Bookmark & share

By Liu Hongli

The rise in broadband services has created a requirement for more powerful metro optical networks. Carriers are trying to reshape their existing network structure to establish an open, flexible, cost-effective, dynamic reconfigurable metro optical network to transport broadband services. Now, NG Metro WDM offers an opportunity to migrate the TDM-oriented transport network to an IP oriented transport network.


Metro Optical Network Revolution Driven by Broadband Services

Commercial Metro WDM equipment was deployed in 1997, but point-to-point topologies, back-to-back connection, and high networking cost limited the application of first-generation Metro WDM. The subsequent second-generation Metro WDM primarily features static OADM ring networks, with the advantages of tremendous capacity (32/40 2.5/10Gbps wavelengths), strong survivability, and transparency to protocols and bit rate.

However, on existing networks, far less Metro WDM equipment is deployed than SDH/SONET equipment. In a voice-oriented era, the transmission bandwidth is limited. Besides, small-granule services must be converged and groomed before being adapted to the optical layer. Obviously, legacy SDH/SONET is the most suitable transport technology. In this era, the Metro WDM with its main mission of transporting a large-granule service transparently does not have bright prospects.

With the development of IP & network technologies, network subscribers and bandwidth are growing rapidly. The main services of metropolitan area network are changing from TDM services to IP services. The revolution in service networks has inevitably led to a revolution in bearer networks. Previously, the focus was on TDM, SDH, and the grooming of 2M granularity. Currently, the focus is on GE, 10GE, and 40G technologies, OTN (optical transport network) and reconfigurable optical layer networking solutions.

Therefore, carriers are facing challenges in terms of planning, designing, and constructing the next-generation broadband metro optical network.


Reconfigurable Optical Networking as the Development Trend

Today, everything is migrating to IP; Ethernet is rapidly becoming the universal interface of the network. Carriers are faced with a circuit-to-packet evolution, changes to traffic patterns and unpredictable capacity growth. As Gigabit Ethernet-based traffic grows, WDM becomes increasingly more efficient than stacked SONET/SDH for traffic aggregation and transport.

Except the fibre relief, carriers have to reckon with the additional value of the WDM technology in the aspects of service multiplexing, protection, wavelength & sub-wavelength grooming, fiber/wavelength supervision and management, etc.

People think that the traditional Metro WDM is not flexible, the bandwidth is configured statically, and its networking capability is limited (It only supports linear or ring topology. In the situation of two interconnection rings, all services must be terminated).

The next generation Metro WDM system is flourishing, as it features integrated transport and switching capacity in one network element. It is also an automatic and intelligent system that reduces network CAPEX and OPEX.

There are three main aspects in NG Metro WDM, they are ROADM, OTN based switching & management and integrated GMPLS control plane.

ROADM and Multi-degree ROADM provide rapid network reconfiguration as traffic demands change. ROADM is capable of providing any wavelength or wavelength band to any node & any fiber port through remote network management. Multi-degree ROADM provides transparent optical cross connect for true mesh networking. The ROADM and MROADM can work with a full band tunable laser to form the foundation stone of a dynamic optical layer network for auto wavelength routing.

Notably, ROADM or MROADM only changes the optical wavelength routing. The optical wavelength can directly pass through a node, thus reducing the unnecessary optical-electro-optical conversion and reducing networking costs. Therefore, the reconfigurable capability of the optical layer alone is far from enough for an advanced Metro WDM system.

People should further consider multiplexing, converging and grooming services at the electrical layer to avoid wavelength blocking and allow the construction of highly reconfigurable networks. OTN serves as a new digital transport hierarchy (replacing traditional SDH) and an optical transport hierarchy (replacing traditional WDM) and is the perfect choice.

The advantages of OTN are as follows: Extended capacity to meet the need of broadband data and video services (Gbps per channel and Tbps per fiber); Improved service and timing transparency by using G.709"digital wrapper"technology for Ethernet, IP/MPLS, SDH, ATM, etc; Integrated optical (OCh) and electrical (ODUk) switching in a single network element; Standardized physical, control, and management level for multi-vendor and multi-operator networks; Optimized WDM (simple transport vs. networking solution) provides OCh, OMS, OTS management (overhead to lambda and multi-lambda) and end-to-end ODU performance monitoring, fault and degradation detection.

Network management & control system is changing with evolution of transport networks; from centralized management to a distributed GMPLS control plane.

The advantages of the intelligent control plane are as follows: Automatic discovery of nodes, neighbors, topologies, fibers, wavelength and sub wavelength; Automatic end-to-end wavelength and sub-wavelength service creation, thus avoiding manual errors; Dedicated or shared Intelligent protection and restoration; Rapid response to customer's requirements, reduces time to market; Simplified planning and verification and reduced OPEX; Interoperability between multi-vendor networks.

NG WDM incorporates the advantages of ROADM, OTN, and the intelligent control plane to realize efficient transport, efficient grooming, and efficient administration capabilities.


Networking and Application of NG Metro WDM Systems

Future core networks will be based on IP/MPLS. Ethernet (GE/10GE) is the most economical interface for various applications, such as triple-play IPTV, NGN, IP DSLAM, 3G network, and FTTx on the metro access and aggregation layer.

NG transport network will focus on the problems of Ethernet related to transport, multiplexing, distance expansion, traffic grooming, protection/restoration, and performance monitoring and fault diagnosis.

Huawei's opinion is that one can not really talk about "carrier-class Ethernet" without carrier-class Ethernet transport architecture. Huawei's IP oriented Metro WDM system provides the flexible infrastructure that will easily accommodate uncertain future bandwidth, scalability and flexibility demands.

Huawei's IP oriented WDM system provides a three-class traffic grooming model. Agile optical layer optimizes the structure of the core network, and provides automatic wavelength routing. ROADM eliminates the uncertainty in traffic patterns and required capacity, and speeds up service delivery and reduces total cost of ownership. Higher order electrical layer (GE ADM) implements end-to-end automated creation of GE/10GE channels, traffic grooming, sub-lambda protection, performance monitoring, and fault detection between different wavelengths in the true mesh network. Huawei is the first to launch the GE ADM solution in the industry. Lower order electrical layer –embed in L2 switching capacity implements local Ethernet traffic aggregation (n*GE to GE or n*GE to 10GE), QoS assurance with CAR.

GE ADM with the L2 processing function effectively integrates the tremendous capacity and reliability of optics with the simplicity and scalability of Ethernets, hence meeting the need for carrier-class Ethernets in network planning, construction and management.

To sum up, Metro WDM systems are undergoing profound changes. Integration of various technologies (ROADM, EXC, L2 Switch, OTN, ASON/GMPLS and so on) detaches NG Metro WDM from the initial function of fiber relief while highly effective and low-cost transmission of big-granularity services (GE, 10GE, FC, FICON) has become the core value of Metro WDM. Huawei aims to provide optical network solutions with high operability. With IP-oriented Metro WDM, Huawei is advancing closer to this goal.