Insertion Loss vs. Return Loss

What Is the Difference Between Insertion Loss and Return Loss?

Insertion loss is the energy a signal loses as it transmits along a cable link. It’s a natural phenomenon that occurs for all types of signals, optical or electrical. This loss of signal, also called attenuation, increases with length. Connection points along the cable link — including connectors, splitters, and splices — also cause insertion loss. Insertion loss is measured in decibels (dB) as a positive number.

• • In fiber optic cabling systems, excess insertion loss can be caused by inferior-quality components or improper installation, such as contaminated fiber end faces, connector misalignments, or exceeding the fiber bend radius.


• • In copper cabling systems, insertion loss increases with frequency and is largely dependent on the size of the conductor. The larger the conductor gauge, the less the insertion loss. Higher temperatures can also increase insertion loss in copper cabling systems, which becomes more of a concern in bundles of cables carrying more than 60W of Power over Ethernet (PoE). Learn more about insertion loss and its causes.

By contrast, return loss relates to the amount of signal reflected back toward the transmitting source. It is measured by comparing the power of the input signal to the amount reflected. Like insertion loss, return loss is also measured in decibels as a positive number. But unlike insertion loss, higher return loss values are better — if no signal reflects back, there would be an infinite return loss.

• • In fiber cabling systems, return loss is primarily caused by reflections at connection points. That’s why connector manufacturers specify return loss for their specific fiber connectors. Contaminated connector end faces, gaps, and misalignments can also cause signal reflections. Impurities in the fiber introduced during manufacturing, as well as cracks, open ends, and exceeding fiber bend radius can worsen return loss.


• • Like insertion loss, return loss worsens with higher frequencies in copper cabling links. It is caused by impedance mismatches between components or minor impedance variations along a cable’s length. To achieve better return loss, copper cable and connectivity manufacturers design plugs and jacks with matched impedance, as well as control uniformity throughout the cable manufacturing process. Return loss in copper cabling systems can also be caused by kinked or damaged cables or poor termination practices, such as additional unnecessary pair untwist at termination points. Learn more about return loss and its causes.

Why Are Insertion Loss and Return Loss Important?

Insertion loss and return loss are key indicators of the health of a cabling system.

• • If insertion loss is too high, the transmitting signal may not have enough strength at the far end of a link to be accurately interpreted by the active equipment. This can lead to degraded performance or cause the link not to function. That’s why industry standards specify insertion loss limits for specific fiber applications and categories of copper cabling.


• • Return loss is an essential parameter because reflected signals can interfere with transmitting signals. Poor return loss also means less power available at the far end of the cable, which can cause insertion loss. In other words, a higher return loss value generally correlates to a lower insertion loss value. In copper cabling systems, return loss is essentially a noise measurement. Poor return loss in copper systems increases crosstalk, distorts signals, and causes bit error rates.

How to Interpret Insertion Loss

Insertion loss is the primary parameter measured when performing Tier 1 certification testing for fiber systems, as described in TIA and ISO cabling standards and required by cabling and connectivity manufacturers to receive a system warranty. Insertion loss is measured across a link using an Optical Loss Test Set (OLTS) like the Fluke Networks CertiFiber^® Pro. The OLTS emits a light source on one end of a link and uses a power meter at the other end to measure the signal received and compare it to the amount emitted. If insertion loss fails certification testing with an OLTS, an Optical Time Domain Reflectometer (OTDR) like the Fluke Networks OptiFiber^® Pro OTDR can measure the loss of specific events such as breaks, bends, splices, and connectors. This can help determine the cause and exact location of the loss event. Learn more about the relationship between an OLTS and OTDR.

The primary procedure for testing insertion loss in fiber systems is the one-jumper method. This includes the loss of the first and last connectors, which represents how the cabling plant will ultimately be used. In multimode fiber systems, insertion loss testing requires encircled flux (EF) launch conditions, which control how the light is launched into the fiber to prevent an overfilled launch that can lead to pessimistic results, or an underfilled launch that can lead to optimistic results.

Insertion loss is also a key performance parameter required for copper certification testing and acquiring a manufacturer warranty. It is tested using a copper certification tester like the Fluke Networks DSX CableAnalyzer™ Series, which tests insertion loss over the entire frequency range of each pair for the specific type of copper cabling. For example, in a Category 6 system, insertion loss is tested from 1 to 250MHz, while in a Category 6A system, it is tested from 1 to 500MHz.

Learn more about testing insertion loss in fiber and copper cabling systems.

How to Interpret Return Loss

In fiber systems, return loss is measured across a link using an OTDR like the OptiFiber Pro. The OTDR transmits high-power light pulses into the fiber. When these pulses meet reflective events (i.e., connections, breaks, cracks, splices, sharp bends, or the end of the fiber), they are reflected back and measured. The total of all light reflected from all events and the total backscatter of the link results in the total return loss value for the link. Note that an OTDR can also provide reflectance values and location for each individual event. However, reflectance is the inverse of return loss and a negative number. Learn more about the difference between return loss and reflectance.

Return loss is a performance parameter required for copper certification testing and acquiring a manufacturer warranty. Like insertion loss, it is tested over the entire frequency range for each pair using a copper certification tester, like the DSX CableAnalyzer. Failed return loss at only a single frequency point typically indicates a cable issue. When all four pairs are failing (particularly at lower frequencies), it can indicate a low-quality cable or water in the cable.

Learn more about testing return loss in fiber and copper cabling systems.

Keep Learning

• • What Is Insertion Loss?
• • What Is Return Loss?
• • What Can Go Wrong with Insertion Loss After Installation