ZyXEL Communications VMG1312-B Series User Manual page 340

Appendix D Wireless LANs
cannot " hear" each ot her, t hat is t hey do not know if t he channel is current ly being used. Therefore,
t hey are considered hidden from each ot her.
Figure 197
When st at ion A sends dat a t o t he AP , it m ight not know t hat t he st at ion B is already using t he
channel. I f t hese t wo st at ions send dat a at t he sam e t im e, collisions m ay occur when bot h set s of
dat a arrive at t he AP at t he sam e t im e, result ing in a loss of m essages for bot h st at 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 ( bet ween 0 t o 2432 byt es) , t he st at ion
t hat want s t o t ransm it t his fram e m ust first send an RTS ( Request To Send) m essage t o t he AP for
perm ission t o send it . The AP t hen responds wit h a CTS ( Clear t o Send) m essage t o all ot her
st at ions wit hin it s range t o not ify t hem t o defer t heir t ransm ission. I t also reserves and confirm s
wit h t he request ing st at ion t he t im e fram e for t he request ed t ransm ission.
St at ions can send fram es sm aller t han t he specified RTS/ CTS direct ly t o t he AP wit hout t he RTS
( Request To Send) / CTS ( Clear t o Send) handshake.
You should only configure RTS/ CTS if t he possibilit y of hidden nodes exist s on your net work and
t he " cost " of resending large fram es is m ore t han t he ext ra net work overhead involved in t he RTS
( Request To Send) / CTS ( Clear t o Send) handshake.
I f t he RTS/ CTS value is great er t han t he Fr a gm e nt a t ion Thr e sh old value ( see next ) , t hen t he
RTS ( Request To Send) / CTS ( Clear t o Send) handshake will never occur as dat a fram es will be
fragm ent ed before t hey reach RTS/ CTS size.
Not e: Enabling t he RTS Threshold causes redundant net w ork overhead t hat could
negat ively affect t he t hroughput perfor m ance inst ead of pr oviding a rem edy.
Fragmentation Threshold
A Fr a gm e nt a t ion Thr e sh old is t he m axim um dat a fragm ent size ( bet ween 256 and 2432 byt es)
t hat can be sent in t he wireless net work before t he AP will fragm ent t he packet int o sm aller dat a
fram es.
A large Fr a gm e n t a t ion Th r e sh old is recom m ended for net works not prone t o int erference while
you should set a sm aller t hreshold for busy net works or net works t hat are prone t o int erference.
340
RTS/ CTS
VMG1312-B Series User's Guide