Mumbai University > Information Technology > Sem 4 > CN

Marks: 10M

Year: May 2016

• An ISO standard that covers all aspects of network communications is the Open Systems Interconnection model.

• The OSI model is a layered framework for the design of network systems that allows communications between all types of computer systems. It consists of seven separate but related layers, each of which defines a part of the process of moving information across a network.

• Seven layers of the OSI model:

  The seven layers can be thought of as belonging to three subgroups:

  1.Layers 1, 2, and 3-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 reliability).

  2.Layers 5, 6, and 7-session, presentation, and application-can be thought of as the user support layers; they allow interoperability among unrelated software systems.

  3.Layer 4, the transport layer, links the two subgroups and ensures that what the lower layers have transmitted is in a form that the upper layers can use.

Figure1. 7 layers of OSI Model

Figure2. The interaction between OSI Model Layers

Functions of each of the layer are described below:

1. Physical Layer: It is responsible for movements of individual bits from one hop (node) to the next.

   Functions:

   i. To activate, maintain and deactivate the physical connection

   ii. To define voltages and data rates needed for transmission

   iii. To convert digital bits into electrical signal

   iv. To decide whether the transmission is simplex, half-duplex or full-duplex.

   v. A physical layer is concerned with the connection of devices to the media (Line configuration).

   vi. It also defines the physical topology.

   vii. It also helps in synchronization of bits.

2. Data Link layer: It transforms the physical layer, a raw transmission facility to a reliable link. It is responsible for moving frames from one hop (node) to the next i.e Hop-to-Hop delivery.

   Functions:

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

   ii. Physical addressing: It adds a header to the frame to define the physical address of the sender and/or receiver of the frame.

   iii. Flow Control: It provides a flow control mechanism to avoid a fast transmitter from over-running a slow receiver by buffering the extra bits.

   iv. Error control: It is achieved by adding a trailer at the end of the frame. It also uses a mechanism to prevent duplication of frames.

   v. Access Control: The layer determines which device has control over the link at any given time, when two or more devices are connected to the same link.

3. Network Layer: The network layer is responsible for the delivery of individual packets from the source host to the destination host i.e End to End delivery or source to destination delivery.

   Functions:

   i. It translates logical network address into physical machine address i.e. the numbers used as destination IDs in the physical network cards

   ii. It determines the quality of service by deciding the priority of message and then route a message will take if there are several ways a message can get to its destination.

   iii. It breaks the larger packets into smaller packets if the packet is larger than the largest data frame the data link will accept.

   iv. Routers and gateways operate in the network layer.

4. Transport layer: It is responsible for process-to-process delivery of the entire message.i.e. source to destination delivery of the entire message. It ensures that the whole message arrives intact and in order, ensuring both error control and flow control at source destination level.

   Functions:

   i. Segmentation and re-assembly: It divides each message into packets at the source and reassembles than at the destination.

   ii. Service point addressing The transport layer header H4 includes service point to deliver a specific process from source to a specific process at the destination.

   iii. Connector Control: The layer can be either connectionless or connection oriented.

   iv. Flow Control: It provides end-to-end flow control rather than across a single link.

   v. Error Control: It ensures that the entire message arrives at the receiving transport layer without error.

5. Session Layer: It is responsible for dialog control and synchronization i.e it is network dialog controller. It establishes maintains and synchronizes the interaction among communicating systems.

6. Presentation Layer: It is responsible for translation, compression and encryption. It is concerned with the syntax and semantics of the information exchanged between two systems.

7. Application Layer: It is responsible for providing services to the user. It provides services that directly support user application such as database access, e-mail, file transfer.

   Functions:

   i. Network virtual terminal: The layer creates a software emulation of a terminal at the remote host. The user’s computer tasks to the software terminal, then the software terminal talks to the host and vice versa. The remote host feels that it is communicating with one of its own terminal and allows you to log on.

   ii. Directory services: It provides distributed database sources and access to the worldwide information about various objects and services.

Protocols at different layers are shown below: