Application and challenge of 40Gbit / s high-speed optical transmission technology
1 40Gbit / s high-speed optical transmission technology development and application status
1.1 Business background of 40Gbit / s transmission
The emergence and development of 40Gbit / s transmission technology is inseparable from the vigorous development of telecommunication network services and technologies represented by the Internet, especially the explosive growth of Internet traffic in recent years has directly promoted the emergence of 40Gbit / s transmission demand and Internet application.
From the perspective of the operator, there are two main driving forces for the construction of a 40Gbit / s high-speed optical transmission system: one is the high-speed interconnection demand of the core IP network core router; the other is that some large financial institutions, scientific research institutions and government departments use The 40Gbit / s high-speed circuit leased by its supercomputer or data center interconnection needs. For domestic operators, the current demand belongs to the first type, and the second type of demand is concentrated in developed countries and regions such as North America and Europe.
The scale and capacity of China Telecom ’s ChinaNet has reached the top position in the global backbone IP network, and the backbone IP network of China Unicom (formerly China Netcom) also ranks among the world's largest IP networks. Therefore, these two operators are domestic The earliest demand for 40Gbit / s high-speed optical transmission technology, and the earliest construction of a commercial 40Gbit / s WDM transmission system. At present, 40Gbit / s network construction is concentrated on the backbone network, and it will be extended to the metropolitan area network in the future.
1.2 40Gbit / s transmission technology route
Although the early industry had proposed to use four 10Gbit / s channels to transmit 40Gbit / s inverse multiplexing (IMUX, Inverse MulTIplexing) solutions, Huawei, Ericsson (formerly Marconi) and other manufacturers also launched mature commercial equipment . However, with the development of demand and technology, in the end, people still chose the single-channel speed-up route. Just as the 2.5Gbit / s WDM system was upgraded to 10Gbit / s WDM system, 40Gbit / s WDM transmission technology became 40Gbit / s transmission. Mainstream solutions.
From 10Gbit / s to 40Gbit / s, the signal rate has increased by 4 times, but the increase in technical difficulty is far more than 4 times. The stringent transmission performance requirements of 40Gbit / s signals make the use of 10Gbit / s transmission technology to complete the long-distance transmission of 40Gbit / s signals an impossible task. We assume that the traditional NRZ pattern is used: the ONSR (optical signal-to-noise ratio) of 40Gbit / s signal is 6dB higher than that of 10Gbit / s signal, but due to the influence of nonlinear effects, the input fiber power is 1 ~ 2dB lower, so The OSNR restricted distance of a 40Gbit / s signal is only about 1/6 of a 10Gbit / s signal; more seriously, the restricted distance of chromatic dispersion and polarization mode dispersion (PMD) of a 40Gbit / s signal is only 1/10 of a 10Gbit / s signal 16. Therefore, 40Gbit / s WDM transmission requires a series of new technologies to achieve the transmission distance of the electrical relay that is roughly equivalent to 10Gbit / s WDM transmission. Among them, the advanced modulation pattern is the most prominent representative of the 40Gbit / s WDM transmission enabling technology, which will be highlighted below.
The modulation pattern is the most exciting part of the 40Gbit / s WDM transmission technology, and it is also the richest part. At present, there are nearly 10 kinds of commercial code patterns. According to its technical characteristics, it can be simply divided into 3 categories:
(1) Phase-assisted intensity modulation pattern: its characteristic is that the signal is transmitted through intensity modulation, using ordinary direct detection technology, but introducing specific phase adjustment methods to improve transmission performance; representative patterns include CSRZ (carrier suppression Zero code), DRZ (differential return to zero code) and ODB / PSBT (optical dual binary code / phase integer binary transmission code).
