Passive Optical Networks

The demand for broadband networks is growing such that it carries a nearly constant expansion of the capacity of broadband infrastructure. Increasingly, fiber to the home is being most in demand, and users are more demanding with the quality of service they receive.

The deployments of telecommunications networks that carry data, however, require a huge investment by operators. Therefore, passive optical networks (PON) have become popular in recent years, which allow networks to deploy fiber to the home without the need for active components intermediate between the central and the home user. In particular, GPON networks are gaining increasing popularity among network deployments of fiber to the home.

GPON Networks

GPON (Gigabit Passive Optical Network) is the most widespread PON protocol for FTTH networks in Europe and America. It was approved in 2003-2004 by the ITU-T G.984.1 under the recommendations, G.984.2, G.984.3, G.984.4 and G.984.5.

The standard allows GPON networks to serve up to 128 users on the same network and to be extended to up to 60km. However, the own philosophy of lower costs of passive optical networks is also transferred to the cheaper cost of equipment, especially in the user equipment, which must be affordable to market so that operators can get the maximum number of clients and make efficient the network deployment. While limiting the equipment cost, the sensitivity supported for the optical signal is limited and therefore it is needed GPON regenerators to maximize the reach of the signal, both in kilometers and potential users.

Trunking and link aggregation

Following the same policy to reduce costs in the deployment of fiber to the home networks (FTTH), there have been developed different technologies to allow the same fiber can transmit information through various channels multiplexing different wavelengths (xWDM). This form of link aggregation on a single transmission channel is also known as trunking.

WDM Multiplexing

WDM (Wavelength-Division Multiplexing) is one of the ways of transmitting a signal over a single optical fiber. A WDM system consists of multiplexers/demultiplexers equipment at each end of the fibers. These devices allow modulation of the signal at different wavelengths and send them through the fiber. At the other extreme, another equipment will separate the different wavelengths and process them individually.

WDM solutions have become very popular among telecommunications operators, allowing to multiply the capacity of a fiber optic link without having to change or extend the installation and fiber optic cable, but simply replacing the multiplexers equipment at the end of it.

Trunk-PON and decreasing centrals and fiber optics    

Currently GPON deployment is taking place in the urban areas of those cities where they can ensure a mass of potential customers enough to make profitable the PON deployment. To provide these areas it has been installed OLT equipped in the centrals and has been made the deployment of fiber to the home or neighborhood of the end user.

Sometimes it may be wanted to provide GPON coverage to nearby urbanizations or towns where there may be a possible population of users who demand FTTH service. This situation is being extended by the fact that the communications of mobile broadband are beginning to travel from the base station to the backbone via GPON connections.

However, there is the problem that these towns are connected to the main central through a transport network that may not have the bandwidth enough to serve several GPON networks in the new communities, having been dimensioned to provide only DSL.

In that case, the investment required would increase by having to install a new central with OLT GPON equipment at the new location (which implies facility acquisition, conditioning, maintenance costs, electrical installation and initial network, etc.) plus the additional fiber optic lines for the transport network to connect all traffic on the new PON to the main backbone.

What is Trunk-PON solution? Sharing an optical fiber using WDM technology to carry more than one GPON network at once. The benefits are straightforward:
•    On the one hand, it lowers the costs of laying fiber to be shared among multiple networks
•    On the other hand, costs are reduced in plant and equipment due to concentration of GPON equipment on a set of selected plants. (Less power infrastructure, cooling and transportation).
 

What is the role of GPON Extender in Trunk-PON? GPON-Extender is a Reach Extending solution based on interchangeable and colored SFPs. Using different SFPs combinations of non-conventional frequency and WDM filters, we can use GPON-Extenders for creating Trunk-based WDM PON networks. As a bonus, the distance reachable for the PON network is multiplied by the signal processing of the equipment.

Trunk-PON and over-splitting

Another major problem that there are now with fiber to the home networks is that demand is not uniform in the residential market. Therefore, in order to ensure a maximum number of potential users and thus be able to cover the costs of deploying and maintaining the network, they must cover a large area and the maximum number of users that allows the standard.

To extend the reach of GPON networks to the maximum allowed by the standard GPON regenerator equipments are used like GPON-Extender 1:1 or GPON-Extender 1:4. These equips make it possible for GPON to reach up to 60 kilometers and 128 users.

However, in these distances it is possible that the number of 128 users would not be enough. Therefore, and although the GPON standard defines a wavelength for transmission of data, unconventional lambdas might be used and multiplexed into a single fiber using WDM. Thus we have N GPON networks and a number of users 128xN. 

This type of setup is possible in the GPON regenerator equipment GPON-Extender 1:4 thanks to removable SFP (small form-factor pluggable) optical modules that can be configured in the equipment for unconventional lambdas, and can be placed one for each wavelength after the WDM demultiplexing.

Furthermore, by regenerating the signal in 4 different outputs, an additional level of optical division is gained, reducing the optical budget of the network and maximizing both the distance and the number of users that can access the GPON network.