May 25, 2017

5 Things You Should Know About RSL

By Ramon Morales, NOC Operations Team, US

Do you remember the last time your blood pressure was checked?  I remember staring at the results wondering what the numbers meant.  Usually my thoughts are, “Great, does this mean I’m healthy or should I be concerned?”

Recently, one of our customers had a similar experience with an email received from our customer service group.  The customer’s initial impression was that the content received was vague and meaningless.  “What am I supposed to do with this information?”, the customer asked.  The email contained information about RSL (Received Signal Level) readings.  Although the email content was clear to me, the conversation with the customer helped me recognize that not everyone who relies on this technology is familiar with the terminology.

What’s an RSL reading?

RSL readings are an indicator of how well the radio can “hear” information being transmitted from the far end radio.  The numbers are always preceded by a minus (-) sign.  An example of an RSL reading would be -35.  The closer to zero the reading is, the stronger the signal will be.  Caution:  too strong of a signal can damage the receiver; too weak of a signal and the radio will not be able to “hear” the far end transmitter.

Saturation versus threshold

On one end of the RSL spectrum, resides the saturation point. Saturation refers to the point at which the signal received is so strong that it could damage the receiver.  This is akin to the volume in a stereo system. If the volume is too high, sound becomes distorted and eventually the stereo speakers will fail.

Threshold resides on the other side of the RSL spectrum. When a receiver reaches its threshold point, this means the signal is so weak, the receiver is not able to “hear” the information being received. Continuing with our stereo system analogy, the volume is now so low that the sound cannot be heard.

What’s a good RSL reading?

A “good” RSL reading for one radio might be a “bad” reading for a different radio. To know if RSL readings are “good” or “bad”, received signal levels will need to be compared against PCN (Prior Coordination Notice) datasheet.  PCN datasheets contain path-specific information such as antenna height, transmitter and receiver frequencies and signal levels.  The preferred method of validating if a radio is operating within normal RSL readings is to compare current readings against the value listed in the site specific PCN. Design RSLs are documented in the PCN next to Received Level (dBm). During normal operation, RSL readings should be within +/- 2 dBm from designed value. During fading periods, RSL readings will decrease (moving further away from zero), but should not exceed threshold. The difference between Received Level and threshold is known as fading margin.  Threshold information can be found in the product datasheet.  Aviat Networks’ product datasheets describing our radios saturation, threshold, and other valuable content can be downloaded from our AviatCare page at www.aviatcloud.com.

Factors that could adversely impact RSLs

In no particular order, the following is a partial list of events that can negatively affect RSLs:

• Hardware failure – equipment failure such as the transmitter at the far end or the receiver at the site reporting low RSLs
• Antenna misalignment – this can be caused by severe weather conditions
• Environmental – changes in temperature, fog, rain, or snow can cause signal degradation
• Vegetation – overgrown trees can block RF signal, thus resulting in path degradation
• Interference – nearby transmitting sites can potentially cause low RSLs and errors in traffic
• Obstructions – these can be caused by several things such as buildings, ships, airplanes, etc.  Just about anything blocking the line of sight between the two radios can cause low RSLs

Troubleshooting Tips

• Always maintain a copy of the closeout package, PCN, and most recent preventive maintenance results in electronic format at an easily accessible location.  This data will provide you with a baseline to work from.
• In a protected radio, if RSL readings in one receiver are within specifications but readings in the other receiver are not, the problem might be solved by replacing the receiver with low readings.
• If both receivers at the local site are reporting low RSLs but the receivers at the far end reports RSL readings within design values, the problem might be caused by the transmitter at the far end.  Switch traffic to the offline transmitter at the far end radio.  If RSLs return to normal parameters, replace the far end transmitter.
• If RSL readings are low or outside parameters in both directions, the problem might be environmental related or the antennas might be out of alignment.
• Consult the product manual or contact technical support for additional troubleshooting guidance.

Although microwave communication has been around for quite some time, there are many aspects to this technology that can be foreign to newcomers.  Next time anyone provides you with an RSL reading, you will be able to evaluate whether the values are “good” or “bad”.

Ramon Morales, NOC Operations Team, US