Foreword
Fiber-to-the-x (FTTx) fiber access
(FTTx, x = H for home, P for premises, C for curb and N for node or neighborhood) Among them, FTTH fiber to the home, FTTP fiber to the premises, FTTC fiber to the roadside/community, and FTTN fiber to the node.
Fiber-to-the-home (FTTH) is a dream and technological direction that people have been pursuing for 20 years. However, due to obstacles in cost, technology, and demand, it has not yet been widely promoted and developed. However, this slow-moving situation has recently changed considerably. Due to policy support and technology development, FTTH has once again become a hot spot after many years of silence, entering a period of rapid development. The comfort and convenience of life brought by various related broadband applications such as VoIP, Online-game, E-learning, MOD (Multimedia on Demand) and smart home, and the interactive high-definition viewing caused by HDTV The revolution has made optical fiber with excellent characteristics such as high bandwidth, large capacity, and low loss an inevitable choice for the medium that transmits data to the client. Because of this, many insightful people regard FTTx (especially fiber-to-the-home and fiber-to-the-premises) as an important turning point in the recovery of the optical communication market. And it is expected that in the next few years, FTTH network will have greater development. This article will give a comprehensive introduction to the division of FTTx, the main technologies implemented, and the development of FTTx around the world.
1. FTTx division
FTTx technology is mainly used for the fiberization of access networks, ranging from central office equipment in regional telecommunication equipment rooms to user terminal equipment. Unit; ONU) or Optical Network Terminal (Optical Network Terminal; ONT). Classified according to the distance from the fiber to the user, as shown in Figure 1, it can be divided into Fiber To The Cabinet (FTTCab), Fiber To The Curb (FTTC), Fiber To The Building (Fiber To The Building; FTTB) and fiber to the home (Fiber To The Home; FTTH) and other four service forms. American operator Verizon collectively refers to FTTB and FTTH as Fiber To The Premise (FTTP). The above services can be collectively referred to as FTTx.
1.1. FTTC
FTTC is currently the most important form of service. It mainly serves users in residential areas. ONU devices are placed in roadside boxes, and coaxial cables from ONUs are used to transmit CATV signals or twisted pairs to transmit telephone and Internet services.
1.2. FTTB
There are two types of FTTB according to the service objects. One is the user service of the apartment building, and the other is the company line number service of the commercial building. Both of them set the ONU at the distribution box in the basement of the building. It is an extension of FTTC, and the commercial building is for medium and large enterprises, which must increase the transmission rate to provide broadband services such as high-speed data, e-commerce, and video conferencing.
1.3. FTTH
As for FTTH, the ITU considers FTTH to be within 100 meters from the optical-to-electrical converter (or media converter MC) at the end of the fiber to the user's desktop. FTTH extends the distance of optical fiber to the end user's home, so that various broadband services can be provided in the home, such as VOD, shopping at home, taking classes at home, etc., providing more business opportunities. If it is matched with WLAN technology, broadband and mobile will be combined, and the prospect of broadband digital home in the future can be achieved.
2. Classification of FTTx technology
There are two main ways to connect optical fibers to ONUs. One is point-to-point topology (Point to Point; P2P), which uses one fiber from the central office to each user; the other is point-to-multipoint topology (Point to Point; P2P). to Multi-Point; P2MP) passive optical network (Passive Optical Network; PON), its topology is shown in Figure 2. For an unprotected FTTx system with N end users at a distance of M km, if a point-to-point solution is adopted, 2N optical transceivers and NM km of optical fibers are required. However, if a point-to-multipoint solution is adopted, N+1 optical transceivers, one or more (depending on the size of N) optical splitters, and about M km of optical fibers are required. At this point, point-to-multipoint The multi-point solution greatly reduces the number of optical transceivers and fiber consumption, and reduces the rack space required by the central office, which has obvious cost advantages.
2.1. Point-to-point FTTx solution
Point-to-point direct optical fiber connection has the advantages of easy management, no complicated uplink synchronization technology and automatic terminal identification. In addition, all uplink bandwidth can be used by a terminal, which is very beneficial to bandwidth expansion. But these advantages cannot offset its disadvantages in terms of device and fiber costs.
Ethernet + Media Converter is a transitional point-to-point FTTH solution, which uses a media converter (Media Converter; MC) to convert electrical signals into optical signals for long-distance transmission. Among them, MC is a pure photoelectric/electro-optical converter, which does not process signal packets, so the cost is low. The advantage of this solution is that only MC needs to be added to the existing electrical Ethernet equipment. The topological structure of MC mode is shown in Fig. 3 . For the 100 Mbps Ethernet network that has been popularized at present, the rate of 100 Mbps can also meet the needs of the access network. It is not necessary to replace the network card that supports optical fiber transmission, and only need to add MC, so that users can reduce the cost of upgrading. It is a point-to-point FTTH Solutions for networks during program transitions. Because its technical architecture is quite simple, cheap and directly combined with Ethernet, it once became the mainstream of FTTH in Japan. However, in the 2004 OFC meeting, NTT announced that from now on, the Japanese FTTH bid will adopt point-to-multipoint (Point to Multi-Point) , P2MP) architecture of the PON network model is bound to affect the future of MC.
