PON technology - passive optical networks
Expanding the audience of consumers of Internet services and, accordingly, users of broadband networks requires the introduction of new technologies. Data transmission facilities must regularly upgrade 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 also problems of a different kind, which are expressed in an increase in the cost of maintaining more massive networks and expanding the range of end user needs. One of the ways to optimize overall performance is PON technology, which also allows you to save the potential of networks to further expand their capacity and functionality.
Optical fiber and PON technology
The new development facilitates the technical organization and further operation of information data transmission networks, but this is achieved largely due to the advantages of conventional optical lines. Even today, against the backdrop of the introduction of high-tech materials, the use of channels built on aging telephone pairs and xDSL facilities continues. Obviously, the access network based on such elements is significantly inferior in efficiency to fiber-coaxial lines, which also cannot be considered as something productive by today's standards.
An alternative to traditional networks and wireless has long been optical fiber. But if in the past laying such cables was an overwhelming task for many organizations, today optical components have become much more affordable. In fact, even before fiber was used to serve ordinary subscribers, including the next stage of development was a telecommunications network built on the Micro-SDH architecture, which opened up fundamentally new solutions. It is in this system that 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 over a single passive optical fiber. As a result, the organization was named FSAN, bringing together both operators and network equipment manufacturers. 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.
How the network works
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 serving multiple subscribers with information flows. The final receiver is the ONT device, which, in turn, also acts as a transmitter. The number of subscriber points connected to the central receiving and transmitting module depends only on the power and maximum speed of the PON equipment used. The technology, in principle, does not limit the number of network participants, however, for the optimal use of resources, the developers of telecommunication projects still put certain barriers in accordance with the configuration of a particular network. Broadcast of the information flow from the central receiving-transmitting module to the subscriber device is carried out at a wavelength of 1550 nm. Conversely, the reverse data streams from consumer devices to the OLT point are transmitted at a wavelength of about 1310 nm. worth considering separately.
Forward and reverse flows
The main (that is, direct) stream from the central module of the network is broadcast. This means that optical lines segment the overall data stream by highlighting address fields. Thus, each subscriber device "reads" only information intended specifically for him. 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 uses a time-shared multiple access provisioning scheme. To eliminate the possibility of crossing signals from several information receiver nodes, each subscriber's device has its own individual schedule for data exchange, adjusted for delay. This is the general principle by which the PON technology is implemented in terms of the interaction of the receiving-transmitting module with end users. However, the network layout configuration may have different topologies.
Topology "point-to-point"
In this case, a P2P system is used, which can be performed both for common standards and for special projects involving, for example, the use of optical devices. In terms of the security of subscriber point data, this type of Internet connection provides the maximum security possible for such networks. However, the laying of an optical line for each user is carried out separately, so the cost of organizing such channels increases significantly. In some way, this is not a general, but an individual network, although the center with which the subscriber node works can also serve other users. In general, this approach is appropriate for use by large subscribers, for whom line security is especially important.
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 locations of nodes are calculated at the stage of project development, however, access networks do not make it possible to estimate in advance the number of subscriber nodes.
Under the condition of random temporary and territorial connection of subscribers, the ring scheme can be significantly complicated. In practice, such configurations often turn into broken circuits with many branches. This happens when the introduction of new subscribers is carried out through the gap 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 reduces the reliability of the network during operation.
Features of the EPON architecture
The first attempts to build a PON network close in terms of consumer coverage to Ethernet technology were made in 2000. The EPON architecture became the platform for developing the principles of network formation, and the IEEE specification was introduced as the main standard, on the basis of which separate solutions for the organization were developed. PON networks. EFMC technology, for example, served a point-to-point topology using twisted copper pair. But today this system is practically not used due to the transition to fiber optics. As an alternative, ADSL-based technologies are still more promising areas.
In its modern form, the EPON standard is implemented according to several connection schemes, but the main condition for its implementation is the use of fiber. In addition to the use of different configurations, the EPON PON connection technology also provides for the use of some options for optical transceivers.
Features of the GPON architecture
The GPON architecture allows the implementation of access networks based on the APON standard. In the process of organizing the infrastructure, it is practiced to increase the network, as well as creating conditions for more efficient transmission of applications. GPON is a scalable frame structure that allows serving subscribers at information flow rates up to 2.5 Gbps. In this case, the reverse and forward flows can operate both at the same and with different speed modes. In addition, an access network in a GPON configuration can provide any encapsulation in a synchronous transport protocol regardless of the service. If only static band division is possible in SDH, then the new GFP protocol in the GPON structure, while maintaining the characteristics of the SDH frame, makes it possible to dynamically allocate bands.
Technology Benefits
Among the main advantages in the PON scheme are the absence 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, the 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 connecting to one central module, on which the quality of service for all infrastructure participants depends. Separately, it is worth considering the tree-like topology of P2MP, which optimizes optical channels as much as possible. Due to the economical distribution of lines for receiving and transmitting information, this configuration ensures the efficiency of the network, regardless of the location of the subscriber nodes. At the same time, new users are allowed to enter without fundamental changes to the existing structure.
PON Network Disadvantages
The widespread use of this technology is still hindered by several significant factors. The first is the complexity of the system. The operational advantages of this type of network can only be achieved if a high-quality project is initially completed, taking into account many technical nuances. Sometimes the way out is the PON access technology, which provides for the organization of a simple typological scheme. But in this case, you should prepare for another drawback - the lack of the possibility of redundancy.
Network testing
When all the stages of the initial development of the network scheme have been completed and technical measures have been completed, the specialists begin testing the infrastructure. One of the main indicators of a well-executed network is the line attenuation index. Optical testers are used to analyze the channel for problem areas. All measurements are made on the active line using multiplexers and filters. A large telecommunications network is usually tested using optical reflectometers. But such equipment requires special training from users, not to mention the fact that expert groups should deal with the interpretation of reflectograms.
Conclusion
With all the difficulties in the transition to new technologies, companies providing telecommunications services are quickly mastering truly effective solutions. Fiber optic systems, which are not simple in technical design, are also gradually spreading, which include PON technology. Rostelecom, for example, began introducing new format services back in 2013. Residents of the Leningrad Region were the first to gain access to the capabilities of PON optical networks. What is most interesting, the service provider provided even local villages with fiber optic infrastructure. In practice, this allowed subscribers to use not only telephone communications with Internet access, but also connect to digital television broadcasting.
