages, and cable specifications.

The main difference is in cost and convenience of installation and maintenance.

Data Link Layer

Data Link layer is the second layer. It defines the actual format of data on the network. A network data frame, packet, includes checksum, source and destination address, and data.This is the layer at which the bridges and switches operate.

The data link layer handles the physical and logical connections to the packets destination, using a network interface. The largest packet that can be sent through a data link layer defines the Maximum Transmission Unit (MTU). A host connected to an Ethernet would have an Ethernet interface to handle connections to the outside world, and a loopback interface to send packets to itself.

Ethernet addresses a host using a unique, 48-bit address called its Ethernet address or Media Access Control (MAC) address.

Network Layer

The Network layer basically perform establishes,maintains and terminates the network connections. THis layer performs network routing functions, and might also perform segmentation/desegmentation, and report delivery errors.NFS uses Internetwork Protocol (IP) as its network layer interface. IP is responsible for routing, directing datagrams from one network to another.

The network layer may have to break large datagrams, larger than MTU, into smaller packets and host receiving the packet will have to reassemble the fragmented datagram. 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 identifies the host on that network.

Transport Layer

The Transport layers application is transparent transfer of data between end users

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. NFS is built on top of UDP because of its speed and statelessness. Statelessness simplifies the crash recovery.

Session Layer:

The session layer is user to control the session. It establishes, manages and terminates the connections between the local and remote application.

The session protocol defines the format of the data sent over the connections. The NFS uses the Remote Procedure Call (RPC) for its session protocol. RPC may be built on either TCP or UDP. Login sessions uses TCP whereas NFS and broadcast use UDP.

Presentation Layer

The presentation layer performs the data transformation to provide common interface for user application such as reformation data encryption and compression. It converts local representation of data to its canonical form.

Application Layer

It provides services directly to user applications. Such as network services to the end-users. Mail, ftp, telnet, DNS, NIS, NFS are examples of network applications.

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