The New Era of WDM

Release Date:2006-12-15  Author:Zhang Lian  Click:

As an economy steadily improves, it drives growth in demand for the information and communications. There is a clear worldwide trend for telecoms service demand to migrate from voice services to a great variety of integrated services including voice, data and multimedia applications. Especially in recent years, IP services have enjoyed explosive growth worldwide, which pushes telecoms authorities to ask for ever longer transmission distances, and ever higher capacity, switching speed and networking flexibility.

Wavelength division multiplexing (WDM) has been considered as an ideal solution to extend the capacity of optical networks, and commercialisation of it flourished in late 1990s. WDM systems can support increasing data communications volumes, and ever changing IP services demands on transmission capacities and transmission rates.

Meeting development requirements of markets and services

WDM systems have been deployed by the worldwide telecoms sector to upgrade the capacity of existing optical fibre communication systems. Essentially, WDM systems provide a series of compelling benefits including exploiting fibre's huge bandwidth; greatly reducing the cost of large-capacity and long-haul transmission with less optical fibres, regenerative repeaters and optical amplifiers; providing switching functionality; creating a transparent and highly reliable optical transport network; and allowing rapid new services introduction. It is applicable for metro and access networks.

At present, WDM is trending in two directions: towards more wavelengths, higher rates and ultra long-haul transmission; and towards WDM networking. Point-to-point WDM systems only offer increased bandwidth. However, services and market development require on-demand capacity allocation, provision of customised services and low cost. To meet the needs for capacity and bandwidth in the best way possible, WDM equipment must support flexible service deployment and service-specific resources allocation.

New WDM technologies

  • 40G WDM transmission

40 Gb/s requires innovative technologies to overcome fundamental technical challenges, including 40G optical signal modulation, multi/demultiplexer for large capacity networks, non-linearity, group delay dispersion, polarisation, optical amplification, optical signal receiving, high-speed forward error correction (FEC), etc. To date, a host of vendors have grasped the key techniques in developing 40Gb/s WDM system. For example, ZTE's 80×40G WDM equipment has completed successful field trials over distances of 80km×5 and 100km×4, gone through the appraisal and been accepted by China's 863 Program Expert Group. The 863 Program is a national high-tech development programme.

 The 40G per wavelength WDM system holds the future for large-capacity transport and represents development trend of technology. In network application, routers with 10Gb/s interfaces have been largely applied, while those with 40Gb/s interfaces are starting to be employed.

In order to improve the efficiency and function of the core network, the most reasonable approach would be to develop its single wavelength rate so that it is transformed into 40Gb/s transmission. Based on traffic-growth expectations, there are broad applications on the horizon for 40G WDM transport in the metro core as well as backbone network.

  • Ultra long-haul transmission

According to Chinese WDM standards, long-distance transmission can be divided into long-haul that supports spans of less than 1000km, enhanced long-haul that supports spans of fibres between 1000 and 2000 km and ultra long-haul (ULH) that supports spans of over 2000km.

To date, the LH and ELH WDM systems have been widely deployed in China and quite a few Chinese and international vendors have developed and commercialised ULH WDM systems. ZTE's industry-leading ULH WDM system can achieve a a transmission distance of up to 5000km.

 The ULH WDM system's main enabling technologies include Raman amplifier, FEC, dynamic gain equaliser, modulation code and dispersion compensation. In the long term, optical solitons and full-spectrum WDM will be the key solutions for ultra long-haul transmission.

  • ROADM

Traffic continues to grow as people make more use of high-bandwidth applications. Reconfigurable optical add/drop multiplexers (ROADMs) can improve capacity and support maximum flexibility for bandwidth growth when compared with fixed OADMs. The conventional OADM adds and drops predetermined groups of wavelengths, and reconfiguring an OADM network means reengineering the network.

ZTE can offer a complete product chain of ROADM systems. ZTE's ROADM system features full add-drop capability of 40 and 80 wavelengths in full configuration, achieving the goal of "any-wavelength-to-anywhere". The system provides power equalisation for the adding and passing through of wavelength, and supports both manual and automatic configuration. Automatic protection is provided to automatic configuration.

ZTE's ROADM solution enables"pay-as-you-grow" scalability. In the initial ROADM deployment, fixed wavelength add/drop systems are recommended for the fixed distribution of services. Some wavelength-tuneable OTUs will be added to address the growing demand for bandwidth and new services and to increase network agility.

