As fiber optic technology has developed to its current state of maturity, its applications have become widespread, but there is still great room for further development. This article will discuss the current status and advantages and disadvantages from the two major aspects of the promising optical switching technology and fiber-to-the-home (FTTH), and finally introduce the types of optical fibers and methods for selecting optical fibers.
1.Optical Switching: The Trend for Future Development
Optical switching refers to directly switching the input optical signal to any optical output without any optical/electrical conversion. Optical switching is one of the key technologies for all-optical networks. In modern communication networks, the all-optical network is the future direction for the development of broadband communication networks. All-optical networks can overcome the capacity bottleneck limitations of electronic switching, significantly reduce network construction costs, and greatly improve network flexibility and reliability. Optical switching technology can be divided into optical circuit switching and optical packet switching. For technological reasons, the current focus is mainly on the development of optical circuit switching, but the future direction will be optical packet switching.
Optical fibers only solve the transmission problem; the switching of light also needs to be addressed. In the past, communication networks were composed of metal cables, transmitting electronic signals, and switching was performed using electronic switches. Now, except for a small segment at the user end, communication networks are composed of optical fibers, transmitting optical signals. The reasonable approach should be to adopt optical switching. However, due to the current immaturity of optical switch devices, the “optical-electrical-optical” method has to be adopted to solve the switching in optical networks, which involves converting the optical signal into an electrical signal, performing electronic switching, and then converting it back to an optical signal. This is an unreasonable and inefficient approach. Efforts are being made to develop large-capacity optical switches to realize optical switching networks, particularly the so-called Automatically Switched Optical Network (ASON).
The optical switching currently seen on the market is mostly based on optoelectronic and optomechanical technologies. However, optical switches based on thermal, liquid crystal, acoustic, and microelectromechanical systems (MEMS) technologies will gradually be developed. Among these, MEMS technology is currently the most promising technology. Optical switching provides a highly competitive solution for the photonization of IP backbone networks. On the one hand, optical switching can enable the flattening of the protocol layers in existing IP backbone networks, thereby more fully utilizing the bandwidth potential of dense wavelength division multiplexing (DWDM) technology. On the other hand, since optical switching networks are completely transparent to burst packets, without any optical-electrical conversion, optical burst switching fabrics can truly achieve the so-called terabit-level optical routers, completely eliminating the bandwidth expansion difficulties caused by the current electronic bottleneck. Moreover, the quality of service (QoS) support feature of optical switching also meets the requirements of the next-generation Internet. Therefore, optical switching networks have the potential to replace the current IP backbone networks based on ATM/SDH architectures and electronic routers, becoming the backbone network of the next-generation photonic Internet.
2.Development of Fiber-to-the-Home (FTTH)
FTTH (Fiber-to-the-Home), as the name suggests, refers to a single optical fiber reaching directly into the home. Specifically, FTTH refers to the installation of an optical network unit (ONU) at residential or enterprise user premises, representing the closest optical access network application type to the user among the fiber access series, except for fiber-to-the-desktop (FTTD). The notable technical feature of FTTH is not only providing greater bandwidth but also enhancing the network’s transparency to data formats, rates, wavelengths, and protocols, relaxing requirements for environmental conditions and power supply, and simplifying maintenance and installation.
FTTH can provide users with extremely rich bandwidth, and as such, it has long been considered the ideal access method, playing an important role in realizing the information society. However, large-scale promotion and construction are still needed. The amount of optical fiber required for FTTH is 2-3 times the existing deployed optical fiber. In the past, due to the high cost of FTTH, lack of broadband video services and broadband content, FTTH has not been able to develop on a large scale, with only a limited number of trials. In recent years, with the advancement of optoelectronic components, the prices of optical transceivers and optical fibers have dropped significantly. Additionally, the increasing enrichment of broadband content has accelerated the practical implementation of FTTH.
