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PON-technology - passive optical networks

Expansion of the audience of consumers of Internet services and, accordingly, users of broadband networks requires the introduction of new technologies. Means of data transmission should regularly increase the capacity of communication lines, which forces service companies to update transport information channels. But, in addition to the growth in the volume of transmitted data, there are other problems that are expressed in the increase in the cost of servicing more massive networks and expanding the range of end-user needs. One of the ways to aggregate optimization of the characteristics of telecommunications systems is the PON-technology, which also allows preserving the potential of networks for further expansion of their capacity and functionality.

Optical fiber and PON technology

The new development facilitates the technical organization and further operation of information networks of data transmission, but this is achieved largely due to the advantages of conventional optical lines. Even today, with the introduction of high-tech materials, the use of channels based on aging telephone pairs and xDSL facilities continues. It is obvious that the access network on such elements is substantially losing in efficiency to fiber-coaxial lines, which also can not be regarded as something productive by the standards of today.

An alternative to traditional networks and wireless communication channels has long been an optical fiber. But if before laying such cables was an impossible task for many organizations, today optical components have become much more accessible. Actually, before, the fiber was used to serve ordinary subscribers, including Ethernet. The next stage of development was the telecommunications network, built on the architecture of Micro-SDH, which opened fundamentally new solutions. Just in this system the concept of PON networks has found its application.

Network standardization

The first attempts to standardize the technology were made back in the 1990s, when a group of telecommunications companies set out to put into practice the idea of multiple access to a single passive optical fiber. As a result, the organization was named FSAN, bringing together both operators and manufacturers of network equipment. The main goal of FSAN was to create a package with general recommendations and requirements for the development of PON hardware, so that equipment manufacturers and providers can work together in the same segment. To date, passive communication lines based on PON technology are organized in accordance with ITU-T, ATM and ETSI standards.

Principle of network operation

The main feature of the PON idea is that the infrastructure operates on the basis of one module, which is responsible for the functions of receiving and transmitting data. This component is located in the central node of the OLT system and allows a lot of subscribers to serve information flows. The ultimate receiver is an ONT device, which, in turn, also acts as a transmitter. The number of subscriber points connected to the central receiving and transmitting unit depends only on the power and maximum speed of the PON equipment used. Technology, in principle, does not limit the number of network participants, however, for optimal use of resources, telecommunications project developers still place certain barriers in accordance with the configuration of a particular network. The transmission of the information flow from the central receiving-transmitting module to the subscriber unit is performed at a wavelength of 1550 nm. Conversely, reverse data streams from consumer devices to the OLT point are transmitted with a wavelength of about 1310 nm. These flows should be considered separately.

Forward and Backward Flows

The main (i.e., direct) stream from the central module of the network refers to the broadcast. This means that optical lines segment the total data flow, allocating address fields. Thus, each subscriber device "reads" only information intended specifically for it. This principle of data distribution can be called demultiplexer.

In turn, the reverse stream uses one line to broadcast data from all subscribers connected to the network. This is how the multiple access scheme with time-sharing is used. To exclude the possibility of crossing signals from several information receiving nodes, the device of each subscriber has its own individual schedule for data exchange with delay correction. This is a general principle by which PON-technology is realized in terms of interaction of the receiving-transmitting module with end-users. However, the configuration of the network layout may have different topologies.

Point-to-point topology

In this case, a P2P system is used, which can be performed for common standards, and for specific projects involving, for example, the use of optical devices. In terms of data security of subscriber points, this type of Internet connection provides the maximum security possible for such networks. However, the laying of the optical line for each user is carried out separately, so the cost of organizing such channels is significantly increased. In some ways, this is not a general, but an individual network, although the center with which the subscriber node operates, can also serve other users. In general, this approach is suitable for use by large subscribers, who are particularly important in line security.

Topology "ring"

This scheme is based on the SDH configuration and is best deployed in backbone networks. Conversely, ring-type optical lines are less efficient in the operation of access networks. So, when organizing a city highway, the nodes are located at the project development stage, but access networks do not provide an opportunity to estimate the number of subscriber nodes in advance.

