- IEEE 802.11 MAC protocol
for an Access Point:
- Transmitting
a data/poll frame:
- Access Point
must defer
PIFS time.
- To allow an ACK frame to be transmitted as early as possible !!!
- When the channel is
idle for
PIFS time:
- The Access Point can transmit one poll/data frame
Otherwise:
- Back off and re-try
- Access Point
must defer
PIFS time.
- Transmitting
a ACK frame:
- Access Point
must defer
SIFS time.
- When the channel is
idle for
PIFS time:
- The Access Point can transmit the ACK frame
Otherwise:
- Back off and re-try
- Access Point
must defer
SIFS time.
- Transmitting
a data/poll frame:
- IEEE 802.11 MAC protocol for an
user node (station):
- Transmitting a
data frame:
- The station
must
wait for
a poll message
from the Access Point
- When the station received the poll frame, the station can immediately (without waiting) tranmsmit the data frame
- The station
must
wait for
a poll message
from the Access Point
- Transmitting an
ACK frame:
- A station can
transmit an
ACK frame
without
being
polled !!!
(Because the ACK frame must be transmitted ASAP !!!)
- The user node (station)
must defer
SIFS time.
- When the channel is
idle for
PIFS time:
- The Access Point can transmit the ACK frame
Otherwise:
- Back off and re-try
- A station can
transmit an
ACK frame
without
being
polled !!!
- Transmitting a
data frame:
- Example:
Suppose A and
C
want to send a frame to B
Sequence of events:
NOTE:
- Node B can send its
ACK frame
before
the
AP
sends
the next poll frame because:
- SIFS duration is shorter than the PIFS duration !!!
- This will prevent the node A from timing out (and re-transmit the data frame) !!!
- Node B can send its
ACK frame
before
the
AP
sends
the next poll frame because: