Optical network technologies converge on broadband China


By Hans-Jürgen Schmidtke and Wolfgang Becker, Siemens AG

Being the only fully licensed nationwide operator for voice, data and mobile in China, China Unicom is the only provider of an array of fixed and mobile telecommunications services. Seven years after the Chinese government opened the market to competition, the company is one of six providers in China, with annual revenues approaching USD3bn. It boasts the largest voice over IP (VoIP) network in the world, connecting well over 300 cities throughout the country to its network hubs in Guangzhou and providing international IP gateways in Beijing, Guangzhou and Shanghai.

Considering that Chinese businesses and individuals generate at least 65 billion long-distance minutes of voice and data communications annually, the commercial potential for a forward-thinking service provider is unprecedented.

To grasp this opportunity, an advanced fibre backbone is a pre-requisite. China Unicom's core fibre-optic network serves as the common platform for voice, cellular, paging, long-distance and data/Internet networks. The convergence of voice, data, video and IP services is an important industry trend. Siemens has developed a network strategy that converges the different networks using a common infrastructure to rationalise network resources and deliver integrated, cost-effective communications services to their customers.

Siemens' telecommunications equipment has been adopted for China Unicom's cellular network — the second-largest in China — with over 60 million wireless subscribers that contribute over 70% of the company's revenues. This involves DWDM and 10Gbit/s SDH solutions spanning up to 40,000km.

In a USD50m-plus contract, Siemens was asked to deliver a next-generation future-proof backbone optical network with management and maintenance points on the network from which smaller loops and extensions can be generated. The largest optical network contract from China Unicom ever, with a total length of over 15,000km, and over 200 stations, the network covers 19 of China's 31 provinces. The DWDM network of China Unicom consists of long point-to-point links such as Beijing–Shanghai–Guangzhou, Shanghai–Zhengzhou, and Beijing–Zhengzhou–Guangzhou.

There are also other cities with 3R-regeneration sites, within these long links, where local traffic is added or terminated. All these nodes can easily be transferred to optical add-drop modules (OADM) or optical cross connects (OXC), which avoid 3R regeneration.

All DWDM point-to-point links are used by an SDH network, which forms huge rings with up to 6000km circumference. This SDH network is connected to the provincial networks of China Unicom, which also forms rings via DWDM regional networks.

At the heart of the DWDM network is Siemens' TransXpress Infinity platform, which can provide up to 20 million simultaneous ISDN-quality telephone calls over a single fibre pair. It unifies a long-haul and ultra-long-haul platform. The TransXpress Infinity's modular, "pay-as-you-grow" design means the client can begin with a few channels, but could grow to 160 channels, each carrying 10Gbit/s over a single fibre pair.

The platform provides an automatic power balancing method based on various monitoring modules such as an optical performance analyser, gain tilt monitors and power monitors. This fully automated system enables controlling, easy up- and downgrading of the system, an easy failure tracing and recognition. The platform applies a special method for dispersion compensation that uses pre- and post-compensation that enables ultra-long haul reaches.

The introduction of remotely and static configurable OADMs allows a significant reduction of overall network costs and, at the same time, provides state-of-the-art network architectures. Often, the provider does not want to fix every link at the beginning of the installation.

Several elements of the system enable long-haul and ultra-long-haul transmission. Transponders (OCR10T-card) prepare the client signal from the SDH network to be transmitted over a DWDM channel. The regenerators (OCR10R-card) electronically re-shape, re-time and re-amplify the signals. All the DWDM channels are capable to transmit 10Gbit/s on a single wavelength. The 10Gbit/s transmitters use a forward error correction (FEC) according to the ITU G709 standard. And the design of the receivers allows a permanent optimisation of the decision circuit and therefore reduction of the bit error rate (BER).

In order to meet service-level agreements, the 10Gbit/s SDH client signal is supervised by non-intrusive monitoring of B1 bytes and path trace. The client signal itself is mapped without any changes into an optical data unit (ODU) frame according to ITU G.709. Additionally, a new path and section layer is introduced on the base of ODU2 according to ITU G.709. The path layer supervises the connection between two transponders; the section layer supervises the connection between transponder and regenerator or between two regenerators. Here, the transponders provide all performance records, which are necessary to guarantee service-level agreements.