  • GMPLS

Generalised multi protocol label switching (GMPLS) has drawn attention as a network protocol that is applicable to optical networks and has been standardised in the Internet Engineering Task Force (IETF). GMPLS, migrating from MPLS, enables unified control management of the network layers of several types of network: TDM, and Optical in addition to IP, merging the worlds of IP and WDM.

GMPLS's control plane enables  self- discovery of network resources, routing control, path management, link protection and recovery. It also allows for end-to-end connection, bandwidth-on-demand, automatic traffic engineering, additional protection/restoration, and optical virtual private network (OVPN) in multi-vendor environments.

GMPLS will finally replace ATM, SONET/SDH functions such as QoS, recovery, VPN and ring protection. Meanwhile, the IP-over-WDM network using GMPLS is both interoperable and scalable. This allows service suppliers to create a large-capacity network architecture for rapid provisioning of the required connections. GMPLS helps operators to cut operational complexity and costs and enhance the operational performance and flexibility of their networks, without impacting network-level QoS, thus becoming the impetus for future optical network development.

  • IP-over-WDM

IP-over-WDM has been regarded as one of the most attractive architectures for the new Internet when there is explosive growth of Internet traffic. With WDM offering high-bandwidth transport and powerful switching capabilities, IP-over-WDM improves network management and promotes simplicity through coordinating traffic engineering, protection/ restoration and QoS at both the IP and WDM layers.

Carrying IP traffic over the WDM layer to avoid immediate layers such as ATM, SDH can improve bandwidth efficiency, reduce overhead and equipment costs, meet the demands for bursty and unpredictable IP traffic, drastically reducing OPEX costs. IP-over-WDM networks featuring high marketability and survivability are not only compatible with existing communication networks, but also support upgrading to future broadband networks. It is worth noting that the SDH layer will be marginalised and eliminated under development for IP-over-WDM integration, and protection of the IP layer transport will have increasing dependence on the WDM layer. ZTE is the only vendor that can offer six protection modes at the optical layer, including the unique 2-fibre bidirectional line shared ring protection technique.

Application

ZTE has made great breakthrough in ultra long-haul transmission. In the Xinjiang Uygur Autonomous Region, ZTE built an exemplary ultra-large capacity and ultra long-haul WDM transmission project for a domestic mainstream vendor. This project passed expert appraisal and was put into operation.

The project is a provincial trunk line transmission network, with a multiplex section of 1104km and a total capacity of 40×10Gbs. The project adopts a series of advanced and mature technologies such as Advanced FEC, Electrical Return to Zero (ERZ), Raman amplifier, centralised wavelength monitoring, power management and dispersion management. The success of this project means that ZTE's high capacity and ultra long-haul application has reached a new level of quality and capability.

The project enables terabit transmission with multiplex section lengths of more than 1000km, proving ZTE's leading position in ULH and ultra high capacity DWDM transport, and proving the maturity of its products.

ZTE has carried out investigations into relevant technologies independently or cooperatively in recent years, and made a series of notable achievements with its partners.

  • Achieved terabit and 1000km multiplex section transmission in the trunk line network of a mainstream operator under China's 863 Programme;
  • Submitted two recommendations to ITU-T. ZTE had been selected to edit Recommendation G.665 on the evaluation of Raman amplifiers which was published in January 2005.
  • Deduced the world's first semi-analytical solutions to two kinds of noise sources in distributed Raman amplifiers (DRA), which have been verified;
  • Proposed that the DRA can also compensate for an Erbium Doped Fibre Amplifier (EDFA) gain profile to flatten the noise spectrum. This has been verified and applied;
  • Proposed to use the widely-tuneable continuous-wave laser as pump source to achieve ultra-flat gain spectrum amplification of DRA;
  • The emulation software introduces a modelling method for the atomic function model, demonstrating the inner structure of all kinds of complex models simply and clearly.
  • Achieved WDM transmission of
    1.6Tb/s capacity over a distance 5490km without electric regeneration in laboratory tests.

The project is based upon ZXWM M900 DWDM system. The ZXWM M900 is ZTE's flagship product used in DWDM transmission in trunk lines. As a patented and standards-compliant bidirectional WDM system, the ZXWM M900 serves as a mainstream solution for the construction of trunk transmission networks, demonstrating ZTE's competitiveness as a global provider of optical transport.

The ZXWM M900 accounts for a more than 33 percent increase in domestic DWDM market share for ZTE. In the international market, the ZXWM M900 has successfully entered countries such as India, Indonesia, Czech Republic, Bulgaria, Macedonia and Thailand.

ZTE, aiming to establish a greater international presence, is committed to developing cost-effective and highly reliable trunk transmission solutions for its customers.

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