Appendix D Wireless LANs
VMG1312-B Series User’s Guide
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cannot "hear" each other, that is they do not know if t he channel is current ly being used. Therefor e,
they are considered hidden from each ot her.
Figure 197 RTS/ CTS
When stat ion A sends dat a t o the AP, it m ight not know that the st at ion B is already using t he
channel. I f t hese t wo st at ions send dat a at the same tim e, collisions m ay occur when bot h sets of
dat a arrive at the AP at the sam e tim e, resulting in a loss of m essages for both stat ions.
RTS/ CTS is designed t o prevent collisions due t o hidden nodes. An RTS/ CTS defines t he biggest
size dat a fram e you can send before an RTS (Request To Send) / CTS (Clear t o Send) handshake is
invoked.
When a dat a fram e exceeds t he RTS/ CTS value you set (between 0 t o 2432 byt es) , the station
that want s to t ransm it this fram e m ust first send an RTS (Request To Send) m essage t o the AP for
perm ission to send it. The AP then responds with a CTS ( Clear to Send) m essage t o all ot her
st at ions within it s range t o not ify t hem t o defer their t ransm ission. I t also reserves and confirm s
wit h the request ing st ation the t im e fram e for t he request ed transm ission.
St ations can send fram es sm aller t han the specified RTS/ CTS directly to the AP without the RTS
( Request To Send)/ CTS ( Clear t o Send) handshake.
You should only configure RTS/ CTS if t he possibility of hidden nodes exist s on your network and
the " cost" of resending large fram es is m ore t han the extra network overhead involved in t he RTS
( Request To Send)/ CTS ( Clear t o Send) handshake.
I f t he RTS/ CTS value is greater than the Fragm e nt a t ion Thr eshold value (see next) , t hen t he
RTS ( Request To Send)/ CTS ( Clear to Send) handshake will never occur as data fram es will be
fragm ent ed before they reach RTS/ CTS size.
Note: Enabling the RTS Threshold causes redundant network overhead that could
negat ively affect t he throughput perform ance instead of providing a rem edy.
Fragmentation Threshold
A Fragm e nt a t ion Thre shold is t he m axim um dat a fragm ent size (between 256 and 2432 bytes)
that can be sent in the wireless network before the AP will fragm ent the packet int o sm aller dat a
fram es.
A large Fr agm e nt a t ion Threshold is recom m ended for networks not prone to interference while
you should set a sm aller t hreshold for busy net works or networks t hat are prone t o int erference.
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