2.2. Point-to-multipoint FTTx solution
In an optical access network, if the optical distribution network (ODN) is composed entirely of passive devices and does not include any active nodes, then this optical access network is a PON. The architecture of PON is mainly to split the optical signal from the optical fiber line terminal equipment OLT through an optical fiber through a passive device Splitter (optical splitter), and broadcast the optical signal to each user terminal equipment ONU/T, so that It greatly reduces the cost of network equipment room and equipment maintenance, and also saves a lot of construction costs such as optical cable resources. Therefore, PON has become the latest hot technology of FTTH. PON technology began in the early 1980s. The PON products currently on the market are mainly divided into APON/BPON (ATM PON/Broadband PON), EPON (Ethernet PON) and GPON (Gigabit PON) according to the technology they use. , Among them, GPON is the latest standardized and commercialized technology. Different PON technologies have different advantages and disadvantages, as shown in Table 1.
2.3. PON access network technology
As an access network technology, PON is positioned in the so-called "last mile", that is, a solution between service providers, telecom central offices, and business users or home users.
With more and more broadband applications, especially the rise of video and end-to-end applications, people's demand for bandwidth is becoming stronger and stronger. In North America, the bandwidth demand of each user will reach 20-50Mb/s within 5 years, and will reach 70Mb/s within 10 years. Under such a high bandwidth demand, the traditional technology will not be able to do it, but the PON technology can show its talents.
In 1987, researchers from British Telecom Corporation first proposed the concept of PON. Several are introduced below.
APON was proposed in 1995. At that time, ATM was expected to occupy a major position in local area network (LAN), metropolitan area network (MAN) and backbone network. Major telecommunication equipment manufacturers have also developed APON products, and APON products are currently in practical application in North America, Japan and Europe. However, after years of development, APON has not occupied the market very well. The main reason is that the ATM protocol is complicated, the promotion of APON is hindered, and the equipment price is relatively high, which is relatively expensive compared with the access network market. Since APON can only provide ATM services for users, at the end of 2001, FSAN updated the web page and renamed APON as BPON, that is, "Broadband PON". The BPON standard.
In the field of LAN, Ethernet technology develops rapidly. Ethernet has developed into a widely accepted standard, and now there are more than 4 million Ethernet ports in the world, and 95% of LANs use Ethernet technology. Ethernet technology develops rapidly, and the transmission rate increases from 10 Mbit/s, 100 Mbit/s to 1000 Mbit/s, 10 Gbit/s or even 40 Gbit/s, and the application environment is also developing from LAN to MAN and core network.
EPON was proposed by the EFM (Ethernet in the First Mile) research group established by the IEEE 802.3 working group in November 2000. EPON is a combination of several best technologies and network structures. EPON takes Ethernet as the carrier, adopts point-to-multipoint structure, and passive optical fiber transmission mode. The downlink rate can reach 10 Gbit/s at present, and the uplink sends data streams in burst Ethernet packets. In addition, EPON also provides certain operation, maintenance and management (OAM) functions.
EPON technology has good compatibility with existing equipment. And EPON can also easily realize the smooth upgrade of bandwidth to 10 Gbit/s. The newly developed quality of service (QoS) technology makes it possible for Ethernet to support voice, data and image services. These technologies include full-duplex support, priority (p802.1p) and virtual local area network (VLAN). However, the standard for Ethernet to support multiple services has not yet been formed, and it cannot support non-data services, especially TDM services, well. In addition, its transmission efficiency is lower compared with GPON.
In 2001, the FSAN group started another standard work aimed at standardizing PON networks with a working rate higher than 1Gbit/s. This work is called Gigabit PON (GPON). In addition to supporting higher speeds, GPON also supports multiple services with high efficiency, and provides rich OAM&P functions and good scalability. Representatives of operators in most advanced countries proposed a set of "Gigabit Service Requirements" (GSR) documents as one of the standards submitted to ITU-T; in turn, they became the basis for proposing and developing GPON solutions. This shows that GPON is a solution designed according to the precise needs of consumers and driven by operators, and it is worthy of the trust of product users.
3. Optical fiber loop classification
FTTx is divided into three types in the design of the transmission layer, namely Duplex dual-fiber bidirectional loop, Simplex single-fiber bidirectional loop and Triplex single-fiber three-directional loop. Among them, the dual-fiber circuit uses two fiber optic connections between the OLT end and the ONU end, one is the downstream (Downstream), the signal is from the OLT end to the ONU end; the other is the upstream (Upstream), the signal is from the ONU end to the OLT end. The Simplex single-fiber circuit is also called Bidirectional, or BIDI for short. This solution uses only one optical fiber to connect the OLT end and the ONU end, and uses WDM to transmit uplink and downlink signals with optical signals of different wavelengths. Compared with the Duplex dual-fiber loop, the single-fiber loop transmitted by WDM can reduce the fiber usage by half, and can reduce the cost of the ONU user end. However, when using the single-fiber mode, the light splitting and combining unit must be introduced on the optical transceiver module. The architecture is a bit more complicated than the optical transceiver module using dual-fiber mode. The BIDI uplink signal uses laser transmission in the 1260-1360 nm band, and the downlink uses the 1480-1580 nm band. In a dual-fiber loop, both uplink and downlink use the 1310 nm band to transmit signals.
4. Summary
At the 2004 China Optoelectronics Industry Forum, academician Zhao Zisen and many other experts believed that the future broadcasting market will be the main force to promote the development of FTTH in China, so it is more convenient to use three-wavelength PON, one of which (1550nm) transmits broadcasting For TV, two wavelengths (1310/1490nm) transmit uplink and downlink data, which requires the so-called Triplex architecture. And Triplexer has become a key component required by the FTTH system. Fiberhome Technology Group quickly launched single-fiber three-way photoelectric products according to market needs, mainly used in FTTC (fiber to the community), FTTB (fiber to the building), FTTH ( Fiber to the home), FTTD (Fiber to the table).
