We will take the study of access control methods to the next step and learn about the other method called polling. We will learn about what is polling, how transmission of data occurs while polling. Then, We will further look at the advantages and disadvantages of polling to clear our understanding.

What is Polling?

To impose order on a network of independent users to establish one station in the network as a controller that periodically polls all other stations is called polling.

There are two types of polling policies –
(a) Round robin order
(b) Priority order

Working of Polling

Polling Tutorial Example

Polling works with topologies having Primary Station and Secondary Stations.

• The Primary Station Controls the link whereas the secondary station follows its instructions.
• The exchange of data occurs only and only through the primary device even when the final destination of transmission is secondary.
• Whenever primary Station has something to send, it sends the message to each node.
• Before sending the data, it creates and transmits a Select (SEL) Frame. One field of SEL Frame includes the address of the intended secondary station.
• While sending, the primary station should know whether the target device is ready to receive or not.
• Hence, it alerts the secondary station for upcoming transmission and wait for an acknowledgement (ACK) of secondary station’s status.
• Poll Function: When the primary is ready to receive data, it must ask (poll) each device if it has anything to send.

Negative Acknowledge Example

• If the secondary has data to transmit, it sends the data frame. Else, it sends a negative acknowledgement (NAK).
• The primary station then polls the next secondary. When the response is positive (a data frame), the primary station reads the frame and returns an acknowledgement (ACK).
• There are two possibilities to terminate the transmission –
  (a) either the secondary sends all data, finishing with an EOT frame.
  (b) or, the primary station shows the timer up signal.

Notes to Remember:

• In polling time is allotted to a station only if requested.
• If a station does not want to transmit data it will never participate in polling.
• When a station is picked up by polling that gives a chance for the station to transmit.
• Efficiency of polling = $ (\frac {Useful Time}{Cycle Time}) = (\frac {TT}{T_{polling} + TT + TP}) $ .
  Where,
  – TT = Transmission time
  – TP = Propagation time
  – $T_{polling} $ = Time required for polling.

Advantage Of Polling

– No state is ever wasted

Disadvantage Of Polling

– No fair sharing.

Till now we have seen in greater detail about the TDM and Polling methods of access control. All the required points are extensively covered. So, after the TDM and Polling method we will now move ahead to learn about the CSMA/CD in the next tutorial.

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