Given the occasional temporary and territorial connection of subscribers, the ring scheme can be considerably complicated. In practice, such configurations often turn into broken circuits having many branches. This happens when the introduction of new subscribers is performed through the break of existing segments. For example, loops can be formed in the communication line, which are combined in one wire. As a result, "broken" cables appear, which in the process of operation reduces the reliability of the network.

Features of EPON architecture

The first attempts to build a PON network, approximating the degree of coverage of consumers to Ethernet technology, were undertaken in 2000. The platform for developing the principles of network formation was the EPON architecture, and as the main standard, the IEEE specification was introduced, on the basis of which separate solutions were developed for the organization Networks PON. EFMC technology, for example, served the point-to-point topology using a twisted copper pair. But today this system is practically not used in connection with the transition to optical fiber. As an alternative, more promising areas are still technologies based on ADSL.

In the current form, the EPON standard is implemented by several connection schemes, but the main condition for its implementation is the use of fiber. In addition to the use of different configurations, the PON connection technology according to the EPON standard also provides for the possibility of using certain optical transceiver options.

GPON Architecture Features

The GPON architecture allows the implementation of access networks based on the APON standard. In the process of infrastructure organization, it is practiced to increase network bandwidths , as well as to create conditions for more efficient transfer of applications. GPON is a scalable frame structure that allows users to serve subscribers at the speed of information flows up to 2.5 Gbit / s. In this case, the reverse and direct streams can work both on one and with different high-speed modes. In addition, the access network in the GPON configuration can provide any encapsulation to the transport synchronous protocol regardless of the service. If in the SDH it is possible to implement only static division of the bands, the new GFP protocol in the GPON structure, while preserving the characteristics of the SDH frame, enables dynamic allocation of the bands.

Technology advantages

Among the main advantages of optical fibers in the PON scheme are the lack of intermediate links between the central receiver-transmitter and subscribers, economy, ease of connection and ease of maintenance. To a large extent, these advantages are due to the rational organization of networks. For example, an Internet connection is provided directly, so the failure of one of the adjacent subscriber devices does not affect its performance in any way. Although the array of users, of course, is united by connection to one central module, on which the quality of service of all infrastructure participants depends. We should also consider the tree topology P2MP, which optimizes optical channels to the maximum. Thanks to the economical distribution of information transmission and transmission lines, this configuration ensures the efficiency of the network, regardless of the location of subscriber units. At the same time, new users can be entered without radical changes to the existing structure.

Disadvantages of the PON network

A wide use of this technology is still hampered by several significant factors. First of all, this is the complexity of the system. The operational advantages of this type of network can only be ensured if the initial implementation of a quality project takes into account a variety of technical nuances. Sometimes the way out of the situation is access technology PON, which provides for the organization of a simple typological scheme. But in this case it is necessary to prepare for another shortcoming - the lack of the possibility of reserving.

Network testing

When all stages of the initial development of the network scheme are completed and technical activities are implemented, specialists begin to test the infrastructure. One of the main indicators of a high-quality network is the attenuation on the line. To analyze the channel for the presence of problem zones, optical testers are used. All measurements are made on the active line using multiplexers and filters. A large-scale telecommunications network is usually tested using optical reflectometers. But such equipment requires special training from users, not to mention the fact that the interpretation of the reflectograms should be handled by expert groups.

Conclusion

With all the complexities in the transition to new technologies, companies that provide telecommunications services are quickly developing truly effective solutions. Gradually spread and uneasy in the technical performance of fiber-optic systems, which includes the technology PON. "Rostelecom", for example, began to introduce services of a new format as early as 2013. Access to the capabilities of PON optical networks was first received by residents of the Leningrad region. What is most interesting, the service provider has provided fiber-optic infrastructure, even local villages. In practice, this allowed subscribers to use not only telephone communication with Internet access, but also to connect to digital television broadcasting.

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