User guide

Using Link Efficiency to Check FSK Received
Signal Quality
Engineering Your RF Communications
132
pmp-0229 (Mar 2013)
Table 31: Signal quality levels indicated by jitter
Signal
Modulation
Correlation of Highest Seen
Jitter to Signal Quality
High
Quality
Questionable
Quality
Poor
Quality
1X operation
(2-level FSK)
0 to 4 5 to 14 15
2X operation
(4-level FSK)
0 to 9 10 to 14 15
In your lab, an SM whose jitter value is constant at 14 may have an incoming packet efficiency of
100%. However, a deployed SM whose jitter value is 14 is likely to have even higher jitter values
as interfering signals fluctuate in strength over time. So, do not consider 14 to be acceptable.
Avoiding a jitter value of 15 should be the highest priority in establishing a link. At 15, jitter
causes fragments to be dropped and link efficiency to suffer.
Modules calculate jitter based on both interference and the modulation scheme. For this reason,
values on the low end of the jitter range that are significantly higher in 2X operation can still be
indications of a high quality signal. For example, where the amount of interference remains
constant, an SM with a jitter value of 3 in 1X operation can display a jitter value of 7 when
enabled for 2X operation.
However, on the high end of the jitter range, do not consider the higher values in 2X operation to
be acceptable. This is because 2X operation is much more susceptible to problems from
interference than is 1X. For example, where the amount of interference remains constant, an SM
with a jitter value of 6 in 1X operation can display a jitter value of 14 when enabled for 2X
operation. As indicated in Table 31, these values are unacceptable.
OFDM uses a different modulation scheme and does not display a jitter value.
Using Link Efficiency to Check FSK Received
Signal Quality
A link test, available in the Link Capacity Test tab of the Tools web page in an AP or BH,
provides a more reliable indication of received signal quality, particularly if you launch tests of
varying duration. However, a link test interrupts traffic and consumes system capacity, so do not
routinely launch link tests across your networks.