Tag Archives: OLT

Basic Knowledge About Optical Line Terminal (OLT)

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.

What Is OLT?

An OLT (optical line terminal), also known as optical line termination, acting 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.

Optical Line Terminal with 8-PON ports

The Features of OLT

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.
The Functions of OLT

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.
The Role of OLT in GPON Network

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.

Active PON

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.

Conclusion

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.

Passive Optical Network Technology

The tremendous growth in IP traffic badly influenced the access network capacity. It’s believed that the copper-based access networks can’t provide either the minimum bandwidth or the required transmission distance for delivering services of voice, data, and video programs. Passive optical network (PON) is seemed as a promising and cost-effective way to solve this problem.

What’s PON?

PON is a telecommunication network that uses point-to-multipoint fiber to the end-points in which optical splitters are used to enable a single optical fiber to serve multiple end-points. It does not include any electrically powered switching equipment.

Three Devices in PON

There are three distinct devices in the network (as shown in the following picture): the OLT (optical line terminal), the ONUs (optical network units) or ONTs (optical network terminals) and the splitter. Each one has a necessary function in the passive optical network. PON always works under transmission between the OLT and the different ONT’s through optical splitters, which multiplex or demultiplex signals based on their origin and destination.

PON-devices

  • OLTs are located in provider’s central switching office. This equipment serves as the point of origination for FTTP (Fiber-to-the-Premises) transmissions coming into and out of the national provider’s network. An OLT, is where the PON cards reside.
  • ONU converts optical signals transmitted via fiber to electrical signals. These electrical signals are then sent to individual subscribers. ONUs are commonly used in fiber-to-the-home (FTTH) or fiber-to-the-curb (FTTC) applications. Using different wavelengths for each service makes it possible to transmit high-speed Internet and video services at the same time. Wavelength multiplexing is performed at the central office and a wavelength demultiplexing mechanism is provided at the customer’s house.
  • PON splitter is used to split the fiber optic light into several parts at a certain ratio. For example, a 1X2 50:50 fiber optic splitter will split a fiber optic light beam into two parts, each get 50 percent of the original beam.
Advantages of PON

There are many advantages given by the use of fiber and the passive elements that compose the network. The following will tell about the advantages of PON.

  • High bandwidth The bandwidth allowed by systems based on PON can reach the 10 Gbps rate down to the user. The need to increase the bandwidth and the speed is another justification for the use of PON.
  • Long distance A PON allows for longer distances between central offices and customer premises. While with the Digital Subscriber Line (DSL) the maximum distance between the central office and the customer is only 18000 feet (approximately 5.5 km), a PON local loop can operate at distances of over 20 km.
  • Low cost On one hand, the cost of passive elements is low. On the other hand, the installation of these PON elements is much more economic. And it avoids operation and maintenance costs, such as absence of falls or maintenance of the network feeds.

Of course PON has some disadvantages. Compared with an active optical network, it has less range. That means subscribers must be geographically closer to the central source of the data. PON also make it difficult to isolate a failure when they occur. However, these disadvantages can not avoid choosing PON as the best possible configuration. Because it saves the cost of deploying PON networks regarding other two configurations (point to point and active optical network). And the flexibility of the network allows the usage of a channel by a large number of users.