ONU / ONT

In ITU-T terminology, an ONU (Optical Network Unit) is any end unit attached to an OLT in a PON, while an ONT (Optical Network Terminal) is an ONU located at the customer premises serving a single subscriber. In practice the two terms are mostly used interchangeably. The device receives the downstream signal at 1490 nm (GPON) or 1577 nm (XGS-PON) and transmits at 1310 nm or 1270 nm in the time slot the OLT assigns to it.

Common types are as follows. The SFU (Single Family Unit) is a pure bridge with a separate router behind it. The HGU (Home Gateway Unit) combines router, Wi-Fi and telephone ports in one box. The SFP ONU (ONU stick) plugs straight into the SFP cage of a switch or router. MDU/MTU models serve several subscribers in apartment or office buildings.

An ONU is registered on the OLT by serial number (a four-letter vendor code plus a hexadecimal number), password or LOID. For GPON class B+, ONU receiver sensitivity is typically −27 dBm and overload about −8 dBm. The received power measured in the field should sit within this window with adequate margin. HGU devices are usually configured remotely via TR-069, and the PPPoE or IPoE client generally runs on them as well.

Frequent problems include OMCI incompatibility with OLTs from other vendors and low signal due to dirty or damaged connectors. A faulty ONU that transmits light outside its time slot (a rogue ONU) disrupts every subscriber on the same port. Before bulk purchases, the chosen ONU model should be tested against the existing OLT.