User's Manual

Appendix D Wireless LANs
B222s User’s Guide
232
cannot "hear" each ot her, t hat is t hey do not know if t he channel is currently being used. Therefore,
they are considered hidden from each ot her.
Figure 165 RTS/ CTS
When st at ion A sends dat a t o t he AP, it m ight not know t hat t he st ation B is already using the
channel. I f t hese t wo stations send dat a at t he sam e t im e, collisions m ay occur when both set s of
dat a arrive at the AP at t he sam e t im e, resulting in a loss of m essages for both stat ions.
RTS/ CTS is designed to prevent collisions due t o hidden nodes. An RTS/ CTS defines t he biggest
size data fram e you can send before an RTS ( Request To Send)/ CTS ( Clear to Send) handshake is
invoked.
When a dat a fram e exceeds t he RTS/ CTS value you set ( between 0 to 2432 byt es) , the st at ion
that want s to 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 notify t hem to defer t heir t ransm ission. I t also reserves and confirm s
wit h t he request ing stat ion the tim e fram e for t he request ed t ransm ission.
St ations can send fram es sm aller t han t he specified RTS/ CTS direct ly to t he AP wit hout the RTS
( Request To Send) / CTS ( Clear t o Send) handshake.
You should only configure RTS/ CTS if t he possibilit y of hidden nodes exists on your net work and
the "cost " of resending large fram es is m ore than 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 Fra gm ent at ion Thr e shold value ( see next ) , t hen the
RTS ( Request To Send) / CTS ( Clear t o Send) handshake will never occur as dat a fram es will be
fragm ented before t hey reach RTS/ CTS size.
Note: Enabling the RTS Threshold causes redundant net work overhead t hat could
negat ively affect the throughput perfor m ance instead of providing a rem edy.
Fragmentation Threshold
A Fr a gm e nt at ion Thr eshold is the m axim um dat a fragm ent size ( between 256 and 2432 byt es)
that 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 Fra gm e n t at ion Thr eshold is recom m ended for networks not prone t o interference while
you should set a sm aller t hreshold for busy net works or net works t hat are prone t o interference.