User Manual

Appendix
BreezeNET PRO.11 Series 9-28 User’s Guide
Figure 9.16: Frame Fragmentation
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 fol-
lowing paragraph), i.e. 78 microseconds.
• DIFS - Distributed IFS, is the Inter Frame Space used for a station will-
ing 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.