Expanding 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, other kinds of problems arise, which are expressed in increasing the cost of servicing more massive networks and expanding the range of needs of end users. One of the ways to jointly optimize the characteristics of telecommunication systems is PON-technology, which also allows you to preserve the potential of networks to further expand their capacity and functionality.
Fiber Optic 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, amid the introduction of high-tech materials, the use of channels based on aging telephone pairs and xDSL facilities continues. Obviously, the access network on such elements is significantly less efficient in fiber-coaxial lines, 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 earlier laying such cables was an impossible task for many organizations, today optical components have become much more affordable. Actually, earlier fiber was used to serve ordinary subscribers, including Ethernet technology. The next stage of development was the telecommunication network, built on the Micro-SDH architecture, which opened up completely new solutions. It is precisely 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 decided to put into practice the idea of multiple access over a single passive optical fiber. As a result, the organization was named FSAN, combining 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 one segment. Today, 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 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 optimal use of resources, telecommunication project developers nevertheless put certain barriers in accordance with the configuration of a particular network. The information stream is transmitted from the central receiving and transmitting module to the subscriber unit at a wavelength of 1550 nm. Conversely, reverse data streams from consumer devices to the OLT point are transmitted at a wavelength of about 1310 nm. These streams should be considered separately.
Forward and reverse flows
The main (i.e. direct) stream from the central network module is broadcast. This means that the optical lines segment the overall data stream, highlighting the address fields. Thus, each subscriber device "reads" only information specifically designed for him. This principle of data distribution can be called demultiplexer.
In turn, the reverse flow uses one line to transmit data from all subscribers connected to the network. This uses a time-division multiple access scheme. To eliminate the likelihood of signals crossing from several nodes-receivers of information, the device of each subscriber has its own individual schedule for data exchange adjusted for delay. This is a general principle according to which PON technology is implemented in terms of the interaction of the transmitting and receiving module with end users. However, the configuration of the network cabling scheme 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 special projects involving, for example, the use of optical devices. In terms of data security of subscriber points, an Internet connection of this type 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 increases significantly. In a way, this is not a common, but an individual network, although the center with which the subscriber unit works can also serve other users. In general, this approach is suitable for use by large subscribers, for whom line safety is especially important.
Ring topology
This scheme is based on the SDH configuration and is best disclosed in backbone networks. Conversely, ring-type optical lines are less efficient in operating access networks. So, when organizing a city highway, the locations of nodes are calculated even at the stage of project development, 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 circuit can be significantly complicated. In practice, such configurations often turn into broken circuits with many branches. This happens when new subscribers are introduced through breaking 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 during operation reduces network reliability.
EPON Architecture Features
The first attempts to build a PON network, similar in terms of consumer coverage to Ethernet technology, were made in 2000. The platform for developing the principles of networking was EPON architecture, and the IEEE specification was introduced as the main standard, based on which individual solutions for 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 in connection with the transition to optical fiber. As an alternative, ADSL-based technologies continue to be 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 PON connection technology according to the EPON standard also provides the possibility of using 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, an increase in network bandwidth is practiced, as well as the creation of conditions for more efficient application transfer. GPON is a scalable personnel structure that allows you to serve subscribers at speeds of information flows up to 2.5 Gb / s. In this case, the reverse and forward flows can work both on one and with different speed modes. In addition, the access network in the GPON configuration can provide any encapsulation in the transport synchronous protocol regardless of the service. If only static division of bands is possible in SDH, then the new GFP protocol in the GPON structure, while maintaining the characteristics of the SDH frame, allows dynamic band allocation.
Technology benefits
Among the main advantages of optical fibers in the PON scheme, the absence of intermediate links between the central receiver-transmitter and subscribers, the economy, ease of connection and ease of maintenance are highlighted. 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. Although the array of users, of course, is combined by connecting to one central module, which determines the quality of service for all participants in the infrastructure. We should also consider the P2MP tree topology, which optimizes optical channels as much as possible. Due to the economical distribution of information transmission and reception lines, this configuration ensures network efficiency regardless of the location of subscriber nodes. At the same time, it is allowed to introduce new users without radical changes to the existing structure.
PON Network Disadvantages
The widespread use of this technology is still hampered by several significant factors. First of all, it is the complexity of the system. The operational advantages of this type of network can be ensured only if the initial implementation of a high-quality project, taking into account many technical nuances. Sometimes the PON access technology, which provides for the organization of a simple typological scheme, becomes a way out. But in this case, one should prepare for another drawback - the lack of redundancy.
Network testing
When all the stages of the initial development of the network diagram 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 attenuation indicator on the line. To analyze the channel for the presence of problem areas, optical testers are used. All measurements are made on the active line using multiplexers and filters. A large-scale telecommunication network is usually tested using optical reflectometers. But such equipment requires special training from users, not to mention that expert groups should deal with the decoding of reflectograms.
Conclusion
With all the difficulties in the transition to new technologies, companies that provide telecommunications services are quickly mastering truly effective solutions. Fiber-optic systems, which are difficult in technical execution, are gradually spreading, including PON technology. Rostelecom, for example, began to introduce services of a new format back in 2013. Residents of the Leningrad Region were the first to gain access to the capabilities of PON optical networks. Most interestingly, the service provider has provided fiber-optic infrastructure even to local villages. In practice, this allowed subscribers to use not only telephone communications with Internet access, but also connect to digital television broadcasting.