RSSI vs SNR for Wireless Links

The bridge is mounted, the link is up, and the radio’s status page shows a column of numbers: signal, noise, a ratio, a modulation rate and a reading for each chain. Knowing RSSI vs SNR is what turns that screen into a verdict on the install. This guide explains what each figure means, which one decides the speed, and what to do when the numbers disagree.

What the radio status screen is telling you

Labels differ, but most point to point radios and access points report the same figures:

• Signal or RSSI: the received power from the far end, in dBm. It is negative, and -55 dBm is stronger than -70 dBm.
• Noise floor: the background energy the receiver hears across its channel, also in dBm.
• SNR: the gap between the two, in dB. Some radios also show a version that counts interference.
• Per-chain signal: one reading for each antenna chain, usually one per polarization.
• Modulation or rate: the scheme the radio has chosen, and the throughput that comes with it.

Read them together, at both ends.

RSSI vs SNR: loudness against clarity

RSSI tells you how loud the far end is. It comes from the link budget, so transmit power, antenna gain, path loss and cable loss all set it; our guide to planning a point to point wireless link works through that budget, and antenna gain in dBi explained covers the gain side.

SNR tells you how clearly the receiver can hear it. Because both readings are in dBm, the ratio in dB is a subtraction: a Princeton University lecture on adapting to the wireless channel defines SNR as 10 log10 of signal power over noise power, which is the same thing worked in decibels.

That is why a strong signal can still give a poor link. The figures below are assumed, to show the arithmetic:

Where the floor sits in the first place depends on channel width. Our guide to channel width for a point to point link covers that arithmetic, so it is not repeated here.

Why SNR decides the modulation rate

A radio carries more bits per symbol by packing its constellation points closer together, and closer points are easier for noise to confuse. The Princeton lecture gives the theoretical SNR each scheme needs to keep the bit error rate below 1 in 100,000 on a clean channel. Higher bit rates require more SNR:

These are textbook figures. Real radios publish their own minimums for each rate and width; work to the data sheet.

Rate adaptation watches the SNR, not the RSSI. When the margin disappears, the radio steps down and throughput falls.

What is a good SNR for a wireless link?

There is no single number, because the answer depends on the modulation you need. Work it out from the radio’s data sheet: find the SNR or sensitivity listed for the rate you need, then add fade margin for rain, foliage and seasonal change.

Installer forums offer rules of thumb rather than standards. In a long installer forum thread on signal strength, posters suggest a signal at least 20 dB above the noise for a reliable link, an SNR above 25 dB with at least 12 dB of margin at the chosen rate, and roughly 30 dB for 64-QAM 5/6. They are field opinions, not standards.

After install, note the SNR and modulation at both ends and check again over a few days. A link that holds its rate through a wet evening has the margin it needs.

Signal too strong on a wireless bridge

More is not always better. A receiver has a maximum input level, and above it the front end compresses and distorts the signal, so errors rise even though RSSI looks excellent. The IEEE 802.11 standard sets that maximum, for OFDM, at -30 dBm in the 5 GHz band and -20 dBm at 2.4 GHz, and those levels have carried forward unchanged through 802.11n, ac and ax.

Short hops with high gain dishes can exceed it. In the same forum thread one poster reports a link that ran faster at -50 dBm than at -38 dBm, and others suggest staying below about -40 dBm. The data sheet gives the real ceiling.

The fix is to lower the level at the source, not to spoil the aim:

1. Reduce transmit power at both ends, in the radio’s settings, until the signal sits comfortably below the maker’s maximum.
2. If the radio cannot go low enough, fit the attenuator the maker specifies for that model.
3. Keep the antennas properly aligned. Deliberate misalignment makes the link unpredictable and widens the beam onto other links.

Interference and a rising noise floor

The noise floor is not fixed. Another link, an access point or a sector on a nearby tower working in or near your channel adds energy the receiver cannot tell from noise, so the floor rises and the SNR shrinks while RSSI stays the same.

Signs to watch for:

• SNR and modulation that dip at particular times of day.
• One end reporting a much higher floor than the other, since each receiver hears its own surroundings.

Run the radio’s spectrum scan at the affected end, move to a cleaner channel or a narrower width, and recheck. On a crowded tower, a band change may help more; see 5 GHz vs 60 GHz wireless bridge.

Chain imbalance between polarities

A dual polarized radio reports one signal for each chain. On a clean, well aligned path the two readings should be close. A large gap between them points to the install rather than the path.

• Check the cabling first: a loose, wet or damaged connector or jumper on one chain drops only that chain.
• Check the chain mapping: chain 0 and chain 1 must land on the same polarization at both ends, as set out in antenna polarization explained.

Forum threads report gaps of 10 and 16 dB traced to alignment or a faulty antenna, with a replacement antenna fixing one. Read the gap at both ends: if the same chain is low at both, suspect the mapping or the path; if only one end is low, start at that end’s cabling.

Where to find point to point radios

The point to point and wireless radios category and the wireless network antenna category sit in the Broadband and WiFi division. For data sheets, pricing and availability, ask your sales representative through the Bursma contact page, call the Grand Rapids area low voltage branch, or open a Bursma dealer account.

Frequently asked questions