The main advantages of FTTH are: First, it is a passive network, and from the central office to the user, it can essentially be passive in between. Second, it has a relatively wide bandwidth, which is well-suited for large-scale deployment by operators over long distances. Third, since it carries services on optical fibers, there are no significant issues. Fourth, due to its relatively wide bandwidth, it supports flexible protocols. Fifth, with technological developments, including point-to-point, 1.25G, and FTTH modes, relatively complete standards have been established.
Developed countries have different views on FTTH: Operators like Verizon and Sprint in the United States are relatively proactive, aiming to adopt FTTH to upgrade their networks within 10-12 years. NTT in Japan was the earliest to develop FTTH, starting as early as 1997. After 2000, the number of users increased significantly due to cost reductions. In the United States, FTTH installations increased by over 200% around December 2002.
Currently, the widely adopted ADSL technology still has certain advantages compared to FTTH: ① Low cost, ② Simple engineering construction by utilizing the existing copper wire network, and ③ Ability to meet the demand for transmitting current video programs and files. These reasons have constrained the large-scale promotion of FTTH.
The high cost of equipment, leading to low investment returns, is a key factor hindering the development of FTTH. Currently, FTTH equipment prices are still exorbitantly high, often with retail prices approaching $1,000. However, FTTH has seen relatively good development in developed countries like Japan and the United States, partly because their telecommunications operators can charge users higher service fees. It is understood that in Japan, telecommunications operators charge FTTH users a monthly service fee of 5,000-6,000 yen, equivalent to approximately 400-500 Chinese yuan, while in the United States, the monthly service fee for FTTH users is also around $80-100. Generally, telecommunications operators can recover their investment in FTTH networks within 2-3 years, which is certainly a decent return on investment. However, the situation in China is completely different. In many cities in China, due to intense market competition, the monthly usage fees for ADSL and LAN broadband access based on Category 5 cabling have already dropped below 50 Chinese yuan, and in some regions with higher usage fees, such as Shenzhen, the monthly usage fee is only around 100 Chinese yuan. Based on this level of broadband access service fees, it is simply impossible to support the construction and operation of FTTH networks, with an investment recovery period of up to 10 years, which is clearly not attractive enough for telecommunications operators. It is evident that what the broadband access market needs is low-cost FTTH, as only low-cost FTTH will have the opportunity for application and development, and it is certain to have the opportunity for development.
Optical fibers themselves also have disadvantages, such as their brittle nature and low mechanical strength, which are critical weaknesses. If not handled carefully, they can easily break within the optical cable sheath. Additionally, the splicing of optical fibers is relatively difficult, requiring construction personnel to have good skills in cutting, connecting, branching, and coupling.
FTTH Solutions: Currently, the main FTTH access technologies are divided into two major categories: Ethernet Passive Optical Network (EPON) and Gigabit Passive Optical Network (GPON) based on Passive Optical Network (PON) access technology, and Fiber P2P technology based on Active Optical Network (AON) access in small areas.
P2P Solution – Advantages: Independent transmission for each user without mutual interference, flexible system changes; inexpensive low-speed optoelectronic modules can be used; long transmission distance. Disadvantages: An active node needs to be placed in the user area to consolidate users, reducing the number of optical fibers and ducts directly reaching the central office.
PON Solution (continued) – Disadvantages: Expensive high-speed optoelectronic modules are required; electronic modules that distinguish different user distances must be used to avoid upstream signal conflicts between users; transmission distance is shortened by the PON splitting ratio; the downstream bandwidth of all users is shared, and if user bandwidth cannot be guaranteed, not only network expansion but also replacement of the PON and user modules are needed to resolve the issue.
There are several types of PON: (1) APON: ATM-PON, suitable for ATM switching networks. (2) BPON: Broadband PON. (3) OPON: Using the Open Frame Processing (OFP) protocol. (4) EPON: PON using Ethernet technology, with GEPON being the Gigabit Ethernet PON. (5) WDM-PON: PON using wavelength division multiplexing to distinguish users, but rarely used in FTTH due to maintenance inconvenience associated with wavelength-dependent users.
