Overview
In everyday life, accessing news, social apps, and online shopping all rely on the access network. The access network refers to all equipment between the backbone network and the user terminal. As technology advances and IP traffic grows, optical access network technology has become a key area of access network development.
Common optical access deployments include FTTC (fiber to the curb), FTTB (fiber to the building), and FTTH (fiber to the home).
Below are the three "O"s of the optical access network.
Relationship Among OLT, ONU, and ODN
The relationship among OLT, ONU, and ODN is illustrated below.
- OLT processes information received from the core network and forwards it to the ODN, and it also collects information from ONUs via the ODN and forwards it to the core network.
- ODN provides the optical transmission channel between OLT and ONU.
- Each ONU analyzes the signal transmitted from the ODN, extracts the portion intended for that ONU, and schedules user information to send back via the ODN.
ONU
Concept
ONU (Optical Network Unit) is the customer-premises device in an optical access network, deployed at the user end to work with the OLT. An ONU provides services such as data, IPTV (interactive television), and voice (Integrated Access Device), and implements Ethernet layer 2 and layer 3 functions.
Functions
- Selectively receive data sent by the OLT.
- Respond to commands issued by the OLT and make corresponding adjustments.
- Buffer user Ethernet data and transmit user Ethernet data to the OLT.
- User and service multiplexing: aggregate information from multiple users and connect individual service interfaces.
- Transmission multiplexing: analyze signals received from the ODN, extract the portion belonging to this ONU, and schedule transmissions to the ODN.
- ODN interface functions: provide optical/electrical and electrical/optical conversion interfaces and related physical optical interfaces.
Types
Based on the three basic optical access deployment types, ONUs can be classified as:
- Card-type ONU: Mainly used for FTTC deployments. The ONU is placed at distribution points such as street cabinets or pole-mounted enclosures and connects to users via twisted-pair copper lines.
- Box-type ONU: Mainly used for FTTB deployments. The ONU is installed inside a building and distributes services to multiple users via multiple twisted-pair lines.
- Residential ONU: Mainly used for FTTH deployments. The ONU is installed inside the user premises to provide interactive broadband services, with the local access up to the user terminal implemented in fiber.
ODN
Concept
ODN (Optical Distribution Network) is the fiber-cable network based on PON equipment. ODN commonly uses a P2MP (point-to-multipoint) topology. Because ODN networks contain many nodes and involve complex management, construction costs can be relatively high, accounting for up to 50% to 70% of total investment.
Function
Provide the optical transmission channel between OLT and ONU.
Structure
ODN consists of five parts: feeder segment, optical cable distribution point, distribution segment, optical cable access point, and drop segment.
- Feeder segment: From the central office ODF (optical distribution frame) to the optical cable distribution point. The feeder segment serves as the trunk fiber for long-distance coverage.
- Distribution segment: From the optical cable distribution point to the user access point. The distribution segment allocates fiber to nearby user areas along the route.
- Drop segment: From the user access point to the terminal. The drop segment implements fiber-to-the-home connections.
OLT
Concept
OLT (Optical Line Terminal) is the terminal device that connects to the fiber trunk. The OLT is analogous to a switch or router in a traditional telecommunications network. It provides element-level management and business-level security and configuration management, and it allocates bandwidth according to users' QoS/SLA requirements.
Functions
- Uplink to higher-layer networks and provide uplink access for the PON.
- Aggregate various service signals and transmit them to end users in the appropriate format.
- Downlink users via the ODN to ONUs, route incoming user signals to the appropriate service networks, and provide ONU control, management, and ranging functions.
Architecture
The OLT can be divided into three parts: core, service, and common.
- Core: aggregation and distribution, service processing, and ODN adaptation functions.
- Service: provides service interface functions, including interface adaptation and protection, and, when required, signaling processing and media format conversion for specific services.
- Common: includes power supply and OAM functions.
Summary
In an optical access network, the OLT serves as the central controller, the ODN provides the transport medium, and the ONU represents the distributed edge devices that deliver services to users. With ongoing developments in mobile communications such as 5G, optical access technologies continue to evolve. Future work includes further broadband deployment and integration of optical access with other access methods.
