China's optical-network evolution

Since 1990, the capacity of China's public switched network has increased from 12 million lines to more than 400 million lines (as of December 2001).¹ The corresponding telephone penetration has increased from 1.1% to about 28%. In 2001, the approximately 34 million new wired phone subscribers in China accounted for almost half of all new wired phone subscribers worldwide. The total number of Internet subscribers was 33.7 million at the end of 2001. Such growth creates great demand for transport capacity that can be met only by optical networking.

The history of optical fiber networks in China is not very long, but the amount of change has been tremendous. Over the past decade, China has made several strategic decisions that have fundamentally impacted the design of its optical fiber networks.

In the late 1980s and early 1990s, China started to deploy optical fiber-based transport systems. Initial projects were all pleisosynchronous digital hierarchy (PDH)-based systems (see sidebar on page 24). The continuous traffic growth of 40% to 50% annually in the national backbone network, however, put great pressure on the network. Around 1994, China made its first strategic move, transitioning from PDH to synchronous digital hierarchy (SDH) for the national backbone network. Since then, the mass deployment of 2.5 Gb/s systems in the national backbone level has resulted in the installation of tens of thousands of SDH network elements in the network. China's optical-fiber backbone has become a pure SDH-based network implemented wholly over fiber. In fact, it is the largest SDH-based transport network in the world.

China's backbone networks incorporate approximately 500,000 km of installed fiber cable. The total fiber cable installed in the country exceeds 1.5 million km. Careful traffic projections and network performance analysis, coupled with China's network evolution strategies, led to the 1995 implementation of G.652 fibers for the backbone networks of western China for the foreseeable future. At the same time, G.653 fibers were ruled out for wavelength division multiplexing (WDM) applications based on four-wave-mixing effects and have not been installed since that time; moreover, existing G.653 fibers are being retired. G.655 non-zero-dispersion fibers are judged to be the best choice for high-traffic areas, especially in eastern China. However, we continue to seek fibers that provide a lower dispersion slope, large effective area, and appropriate relative higher dispersion value. In order to reach optimum performance in the coming years, fiber performance must continue to improve.

Network capacity has been and will continue to be a major issue for network planning due to the high traffic growth rate. A network upgrade based on WDM technology is the favored choice for China's transport networks. Many 2.5 Gb/s WDM systems with eight to 32 channels already exist in the national and provincial backbone networks. Tens of thousands of WDM systems have been deployed in the national backbone network.

After a two-year field trial with both point-to-point- and MSPring-based 10 Gb/s WDM systems with eight to 40 channels, China Telecom made another strategic move in 2000: We began to deploy three ultra-long MSPring-based 1.6 Tb/s WDM systems with 160 channels in the national backbone networks. The installations will soon be complete. At almost the same time, China Telecom started deploying digital cross-connect (DXC) equipment in major cities to form a highly responsive backbone network with restoration capabilities. This installation will also be completed soon. These systems will cover major economic and politically important areas.

China Telecom is a state-owned company that provides all wired telecom services. Other big operators such as China Unicom (Beijing) and China Railway Telecommunications Company (CRTC; Beijing) followed the same thinking and are starting to deploy the same MSPring-based 10 Gb/s WDM system. China's backbone networks will soon consist of modern WDM-system-dominant networks.

In the last five years, the traffic growth rate in China has been extremely fast. By 2005, the number of wired telephone customers is estimated to reach 220 to 260 million, while the number of expected cellular customers will reach 260 to 290 million; an estimated 40% of the population will have telephone access. We expect the number of data (including IP and multimedia) subscribers to exceed 200 million. With the recent explosive growth of data traffic and the potential for fast growth of video traffic, it is beyond our capability to project the total demand for optical transport capacity.

To meet these challenges, we will continue evolving with more wavelengths and higher speed. We are considering three options. The first is to use ultra-longhaul transmission systems, which offer non-electrical regeneration distances stretching to more than 3000 km to directly connect major developed areas. The second is to upgrade to 40 Gb/s–based WDM systems for backbone networks, which would allow us to accommodate potential demands from router networks with 10 Gb/s interfaces. This move, however, cannot start until major issues with 40 Gb/s devices and fiber, including chromatic dispersion, cost, and polarization mode dispersion, are addressed.

