User`s guide

Appendix
User’s Guide 9-32 BreezeNET PRO.11 Series
9.5.3.5 Inter-Frame Spaces
The Standard defines 4 types of Inter Frame Spaces, which are use 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, therefore
giving it 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 called in this case), to gain access to the medium before any
other station. This value is SIFS plus a Slot Time (defined in the following
paragraph), 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.
9.5.3.6 Exponential Back-off Algorithm
Back-off 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.