The Internet is just an ordinary thing in the lives of billions of people, and emails, file sharing, messaging, cloud services, video calls, online gaming and online movie streaming are taken for granted. In some places, people do not have access to drinking water but have access to the Internet. This access is made possible by the enormous transmission capacity that is available in the core fiber networks using coherent optical transmission equipment, offering hundred gigabit per second transmission speeds (400 Gbps in 2018); with the help of wavelength multiplexing, the total capacity can reach tens of terabits per second. The maximum capacity of the core Internet fiber networks is just one part of the story—it is even more important that end-users can access that massive volume of data [1
]. Wireless networking with mobile phones, tablets and other wireless devices is responsible for tremendous growth in Internet traffic, but optical fiber is still the most promising candidate to satisfy any foreseeable data traffic [2
Since the inception of the Internet and the World Wide Web (WWW), bandwidth consumption among common users has continually increased. Video applications demand the highest consumption because high-definition television (HDTV) has become almost obsolete and modern users are seeking super HDTV, which is also called 4k, while TVs with even better resolution, such as 8k, are available on the market [5
]. However, 8k is not as widespread due to a lack of real content (i.e., movies available in these super HD resolutions). HDTV streams may consume megabits per second, while 8k may consume a maximum of one hundred megabits per second. Therefore, Internet services must be able to offer additional bandwidth, especially end-user demand for bandwidth, which is continuously increasing both in access networks and core networks [7
A well-known and respected visual networking index (VNI) by Cisco is available online [7
] and confirms the substantial data growth: global Internet Protocol (IP) traffic will increase nearly threefold over the next 5 years and will have increased 127-fold from 2005 to 2021. IP traffic will increase at a compound annual growth rate (CAGR) of 24% from 2016 to 2021. Annual global IP traffic will reach 3.3 ZB (a zettabyte is 1000 exabytes) by 2021. In 2016, global IP traffic was 1.2 ZB per year or 96 EB per month. By 2021, global IP traffic will reach 3.3 ZB per year, or 278 EB per month. Although these data are predictions, IP traffic is significantly increasing every year. Note that some parts of Africa with a very young population do not have access to the Internet, which indicates that stagnation in growth is not expected.
Although the price of fiber is low, underground installation is the most expensive part and is usually linked to legal issues. Thus, when fiber is laid/buried in the ground, one fiber must be used multiple times without installing any active devices along the way. Passive optical networks can solve this problem.
Bandwidth consumption is dependent on many factors. An interesting resolution for introverted and extroverted people, nations and continents is explained in [8
]. Bandwidth consumers are described as Telhomers in the United States of America (USA) and Canada. Telenese live in Asia and Europeans are described as Cafeneans. Although it may not appear to be a serious approach, the difference between climate and temperament among different continents and nations must be respected to achieve the best results. One example pertains to rural remote areas of Sweden [9
]. Even young people are willing to return to the empty countryside if they can use a broadband Internet connection to communicate with friends and families, watch the news and work in home offices.
Passive optical networks (PONs) have been attracting attention as a promising access technology for more than 20 years with the first International Telecommunication Union (ITU) Recommendation G.983.1, which was published in 1998 [10
], featured (then) popular asynchronous transfer mode (ATM) technology and related ATM-based PONs.
The first mention of optical fibers usage in passive access networks was published in 1987 by Stern and colleagues [11
]. The change in the transmission medium from copper to silica fiber required the connection of active devices to provide the conversion of optical signals to electrical signals, which is not used in contemporary PONs.
This concept was referred to as an optical distribution network (ODN). ODN concepts that were developed in 1987 remain preserved, with some changes. PONs are assembled from passive devices, such as optical fibers, connectors and power splitters, with active elements such as optical line termination (OLT) devices and optical network units (ONUs). Technologies and principles of PONs have been actively researched and developed over the last three decades and remain a focus of researchers. As a result, different PONs are available that encompass not only access but also metropolitan distances and multigigabit data rates.
PONs are similar to another technology: fiber to the home (FTTH). The FTTH Council was formed in 2001 to promote FTTH concepts in North America, especially the USA and Canada. Although Europe as whole was not as active in this area of networking (some countries were), the importance of PONs and FTTH was recognized by the European Union (EU). The EU aims to financially support the commissioning of fast optical networks, at least 30 Mbps for each customer and 100 Mbps for new connections until 2020. The EU gives more attention to next-generation (NG) optical access networks [12
]. Despite the immense popularity of wireless technologies, fiber is recognized as the important part of many broadband initiatives, for example, in large international corporations such as Google [13
] or AT&T [14
Many current solutions specify how to terminate optical fiber at an end-user, and FTTH is not considered to be the only solution. Generally, fiber to the x (FTTx) is the name of the convention, where the letter “x” expresses a technology. FTTH is only a part of the generalized term FTTx, and these technologies are very popular and frequently deployed; Internet services providers (ISPs) have to find a trade-off between price and penetration [15
]. For example, for fiber to the building (FTTB), optical fiber is installed in optical splice boxes in buildings, and then, different transmission mediums (such as copper wire) are used to carry the signal into customer apartments. Other solutions employ edge boxes (e.g., switches or routers) in a building that are powerful, small and have low power consumption. These boxes provide optical interfaces with optical cables that reach individual apartments equipped with equipment that have optical receivers and usually use Ethernet for services such as Internet protocol television (IPTV).
