Building Broadband Optical Access Network with CWDM

Release Date:2008-07-24  Author:Luo Lairong  Click:

Replacing Copper with Fiber
When fixed operator's voice business is declining, both China Telecom and China Netcom, China's fixed-line operators, adopt the strategy of developing broadband services. With the approach of 3G, China is set to restructure its telecom industry. The existing fixed-line operators are pressed to build broadband access networks for future initiative.

Transmission bandwidth on copper lines is in inverse proportion to transmission distance. Thus, when optical broadband access network is being built, the copper cables will be reduced in length or avoided, that is, optical nodes should be placed as close to end users as possible. This is also called "replacing copper cables with optical cables".

China Telecom takes "replacing copper cables with optical fibers" as an important measure to transform its network and improve its network operation and maintenance quality. This marks the change from the focus on narrowband voice based on copper cables to focusing on broadband data based on optical cables.

The actual application of replacing copper cables with optical fibers is fiber to the x (FTTx). According to where fiber ends, FTTx can be further categorized into: fiber to the home (FTTH), fiber to the building (FTTB), fiber to the node (FTTN). FTTH, which deploys an optical network unit (ONU) in the end user's living or office space, is the closest to the end user among all FTTx solutions, and is the ultimate solution to broadband access.

Using CWDM for Optical Access Network
There are two solutions for replacing copper with fiber: using EPON or moving down Point of Presence (POP) like MSAG, MSAN and DSLAM. In these two solutions, the uplink traffic of EPON or POP increases greatly, and the bandwidth of the corresponding uplink interface is usually FE or GE.

The existing MSTP network is insufficient to process large-granule GE service, and with the increase of access bandwidth, the traditional STM-1/4/16 MSTP ring bandwidth is obviously inadequate for use. A good metaphor is that roads are available between towns and villages, so the bottleneck roads between towns and counties also need to be reconstructed.

The low-cost CWDM equipment is the best way to reconstruct and upgrade the existing transmission network access layer.



Figure 1 illustrates using CWDM to build the bear network of the transmission network access layer. After reconstructing the network with CWDM equipment, the existing GE and FE services are converged and carried via some CWDM wavelengths, while the original MSTP ring services are carried via a CWDM wavelength. The solution has the following advantages:

  • IP over WDM, complying with the network evolution trend, simplifies the network hierarchy, avoids wasting resources at the SDH layer, and meets urban user demands for various services by utilizing the transparent CWDM and DWDM transmission.
  • The transmission network access layer, built with high performance-cost ratio CWDM, features low initial investment and smooth upgrade. It can carry the original MSTP ring services via a CWDM wavelength, making full use of the original equipment.
  • With the complete protection mechanisms and mature carrier-class applications of WDM network, the solution can meet urban user's demands for high reliability and QoS, and has natural advantages in carrying Storage Area Networking (SAN) services in urban areas. It is an integrated platform for carrying NGN/IPTV/3G/VIP services. 

Introduction to ZXMP M600
ZTE's independently developed CWDM product, ZXMP M600, in strict compliance with CWDM international standards. It has the following characteristics:

High system integration

  • An Optical Terminal Multiplexer (OTM) node formed by a CWU box (1U high) can implement the bidirectional transmission on 4+1 wavelengths. +1 refers to 1310nm Optical Supervision Channel (OSC). 
  • An Optical Add/Drop Multiplexer (OADM) node formed by a CWU box (1U high) can implement the bidirectional transmission on 2+1 wavelengths.
  • A CWU box (6U high) can implement the bidirectional transmission on 18 wavelengths at most.


Powerful transmission and multiservice access
ZXMP M600 supports a transmission capacity of 45Gb/s (2.5Gb/s×18), the highest in the industry. By performing O/E/O wavelength conversion, it converts the accessed optical signals into G.694.2-compliant wavelength signals for output. It can access services of different rates, including STM-1, STM-4 and STM-16, or services of continuous rates (12.5Mbit/s-2.7Gbit/s). It can also converge multiple services, for example, converge 2×GbE into STM-16.

Flexible networking
It supports multiple networking applications including chain, ring and tangent ring. With a modular structure, it provides modular upgrade path and supports flexible add/drop of 1 to 18 wavelengths.

Powerful protection
It provides optical multiplex section 1+1 protection and optical path 1+1 ring protection.

Complete supervision information transmission function
It provides two pairs of OSC interfaces for 100Base-FX applications and two 10/100M Ethernet interfaces. The ETM board can also offer the electronic supervision channel. The OSC can transmit signals over a distance of 80 km.

Flexible box design
A CWU is 1U high (44 mm) and has 6 board slots. It can be mounted on desk or installed in the 19-inch IEC/ETSI cabinet at front or back. A CWE is 6U high (265.9 mm) and has 18 board slots. It can be installed in the 19-inch IEC/ETSI cabinet at front or back.

Multiple power supply modes
A CWU box supports -48V/-60V DC power supply or 220V (50Hz)/110V (60 Hz) AC power supply. A CWE box supports -48V/-60V DC power supply. Each box provides two power boards for 1+1 hot backup, ensuring reliable power supply of the system.

Convenient maintenance
Except the power and NCP boards, the boards for CWU and CWE support mixed insertion, hot swap, replaceable SFP optical module and front installation. The box fans support hot swap and front installation.

ZXMP M600 Application
As Figure 2 shows, ZXMP M600 is applied to the transmission network reconstruction for the fiber replacement project in a Chinese county. In this fiber replacement project, some access nodes in county network 1 and 2 need to deploy IPTV services, and use GE as DSLAM uplink interface, but the original MSTP network cannot meet such demands. After careful evaluations and tests, the ZXMP M600 was chosen for the transmission network reconstruction. In Figure 2, CWDM rings are built for county network 1 and 2, the original MSTP network transmits services via the CWDM network, and new services from DSLAM are directly converged to the local DWDM network via CWDM wavelengths. The network has been running smoothly since the operation.



When fixed operators are carrying out the fiber replacement strategy, CWDM becomes an important means to solve the bandwidth bottleneck in the transmission network access layer. ZTE's CWDM equipment ZXMP M600, featuring large capacity, high system integration, multi-service access, high performance ratio, has found wide applications in the transmission network reconstruction of various operators such as Shanghai Telecom, PCCW, Zapp Portugal and Wharf T&T. In a word, it has become the best choice for accelerating the installation of fiber in the access network. 

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