i. An International Standards Organization (ISO) standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model.

ii. ISO is the organization; OSI is the model.

iii. An open system is a set of protocols that allows any two different systems to communicate regardless of their underlying architecture.

iv. The purpose of the OSI model is to show how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software.

v. It consists of seven separate but related layers, each of which defines a part of the process of moving information across a network.

Fig8: The OSI model

Layered Architecture:

i. The OSI model is composed of seven ordered layers: physical (layer 1), data link (layer 2), network (layer 3), transport (layer 4), and session (layer 5), and presentation (layer 6), and application (layer 7).

ii. The seven layers can be thought of as belonging to three subgroups. Layers 1, 2, and3; physical, data link, and network are the network support layers; they deal with the physical aspects of moving data from one device to another (such as electrical specifications, physical connections, physical addressing, and transport timing and

iii. reliability). Layers 5, 6, and 7; session, presentation, and application, can be thought of as the user support layers.

iv. In Figure9, which gives an overall view of the OSI layers, D7 data means the data unit at layer 7, D6 data means the data unit at layer 6, and so on. The process starts at layer 7 (the application layer), then moves from layer to layer in descending, sequential order.

v. At each layer, a header can be added to the data unit. At layer 2, a trailer may also be added.

vi. When the formatted data unit passes through the physical layer (layer 1), it is changed into an electromagnetic signal and transported along a physical link.

Fig9: An exchange using the OSI model

• Physical Layer :

  The physical layer coordinates the functions required to carry a bit stream over a physical medium.

  It deals with the mechanical and electrical specifications of the interface and transmission media. It also defines the procedures and functions that physical devices and interfaces have to perform for transmission to occur.

  The physical layer is also concerned with the following:

  i. Physical characteristics of interfaces and media: The physical layer defines the characteristics of the interface between the devices and the transmission media. It also defines the type of transmission media.

  ii. Representation of bits: The physical layer data consists of a stream of bits (sequence of 0s or 1s) with no interpretation. To be transmitted, bits must been coded into signals, electrical or optical. The physical layer defines the type of encoding.

  iii. Data rate: The transmission rate, the number of bits sent each second is also defined by the physical layer. In other words, the physical layer defines the duration of a bit, which is how long it lasts.

  iv. Synchronization of bits: The sender and receiver must not only use the same bit rate but must also be synchronized at the bit level. In other words, the sender and the receiver clocks must be synchronized.

  v. Physical topology: The physical topology defines how devices are connected to make a network. Devices can be connected using a mesh topology, a star topology, a ring topology or a bus topology.

  vi. Transmission mode: The physical layer also defines the direction of transmission between two devices: simplex, half-duplex, or full-duplex.

• Data Link Layer :

  The data link layer transforms the physical layer, a raw transmission facility, to a reliable link. Other responsibilities of the data link layer include the following:

  i. Framing: The data link layer divides the stream of bits received from the network layer into manageable data units called frames.

  ii. Physical addressing: If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the sender and/or receiver of the frame.

  iii. Flow control: If the rate at which the data is absorbed by the receiver is less than the rate produced at the sender, the data link layer imposes a flow control mechanism to prevent overwhelming the receiver.

  iv. Error control: The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames. Error control is normally achieved through a trailer added to the end of the frame.

  v. Access control: When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.

• Network Layer :

  The network layer is responsible for the source-to-destination delivery of a packet. The network layer ensures that each packet gets from its point of origin to its final destination. Other responsibilities of the network layer include the following:

  i. Logical addressing: The network layer adds a header to the packet coming from the upper layer that, among other things, includes the logical addresses of the sender and receiver.

  ii. Routing: When independent networks or links are connected together to create internetworks or a large network, the connecting devices route or switch the packets to their final destination. One of the functions of the network layer is to provide this mechanism.

• Transport Layer :

  The transport layer is responsible for process-to-process delivery of the entire message. The transport layer, ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-to-destination level.

  Other responsibilities of the transport layer include the following:

  i. Segmentation and reassembly: A message is divided into transmittable segments, with each segment containing a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission.

  ii. Connection control: A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine. A connection oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets.

  iii. Flow control: The transport layer is responsible for flow control.

  iv. Error control: Like the data link layer, the transport layer is responsible for error control. However, error control at this layer is performed process-to-process rather than across a single link.

• Session Layer :

  It establishes, maintains, and synchronizes the interaction between communicating systems. Specific responsibilities of the session layer include the following:

  i. 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 duplexor full-duplex mode.

  ii. Synchronization: The session layer allows a process to add checkpoints into a stream of data.

• Presentation Layer :

  The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems. Specific responsibilities of the presentation layer include the following:

  i. Translation: The processes in two systems are usually exchanging information in the form of character strings, numbers, and so on. The information should be changed to bit streams before being transmitted. The presentation layer is responsible for interoperability between these different encoding methods.

  ii. Encryption: Encryption means that the sender transforms the original information to another form and sends the resulting message out over the network.

  iii. Compression: Data compression reduces the number of bits contained in the information.

• Application Layer :

  The application layer enables the user, whether human or software, to access the network. It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services.

  Specific services provided by the application layer include the following:

  i. Network virtual terminal: A network virtual terminal is a software version of a physical terminal and allows a user to log on to a remote host.

  ii. File transfer, access, and management (FTAM): This application allows a user to access files in a remote host, to retrieve files from a remote computer for use in the local computer, and to manage or control files in a remote computer locally.

  iii. E-mail services: This application provides the basis for e-mail forwarding and storage.

  iv. Directory services:- This application provides distributed database sources and access for global information about various objects and services.

  Encapsulation:

  i. Fig9 reveals another aspect of data communications in the OSI model i.e. encapsulation.

  ii. A packet at level 7 is encapsulated in the packet at level 6. The whole packet at level 6 is encapsulated in a packet at level 5, and so on.

  iii. In other words, the data part of a packet at level N is carrying the whole packet (data and overhead) from level N+1.

  iv. The concept is called encapsulation because level N is not aware what part of the encapsulated packet is data and what part is the header or trailer.

  v. For level N, the whole packet coming from level N+1 is treated as one integral unit.