FTTx technology

FTTx technology

Foreword

Fiber-to-the-home (FTTH) is the dream and technology direction people have been pursuing for 20 years. However, due to obstacles in cost, technology and demand, it has not yet been promoted and developed on a large scale. However, this slow progress has greatly improved recently. Due to policy support and the development of technology itself, after many years of silence, FTTH has once again become a hot spot and entered a period of rapid development. Various related broadband applications such as VoIP, Online-game, E-learning, MOD (Multimedia on Demand), and smart home have brought about the comfort and convenience of life, and the interactive high-definition viewing of HDTV The revolution has made optical fibers with excellent characteristics such as high bandwidth, large capacity, and low loss as an inevitable choice for media to transmit data to clients. Because of this, many people of insight regard FTTx (especially fiber to home and fiber to station) as an important turning point in the recovery of the optical communications market. And it is expected that in the next few years, the 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 in various parts of the world.

1. FTTx division

FTTx technology is mainly used for access network fiberization, ranging from the central office equipment of the regional telecommunications room to the user terminal equipment, the central office equipment is the optical line terminal (Optical Line Terminal; OLT), and the user terminal equipment is the optical network unit (Optical Network Unit) Unit; ONU) or optical network terminal (Optical Network Terminal; ONT). According to the distance from the fiber to the user, as shown in Figure 1, it can be divided into fiber to the cabinet (Fiber To The Cabinet; FTTCab), fiber to the roadside (Fiber To The Curb; FTTC), fiber to the building (Fiber To The Building; FTTB) and Fiber to The Home (FTTH) and other 4 service forms. US operator Verizon refers to FTTB and FTTH as fiber to the premises (Fiber To The Premise; FTTP). These 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. The ONU equipment is placed in the roadside chassis, and the coaxial cable from the ONU is used to transmit CATV signals or twisted pair to transmit telephone and Internet services.

1.2.FTTB

There are two types of FTTB according to the service target, 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 in the basement wiring box of the building, just the ONU of the apartment building It is an extension of FTTC, and the commercial building is for medium and large enterprise units, and it must increase the transmission rate to provide high-speed data, e-commerce, video conferencing and other broadband services.

1.3.FTTH

As for FTTH, ITU believes that it is FTTH from the optical fiber converter (or media converter MC) at the fiber end to the user's desktop no more than 100 meters. 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, class at home, etc., providing more business opportunities. If combined with WLAN technology, broadband and mobile will be combined, you can achieve the future vision of broadband digital homes.

2. FTTx technology classification

There are two main ways to connect the ONU with optical fiber. One is the point-to-point topology (Point to Point; P2P), and one fiber is used from the central office to each user; the other is the point-to-multipoint topology (Point 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 fiber are required. However, if a point-to-multipoint solution is adopted, N eleven optical transceivers, one or more (depending on the size of N) optical splitters, and approximately M km of optical fiber are required. At this point, point-to-point The multi-point scheme greatly reduces the number of optical transceivers and the amount of optical fiber, 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 fiber connection has the advantages of easy management, no complicated uplink synchronization technology and automatic terminal identification. In addition, the entire upstream bandwidth can be used by a terminal, which is very conducive to the expansion of bandwidth. But these advantages do not offset its disadvantages in terms of device and fiber costs.

Ethernet + Media Converter is a transitional point-to-point FTTH scheme. This scheme uses Media Converter (MC) to convert electrical signals into optical signals for long-distance transmission. MC is a simple photoelectric / electro-optical converter, it does not process the signal package, so the cost is low. The advantage of this solution is that you only need to add MC to the existing electrical Ethernet equipment. The topological structure of MC way is shown as in Fig. 3. For the currently popular 100 Mbps Ethernet network, the 100 Mbps rate can also meet the needs of the access network. There is no need to replace the network card that supports fiber transmission. Only the MC needs to be added, so that users can reduce the cost of upgrades. It is a point-to-point FTTH Network solutions during the transition period. Because its technical architecture is quite simple, cheap and directly integrated with Ethernet, it once became the mainstream of FTTH in Japan. However, at the 2004 OFC meeting, NTT declared that the FTTH bid for Japan will take point to multi-point from now on (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 PON (Figure 2). The architecture of the PON is mainly to distribute the optical signal from the optical fiber line terminal equipment OLT through a fiber through the passive device Splitter (optical splitter) to broadcast the optical signal to each user terminal equipment ONU / T Significantly reduce the cost of network equipment room and equipment maintenance, and save a lot of optical cable resources and other construction costs, so PON has become the latest hot technology of FTTH. PON technology began in the early 1980s, and 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 adopted. Among them, GPON is the latest standardized and productized 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 at the "last mile" that is often said, that is, a solution between service providers, telecommunications bureaus, and commercial 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. In North America, the bandwidth requirements of each user will reach 20-50Mb / s within 5 years, and 70Mb / s within 10 years. Under such high bandwidth requirements, traditional technologies will not be competent, but PON technology can show its talents.

