Installation of FTTH active equipment
Once the PON cable plant is installed and tested, the network electronics can be installed. The OLT is installed at the headend and each OLT port connected into the fiber to the designated service area and the splitters installed to serve the intended users. At each user location, an ONU or ONT is installed to complete the network and connect to a wired or wireless network in the user’s location.
In this paper, engineer Vladimir Grozdanovic explains the different types of equipment and how they are installed to create an operating PON.
An OLT (Optical Line Terminal) is the main device in a PON system that connects ONUs through the ODN segment, enabling services to subscribers. OLT is typically located in the Central Office/Headend (CO/HE), but there are also solutions where OLT is remotely located – for example the mini OLTs used in rural networks.
Fig. 1. The different OLTs
The main components of OLT are:
· power unit,
· FAN module,
· control or uplink board, and
· service or line board.
Fig. 2. The main components of OLT
OLTs usually use -48 VDC for power supply, a telco standard, with battery backup. Typically, the chassis contains two power units that operate simultaneously for backup.
Control boards are the most crucial components forconfiguration, management, authentication, QoS, network control and monitoring, etc. These boards establish a connection with the core network, and enable services to subscribers. They usually have 4 slots for SFP modules for uplink connections and use UTP cables, simplex or zip cord cables (multimode or single mode) to connect to switches or routers. Typically, there are two control boards in the chassis, with one being active and the other on standby. This is important for backup.
Fig. 3. Block diagram of OLT connection with core equipment in HE/CO
Service boards enable the connection of ONUs (Optical Network Unit) at the user location.. They often have 8 or 16 slots for installing SFPs. The types of service boards and SFP modules depend on the PON standard. There are boards which support 2 or more PON standards – combo or flex PON boards (e.g. GPON and XGS-PON). All types of SFP modules use single mode fibers with SC/PC connectors. For example, for GPON standard, SFP modules of class B+, C+, and C++ are usually used. The output optical power (Tx) and received optical power (Rx) depend on the type of module.
The number of subscribers on a single PON port depends on the distance, FTTx scenarios, and PON standards. For example, for FTTH XGS-PON, a maximum of 256 users can be connected. The number of PON ports or service boards in the chassis depends on the chassis size. Thus, there are OLT variants with integrated PON ports (pizza and outdoor OLT), small chassis with 2 slots, medium ones with 7 or 8 slots, and large ones with 15 or 17 slots for service boards.
The same OLT can be used for different FTTx scenarios, provided that they are not mixed on PON ports. In other words, one PON port corresponds to one FTTx scenario.
OLT requires sufficient space in the rack or cabinet for normal operation. If two or more OLTs or other active equipment are mounted in the same rack or cabinet, compliance with prescribed distances must be ensured.
Fig. 4. The example of installation of OLT
Additionally, OLT requires optimal temperature, grounding, and continuous power supply. This type of communication equipment requires connection to a UPS with a minimum of 8 hours of autonomy.
If dislocated OLT solutions are used, it is necessary to provide adequate space and conditions for the active equipment – sufficient room for installation and uninterrupted operation, power supply, UPS, optimal temperature, etc.
Fig. 5. The example of remote OLT
If OLTs for indoor installation are used, specially designed cabinets are used for them. Another solution is the use of OLTs for outdoor installation. The cabinet or OLT for outdoor installation has suitable IP (usually IP68) to prevent the ingress of water, moisture, dust, etc.
Fig. 6. The example of cabinet for indoor OLT
ONU (Optical Network Unit) is the final segment of the PON network located near the customer. Depending on its location, there are different scenarios which are often given unique names:
· FTTC (Fiber To The Curb/Cabinet),
· FTTB (Fiber To The Building),
· FTTDp (Fiber To The Distribution point),
· FTTH (Fiber To The Home),
· FTTR (Fiber To The Room in MDUs),
· FTTM (Fiber To The Mobile base station/Machine), etc.
ONU is responsible for optoelectronic conversion and enables services on its interfaces. In FTTH and FTTR solutions, ONU is called ONT (Optical Network Terminal), while in other scenarios, it still retains the name ONU. MDU (Multi-Dwelling Unit) is another term used in FTTB and FTTC solutions.
The main differences between ONU and ONT devices lie in the types and number of interfaces, as well as the adaptability of the devices to the environment in which they operate.
ONT is a device designed for a single household, and accordingly, it has a smaller number of ports. They can be divided into two groups: bridge ONT and gateway ONT. Gateway ONT is mostly used. Modern ONT models are router variants (gateway ONT) that support WiFi 5 or WiFi 6, with 4 GE ports, 1 or 2 POTS ports, and rarely 1 RF port. These terminal devices generally do not require IP protection, operation in extreme temperature conditions, and remote power supply - PoE.
Fig. 7. Brigde ONT (left side) and gateway ONT (right side)
During the installation of the ONT device, it is necessary to coordinate with the subscriber to determine its location, ensuring optimal connectivity for user equipment, WiFi coverage, power supply, and suitable operating conditions for the ONT. It is most commonly installed in the living area or hallway, but there are situations where it is installed in the attic, garage, or on the house wall due to existing infrastructure. Some of these solutions may require the use of G.fast, G.hn, or MoCA technology to enable gigabit services over existing copper cables.
Fig. 8. ONT installation on the facade of the house
Traditional telephone services used to receive power from the Central Office, but this is not the case with FTTx solutions. In the event of a power outage at the user's location, they will not be able to use telephone services. Therefore, some operators have started to implement solutions such as backup power through batteries or utilizing old telephone installations.
Vladimir Grozdanovic is a graduate electrical engineer for telecommunications with more than 10 years of experience in access networks (HFC and FTTH) in large cable operators in Serbia (SBB and Jotel).
Ó2024 The Fiber Optic Association Inc.