(2) Intensity-assisted phase modulation pattern: its characteristic is that the signal is transmitted through the phase modulation method, using differential or coherent receiving technologies, which has good transmission performance, and at the same time introducing NRZ, RZ and other intensity modulation methods to achieve improved transmission performance 〠Use 50GHz interval and other purposes; representative code types include RZ-DPSK (return to zero-differential phase shift keying code), NRZ-DPSK (non-return to zero-differential phase shift keying code) and RZ-DQPSK (return to zero- Differential four-phase phase shift keying code), P-DPSK (partial differential phase shift keying code), which is widely used in the current network, is also a special NRZ-DPSK code, which is characterized by controlling the amplitude of the difference to cancel The effect of filtering effect, so as to achieve 50GHz interval transmission at a small cost.
(3) Polarization multiplexing modulation pattern: its characteristic is to use two orthogonal polarization states to transmit different information, improve the system spectrum utilization rate, and reduce the single channel signal rate. The modulation mode of each polarization channel can be Any one of the above modulation patterns; currently the only polarization multiplexing modulation pattern commercially available in the 40Gbit / s WDM transmission system is Nortel ’s DP-QPSK (bipolar four-phase phase shift keying code).
The role of advanced modulation patterns in the 40Gbit / s WDM transmission system is all-round, for example: to extend the transmission distance, the current transmission distance of the 40Gbit / s WDM system without electrical relay has exceeded 1000km or even 1500km; meet the 50GHz interval transmission, Improve spectrum utilization; increase the PMD tolerance, reduce the PMD performance requirements of the optical cable, and expand the scope of the existing network. Table 1 lists the technical characteristics and application scenarios of the 40Gbit / s WDM transmission equipment used by manufacturers that are currently active in the domestic transmission equipment market.
Table 1 Comparison Table of Common Patterns for 40Gbit / s WDM System
In addition to the modulation pattern, adjustable dispersion compensation technology (used to compensate for the 40Gbit / s signal dispersion tolerance is too low), high-speed chip technology (40Gbit / s FEC, Framer, SerDes and other core chips), high-speed modulation / decoding Tuning technology is also an important enabling technology for 40Gbit / s WDM transmission system.
1.3 Development and application of 40Gbit / s transmission equipment
In terms of 40Gbit / s applications, the lag of transmission equipment has actually become a bottleneck factor that 40Gbit / s cannot be widely used in previous years. The mainstream router manufacturers such as Cisco and Juniper launched commercial 40Gbit / s POS boards as early as 2006, but the popularity of 40Gbit / s WDM transmission equipment was after 2007. Especially in 2008, mainstream transmission equipment manufacturers have released 40Gbit / s WDM transmission equipment. The more active domestic manufacturers include Huawei, Fiberhome, ZTE, Nortel, Shanghai Bell, Ericsson, etc.
At present, mainstream manufacturers' 40Gbit / s WDM transmission equipment has been serialized, including ODB / PSBT and other code patterns that support short- and medium-distance transmission, DPSK code patterns that support medium and long-distance transmission, and even more complex DQPSK, DP-QPSK, etc. Code type, the applicability of the equipment has been greatly improved. Mainstream telecom operators have also widely recognized the maturity of 40Gbit / s WDM transmission technology and equipment. According to the statistics in the latest industry report published by the well-known consulting company Ovum in late November 2008, as of 2008, more than 30 An operator has deployed a 40Gbit / s transmission network, including China Telecom and China Unicom (formerly China Netcom). In the report, Ovum believes that 40Gbit / s transmission technology has entered the "Generalized Deployment Phase" and will usher in a healthy and sustainable development period.
