- The optional
point coordination function
provides contention-free frame transfer.
- Initially, the
802.11 system is operating in the
Distributed Mode
- An Access Point can
make stations enter the
Point Coordinated Function mode:
- The AP (coordinator)
sends a beacon frame that
contains the
duration of
the contention-free period
(The duration parameter is called CFPMaxDuration)
- When stations receive the beacon, they update their
Network Allocation Vector (NAV)
with the CFPMaxDuration
value.
- The net effect of this is preventing stations from transmitting on the medium until the end of the contention-free period.
(More on the NAV when we discuss DCF)
- During this period, a station will only transmit when it is given permission
Example:
- The AP (coordinator)
sends a beacon frame that
contains the
duration of
the contention-free period
- After sending the beacon,
the 802.11 system enters into the
Point Coordinate Function mode
- Operation of the PCF mode:
- Access Point
will sense PIFS time
before transmitting a frame
- A station that wants to
transmit/retranmit a data frame
must wait for a poll message
from the AP
- A station that wants to transmit a ACK frame will sense SIFS before transmitting the ACK frame
- Access Point
will sense PIFS time
before transmitting a frame
- Example:
Suppose A and
C
want to send a frame to B
Sequence of events:
NOTE:
- Node B will be able to
send its ACK
before AP sends the next poll
- This will prevent node A from timing out !!!
- Node B will be able to
send its ACK
before AP sends the next poll
- The AP can send the following
frames in the PCF mode:
- Data frame
- Frame contains data intended for one of the station
- CF Poll frame
- contains a poll message
- The polled station must transmit a message (use a null message if you don't have anything to send)
- contains a poll message
- Data + CF Poll frame
- contains a message
and a
poll
to the same station
- The polled station will receive the message and simultaneously allow to transmit a message (use a null message if you don't have anything to send)
- contains a message
and a
poll
to the same station
- CF End frame
- terminates the PCM mode
- This frame is sent when:
- CFPDurRemaining time
expires, or...
- The point coordinator has
no further frames
to transmit
and
no stations to poll...
(Stations have an option of being pollable.
A station can indicate its desire for polling using the CF Pollable subfield within the Capability Information field of an association request frame.)
- CFPDurRemaining time
expires, or...
- terminates the PCM mode
- Data frame
- When there are multiple APs
operating as coordinators,
there is a risk of collisions
when their cells overlap
- To minimize these collisions,
the point coordinator utilizes
a random backoff time
if the point coordinator when
they collide
(Collision is "detected" (assumed ?) when a coordinator sends a data frame and does not receive the corresponding ACK)