Evaluation of Different Cable Termination Solutions for FTTH Deployment in China

Mr. Hongwei Yang, Mr. Yu Wu Jiangsu Posts & Telecommunications Planning and Designing Institute Co., Ltd (JSPTPD).

Nanjing, Jiangsu Province, P.R. China +86-25-5286-8681 · yhw@jsptpd.com; +86-25-5286-8797 · wuyu@jsptpd.com

Abstract The massive deployment of FTTH in China has made the termination of optical cable at the subscriber end a tough task with high failure rates in the performance of the ODN. In this paper we study the cable termination solutions which have been used in FTTH deployment in China, and introduce the development of cable termination technologies for the subscriber end in China. Finally we propose solutions for cable termination at the subscriber end for the massive FTTH deployment in China.

Keywords: FTTH; massive deployment; cable termination at the subscriber end; fusion splicing; mechanic splicing; pre-termination.

1. Introduction In 12th 5-year-plan, China plans to construct a next generation information infrastructure, deploy next generation wireless networks, next generation Internet, digital broadcasting and television systems & satellite communication. Governments will advocate city-wide broadband wireless coverage, urban FTTH and speed up broadband deployment in rural communities to improve broadband access penetration and access bandwidth. China Telecom officially initiated the “Broadband China OptiNet City” project on 16th February 2012 to open the prelude of massive FTTH deployment. This project’s objective is to realize fiber coverage for all families, municipal & commercial subscribers in the urban of South China by the end of 12th 5-year-plan.i.e. 100 million households shall be covered by FTTH; This will put China Telecom in the list of top carriers for FTTH deployment. Besides China Telecom, other carriers in China are also starting to deploy FTTH. As the key section of FTTH, ODN (Optical Distribution Network) has become the focus of carriers with regard to ODN cost and quality.

With elements in ODN getting more and more mature, cable termination at the subscriber end has replaced splitters as the 1st component in the total cost of ODN due to termination’s huge quantity and high failure rates at the subscriber end, drawing much attention. The industry continues exploring how to achieve reliable, efficient & economic termination solutions.

2. Analysis of Different Cable Termination Solutions at The Subscriber End In the early stage of deploying ODN in FTTH, the bow-type cable or other types of cables were used as drop cable, and the

on-site termination was preferred. The termination solutions are listed below:

Solution 1: The drop cable is terminated with field assembly (FA) in 86x86 wall outlet (see Figure 1).

Figure 1. Solution 1

Solution 2: The drop cable is terminated with field installable connector (FIC) (see Figure 2).

Figure 2. Solution 2

Solution 3: The drop cable is terminated with fiber socket by mechanic splicer in 86x86 wall outlet (see Figure 3).

Figure 3. Solution 3

Solution 4: The drop cable is terminated with pigtail by mechanic splicer in 86x86 wall outlet (see Figure 4).

Figure 4. Solution 4

Solution 5: The drop cable is fusion spliced with fiber socket in 86x86 wall outlet (see Figure 5).

FA Drop Cable

Drop Cable

FIC

Drop Cable Fiber Socket

Mechanic

Pigtail

Drop Cable

Mechanic Splicer

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Figure 5. Solution 5

Solution 6: The drop cable is fusion spliced with pigtail in 86x86 wall outlet (see Figure 6).

Figure 6. Solution 6

With the higher price of FIC in the early stage, mechanic splicer or pigtail were used to save cost. But with FIC price reduction, solution 1&2 have gradually substituted solution 3&4 and dominated the field termination (connection).

Cable termination at the subscriber end has become the point with the highest failure rates during the operation of current network. The table below (see Table 1) shows the statistics of FTTH malfunction reported by China Telecom in a developed province in December 2010.

Table 1. Statistics of FTTH malfunction

Item Cause Frequency Share

1 Power down at central office 329 7.6%

2 Feeder or distribution cable failure 179 4.1%

3 Splitter failure 93 2.2%

4 Drop cable failure 425 9.8%

5 Pigtail or connector failure 193 4.5%

6 Project cutover 153 3.5%

7 Termination failure at subscriber end 2180 50.4%

8 ONU failure 473 10.9%

9 Power down at user terminal 54 1.2%

10 Others 251 5.8%

11 Total 4330 100%

The cable termination problem at the subscriber end takes more than half of total malfunctions. In another developed province, the cable termination problem takes high ratio in total malfunctions in the major cities from December 2011 to January 2012 such as 43.8% in Foshan 41.4% in Guangzhou, 35.7% in Zhongshan, 31.7% in Yangjiang, 30.4% in Shanwei,. Average is 29.5%.

The major field mechanic termination technologies used are listed below:

2.1 Pre-stubbed Field Installable Connector (FIC) With this technology, a small piece of fiber is pre-stubbed in the ferrule, the fiber in drop cable is connected with pre-stubbed fiber through index matching material. Index matching material is used to fill in between the two fiber ends to minimize Fresnel reflection. The index matching material can be liquid or gel with similar index as the fiber core. For high quality index matching material, its optical absorption rate is less than 0.0005%, which results in 0.001dB loss. In a pre-stubbed FIC, a fiber is embedded in the front section of ferrule, and its end face is precisely polished and tested. In the field, the fiber is exposed from the drop cable, stripped and cleaved; Then it is connected

with embedded fiber through an internal alignment mechanism (see Figure 7).

