Polling is a method for controlling access to a shared medium where a primary station periodically polls or queries secondary stations on the network to see if they have any data to transmit. The primary station controls the link and knows when the medium is idle, but does not know the status of the receivers. It alerts receivers when it is ready to transmit and waits for an acknowledgement before sending data. The primary station must also poll each secondary station in turn when it is ready to receive data to see if any station has data to transmit. Polling allows for predictable access times but has drawbacks such as high dependence on the reliability of the primary station and reduced throughput under low loads.

1. Presented By
Saddam Hussain
Presentation on Controlled access
protocol: Polling

2. Contents
 What is polling?
 Functions of polling: Select and Poll
 Advantage and Drawbacks of polling
 References

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

4.  It works with topologies in which one device is
designated as a Primary Station and the other
devices are Secondary Stations.
 The Primary device controls the link, where as
the secondary follows it’s instructions.
 Exchange of data must be made through the
primary device even though the final destination
is secondary.

5. SELECT FUNCTION:
 Primary station controls the link and knows
medium is idle.
 But does not know whether receiver is ready or
not.
 Alerts the receiver, gets ack, sends the data and
get ack.

6.  Hence, it alerts the secondary for the upcoming
transmission and wait for an acknowledgement
(ACK) of secondary’s status.

7. POLL FUNCTION:
 When the primary is ready to receive data, it
must ask (poll) each device if it has anything to
send.
 If the secondary has data to transmit, it sends
the data frame. Otherwise, it sends a negative
acknowledgement(NAK) .
 The primary then polls the next secondary.
When the response is positive (a data frame),
the primary reads the frame and returns an
acknowledgment (ACK).

8.  There are two possibilities to terminate the
transmission: either the secondary sends all
data, finishing with an EOT frame, or the primary
says timer is up.

9. Advantages:
 Priorities can be assigned to ensure faster
access from some secondary .
 Maximum and minimum access times and data
rates on the channel are predictable and fixed.
Drawbacks:
 High dependence on the reliability of the
controller.
 Increase in turn around time reduces the
channel data rate under low loads and it’s
throughput.

10. Reference
 www.wikipedia.com/polling
 www.slideshare.net/search?=polling
 Data communications and networking by A.
Forouzan