Computer network assignment

     Assignment:-01

Q 1. What is polling in control access ?

    Ans:-Polling is the process of sending a signal to a client (a computer or terminal) that gives it permission to transmit or asks it to receive. With polling, the clients store all messages that need to be transmitted. Periodically, the server (usually a mainframe computer) polls the client to see if it has data to send. If the client has data to send, it does so. If the client has no data to send, it responds negatively, and the server asks another client if it has data to send.

Polling process is similar to the roll-call performed in class. Just like the teacher, a controller sends a message to each node in turn.

• In this, one acts as a primary station(controller) and the others are secondary stations. All data exchanges must be made through the controller.
• The message sent by the controller contains the address of the node being selected for granting access.
• Although all nodes receive the message but the addressed one responds to it and sends data, if any. If there is no data, usually a “poll reject”(NAK) message is sent back.
• Problems include high overhead of the polling messages and high dependence on the reliability of the controller.

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