Figure 7. Configuration of pre-stubbed FIC

With its easy handling advantage, pre-stubbed FIC was highly accepted by the customers in FTTH deployment after its development and introduction. The performance of pre-stubbed FIC depends on the quality of index matching material. In the early stage of its adoption, the quality and reliability of index matching materials from some suppliers were not good enough due to a poor manufacturing process and/or the index matching liquid/gel either volatilized or deteriorated over time, resulting in increased loss. In some cases, the index matching materials deteriorated due to contamination during installation at site. It remains a concern for carriers to deploy FTTH in China.

2.2 Direct Cleaved Field Installable Connector (FIC) It’s not like pre-stubbed FIC, there is no fiber embedded in ferrule when direct cleaved FIC is manufactured in the factory (see Figure 8). Instead the fiber will be inserted into ferrule in the field after it’s cleaved. For most of direct cleaved FIC, the end faces are not polished and tested in the field, they are directly connected with other connectors.

Figure 8. Configuration of direct cleaved FIC

It is a low cost solution and easy to handle, and it is also free from problems induced by the poor quality index matching materials such as dry-out, deterioration, and contamination. It got some application in China when commercially available. But our study found it did not fully comply with the industry standard of

Ferrule (Polished in factory)

Embedded fiber V groove

Drop cable Index matching gel

Drop cable

Direct pass cavity

Fusion Splicing

Drop Cable

Fiber Socket

Drop Cable

Pigtail

Fusion Splicing

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China, for instance, its return loss (RL) is 8~40 dB ( 40dB is required in Chinese industry standard). Additionally the fiber is slightly movable in ferrule. When we secure the cable, it will change the fiber position in both radial & longitudinal directions. At the same time, unstable optical power coupling were observed during push-pull. Especially the optical parameters deteriorated over time due to the deposition of moisture or dust on the end face.

2.3 Mechanic Splicer Due to the high cost of pre-stubbed FIC and poor performance of direct cleaved FIC, some carriers tried mechanic splicer to connect drop cables to pigtails. The index matching material is also used in a mechanic splicer to minimize Fresnel reflection. The mechanic splicer is composed of V groove with index matching material and fixing parts. Although it’s a low cost solution with better flexibility, it is still not trust-worthy with its Index Matching Material Dependent feature as well as the insufficient protection to fiber, that is commonly encountered issue in this application.

2.4 Fusion Splicing Fusion splicing is based on the mechanism that two fibers will partially melt and join with each other under arc induced by high voltage in a fusion splicer. No extra material is needed. It’s a solution with low loss and high reliability. When a fusion splicing is made, a point with low strength & flexibility at splicing point is created, requiring additional protection. Typically a heat shrink tube is used. Although fusion splicing can achieve low loss and high reliability, the Capex is high, operation is complex and time-consuming. Besides, the legacy fusion splicer (see Figure 9) is big, it takes much time to get the splicer ready and usually local power supply is needed. It is quite inconvenient for FTTH deployment with its highly distributed construction sites. In legacy fusion splicing, the splice protection is required to protect the fiber joint, but in the early stage of FTTH deployment in China, because the splice protection was not designed well, reliability was not high and finally it didn’t become the mainstream solution.

Figure 9. Legacy splicer & tool kit

3. Evolution of Cable Termination Technology at The Subscriber End Since there are more or less weak points for the solutions mentioned above, the more solutions with rational price and reliable performance are under exploration for massive FTTH deployments.

3.1 Pre-Terminated Solution In China construction of ODN is on big scale. In most cases, ODN is simultaneously deployed together with construction of new communities. At the subscriber end, the environment is quite similar, only several lengths are required for drop cable. It’s not a must to do termination in the field, instead the termination can be done in the factory; one or both ends of the drop cable are terminated with connectors required (such as SC/FC/LC), and the cable length, fiber count and cable type can be tailored based on the application. Usually bow-type cable with single fiber is used in China. The pre-terminated cable is ready for plug & play, it simplifies installation and maintenance, and it also cancels the fiber splicing or mechanic splice in the field, so it improves yield and reliability a lot. It can be quickly installed with less space required, plus both training time for the installer and installation cost are reduced. In real practice, the pre-terminated drop cable can be produced in the factory with specified lengths. The large scale and well-controlled production in the factory achieves high efficiency, low cost and high reliability. So when issues in manufacturing were solved, this solution was quickly and widely adopted. There is some limitation about pre-termination solution. With FTTH extending to rural areas, because homes are scattered, the drop cable length required is different case by case, and it is difficult to realize standard production in the factory. Even in the urban environment, to make installation easy, usually only end of drop cable is pre-terminated, and the other end still needs to be terminated in the field.

