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APPENDIX E: IEEE 802.11 Technical Tutorial
E.3.5 I
NTER
F
RAME
S
PACES
The Standard defines four types of Inter-Frame Spaces, which are used to provide
different priorities:
•
SIFS
,
Short Inter-Frame Space
, separates transmissions belonging to a single
dialog (e.g. Fragment-Ack), and is the minimum Inter-Frame Space. There is
always at most one single station to transmit at any given time, so it has priority
over all other stations.
This value is a fixed value per PHY and is calculated in such a way that the
transmitting station will be able to switch back to receive mode and be capable
of decoding the incoming packet. On the 802.11 FH PHY, this value is set to 28
microseconds.
•
PIFS
,
Point Coordination IFS
, is used by the Access Point (or Point
Coordinator, as it’s called in this case), to gain access to the medium before
any other station. This value is SIFS plus a Slot Time (defined in
Section
E.3.6
), i.e. 78 microseconds.
•
DIFS
,
Distributed IFS
, is the Inter-Frame Space used for a station willing to
start a new transmission, which is calculated as PIFS plus one slot time, i.e. 128
microseconds.
•
EIFS
,
Extended IFS
, which is a longer IFS used by a station that has received a
packet that it could not understand. This is needed to prevent the station
(which could not understand the duration information for the Virtual Carrier
Sense) from colliding with a future packet belonging to the current dialog.
E.3.6 E
XPONENTIAL
B
ACKOFF
A
LGORITHM
Backoff is a well-known method used to resolve contention between different
stations wanting to access the medium. The method requires each station to
choose a random number (n) between 0 and a given number, and wait for this
number of Slots before accessing the medium, always checking if a different station
has accessed the medium before.
The Slot Time is defined in such a way that a station will always be capable of
determining if another station has accessed the medium at the beginning of the
previous slot. This reduces collision probability by half.
Exponential Backoff means that each time the station chooses a slot and happens
to collide, it will increase the maximum number for the random selection
exponentially.
Summary of Contents for LW0050A
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