What is the difference between LOS and PON on a router modem?

LOS and PON refers to two different indicators on fibre broadband equipment: PON relates to the optical network connection, while LOS means Loss of Signal.

PON stands for Passive Optical Network. A PON system uses an optical line terminal in the provider network and optical network equipment at customer premises, with passive optical infrastructure between them.

The PON indicator on an optical terminal can show whether the device has registered with the provider's optical line terminal. TP-Link's GPON documentation, for example, describes a PON LED that is on when the terminal is registered with the OLT.

LOS means Loss of Signal. TP-Link's documentation describes the LOS indicator as showing when the equipment is unable to transmit or receive the expected optical signal, depending on the LED state.

If the LOS indicator is showing an alarm, the problem may involve the fibre connection, connector, cable, optical signal or the provider's network.

The exact LED behaviour depends on the equipment manufacturer, so the device manual should always be checked before interpreting a specific light.

For a home user, an active LOS warning is more significant than simply seeing that the internet is unavailable. If the fibre cable has been disturbed, bent sharply or disconnected, the optical terminal may no longer receive the required signal.

Do not repeatedly disconnect or bend the fibre connector while troubleshooting. Check that the cable has not been accidentally disturbed and then contact the ISP if the optical alarm remains.

The easiest way to remember the difference is:

PON indicates the optical-network connection or registration status, while LOS indicates a problem with the optical signal.