Explain the purpose of the Medium Access Control (MAC) timing parameters Short InterFrame-
Space (SIFS), Point (coordination function) Interframe Space (PIFS), and Distributed
(coordination function) Interframe Space (DIFS). Explain why they should be carefully chosen.
Why has a station at least to wait one DIFS prior to transmission?
Solution
Answer
The purpose of the given MAC timing parameters are as below:
SIFS : Short Interframe Space, is the amount of time in microseconds needed by a wireless
interface to process a received frame and to respond with a response frame. It is given by the
difference in time of the first symbol of the response frame in the air and the last symbol of the
received frame in the air. A SIFS time comprises of the delay in receiver RF, PLCP delay and the
MAC processing delay, which depends on the physical layer used.
Point (co-ordination function) Interframe Space: it co-ordinates the communication within a
network.It is the time or duration for which PCF waits to grasp a channel.
Distributed (co-ordination function) Interface Space: it controls the access to the physical
medium. If DFC protocol finds that the medium is continuously idle for DIFS duration, it is then
permitted to transmit a frame. If during the DIFS interval the channel is found busy then that
channel is discarded and another channel is looked for.
A station has to wait for atleast one DIFS prior to transmission because generally it waits for
PIFS duration which is of shorter duration than DIFS and if PIFS is not available it moves to
DIFS duration which gets lapsed if a channel is not found. Therefore, atleast one DIFS duration
wait is required to know if a channel is found for the transmission or not..
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Explain the purpose of the Medium Access Control (MAC) timing paramet.pdf
1. Explain the purpose of the Medium Access Control (MAC) timing parameters Short InterFrame-
Space (SIFS), Point (coordination function) Interframe Space (PIFS), and Distributed
(coordination function) Interframe Space (DIFS). Explain why they should be carefully chosen.
Why has a station at least to wait one DIFS prior to transmission?
Solution
Answer
The purpose of the given MAC timing parameters are as below:
SIFS : Short Interframe Space, is the amount of time in microseconds needed by a wireless
interface to process a received frame and to respond with a response frame. It is given by the
difference in time of the first symbol of the response frame in the air and the last symbol of the
received frame in the air. A SIFS time comprises of the delay in receiver RF, PLCP delay and the
MAC processing delay, which depends on the physical layer used.
Point (co-ordination function) Interframe Space: it co-ordinates the communication within a
network.It is the time or duration for which PCF waits to grasp a channel.
Distributed (co-ordination function) Interface Space: it controls the access to the physical
medium. If DFC protocol finds that the medium is continuously idle for DIFS duration, it is then
permitted to transmit a frame. If during the DIFS interval the channel is found busy then that
channel is discarded and another channel is looked for.
A station has to wait for atleast one DIFS prior to transmission because generally it waits for
PIFS duration which is of shorter duration than DIFS and if PIFS is not available it moves to
DIFS duration which gets lapsed if a channel is not found. Therefore, atleast one DIFS duration
wait is required to know if a channel is found for the transmission or not.
