RTS/CTS
The 802.11 accepted defines request-to-send/clear-to-send (RTS/CTS) as an alternative action of
802.11 to adapt the manual of abstracts on the wireless LAN. RTS/CTS can be set in the access
point or a radio agenda individually, or on both accessories at the aforementioned time. In best cases, the RTS/CTS
function is accessible in antidotal collisions amid hidden nodes. To accretion admission to the shared
wireless medium, a base can alone address if no added base is transmitting. Aural a particular
access point radio cell, it is accessible that two stations associated with the aforementioned admission point might
be out of ambit of anniversary other.
For example, Figure G-8 illustrates the book in which either Base A is far abroad from Station
B or a barrier is blocking the radio signals amid the two stations. The botheration is that Station
A ability be in the average of transmitting a anatomy to the admission point back Base B wants to send
a frame. Base B will accept to the average to actuate whether accession base is already
transmitting. Because Base B cannot apprehend Base A, Base B starts transmitting the frame. A
collision again occurs at the admission point, which destroys both frames. As a result, the admission point
will not accede accession of the frames. Both stations will accept to retransmit their respective
frames, which will acceptable aftereffect in accession collision.
Figure G-8 Hidden-Node Botheration in Wireless LANs
RTS
CTS
Data
Barrier
Station A
Station B
Access Point
CTS
2
2
1
3
Wireless Configuration Parameters 807
The RTS/CTS action is a handshaking action that minimizes the accident of collisions when
hidden nodes are operating on the network. In addition, aegis mechanisms can use RTS/CTS
to abstain collisions amid 802.11b and 802.11g radio cards. If hidden nodes are not causing
significant retransmissions or hidden nodes are not present, again RTS/CTS is about not
necessary (unless aegis mechanisms are in use).
RTS/CTS works by enabling anniversary base to absolutely appeal a time aperture for abstracts transmission.
For example, if RTS/CTS is enabled in the radio cards of Base A and Base B, again Station
A will aboriginal accelerate an RTS anatomy to the admission point afore attempting to address a abstracts frame.
The admission point receives the RTS anatomy and responds with a CTS frame. Both stations receive
the CTS frame. This gives approval for Base A to address a abstracts frame. The CTS frame
carries a continuance bulk that informs all added stations, including Base B, to not transmit
during the defined time interval. This adjournment equals the bulk of time that Base A requires
to accelerate the abstracts anatomy and for the admission point to acknowledge with an acknowledgement. As a result,
collisions due to hidden nodes are abundant beneath likely. The alone time it is accessible with RTS/CTS
enabled is back hidden nodes additionally absence CTS frames from the admission point because of
interference or anemic signals.
Enabling RTS/CTS requires ambience RTS/CTS to a specific beginning bulk in the admission point or
radio card. The beginning is the anatomy breadth that invokes the RTS/CTS process. If the abstracts frame
that needs manual is beyond than the threshold, again the base implementing RTS/CTS uses
the RTS/CTS process.
If you doubtable hidden nodes ability be causing collisions on the arrangement and aspersing network
performance, accede activating RTS/CTS. It is best to aboriginal set the beginning to approximately
750 bytes, which is almost the center point (50 percent of the best anatomy size), and
assess changes in throughput. If throughput increases, again the use of RTS/CTS is beneficial. If
throughput decreases, again RTS/CTS acceptable is not worthwhile. If RTS/CTS is in actuality improving
performance, accede tweaking the beginning bulk to aerate throughput.
Keep in apperception that the use of RTS/CTS adds aerial to the arrangement because the transmission
of abstracts frames ability crave RTS and CTS frames as well. As a result, the abbreviation of
collisions by application RTS/CTS ability not action abundant advance in throughput to compensate
for the added aerial of RTS/CTS frames. The ultimate ambition of application RTS/CTS is to
improve throughput. If RTS/CTS causes throughput to abatement due to the added overhead,
then do not use it. Alike if the use of RTS/CTS improves performance, changes aural the
facility and users that are adrift to added areas will change the hidden-node situations. Thus,
RTS/CTS ability not be necessary, and can alike account abasement in throughput as conditions
change. 802.11 does not action adaptive mechanisms that automatically acclimatize RTS/CTS
thresholds.