China Telecom is the earliest telecom operator in China that focuses on 40Gbit / s transmission. As early as 2004, China Telecom started the 40Gbit / s transmission technology research work, and cooperated with the National Science and Technology Ministry's "Eight-Three-Three" plan. In 2005, the "Shanghai-Hangzhou 40Gbit / s WDM Experimental Transmission System" was built and has been running. This is the first in China, and also belongs to the earlier 40Gbit / s live network experimental transmission system in the world. Since then, based on years of continuous tracking and research on 40Gbit / s transmission technology and equipment, China Telecom built the first commercial 40Gbit / s WDM transmission system in China in 2008, namely "Shanghai-Wuxi 80 × 40Gbit / s WDM system", and In the second half of 2008, 40Gbit / s WDM transmission equipment and system verification tests participated by multiple manufacturers were conducted, which effectively promoted the development of the domestic 40Gbit / s transmission industry. Starting from 2009, China Telecom will step up the deployment of backbone 40Gbit / s transmission networks according to its business development. The first batch of 40Gbit / s WDM transmission networks covering the Yangtze River Delta, Pearl River Delta and other well-developed businesses, 40Gbit / s Areas with urgent application needs. China Unicom (including the former China Netcom) also started construction of the first commercial 40Gbit / s WDM transmission network in 2008, covering major cities in North China.
As the industry chain matures, 40Gbit / s transmission related technical standards work is becoming more and more perfect. ITU-T, OIF and domestic CCSA have formulated and issued a series of technical standards, which effectively promotes the application of 40Gbit / s transmission equipment on the existing network.
2 Challenges faced by 40Gbit / s high-speed optical transmission technology
Although 40Gbit / s high-speed optical transmission technology has entered the stage of large-scale commercial use, in order to cope with the complex existing network application environment and the further needs of future business development, 40Gbit / s transmission technology still faces some challenges. These challenges are in the technical field, such as the limitation of the existing network fiber PMD to 40Gbit / s transmission; there are also cost aspects, such as continuously reducing the cost of the 40Gbit / s WDM transmission system to achieve a single-bit × km transmission cost less than 10Gbit / s WDM System; and challenges brought by the development of next-generation 100Gbit / s transmission technology. This chapter will analyze these challenges in detail, and form the future development direction and prospect of 40Gbit / s high-speed transmission technology.
2.1 40Gbit / s transmission technology suitable for large PMD fiber
For the successive solution of the 40Gbit / s transmission limitation factors such as OSNR and dispersion, PMD has become the main limiting factor currently affecting the transmission distance of the 40Gbit / s WDM system without electric relay. The PMD tolerance of ordinary 40Gbit / s signals is only about 2 ~ 2.5ps. Even if the PMD brought by other optical components of the system is not considered, it can only be practical in optical fibers with PMD coefficients better than 0.1ps / sqrt (km) Value, only when the PMD coefficient is better than 0.05ps / sqrt (km) can the advantages of long-distance transmission be brought into play. This is very demanding for the fiber selection of the existing 40Gbit / s WDM system construction. The biggest technical challenge facing the future 40Gbit / s WDM transmission system is how to apply it to large PMD fiber.
The industry has made many efforts to increase the PMD transmission distance of 40Gbit / s WDM system, and has proposed various solutions. These solutions can be summarized as the following three types:
(1) PMD compensation method: the idea is to follow the idea of ​​dispersion compensation, to track the change of line PMD through certain technical means and to achieve PMD compensation by introducing the opposite polarization delay; this way of thinking is simple and clear, but because of PMD The dynamic characteristics of PMD compensation technology are much more difficult to achieve than dispersion compensation technology. Currently, only a first-order PMD compensation scheme has made some progress. Some manufacturers claim to have launched commercial modules, but there are no reports of large-scale commercial deployment, and because of the principle There is no breakthrough in the compensation mechanism of high-order PMD at present; therefore, the PMD compensation method currently seems to be unsuccessful.
(2) Advanced modulation pattern improves signal PMD tolerance: The idea is to reduce the signal baud rate under the condition that the 40Gbit / s signal bit rate remains unchanged through a complex modulation pattern, thereby improving the signal's own PMD tolerance At present, the most common modulation codes with the function of improving PMD tolerance are mainly RZ-DQPSK and DP-QPSK. The former only depends on the modulation pattern, while the latter also involves the third method (electrical domain equalization) ; At present, it is the most widely used way to improve the 40Gbit / s signal PMD tolerance through the RZ-DQPSK pattern. It can increase the PMD tolerance from 2 ~ 2.5ps to 6 ~ 8ps for other patterns, and the effect is very obvious.
