User's Manual

Appendix
BreezeNET PRO.11 Series 8-12 User’s Guide
Fragmentation
Fragmentation of packets into shorter fragments adds protocol overhead and
reduces protocol efficiency when no errors are expected, but reduces the
time spent on re-transmissions if errors are likely to occur. No
fragmentation or longer fragment length adds overhead and reduces
efficiency in case of errors and re-transmissions (multi-path).
Collision Avoidance
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).
Channelization
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.
8.4. Radio Signal Propagation
8.4.1. Introduction
This section explains and simplifies many of the terms relating to antennas
and RF (Radio Frequency) used when dealing with an RF installation
system.