3.2 Quick Fusion Splicing Solution Pre-termination solution improves the reliability of ODN, but termination in the field is still needed. Fusion splicing technology is trusted for its reliability. The disadvantage is weight of legacy fusion splicer and its complex operation. To change the situation, some manufacturers developed new compact & quick splicers. New splicers integrate features like fiber thermal stripping, cleaning, cleaving, splicing & tubing. Compared with legacy splicers, the new splicers are smaller and easy to handle. They can be put on the ground, or fixed around a wrist or on a pole. In general, new splicers can be used in all installation sites. They use battery as power supply and continuous working time is more than 24 hours with a weight less than 2kg. All processes are included in one piece of equipment, and this shortens operation time (see Figure 10).

Figure 10. Quick fusion splicer & its components

Connector

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With different clamps, the new splicers can be used to splice bow-type cable, 0.9/2.0/3.0mm pigtail. Even in North China where big temperature variances occurs across the year, this method can still guarantee the reliability for cable termination at the subscriber end, so it is used more frequently. But the price is the factor to block its massive application. Take one medium size city in China as example, there are around 500 technicians in charge of installation and maintenance, if one technician is equipped with one splicer, over hundreds of thousands splicers are needed. This is a huge investment for carriers and not acceptable.

3.3 Improved Mechanic Splicing Technology Mechanic splicing doesn’t melt fiber ends vs. fusion splicing. It is easy to handle and almost no extra operation space is needed, making this a good solution for FTTH deployment and maintenance and is warmly welcomed by technicians in the field. Mechanic splicing solution providers have made some modifications to improve performance. 3.3.1 Improved Pre-stubbed FIC. Currently the pre-stubbed FIC has been improved, and the performance of index matching liquid/gel has been improved significantly. For the index matching gel from some suppliers, light transmittance is kept over 97% @ 136*24 hours 0~80 C temperature cycling test. Both oil separation rate & volatile rate are controlled under 0.2%. And the improvement is continued (One study from Corning® reported that the thermal stability, volatility & contamination resistance of their index matching gel had been improved dramatically and the forecasted lifetime is 203 years @ 40 C.). At the same time, the more reliable V groove is used, and it is tailored based on the application environment in China. All these modifications reduce the failure in the field. 3.3.2 Improved Mechanic Splicer. The improved mechanic splicer has been provided by the suppliers, it’s called pig-tail mechanic splicer. The pig-tail is terminated with a mechanic splicer in the factory. In the field, only the operation on bow-type cable is needed such as removing jacket, prepare fiber end etc. The cavity accommodating index matching material is located in the middle section of V-groove where the fibers are spliced and it is sealed very well, this structure prevents index matching material from degradation (see Figure 11 & 12).

Figure 11. Improved pre-terminated mechanic splicer

Figure 12. Configuration of pig-tail mechanic splicer

4. Conclusions In massive FTTH deployment, cable termination at the subscriber end is the concern of design, construction, operation & maintenance personnel. Failures have decreased dramatically with effort and the reliability of ODN is improved. But the application environment is complicated, especially for China. There is not only one right termination technology, instead it shall be a combination of multiple solutions. Pre-termination technology, improved mechanic splicers and quick field fusion splicing will be the major solutions for cable termination at the subscriber end. When termination must be done in the field, for business or quality-oriented customers, splicing will be the right choice. For household, the improved mechanic splicer will be the right solution. For example, in Jiangsu, one of the most developed provinces in China, pre-termination, improved mechanic splicer, and fusion splicing take 40.24%, 54.09%, 5.67% shares, respectively. The latest data indicate that the share of per-termination and splicing is increasing. Currently fusion splicing still has some gap vs. mechanic splicing on convenience & cost. At the same time, mechanic splicing needs to improve reliability vs. fusion splicing. We forecast the gap will be reduced and this is happening.

5. Acknowledgments Special thanks to China Telecom & China Telecom Jiangsu Branch for providing opportunities to do field investigation.

Special thanks to Mr. Shijie Cai for his suggestions and help.

6. References [1] Xuedong Qi, Min Shen, Qiaoping Zhou, Yi Zhou, “Drop

Cable Termination Solutions for FTTH”, Paper Collection of Optical & Copper Cable Symposium 2011, China Institute of Communications, P 9-10 (2011).

[2] Yunfa Liao, Bin Yu, Bingxiao Hu, Genliang Lv, “Analysis of Fiber Mechanic Termination Technology”, Telecommunication Technology, p.52 (July, 2010).

Sealed index matching gel

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7. Pictures of Authors

.

Mr. Hongwei Yang, 34 years old, communication engineer & registered consultant, deputy director of Network Communication Planning and Designing Department of JSPTPD.

Mail address: No. 371, Zhaongshan South Road, Nanjing, Jiangsu Province, P.R.China.

Mr. Yu Wu, 29 years old, assistant communication engineer. Mail address: No. 371, Zhongshan South Road, Nanjing, Jiangsu Province, P.R.China.

843 International Wire & Cable Symposium Proceedings of the 61st IWCS Conference