Optical Line Terminals Information

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Optical Line TerminalsOptical line terminals, also called optical line terminations (OLTs), serve as endpoints for passive optical networks (PONs). They convert electrical signals from equipment managed by a service provider to fiber optic signals readable by a PON.


This system facilitates multiplexing of data streams between signal conversion instruments at the opposite ends of the network, known as optical network units (ONUs) as per IEEE, or optical network terminals (ONTs) as per ITU-T.




Passive optical networks are telecom systems that route data through fiber optic connections. They are referred to as passive due to their use of unpowered splitters to transferring data from a central point to multiple destinations. As a result, the primary transmission line divides into 32 individual branches connected to the main OLT.


In comparison to point-to-point architecture, the point-to-multipoint (P2MP) link supported by PONs requires less physical infrastructure. This minimizes the need to build individual lines for each endpoint, which makes it a more economical Internet access solution for ISPs and NSPs.


Data sent from an OLT module to an ONU is transmitted over fiber optics lines. Fiber strands that end outside the premises are referred to as fiber-to-the-curb (FTTC) or fiber-to-the-neighborhood (FTTN). On the other hand, the strands extending right up to a premise are called fiber-to-the-building (FTTB) or fiber-to-the- home (FTTH).


In addition to PON cards, an OLT holds other uplink cards such as CPU, GWR (gateway router), and VGW (voice gateway) cards. Each OLT component has up to several dozen of such cards that send and receive laser-generated signals.


Technological developments in the field of passive optical network systems focus on meeting the growing demand for broadband solutions. A standard PON comprises an OLT interface and multiple ONUs that are connected by fiber optic cables and splitters. The OLT device allows for integrated management of a PON and also aggregation and distribution of optical signals from all ONUs. PONs are further classified as follows:


  • APON: This early PON model debuted in the 1990s and transfers data via asynchronous transfer mode (ATM).

  • BPON: This was the earliest version of the broadband network. It supported data transfer at a rate of 622 Mbps, equal to the OC-12 (STM-4) rate.

  • GPON: This network is capable of gigabit speeds, both upstream and downstream.

  • EPON: Also known as the IEEE 802.3 standard, this is the most widespread PON system. It supports both upstream and downstream data transmission as Ethernet frames with a maximum bandwidth of 10 Gbps.

Data Collection


Data collected from network subscribers and sent to an OLT is classified as heading upstream; data collected from a metro network or long-haul network and then sent to subscribers via ONTs is classified as heading downstream. Modern consumer broadband services based on FTTH networks rely on GPON, which offers varying up and downstream capacities of 622 Mbps to 2.5Gbps.


The OLT module controls an ONU's start time as well as the duration of data transmission. It is installed in the main data center of the service provider. It serves as the origination point in such systems and controls the process of sending and receiving data to and from the user terminals.


An ONT unit will convert fiber-optic signals into copper/electric signals for transmission. Each unit has power cords connected to a site through a power supply unit.


Different wavelengths of light apply to data transmissions. Voice and data transmit 1490 nm and receive 1550 nm. Video receive 1550 nm. ONTs can deliver traditional telephone service lines, Internet data, and video.


Optical Line Termination Data Types


All data types are transferred at the same time, but each uses a unique wavelength. In some systems splitters are incorporated between the OLT and ONUs.


The splitters permit an individual PON network interface to serve many subscribers. Every end-user connection added via a splitter adds attenuation to the network. Therefore, the use of one splitter is preferred to the use of multiple linked ones.


An ONT optimizes performance by aggregating different categories of data to send upstream where an OLT system receives it. It can modify the data by augmenting and reorganizing it for a more efficient delivery.


The OLT performs bandwidth allocation procedures to smooth delivery of data arriving in bursts from customers.


Linking the OLT components to available IP/Ethernet ports minimizes energy consumption and space requirements, thereby reducing capital expenditure.




Optical line terminals possess a myriad of options, including:


  • Ethernet transmision

  • DCOSIS® provisioning

  • Manual CLI (command line interface) provisioning

  • Upgradeable switching modules

  • VoIP

  • IP video

  • Management features:


    • TFTP/SFTP client

    • Syslog

    • SNTP client

    • RADIUS/TACACS+ client

    • ACL

    • Remote firmware upgrade (OLT, ONU)

  • Routing:

    • ECMP

    • Static routing

    • IP

    • L2 switching

    • Link aggregation

    • Ethernet/IP direct optical interface

    • EPL service


  • Security:

    • PON encryption

    • ONU authentication

    • Secure software download (SSD)

  • QoS support

  • DBA (Dynamic Bandwidth Allocation)

  • Diagnostic:

    • Statistics counters

    • Ping and traceroute

    • ONU loopback

    • Optical monitoring

    • Optical shutdown

    • Remote DiagnosisApplications

Optical line terminals convert and send electrical signals over passive optical networks including:


  • APONs
  • BPONs
  • GPONs
  • EPONs



Agencies and groups establish several standards and qualifications for optical line terminals, including:


  • IEEE (Institute of Electrical and Electronics Engineers)
  • ITU-T (Telecommunication Standardization Sector of the International Telecommunications Union)
  • CableLabs®
  • MEF (Metro Ethernet Forum)



ISO 9144 - Securities - optical character recognition line - position and structure first edition.


ITU-T G.981 - PDH optical line systems for the local network - digital sections and digital line systems (study group 15) 12 pp.


JIS Optical Instruments - JIS optical instruments handbook.


Image Credits: 

Photo by Vivien Guéant / CC By-SA 3.0 | Photo by Riick / CC By-SA 3.0


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