(3) Electric domain equalization technology based on coherent reception: The principle is to use the optical domain phase information retained by the electrical signal after coherent reception to separate the signal distortion caused by PMD, using a special electric domain equalization algorithm (implemented on hardware by high-speed ADC and DSP ) Correct the signal distortion to achieve the purpose of eliminating the impact of PMD; Nortel introduced the first commercial solution in the industry, and its DP-QPSK pattern 40Gbit / s signal average PMD tolerance can reach 25ps, even exceeding 10Gbit / s The level of the signal.
The technical complexity and application range of the above three methods are all different. The author believes that:
â— PMD compensation technology is unlikely to become a large-scale commercial solution due to its principle limitation.
â— DQPSK is a high PMD tolerance modulation pattern that needs to be focused on recently. It achieves an average PMD tolerance of 6-8ps with moderate complexity, reducing the fiber PMD coefficient requirements of the 40Gbit / s WDM system to better than 0.2ps / sqrt (km), the domestic operator's optical cable network construction time is relatively late, and most regions can find fiber optic cables that meet this requirement.
â— The electric domain equalization scheme based on coherent reception has better performance, and can be said to be the ultimate solution for PMD limitation. The author believes that this scheme is a solution for 100Gbit / s WDM transmission, but for 40Gbit / s WDM system, Need to judge according to its future development and the cost-effective relationship with the current program.
2.2 The need to continuously reduce costs
At present, the transmission cost per bit per kilometer of the 40Gbit / s WDM transmission system is still higher than that of the 10Gbit / s WDM system. There are three main reasons: First, the 40Gbit / s WDM transmission technology itself has a higher complexity, and the R & D cost is more shared. More, the cost of components is also higher; second, the device shipments of the 40Gbit / s WDM system are still far less than the 10Gbit / s WDM system, and a large scale effect cannot be formed to effectively reduce costs; third, 40Gbit / The wireless relay transmission distance of s WDM system is not as good as that of 10Gbit / s WDM system, especially in some backbone network ultra-long-distance application scenarios, more OEO regeneration is bound to increase the construction cost of 40Gbit / s WDM transmission system.
Therefore, continuing to reduce the cost of 40Gbit / s WDM system should also start from the above aspects. First of all, operators need to build a 40Gbit / s WDM system in advance according to business needs. Only a large amount of equipment purchase can form a scale effect and reduce the construction cost per bit × km. Secondly, the technology and performance of the 40Gbit / s WDM transmission system need to be further improved, especially in terms of the regeneration distance of the non-electric relay, which needs to reach or exceed the level of the 10Gbit / s WDM system; the PMD limitation problem analyzed in the previous section is also part Scenario 40Gbit / s WDM system is an important reason for the high cost. Effectively solving the PMD problem also helps reduce the cost of the 40Gbit / s WDM system.
In short, the cost challenge of the 40Gbit / s transmission system is to achieve a WDM system lower than 10Gbit / s. With the reduction of OEO regeneration costs and the scale effect of increased equipment shipments due to technological progress, it is optimistic to estimate that the transmission cost per bit per kilometer of a 40Gbit / s WDM system is close to or lower than 10Gbit / s WDM in the next two years or so system.
2.3 Challenges of 100Gbit / s transmission technology development
Although 40Gbit / s is a leap forward compared to 10Gbit / s, 40Gbit / s is far from the end of the high-speed transmission rate. In fact, the 100Gbit / s high-speed transmission technology driven by 100GE (100Gbit / s Ethernet) technical standards and interfaces has received widespread attention in the industry and has become a new hotspot in the field of high-speed optical transmission.
