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 increase the bandwidth 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 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 of cumulative optimization of the characteristics of telecommunication systems is PON technology, which also allows you to save the potential of networks for further expansion of their power and functionality.
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. It is obvious that the access network based on such elements significantly loses in efficiency to fiber-coaxiallines, which also cannot be considered as something productive by today's standards.
Optical fiber has long been an alternative to traditional networks and wireless communication channels. But if in the past laying such cables was an overwhelming task for many organizations, today optical components have become much more affordable. Actually, fiber optics was used before to serve ordinary subscribers, including using Ethernet technology. The next stage of development was a telecommunications network built on the Micro-SDH architecture, which opened up fundamentally new solutions. It was in this system that the concept of PON networks 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 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.
Network principle
The main feature of the PON idea is that the infrastructure works on the basis of a single module that is responsible for the functionsreceiving 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. The transmission 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. These flows should be considered separately.
Forward and reverse flows
The main (that is, direct) stream from the central network module 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 demultiplexing.
In turn, the reverse stream uses one line to broadcast data from all subscribers connected to the network. This is how the multiple collateral scheme is usedtime-shared access. 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.
Point-to-point topology
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.
Ring topology
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 placementnodes are calculated at the project development stage, however, access networks do not provide an opportunity to estimate the number of subscriber nodes in advance.
Under the condition of random temporary and territorial connection of subscribers, the ring scheme can be much more 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.
EPON architecture features
The first attempts to build a PON network close in consumer coverage to Ethernet technology were made in 2000. The EPON architecture became the platform for developing networking principles, and the IEEE specification was introduced as the main standard, on the basis of which separate solutions for organizing PON networks have been developed. 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 applying different configurations, EPON standard PON connection technology alsoprovides for the use of some variants of optical transceivers.
GPON architecture features
The GPON architecture allows implementing access networks based on the APON standard. In the process of organizing the infrastructure, it is practiced to increase network bandwidths, as well as create 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.
Advantages of Technology
Among the main advantages of optical fibers in the PON scheme, there are no 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 is, of course, combined by connecting to one central module, fromwhich depends on the quality of service for all infrastructure participants. Separately, it is worth considering the tree-like topology of P2MP, which optimizes optical channels as much as possible. Thanks 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.
Disadvantages of the PON network
Wide application 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 reservation.
Network testing
When all stages of the initial development of the network scheme have been completed and technical measures have been completed, 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 usingoptical 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
For all the challenges of migration to new technologies, telecommunications companies are quickly adopting 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.