The third option is to transition from point-to-point WDM to optical networking, which is an even more important strategic move. In order to convert the enormous original bandwidth provided by traditional point-to-point WDM systems into bandwidth that can be flexibly applied in actual networking, we will have to introduce flexible optical nodes to offer optical layer networking using semi-dynamic or dynamic approaches; therefore, automatic-switched-optical-network (ASON) systems are attracting a great deal of attention. In concept, ASON is only a new type of optical transport network (OTN) that puts intelligence into optical nodes to permit dynamic provisioning that can address changing traffic patterns.

By evolving point-to-point WDM systems to OTN/ASON systems, we can eliminate the throughput bottleneck of network nodes caused by electronics, provide optical-layer bandwidth-management capability, provide scalability (which allows continuous traffic growth and network expansion), and provide reconfigurability (which allows semi-dynamic and dynamic optical networking). We can also simplify and speed up provisioning of high-speed circuits and services and offer fast network protection and restoration on the order of tens or hundreds of milliseconds to guarantee excellent network and service survivability. This networking approach provides optical transparency, which enables any new systems to connect to exiting systems and enables the network to transport signals with any format. It also links optical network resources to data traffic patterns automatically, thus creating a highly responsive and cost-effective transport network.

Figure 1. A vision of China's future optical network architecture. The national backbone is a mesh topology that uses OXCs (white cubes). Provincial and metropolitan area networks, nodes, and leased electrical or optical lines can be connected to the national backbone using SDXCs (green cubes), standard routers (red disks), or OADMs (blue cylinders). The metropolitan networks may use optical schemes such as SDH or WDM, or they may use some other packet-driven scheme. These may connect to customer nodes or ATM networks.

Figure 2. A trend in China's optical networks is reducing the number of communications protocol layers in the network. At the moment, optical networks contain four layers below data. From the bottom, they are the WDM layer, SDH layer, ATM layer, and Internet Protocol layer. In the future, these are expected to change and merge into a two-layer system, which combines the WDM and SDH layers into one combined network layer, and IP and ATM in another combined IP and MPLS layer.

As OTN technologies mature, all major nodes in the national backbone network will be equipped with optical nodes. These nodes will be connected with each other by existing and future WDM systems. The architecture is similar to the electrical layer for SDH networks. The network will use an optical cross-connect-based (OXC) mesh topology, with multiple optical add/drop multiplexer-based rings connected by OXCs (see figure 1). The whole network will experience a delayering process: The existing four-layer architecture will eventually evolve into a two-layer architecture (see figure 2). The technology for OTN is still evolving; issues such as optical signaling, routing, and automatic dispersion and PMD compensation must be resolved before we can define requirements and a detailed architecture for real networks.

To implement optical intelligence, two evolution architectures are being considered that place control in different layers (see "Understanding the Layers"). The client-server model, backed by the telecommunications community, would place optical-domain-specific control intelligence entirely at the optical layer. The peer model, preferred by the computer community, would place all control intelligence at the IP layer. Most network operators, including China Telecom, prefer the client-server model because it provides an open and transparent platform to serve all client signals, and it allows the optical and client layers to evolve independently. The whole infrastructure, therefore, is not limited by the technology of the electrical client layer. The client layer can only request optical bandwidth service through a well-defined user network interface, and hence the internal structure detail of the optical layer is hidden from the client layer. The interconnection between subnets would occur at network node interfaces, allowing each subnet to evolve independently.

Both models use the same simplified IP-based control plane, which is based on multiprotocol-label-switching (MPLS) control architecture. This provides a simple and mature set of protocols. The use of these protocols for optical layer control leads to the multiprotocol-lambda-switching (MPLmS) protocols. In this scenario, IP routing protocols such as open shortest path first (OSPF) are leveraged for topology discovery, in which equipment on the network is able to query other equipment on the network to determine operating parameters, and MPLS signaling protocols such as constrained based routing label distribution protocol (CR-LDP) are used for automatic provisioning. We believe that as standards move forward and technologies mature, ASON will increase in popularity, and the client-server model will become the dominant evolution architecture over the near and middle term. In the long run, or in regions where the IP layer and transport layer are managed by the same operator, the peer model may become an attractive solution.

