Osi(1)

The OSI Reference Model

This layered network architecture model was developed by International Organization for standardization (ISO) located in Geneva, Switzerland.

It (OSI) was designed to develop common standards of network architecture throughout the world.

It provides a general concept of inter process communication in such a way that any open system may communicate with another open system technically without any problems.

It is a seven layered architecture model .

The model is designed in a highly structured way. It defines a separate set of protocols for each layer and hence each layer has a specific independent function.

The OSI model does not perform any function in the communication process. The actual work is done by the appropriate software and hardware. The OSI model simply defines which tasks need to be done and which protocols will handle those tasks.

Fig. shows the seven layered Architecture of OSI models along with protocols and interfaces.

In actual implementation of the seven layers, the first three layers are likely to be in hardware, the next two layers in the operating system, the presentation layer in library subroutines in the user's address space, and the application layer in the user's program.

Physical Layer

This is the lowest layer in the model. This layer is responsible for activating, maintaining and deactivating a physical circuit between two end systems.

The Physical Layer is responsible for sending bits (bits means binary digits, i.e. 1's and O's) from one computer to another. That is, it may convert the sequence of bits into electric signals, light signals, or electromagnetic signals, depending on whether the two nodes are on a cable circuit, fiber-optic circuit, or microwave/radio circuit, respectively.

Even electrical details, such as how many volts to use for 0 and 1, how many bits can be sent per second, and whether transmission can take place only in one direction or in both directions simultaneously, are decided by the physical layer protocols.

In addition, the physical layer protocols also deal with the mechanical details, such as the size and shape of the connecting plugs, the number of pins in the plugs, and the function of each pin.

Hubs, terminators, couplers, cables, connectors, repeater, multiplexers, transmitter and receives are devices associated with physical layer. The position of the physical layer with respect to the transmission medium and the data link layer is shown in Fig.

Summary of Functions of the physical layer  

1. Physical layer is responsible for sending bits from source computer to destination computer.

 2. It defines the bit transmission encoding i.e. how 0's and 1's are changed to signals.

3. It defines the transmission rate (or data rate) i.e. the number of bits transmitted per second.

4. It deals with line configuration

(a) Point-to-point (b) Multipoint

5.  It defines the transmission mode between two devices.

(a) Simplex (b) Half duplex (c) Full duplex

6.    Topology

It deals with physical topologies i.e. star, ring, bus, hyprid or mesh etc.

7.    Multiplexing

It deals with combining several data channels into one.

8.    Media bandwith (Data transmission speed)

9. It deals with the synchronization of the transmitter and receiver

Data Link Layer

The main task of the data link layer is to provide error free transmission. The physical layer simply transmits the data from the sender's node to the receiver's node as raw bits. It is the responsibility of the data-link layer to detect and correct any errors in the transmitted data. It accepts packets from the network layer and splits the packets into frames which are transmitted by sender through physical layer as shown in fig.

It also provides flow control and prevents overburdening of receiver with two much data at any instant.

The data link layer can also detect when frames are lost and request that those frames be sent again.

Bridges, intelligent hubs and network interface cards are devices typically associated with data link layer.

Functions of data link layer are summarised as :

1. Framming

It divides packets received from the network layer into manageable data units called frames.

2. Physical addressing

It adds a header to the frame to define the physical address of the sender and receiver of the frame.

3. Error control

Error control is main task of the data link layer. It uses the protocols to detect errors in frames and to ensure

transmission of correct frames.

4. Flow control

It provides a flow control mechanism to avoid a fast transmitter from overrunning a slow receiver by buffering the extra bits.

5. Access control

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

Network Layer

The network layer establishes a logical connection between sender and user by providing a logical path between them. This layer switches and routes message packets as necessary to get them to their destination.

The layer is responsible for addressing and delivering message packets.

Network layer makes routing decisions and forwards the packets for devices that are farther away than a

single link.

The network layer also may break large packets into small chunks if the packet is larger than the largest data frame the data link layer accept. The network reassembles the chunks into packets at receiving end.

Routers and gateways operate in the network layer.

The relationship of the network layer to the data link and transport layer is shown in Fig.

 The functions of the Network Layer are

1. Internetworking

This is the main duty of network layer. It provides. the logical connection between different types of networks.

 2. Addressing

Addressing is necessary to identify each device on the network uniquely. This is similar to a

telephone system.

3.             Routing

In a network, there are multiple roots available from a source to a destination and one of them is to be chosen. Routing algorithms are designed to find the most efficient paths between the source and destination nodes of a message.

4.             Packetizing

It breaks the larger packets into smaller packets if the packet is larger than the largest data frame the data link layer accept. This is called as packetizing.

5.             Data Switching

The process of transferring packet (data blocks) from source to destination node is called data switching. Hence, network layer is concerned with the circuit, message or packet switching.

6.  Congestion control

Congestion in a network may occur when the load on the network i.e. the number of packets sent to the network is greater than the capacity of the network. Network layer controls the congestion.

7.   It provides connection services, including network layer flow control, network layer error control and packet sequence control.