Recently, with the rapid development of wireless access technologies, such as the IEEE 802.11g protocol for WLAN with a transmission bandwidth of up to 54 Mbps and a coverage range of over 100 meters, which is now commercially available. If wireless WLAN access is adopted for user data transmission, and considering that the upstream data volume for general users is not large, IEEE 802.11g can be sufficient. On the other hand, FTTH mainly addresses the large downstream data transmission of broadband video such as HDTV, and can also include some downstream data as needed. This forms a home network of “Fiber-to-the-Home + Wireless Access” (FTTH + Wireless Access). For such a home network, if the PON approach is adopted, it becomes particularly simple because this PON does not require upstream data, eliminating the need for ranging electronic modules, thereby significantly reducing cost and simplifying maintenance. If the user group served by the PON is covered by a wireless metropolitan area network (WiMAX, IEEE 802.16), there is no need to build a dedicated WLAN. While wireless access is a trend, wireless access networks still require a densely deployed optical fiber network in the vicinity of users to support them, essentially equivalent to FTTH. FTTH + Wireless Access is the future trend for network development.
3.Proper Selection and Usage of Optical Fibers
Next, let’s discuss the proper selection and usage of optical fibers. Optical fibers can be broadly classified into multimode fibers and single-mode fibers.
Multimode fibers are optical fibers that can transmit multiple propagation modes of light. In the early days of fiber optic communication, multimode fibers (G.651 fibers) were used, with operating wavelengths of 850 nm or 1300 nm, attenuation constants of <4 dB/km and <3 dB/km, respectively, and dispersion coefficients of <120 ps/(nm.km) and <6 ps/(nm.km), respectively. Due to their high attenuation and dispersion, they can only be used for short-distance communication. However, their large core diameters have less stringent requirements for connectors and splices, making them more convenient to use than single-mode fibers. Currently, they are mainly used in local area networks.
Single-mode fibers are optical fibers that transmit only one propagation mode (the fundamental mode). Their main advantages are lower attenuation, longer transmission distances, and larger transmission capacities, with widespread applications in long-haul backbone networks, metropolitan area networks, access networks, and more. Since single-mode fibers only transmit the fundamental mode, they do not experience modal dispersion, resulting in much larger bandwidths than multimode fibers, with bandwidths reaching tens of GHz or more. Therefore, single-mode fibers are particularly suitable for long-distance, high-capacity communication systems. With the continuous development of optical fiber manufacturing and communication technologies, the types of single-mode fibers have also evolved. Common single-mode fibers include: G.652 fibers, G.653 fibers, and G.655 fibers.
When selecting optical fibers, the following three parameters should be considered: ① Maximum unrepeated transmission distance, ② Maximum bit rate at the operating wavelength, and ③ Number of wavelengths for the optical fiber. All of these parameters must take into account the requirements at the end of the optical fiber deployment. If the maximum unrepeated transmission distance is 50-100 km, it is recommended to choose the G.652 conventional fiber, which is inexpensive and suitable for short-distance transmission. If the distance is longer but only requires a single wavelength above 10 Gbit/s, the G.653 dispersion-shifted fiber can be selected. If not only is the distance long but multiple wavelengths carrying 10 Gbit/s or higher rates are required, then the G.655 fiber is the best choice.
From this, the following principles for optical fiber selection can be summarized: 1. For short distances, G.652 conventional fibers should be selected, as the increased investment due to the use of more fiber cores is not significant. 2. For long-distance optical cables, due to the long transmission distance, high-rate and multi-wavelength wavelength division multiplexing techniques must be employed, making the G.655 dispersion-shifted fiber the ideal choice.
4.Conclusion
Fiber optic communication technology has now become one of the important modern information transmission technologies and has seen widespread application in the current information society context. Even when the global communication industry and related fields were in a state of severe depression, fiber optic communication technology still saw some development. According to the current development model in the communication technology field, the application of fiber optic communication technology will inevitably replace all other information transmission methods and become the mainstream technology leading the development of the communication field, ushering in the all-optical era for humankind!