In the field of standards, ITU-T, IEEE and OIF actively promote the formulation of relevant technical standards in the three areas of 100G OTU3, 100GE and 100G DWDM respectively. It is expected that the main technical standards of the three organizations will be completed by the end of 2010. In the field of equipment R & D and application, leading equipment manufacturers have launched research work on 100Gbit / s WDM transmission technology, and some manufacturers have released prototypes and cooperated with some operators (focused on Europe and North America) for multiple 100Gbit / s transmissions. Presentation. Therefore, the development of 100Gbit / s transmission technology is rapid, and there is also an argument in the industry that 40Gbit / s is only a transition technology, and 100Gbit / s is the standard rate for next-generation high-speed networks. The increase in network rate can exceed 40Gbit / s s, from 10Gbit / s to 100Gbit / s directly.
One supporting evidence supporting the above point is the development route of Ethernet. There is no doubt that the main business of the future WDM transmission system is the interconnection of various rates of Ethernet interfaces. From 10M Ethernet to 100GE, IEEE has been increasing the rate of this Ethernet in units of 10 times. The integer of 10 times is the mainstream of Ethernet. 40Gbit / s only exists as a transition technology between 10Gbit / s and 100Gbit / s.
According to the application requirements of the router's 40Gbit / s interface, the technical characteristics of WDM transmission, the current development status of 100Gbit / s equipment and the analysis of cost factors, the author's point of view is: due to the rapid development of 100Gbit / s, the market window of 40Gbit / s WDM transmission It will be affected to some extent, but it cannot be surpassed. In the next 4 to 5 years, high-speed network construction will still be dominated by 40Gbit / s, and will gradually evolve to 100Gbit / s in the future. The reasons are as follows:
(1) From a technical point of view: the current 100Gbit / s transmission technology is still in the laboratory stage, and its maturity is only equivalent to the 40Gbit / s transmission technology around 2005; if there is no support for 100Gbit / s transmission, even if the 100GE interface appears, It can only be used to communicate with equipment in the equipment room, but not to the backbone network.
(2) From the perspective of the industry chain: 100Gbit / s transmission upstream and downstream industry chains have not yet formed. There are many short boards such as core chips and test instruments. Without the support of the industry chain, it is difficult to form a mature 100Gbit / s transmission market.
(3) From the perspective of the expected market size: the higher the rate of transmission technology, the more limited the application scenarios that can be expected, and the relatively abundant transmission methods of 100GE services, especially the future combination of WDM transmission technology and OTN scheduling technology, 40Gbit / s or even 10Gbit The / s line rate can effectively support 100GE service interfaces, so there are uncertainties in the overall market size of 100Gbit / s transmission.
(4) From the perspective of cost performance: 40Gbit / s transmission has achieved a certain scale of application, which will bring obvious cost advantages to it. In the next few years, the cost performance of 100Gbit / s transmission is still difficult to exceed 40Gbit / s transmission.
3 Prospect of high-speed optical transmission technology
At the time of writing this article, I heard the good news: Dr. Charles C. Kao, a British Chinese scientist known as the "father of fiber optics", was announced to award the 2009 Nobel Prize in Physics, and Gao Kun became the three winners One of the winners and get a 1/2 bonus. This is exciting news for everyone in the optical communications industry. The contribution of optical fiber communications in the process of informationization is obvious to all. This achievement is fully qualified to be written into the history of human development. The Nobel Prize is only a review and affirmation of history. The author also hopes that Dr. Gao ’s award will become a symbol, and that optical communication technology and industry will be able to develop in the future.
No matter at what stage, high-speed large-capacity WDM transmission is the most representative of optical communication technology, from 2.5Gbit / s to 10Gbit / s to the current 40Gbit / s, the single wave rate has been increased by 16 times; From the initial 8 × 2.5Gbit / s to the current 80 × 40Gbit / s, the system capacity has increased by 160 times; 100Gbit / s WDM transmission technology has also moved to the foreground, becoming the representative of the next generation of high-speed optical transmission technology.
With the development of business requirements and related technologies, we have sufficient reasons to believe that high-speed optical transmission technology still has a broad space for development: on the one hand, we must continue to increase the single-wave rate and system capacity; on the other hand, we need to further reduce costs and improve cost performance , Expand the scope of application. In short, the only value of the existence and development of high-speed optical transmission technology is to better meet people's information and communication needs.
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