With increasingly advanced and matured technologies in telecommunication network, Fiber to the Home (FTTH) has drawn much more attention of companies specialized in telecommunication nowadays. Generally, the FTTH broadband connections consist of two types of systems, known as Active Optical Networks (AON) and Passive Optical Networks (PON). And most of FTTH deployments are inclined to use a PON due to its low cost and high performance that can help to save a certain amount of money on fiber costs. A Gigabit Passive Optical Network (GPON) system generally contains an optical line terminal (OLT) at the service provider’s central office. As one of the indispensable components of PON, optical line terminal thus plays an essential role in the performance of the whole network connection.
An OLT (optical line terminal), also known as optical line termination, acts as the endpoint hardware device in a passive optical network. The OLT contains a central processing unit (CPU), passive optical network cards, a gateway router (GWR) and a voice gateway (VGW) uplink cards. It can transmit a data signal to users at 1490 nanometers (nm). That signal can serve up to 128 ONTs at a range of up to 12.5 miles by using optical splitters.
The OLT sends Ethernet data to the ONU, initiates and controls the ranging process, and records the ranging information. It provides numerous prominent features listed as follows.
- A downstream frame processing means for receiving and churning an asynchronous transfer mode cell to generate a downstream frame, and converting a parallel data of the downstream frame into a serial data thereof.
- A wavelength division multiplexing means for performing an electro/optical conversion of the serial data of the downstream frame and performing a wavelength division multiplexing thereof.
- An upstream frame processing means for extracting data from the wavelength division multiplexing means, searching an overhead field, delineating a slot boundary, and processing a physical layer operations administration and maintenance (PLOAM) cell and a divided slot separately.
- A control signal generation means for performing a media access control (MAC) protocol and generating variables and timing signals used for the downstream frame processing means and the upstream frame processing means.
- A control means for controlling the downstream frame processing means and the upstream frame processing means by using the variables and the timing signals from the control signal generation means.
OLT is generally employed for terminal connected to the fiber backbone. An OLT has two primary functions:
- Converting the standard signals use by a FiOS service provider to the frequency and framing used by the PON system;
- Coordinating the multiplexing between the conversion devices on the optical network terminals (OLTs) located on the customers’ premises.
As it was mentioned above there are two functions performed by OLT, and the main function of OLT is to control the information float across the optical distribution network (ODN), going both directions, while being located in a central office. Maximum distance supported for transmitting across the ODN is 20 km. OLT has two float directions: upstream (getting an distributing different type of data and voice traffic from users) and downstream (getting data, voice and video traffic from metro network or from a long-haul network and send it to all ONT modules on the ODN.
As we see from the picture above, OLT is designed for controlling more than one PON (in this example it serves for four independent networks). We can see that if every PON has 32 connections, OLT can distribute data to 128 ONTs. OLT has specific standard, so it would work with ONT from different manufacturers.
The OLT now has been widely adopted in fiber optic network access in counties, towns and villages. It can help efficiently reduce network construction cost, while simultaneously providing a guarantee on high bandwidth and high integration. And it is proved to be an ideal and constructive solution to FTTx projects.