We will soon complete national DXC-based restoration networks and overlay SDH MSPring-based protection networks. Ultra-long transmission and advanced ASON networks are under serious consideration, and we may start field trials in the near future. China has already faced many unique challenges. In a few years, the magnitude of China's optical fiber transport network, coupled with its diversity and sophistication, will make it the world telecom industry model in many ways. oe

Reference

1. Statistical data from China Telecom, January 2002.

Leping Wei was teaching electrical engineering at Tsinghua University in Beijing when the optical fiber communication industry started to develop. Intrigued by the articles he read on the subject, Wei decided to choose optical communication as his major when he started his graduate work in 1977 at the China Academy of Post and Telecommunication Science. After graduation, Wei went to work for the Research Institute of Telecommunication Transmission of the Ministry of Post and Telecommunication (MPT), which is responsible for setting up technical standards and specifications in China.

In the early 1990s, Wei wrote the generic requirements for SDH transport networks and a series of technical specifications. Those documents became the major guidelines for building China's advanced SDH networks. For the past 20 years, Wei has greatly influenced the shape of optical networking in his country through his involvement in optical fiber communication, SDH, access network, broadband communication, and network evolution strategy. He served as head of the leading experts group of 863 high technical programs. He has published more than 100 papers and six books.

China recognized Wei's contributions by honoring him as a National Level Expert. He also has been awarded various first-, second-, and third-degree Science and Technology Achievement Prizes from the MPT and Ministry of Information Industry.

"I enjoy this field so much because it has been so dynamic over the past 10 years," says Wei. "Technical innovation and change is so great that it makes me constantly explore new and exciting things."

—Laurie Ann Toupin

alphabet soup

ASON—automatic switched optical network

CR-LDP—constrained-based routing label distribution protocol; a protocol that can supply dynamic traffic engineering and quality of service in networks using MPLS

DXC—digital cross-connect; part of a digital cross-connect system that has access to lower-rate channels in higher-rate multiplexed signals and can electronically rearrange (cross-connect) those channels

G.652, G.653, G.655—International Telecommunications Union standards for single-mode optical fibers

IP—Internet protocol; a protocol for many computer links, including the Internet

MSPring—multiplex section protection for ring network topologies

MPLS— multiprotocol label switching; a framework that provides for efficient designation, routing, forwarding, and switching of traffic flows through the network; MPLS provides a means to map IP addresses to simple, fixed-length labels used by different packet-forwarding and packet-switching technologies.

MPLmS—multiprotocol lambda switching; optical version of multiprotocol label switching; it fits well with WDM when wavelengths are used as labels.

NNI—network node interface; a defined interface between nodes in the public network

OADM— optical add/drop multiplexer

OSPF—open shortest path first; a link-based distributive routing protocol defined by the Internet engineering task force.

OTN—optical transport network

OXC—optical cross-connect; OXCs allow advanced optical routing capabilities and thus create a system that can be controlled by software and managed centrally or distributed. By using the capabilities that OXCs provide, a mesh network can be built using myriad point-to-point solutions, managed entirely at the optical layer.

PDH—pleisosynchronous (nearly synchronous) digital hierarchy; widely used before SDH. Unlike SDH, PDH used point-to-point transmission and a multiplexing method based on bit interleaving.

SDH—synchronous digital hierarchy. An ITU-standardized family of digital carrier rates. SDH topologies use ring architectures, similar to the ANSI SONET standard common in the United States. SDH rates, denoted by STM, correspond to SONET OC-XX rates. For example, STM-16 operates at 2.5 Gb/s, just as SONET's OC-48. The standard defines the packaging of data within SDH frames, the encoding of signals on a fiber-optic cable, and the management of the link.

UNI—user-network interface; a defined interface between the user and the public network

WDM—wavelength division multiplexing

—Yvonne Carts-Powell

The Open Systems Interconnect (OSI) seven-layer model is a global ISO telecom standard containing protocol layers that define hardware and software required for multivendor information-processing equipment to be mutually compatible. The layers are:

• Layer 7: Application Layer. Connects an application or program to a communications protocol.
• Layer 6: Presentation Layer. Encodes and decodes the data to be transmitted.
• Layer 5: Session Layer. Establishes and maintains connection to the communications processes in the lower layers.
• Layer 4: Transport Layer. Responsible for error correction and direction of data flow.
• Layer 3: Network Layer. Switching and routing layer.
• Layer 2: Data-link Layer. Receives and transmits data over the physical layer.
• Layer 1: Physical Layer. The transmission medium itself, e.g., optical fiber.

-Yvonne Carts-Powell