You Should Know - Issue #1, Using RSSI and Jitter
YOU SHOULD KNOW*
Issue #1 ? March 14, 2003
EMPLOYING THE USE OF THE RECEIVED SIGNAL STRENGTH INDICATOR (RSSI),
JITTER AND LINK TESTING IN THE ALIGNMENT & MONITORING PROCESS
Summary
When installing the Canopy system follow the instructions contained in the Canopy User Manuals and incorporate the following recommendations during system initialization. First, use the RSSI setting to establish the strongest signal. Second, after establishing the initial setting, begin using the link test and jitter settings to monitor the signal quality and eliminate the use of the RSSI setting from the monitoring process.
RSSI
The reasons behind using the RSSI settings early on in the alignment process and eliminating them from the long-term monitoring process are very simple. RSSI was not intended to be an absolute measurement instrument but rather a short-term duration tool to help in the establishment of the relative signal strength during the alignment process. Since RSSI was not intended to be an absolute measurement instrument, constant monitoring of the RSSI value over time will not yield much useful information.
The Canopy User Manual advises that a minimum RSSI level of 700 is considered adequate for signal strength. This level may be unnecessarily high. Generally, the higher the RSSI level is the stronger the signal. The operator should maximize RSSI as best as possible and use the jitter and link test to optimize the link. For example, if an installer can only achieve a maximum RSSI of 500 and great jitter and link test then the link is good.
Jitter
On the other hand, it is the representation of jitter that conveys the most important information when one is trying to establish the integrity of an RF link. Jitter is a measure of the variability in temporal position (the variation between the arrival time and the expected time). The scale for Jitter in a 10 Mbps system is 5 to 15 with 5 being the best and 15 being the worst. An operator should install a link with its average jitter reporting between 5 and 9. A consistent level greater than 9 is considered unacceptable and the link should be realigned. The scale for Jitter in a 20 Mbps system is 5 to 15 with acceptable values anywhere in that range.