In 1987, the researchers of British Telecom first proposed the concept of PON. Introduce the following separately.

APON was proposed in 1995, at that time, ATM was expected to occupy the main position in the local area network (LAN), metropolitan area network (MAN) and backbone network. Major telecom equipment manufacturers have also developed APON products, which are currently used in North America, Japan, and Europe. However, after years of development, APON has not captured the market well. The main reason is that the ATM protocol is complicated, the promotion of APON is hindered, and the price of equipment is relatively high, which is relatively expensive compared to the access network market. Since APON can only provide ATM services for users, FSAN updated the webpage at the end of 2001 to rename APON as BPON, or “Broadband PON”. The BPON standard.

In the field of local area networks, Ethernet technology has developed rapidly. Ethernet has developed into a widely accepted standard. There are now more than 4 million Ethernet ports worldwide, and 95% of LANs use Ethernet technology. Ethernet technology has developed rapidly, with transmission rates ranging from 10 Mbit / s, 100 Mbit / s to 1000 Mbit / s, 10 Gbit / s, and even 40 Gbit / s, an order of magnitude improvement; the application environment has also evolved from LAN to MAN and core networks.

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 uses Ethernet as a carrier, adopts point-to-multipoint structure, passive optical fiber transmission mode, the downlink rate can currently reach 10 Gbit / s, 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 achieve a 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 does not support well for non-data services, especially TDM services. In addition, its transmission efficiency is lower than that of GPON.

In 2001, the FSAN group launched another standard work aimed at regulating PON networks with operating rates higher than 1 Gbit / s. This work is called Gigabit PON (GPON). In addition to supporting higher rates, GPON also supports multiple services with high efficiency, providing rich OAM & P functions and good scalability. Representatives of operators in most advanced countries proposed a complete 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 that is designed according to the exact needs of consumers and driven by operators, and is worthy of the trust of product users.

3. Classification of fiber optic circuits

FTTx is divided into three categories in the design of the transmission layer, namely Duplex dual-fiber bidirectional loop, Simplex single-fiber bidirectional loop and Triplex single-fiber tridirectional loop. The dual-fiber loop uses two optical fibers between the OLT and the ONU. One channel is Downstream, and the signal is from the OLT to the ONU. The other channel is Upstream, and the signal is from the ONU to the OLT. Simplex single-fiber loop is also called Bidirectional, or BIDI for short. This solution uses only one fiber to connect the OLT and ONU ends, and uses WDM to transmit upstream and downstream signals with optical signals of different wavelengths. Compared with the Duplex dual-fiber circuit, this single-fiber transmission using the WDM method can reduce the fiber usage by half, which can reduce the cost of the ONU user end. However, when using the single-fiber method, a splitting and combining unit must be introduced in the optical transceiver module. The architecture is a bit more complicated than the dual-fiber optical transceiver module. The upstream signal of BIDI adopts the laser transmission of 1260 to 1360 nm band, and the downstream uses the band of 1480 to 1580 nm. In the dual-fiber loop, both the upstream and downstream sides use the 1310 nm band to transmit signals.

to sum up

At the 2004 China Optoelectronics Industry Forum, many experts such as Academician Zhao Zisen believed that the future broadcast market will be the main force driving the development of FTTH in China, so it is more convenient to use a three-wavelength PON, one of which (1550nm) transmits broadcast TV, 2 wavelengths (1310 / 1490nm) transmit uplink and downlink data, which requires the so-called Triplex architecture. The Triplexer has also become a key component required by the FTTH system. The 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 home).