Alvarion BreezeNET PRO.11 Series User Manual page 172

Appendix
Fragmentation of packets into shorter fragments adds protocol overhead and
reduces protocol efficiency when no errors are expected, but reduces the time spent
on retransmissions if errors are likely to occur. No fragmentation or longer
fragment length adds overhead and reduces efficiency in case of errors and
retransmissions (multi-path).
To avoid collisions with other incoming calls, each station transmits a short RTS
(Request To Send) frame before the data frame. The Access Point sends back a
CTS (Clear To Send) frame with permission to start the data transmission. This
frame includes the time that this station is going to transmit. This frame is received
by all the stations in the cell, notifying them that another unit will transmit during
the following Xmsec, so they can not transmit even if the medium seems to be free
(the transmitting unit is out of range).
Using Frequency Hopping Spread Spectrum (FHSS), different hopping sequences
are assigned to different co-located cells. Hopping sequences are designed so
different cells can work simultaneously using different channels.
Since hopping sequences and hopping timing of different cells cannot be
synchronized (according to FCC regulations), different cells might try to use the
same channel occasionally. Then, one cell uses the channel while the other cell
backs off and waits for the next hop. In the case of a very noisy environment
(multiples and interference), the system must hop quickly. If the link is quiet and
clean, it is better to hop slowly, reducing overhead and increasing efficiency.
User's Guide 9-16 BreezeNET PRO.11 Series