Function of OLT and ONT - Technopediasite


Sunday, February 7, 2021

Function of OLT and ONT

Function of OLT and ONT : By the way, I have written many articles on PON before but often the question is asked what is the OLT and ONT and what are its functions? Today I have prepared a separate article to explain the function of OLT and ONT, you are requested to not try to understand the whole thing just by looking at the topic, read the complete article to get complete information.

Perhaps we all know that a passive optical network always operates under transmission between OLT and individual ONTs via optical splitter, multiplex or demultiplex signals depending on their origin and destination. Therefore, three different devices appear in PON: OLT, ONT and splitters, but the splitter will not be discussed in this article. Each of which has a necessary and prioritized function on passive optical networks. Now I will alternately discuss about the function of OLT and ONT.

Function of OLT


OLT is situated in a central office and it controls the bidirectional flow of information in the ODN (optical distribution network). An OLT needs to be able to support a transmission distance across an ODN of 20 km (presently can be longer with EDFA).

The function of an OLT in the downstream direction is to take in voice, data, and video traffic from a long haul network and broadcast it to all ONT modules on the ODN. In the reverse direction (upstream), the OLT network accepts and distributes all traffic from users.

The simultaneous transmission of different service types over the same fiber in the ODN is enabled using different wavelengths for each direction. For downstream transmission, a PON uses 1490 nm wavelength for combined voice and data traffic and 1550 nm wavelength for video delivery. Upstream voice and data traffic uses 1310 nm wavelength.

Each OLT is tasked to avoid interference between the content of the downlink and the uplink channel using two different wavelengths. For this, techniques WDM (wavelength division multiplexing) is used, and are based on the use of optical filters.

An optical power measurement on the OLT is also required to ensure that sufficient power is provided to the ONT. This must be done during initial activation as it cannot be repeated without interrupting service for the entire network once the network is connected.

Finally, note that OLT does not emit the same light output in all ONTs, but they depend on the distance from that plant. Therefore, a user close to central would require less power, whereas a remote user would require higher power.

Function of ONT


ONT know as  Optical Network Terminal is located in the customer's premises. There it is intended to provide an optical connection to the PON on the top side and electrically interface the client device on the other side. Depending on the communication needs of the customer or users' blocks, ONT typically uses a mix of telecommunication services including various Ethernet rates, T1 or E1 (1.544 or 2.048 Mbps) and DS3 or E3 (44.76 MB / 34.368 Mbps) telephone connections. Supports. , ATM interface (155 Mbps), and digital and analog video formats.

A wide variety of ONT functional designs and chassis configurations are available to accommodate the needs of different levels of demand. The ONT can range in size from a simple box that can be attached to the outside of a home to a fairly sophisticated unit mounted in a standard indoor electronics rack for use in large MDU or MTU applications, such as apartment complexes or office buildings.

Grooming means that the switching device looks inside a time-segmented multiplex data stream, identifies the destinations of individual multiplex channels, and then reorganizes the channels so that they can be efficiently transported to their destination.

In summary, we can say that ONTs are able to filter information associated with a particular user from OLT. They also have the task of encapsulating user information and sending it to the OLT header to redirect it to the appropriate network. Each ONT receives all the signals sent by its respective header ONT, just like the rest of the ONTs in the same phase. Information of OLT is transmitted by broadcasting TDM, and reaches all ONTs alike. However, ONTs have the job of filtering information that only directs itself (at a fixed time interval).

GPON is a point to multipoint access mechanism that allows end users to consolidate multiple services on a single fiber transport network. To realize this technology, many devices are used to support networks, such as optical splitters, ONT, OLT, etc. In this article, I am mainly discussing the functions of ONT and OLT in GPON networks.

Functions of ONT and OLT

The optical network terminal (ONT) is an optical modem that connects to the endpoint with an optical cable. It is used to connect to the PON network on an end-user basis and interface with the user on the other side. Data received from the customer end is sent, collected, and customized by ONT to the upstream OLT.

Here I would like to mention that term ONT is an ITU-T, while ONU is an IEEE term. They both refer to user-side tools in GPON networks. A small difference between them may be the application space. ONU can operate in various temperatures and weather conditions. The optical line terminal (OLT) is the endpoint hardware device located in a central office of the PON network. Its basic function is to control float information in optical distribution networks (ODN) to move in both directions. 

OLT converts standard signals used by Fiber Optic Services (FiOS) to the frequency and framing used by the PON system. In addition, it coordinates multiplexing between ONT conversion devices. There are two float directions for the OLT system. There is an upstream direction to deliver a wide variety of data and voice traffic from users. The other is the downstream direction that receives data, voice, and video traffic from the metro network or from long-thin networks and sends it to all ONT modules on the ODN.

How to add or remove ONT on OLT?

The way to add ONT over OLT is that if ONT password is received, you can run the ONT add command to add ONT offline. However, if the password is unknown, you can run the port auto-find command in GPON port, to enable the ONT auto-function function of the GPON port, and then run the ONT confirmation command to confirm ONT. When ONT is added, you need to run the ONT info command to see the current state of ONT. If the control flag is active.

The way to remove ONT on OLT is when you need to delete ONT on OLT, please use the delete command. ONT configuration data is then deleted with deletion of ONT and forced ONT offline. ONT cannot be removed when it is configured with other services. You must unbind the service before deleting ONT.

How to troubleshoot ONT?

To troubleshoot ONT, you must remember that the most important step is to connect your computer directly to ONT to correct the problem. You can use an Ethernet cable for connection. If the problem still exists, you can reconnect the ONT power supply to clear your internal cache. If the network cannot be restored following the above methods, perhaps you need to consult professionals for help.

Last Word

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