740
21 Wireless Local Area Networks
PLCP
the time required to transmit the physical layer convergence protocol
(PLCP)
D
the frame size
R
the
channel bit rate
CSMA/CA packet transmission time
BO
DIFS
2PLCP
D
R
SIFS
A
R
s
where:
A
the ACK frame size
BO
the
backoff time
DIFS
the distributed inter-frame space
SIFS
the short inter-frame space
The loss of performance strongly depends on
the packet size and data rate,
but a 30% loss is more than likely to occur. The smaller the packets, the larger
will be the impact of CSMA/CA on network performance.
To evaluate the perfor-
mance impact of CSMA/CA it is important to know how the various inter-frame
spaces are defi ned. The 802.11 standard defi nes the following four inter-frame
spaces to provide different priorities.
Short inter-frame space (SIFS): It is used to separate
transmissions belong-
ing 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 fi xed
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. For the 802.11 DSSS PHY the value is 10
s.
Point coordinate inter-frame space (PIFS): It is used by the AP to gain access
to the medium before any other station. This value
is SIFS plus a slot time
(i.e., 30
s).
Distributed inter-frame space (DIFS): It is the inter-frame space used for a
station willing to start a new transmission. It is calculated as PIFS plus one
slot time (i.e., 50
s).
Extended inter-frame space (EIFS): It is the longer
inter-frame space used
by a station that has received a packet which it could not understand. This
is required 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.
