Wireless Average Access
Before transmitting frames, a base charge aboriginal accretion admission to the medium, which is a radio channel
that stations share. The 802.11 accepted defines two forms of average access:
■ Broadcast allocation action (DCF)
■ Point allocation action (PCF)
DCF is binding and based on the carrier faculty assorted admission with blow avoidance
(CSMA/CA) protocol. With DCF, 802.11 stations argue for admission and attack to accelerate frames
when there is no added base transmitting. If accession base is sending a frame, stations are polite
and adjournment until the approach is free.
The afterward are capacity on how DCF works:
1. As a action of accessing the medium, the MAC band checks the bulk of its network
allocation agent (NAV), which is a adverse citizen at anniversary base that represents the amount
of time that the antecedent anatomy needs to accelerate its frame. The NAV charge be aught afore a station
can attack to accelerate a frame. Prior to transmitting a frame, a base calculates the bulk of
time all-important to accelerate the anatomy based on the frame’s breadth and abstracts rate. The base places
a bulk apery this time in the Duration acreage in the attack of the frame. Back stations
receive the frame, they appraise this Duration acreage bulk and use it as the base for ambience their
corresponding NAVs. This action affluence the average for the sending station.
2. An important aspect of the DCF is a accidental Back-off timer that a base uses if it detects a
busy medium. If the approach is in use, the base charge adjournment a accidental aeon of time before
attempting to admission the average again. This ensures that assorted stations that appetite to send
data do not address at the aforementioned time. The accidental adjournment causes stations to adjournment different
periods of time, which avoids the bearings in which all the stations faculty the average at
exactly the aforementioned time, acquisition the approach idle, transmit, and bang with anniversary other. The Backoff
timer decidedly reduces the cardinal of collisions and agnate retransmissions,
especially back the cardinal of alive users increases.
3. With radio-based LANs, a transmitting base cannot accept for collisions while sending data,
mainly because the base cannot accept its receiver on while transmitting the frame. As a
result, the accepting base needs to accelerate an accepting (ACK) if it detects no errors
RTS/CTS Fragmentation
Subset of Abstracts Frames Frames beneath a statically defined
length
Frames beneath a statically authentic length
Overhead Created RTS/CTS action frames Accession of breach headers
Table G-3 Improving Throughput Using RTS/CTS and Breach (Continued)
810 Appendix G: IEEE 802.11 Fundamentals
in the accustomed frame. If the sending base does not accept an ACK afterwards a defined period
of time, it assumes that there was a blow (or RF interference) and retransmits the frame.
To abutment time-bounded commitment of abstracts frames, the 802.11 accepted defines the alternative point
coordination action (PCF), which enables the admission point to admission admission to an alone station
to the average by polling the base during the contention-free period. Stations cannot transmit
frames unless the admission point acclamation them first. The aeon of time for PCF-based abstracts cartage (if
enabled) occurs alternately amid altercation (distributed allocation action [DCF]) periods.
The admission point acclamation stations according to a polling list, and again switches to a altercation period
when stations use DCF. This action enables abutment for both ancillary (for example, video
applications) and asynchronous (for example, e-mail and web-browsing applications) modes of
operation. No accepted wireless NICs or admission credibility on the bazaar today, however, apparatus PCF.
Without able affection of account (QoS), the absolute adaptation of the 802.11 accepted does not
optimize the manual of articulation and video. Currently, no able apparatus exists to
prioritize cartage aural 802.11. As a result, the 802.11e assignment accumulation is currently adorning the
802.11 MAC band to advance QoS for bigger abutment of audio and video (such as MPEG-2)
applications. The 802.11e accumulation should agree the accepted by 2006.
Because 802.11e avalanche aural the MAC layer, it will be accepted to all 802.11 concrete layers and
will be astern accordant with absolute 802.11 wireless LANs. As a result, the abridgement of 802.11e
in abode today does not appulse your accommodation on which concrete band to use. In addition, you
should be able to advancement your absolute 802.11 admission credibility to accede with 802.11e through
relatively simple firmware upgrades already they are available. In the meantime, some wireless LAN
vendors action proprietary QoS mechanisms that advance achievement back there is a mix of
applications with differing QoS needs present on the network, such as articulation and data.