Concept of OSI(Open standard interconnection) – Process of communication between two nodes in a network can be divided into seven layers. Between users, flow of data flow down through the layers in the source computer, and goes UP in the receiving computer. For example PC works on all seven layers.

The OSI reference model has the following seven layers:

Application layer is acting as an interface between the actual application programs. For example Microsoft Word does not reside at the Application layer but instead interfaces with the Application layer protocols. e.g.  FTP and TFTP.

Presentation layer presents data to the Application layer and is responsible for code formatting and data translation. Computers are configured to receive formatted data and then convert the data back into its native format for actual reading (e.g. EBCDIC to ASCII).

Session layer is responsible for setting up, managing, and then tearing down sessions between Presentation layer entities.

Transport layer segments and reassembles data into a data stream. The term reliable communication can be used at this layer. It means that acknowledgments, sequencing, and flow control will be used.

Flow control tells at what speed should data transfer. It prevents a sending host on one side of the connection from overflowing the buffers in the receiving host. Windows are used to control the amount of outstanding, unacknowledged data segments. When you have configured a window size of 1, sending machine waits for an acknowledgment for each data segment it transmits before transmitting another. If you’ve configured a window size of 3, it’s allowed to transmit 3 data segments before an acknowledgment is received.

If a receiving host fails to receive all the bytes that it should acknowledge, the host can improve the communication session by decreasing the window size.

Network layer manages device addressing, tracks the location of devices, and determines the best way to move data.

Data Link layer provides the physical transmission of the data and handles error notification, network topology, and flow control. Flow control means – It tells at what speed should data transfer. Error detection means – Receiver know what value of frame check sequence ,So receiver notified that this is same or not. If not same then frame is corrupted.

Logical Link Control (LLC) header tells the Data Link layer what to do with a packet once a frame is received. e.g. A host will receive a frame and look in the LLC header to find out where the packet is destined—say, the IP protocol at the Network layer. The LLC can also provide flow control and sequencing of control bits.

The switches and bridges both work at the Data Link layer and filter the network using Physical (MAC) addresses.

The biggest benefit of using switches instead of hubs in your network is that each switch port is actually its own collision domain. A hub creates one large collision domain. E.g. 24 port switch- 24 small small collision domain and only one broadcast domain. But router separate broadcast domains are there.

Physical layer sends bits and receives bits. Bits come only in values of 1 or 0—a Morse code. The DCE is usually located at the service provider, while the DTE is the attached device. The services available to the DTE are most often accessed via a modem or channel service unit/data service unit (CSU/DSU). Generally clock rate given on DCE side.

Q. In the following exhibit, identify the number of collision domain and broadcast domains in each specified device.

Ans. (A) Hub creates 1 large collision domain and 1 large broadcast domain.

(A) Bridge Breaks collision domain so 2 collision domain and 1 large broadcast domain.

(A) Switch Breaks collision domain so 4 collision domain and 1 large broadcast domain.

(A) Bridge Breaks collision domain and broadcast domain. So 3 collision domain and 3 broadcast domain.

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