An ONU (optical network unit) is a type of network node used in GPON (gigabit passive optical networks). ONUs are located on the subscriber premises and connect to the central office over fiber optic cables.
ONUs act as the interface between the fiber optic network and the subscriber. The main purpose of an ONU is to convert downstream optical signals coming from the provider into electrical signals that can be used by the subscriber's equipment. It also converts upstream electrical signals from the subscriber into optical signals that can be transmitted back to the provider.
ONUs are powered by the provider network. They do not require a separate power source at the subscriber premises.
Most ONUs support Ethernet and POTS (plain old telephone service) interfaces to connect to subscriber equipment like computers, phones, routers, etc. Some ONUs also include CATV/RF video outputs.
ONUs support multiple types of PON protocols like GPON, EPON, XG-PON, XGS-PON, etc. The specific protocol must match what the provider network uses.
ONUs are assigned a unique identifier by the provider network. This allows the ONU to register and be individually addressed by the optical line terminal (OLT) at the provider central office.
Advanced ONUs support triple play services - the ability to deliver high speed internet, video and voice services over a single fiber drop.
ONU and ONT (optical network terminal) refer to very similar devices. They both provide the interface between the optical fiber network and the subscriber.
However, there are some differences:
ONT is used more often for passive optical networks like GPON and EPON. ONU tends to refer to active optical networks like APON.
ONTs usually have greater service integration - triple play capability is standard. ONUs may be more basic.
ONTs often have more advanced features like WiFi, firewalls, routing, etc. built into them. ONUs are typically more basic.
But in general, ONU and ONT can be used interchangeably.
The latest generation ONUs support XPON (10 gigabit capable passive optical network) architectures. These include:
XG-PON ONUs - Support up to 10 Gbps downstream and 2.5 Gbps upstream in XG-PON networks. Provide high bandwidth for gigabit internet and HD video services.
XGS-PON ONUs - Support symmetric 10 Gbps in both directions. Used for applications like business services that require ultra high upload speeds.
NG-PON2 ONUs - Support multiple wavelengths for very high capacity. Each wavelength operates at 10 Gbps symmetrically. Used for applications like 5G backhaul that demand massive bandwidth.
XPON ONUs are backwards compatible with older GPON and EPON networks. This provides a simple upgrade path for providers to move to 10 gigabit speeds on demand.
Many providers deliver traditional CATV (cable TV) services over their fiber networks in addition to high speed internet.
CATV ONU include an RF overlay port to connect coaxial cables from cable boxes and other set top boxes. This allows providers to offer a complete TV channel lineup alongside data and voice services, all delivered over fiber.
RF video overlay enables fiber-to-the-home deployments without needing to run additional coax cables. Everything can be delivered over a single fiber drop.
EPON (Ethernet passive optical network) is a lower cost alternative to GPON used in some fiber deployments.
EPON ONUs connect subscribers to EPON networks. They function similar to GPON ONUs but are designed specifically for EPON protocol.
Key attributes of EPON ONUs:
Support symmetrical 1 Gbps upstream and downstream.
Use EPON multi-point control protocol for signals.
Lower cost than GPON ONUs.
Integrate Layer 2 switching for Ethernet.
Ideal for fiber to the home, business or cell tower.
EPON and GPON ONUs cannot interoperate. The network protocol must match the ONU protocol. But providers can deploy GPON and EPON ONUs on separate ports of the same OLT to support both types of subscribers.
GPON ONUs are the most widely deployed type of ONU. They connect subscribers to gigabit-speed GPON networks.
Key features of GPON ONUs include:
2.5 Gbps downstream and 1.25 Gbps upstream data rates.
Use GPON encapsulation method for data transmission.
Support high bandwidth HD video, voice and internet access.
4 POTS lines for telephone service.
Triple play service integration - data, video and voice.
802.1Q VLAN tagging for service convergence and security.
ONU management via OMCI (ONU management and control interface).
Power saving sleep mode enabled by the OLT.
GPON ONUs deliver fiber optic broadband to millions of residential and business customers worldwide. They continue to be deployed for new FTTx networks.
In summary, an ONU provides the interface between fiber optic provider networks and the subscriber. ONUs support various PON protocols to deliver voice, video and high speed data over a single fiber connection. They enable reliable, high bandwidth services to homes and businesses.
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