A PON network is usually created by point-to-multipoint (P2MP) infrastructure, as depicted in Figure 1
, where part ”(a)” presents the base topology with a splitter represented by a particular standard, and part ”(b)” presents the most popular solution with the cascade ordering of splitters. The composition can differ with regard to an ISP. The first splitter is used after an active OLT port to provide an effective number of ONUs that are connected to the OLT port. The single port of an ONU has the limitation of time slots, and therefore, full capacity of the ONU is unreachable. Another type of networking is point-to-point (P2P), which is more complex and requires fiber for every aggregation switch. The difference is highlighted in Figure 2
The development of new technical solutions and standards is not limited to transmission speeds in networks. The investments and business models of key investors have to be taken into account [16
]. The gigabit PON (GPON) is well prepared, and the price of GPON devices has decreased. However, newer standards, such as XG-PON or the newest next-generation PON stage 2 (NG-PON2) and Ethernet-based PONs—Ethernet PON (EPON) and 10G-EPON—introduce a new term similar to FTTH. The difference between GPON networks and EPON networks is important because both networks are derived from different backgrounds. GPON was proposed by ITU and employs time division multiplexing (TDM) technology, such as synchronous digital hierarchy (SDH) or the US-equivalent synchronous optical network (SONET). A variant of TDM, which is referred to as a time division multiple access (TDMA) format, is used to allocate time slots to each user to provide downstream bandwidth. TDM technologies such as SDH and SONET (and connection-oriented ATM) are becoming not used in networks due to their complexity and high price. However, an optical transport networks (OTN) is used in core networks and is subject to the reprisal of a software defined network (SDN) because OTN is ITU technology. EPONs are based on the most prevalent networking technology: the Ethernet. EPONs are compatible with other Ethernet devices, such as switches or hubs, and deploying and troubleshooting EPONs are easy because the Ethernet is ubiquitous and well known to many networking engineers. The Ethernet is data link (L2) technology, and Ethernet frames carry IP packets, such as those used for the whole Internet, everywhere.
Another important (perhaps the most important) aspect of EPONs is price—because of massive fabrication of Ethernet chips, 1 Gigabit Ethernet (GE) and even 10 GE are affordable. Some sources indicate that GPONs are 2 times more expensive than EPONs. Other price comparisons are available, but they may be slightly misleading because vendors tend to promote particular technologies, and the GPON/EPON price factor of 10 is most likely exaggerated. The cost of the newest technologies does not allow extensive deployment, and obsolete devices are being replaced by new equipment. This “one by one” strategy is always included in incoming standards for PONs. Two choices are available for selecting an appropriate standard in brownfield or greenfield scenarios. For an incipient ISP, the greenfield scenario is better because it has only fiber infrastructure and no deployed active and passive PON elements (a PON is passive by definition, but active devices are essential and only placed at the end of fibers). An arbitrary standard can be selected for their network. For the brownfield scenario, ISPs operate one standardized technology, and the replacement process becomes more complex. All customers cannot be disconnected at an OLT port during the OLT replacement process because current ONUs will not be able to operate by a new standard; each standard has similar operations but with an additional component of the transmission convergence layer.
PONs are definitively here, and new developments ensure that they will remain for years to come. New standards for high-speed PONs exist, and additional standards are being prepared. We discussed different technologies that range from cost-effective solutions for everyone to current examples that can satisfy even business needs. We described the data communication among PON elements: OLTs and ONUs and schematics of all data messages.
All mentioned PON technologies are similar in some aspects because they share the same fiber network and physical topologies. On the other hand, there are quite large differences in ITU- and IEEE-based approaches, which are already known from ATM and Ethernet switching networking technologies. Although standards are important, the lack of standards should not hinder the creation of new approaches, for example, deployment of optical amplifiers in PONs, including mature technologies such as EDFAs and SOAs.
New trends of open networking promoted by megadata and hyperdata center companies should be considered for new trends in PON deployment to avoid any vendor dependencies and lock-ins. Open networking can ensure that technologies are replaced or migrated to new equipment as needed. These new open trends are not standardized but cannot be disregarded because they are emerging in many parts of the world, especially in North America and Asia. The support of new applications, such as accurate time transfer or distributed fiber sensing, should be considered. This new class of applications may not appear to be appropriate for a PON environment, and current user requirements and new open approaches may prevent well-known failed predictions, such as the famous quotes “I think there is a world market for maybe five computers” or “There is no reason anyone would want a computer in their home”. With open ideas for networking deployment, we may reserve these famous quotes related to PONs for future users.
The article does not cover nonlinear phenomena in passive optical networks. From ODN point of view and based on parameters of current PONs we do not think that nonlinear phenomena can affect signal significantly. If we consider on-off keying (OOK) modulation with 100ps pulse duration and average power of about 5 dBm, the peak power is less than 7 mW. Moreover, the maximum recommended distance for PON is 20 km, which is a relatively short distance. The laser phase is another critical parameter that should not be neglected in a high-speed transmission system, especially in case of high order modulation formats, which is not the case. We do not think that phase noise in simple OOK modulation can influence the performance of the system and its performance.