SDH services of 2.5Gbit/s are transmitted via multiplexing transponders (TEX), which offer the best utilisation of the wavelength by time-domain multiplexing four times STM16 to STM64. To provide transparency, most of the 2.5G SDH overhead is transmitted via 10Gbit/s SDH overhead. Also, service-level agreements on STM16 level can be achieved, since all bit-error related information is transmitted via the STM64 signal. The performance data of all incoming and outgoing signals are monitored and recorded.

The whole network is run by the Telecommunication Network Management System (TNMS).

Siemens offers a range of TNMS to manage SDH and DWDM networks efficiently. Through its wide range of interfaces, it can be integrated into third-party operations support systems. By selecting the NE on the network map, the TNMS offers element management functionality. The TNMS is structured in regional and centralised functionality. All the routes in China's regional areas are controlled by a separate TNMS, while a cross-domain manager controls the long-haul traffic.

Implementation placed tremendous planning and logistical demands on Siemens' team, which began its work under a project manager in Germany before transferring to China for the actual installation, testing and commissioning. "The whole process — from inspection to operational network — was completed in three months," said Li Zhengmao, vice president of China Unicom.

China Unicom also chose the Siemens solution for its operational and lifetime network costs. For example, the China Unicom network has one long link with 12 spans. In March 2002 Siemens announced a world record, carrying live traffic over almost 1,000km across a DWDM link between Tianjin and Shenyang without needing to electronically regenerate the signal.

Some 54 tons of equipment was delivered into China (the company now employs 250 optical networks staff permanently in China) before being prepared for installation at the 200 stations. Local engineers, managed by Siemens, carried out the installation.

The staff was trained in Germany and in China to get the network operating very quickly. The experience of a company such as Siemens that operates globally allows for the day-to-day operational support necessary to ensure a reliable operating network that is managed from a Network Operations Centre in Beijing, along with a back-up facility in Guangzhou.

Now that the TransXpress Infinity range is housed in the China Unicom core network, the portfolio is under consideration as an option for metropolitan and access switching by each individual province — that is, each autonomous unit of China Unicom. Moreover, there are plans to increase the number of channels in the core links as well as in certain extended links.

Additional featurers include:

  • Scalable DWDM and optical amplifier technologies offering the highest available span performance as well as superior cost and performance modules and options.
  • System transparency to transmission format, bit-rate and modulation format.
  • SONET/SDH, IP, ATM, applications with 2.5Gbit/s, 10Gbit/s and even 40Gbit/s channel data rates as well as any arbitrary mixture. Suitable for operation with many fibre types, such as SSMF, NZDSF and DSF, with built-in dispersion compensation.
  • Sophisticated optical add-drop multiplexing (OADM) solutions with remote configurability.
  • Superior multiplexer/demultiplexer scalability, featuring smooth capacity upgrade.
  • Open architecture — a stand-alone optical transport platform offering multi-vendor capability as well as full integration into Siemens' TransXpress Transport Network solutions.
  • Equipment density — a highly compact design, significantly reducing floor space requirements; less equipment means lower support and maintenance costs.

The main challenge of the project was to implement a 15,000km broadband core optical converged network capable of supporting all China Unicom's voice, data, video and Internet services.

The solution devised involved:

  • A DWDM network of up to 10Gbit/s x 160 channels per fibre, based around the TransXpress Infinity MTS system, pre-staged and tested prior to delivery in China;
  • Training of China Unicom staff;
  • Network management based on Siemens' TMN software.

The future should see:

  • Pay-as-you-grow options taking shape in line with business demand.
  • Smooth migration enabled from SDH to DWDM.
  • Extra services such as 3G mobile launched — without fear of fibre exhaustion.

Hans-Jürgen Schmidtke
Product Line Manager
Siemens ICN

Wolfgang Becker
Product Line Manager
Siemens ICN

Hans-Jürgen Schmidtke and Wolfgang Becker are both product line managers for Siemens ICN in Munich, Germany. They are responsible for the definition of DWDM products.

Siemens' TransXpress Infinity MTS is an ultra-high-capacity dense wavelength division multiplexing (DWDM) transmission system. This flexible system can be upgraded channel by channel to 160 channels, without affecting existing traffic. Its range of options includes up to 160 channels at 10Gbit/s — a total capacity of 1.6Tbit/s data throughput on a single fibre; a further upgrade option (80 x 40Gbit/s) can reach 3.2Tbit/s.

The system's re-configurable OADM provides up to 100% add/drop capacity. Moreover it can stretch up to 4000km without needing opto-electrical signal regeneration, amounting to the highest bandwidth and reach commercially available.

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