WHITE PAPER
INTRODUCTION
Traditional dense wavelength-division multiplexing (DWDM) networks, whether hub-and-spoke or mesh architectures, have significant challenges meeting the dynamic-wavelength demands for user services. Service providers and enterprises rely on flexible aggregation devices that can build efficient, dynamic Layer 2 and Layer 3 architectures to manage the difficulty of provisioning unplanned wavelength services at the DWDM transport layer. As wavelength-bandwidth demands continue to increase, there is more direct connectivity of switches, routers, and storage-aggregation devices to the DWDM layer. This is in part accelerated by the demand for high-bandwidth services such as Gigabit Ethernet, 10 Gigabit Ethernet, and SONET OC-192/SDH STM-64.
Current metro DWDM networks meet the bandwidth demands for 2.5- and 10-Gbps wavelength services. However, these DWDM networks are inflexible and unable to meet changing user wavelength-service requirements. They also pose significant challenges in terms of network engineering and planning for future services. Today, large enterprises and service providers often build new DWDM networks to address new service requirements rather than optimizing existing networks that could potentially support additional services. This is in part a result of the extended downtime often required to reengineer current networks to support new services.
Metro DWDM solutions based on fixed optical add/drop multiplexers (OADMs) have not been attractive for mass deployments because they do not provide the operational simplicity and flexibility that service providers have grown to expect with SONET/SDH, leading to high operational costs or simply the unwillingness to take on the high-touch operational model that fixed OADM solutions require (Figure 1).
Metro DWDM networks are commonly based on fixed OADMs, in which optical paths are typically predetermined when the system is engineered and unused capacity cannot be reallocated without reengineering the system. To drop a wavelength at a node requires physical visits by design engineers to essentially redesign the network-changing filters, retuning existing optical interfaces, or possibly adding new ones. Other working services are disrupted during this process. In a dynamic metropolitan carrier environment, where customer turnover and demand for new service turn-up are constant, the high-touch model of fixed OADMs is not feasible. This is also true for high-end enterprise networks where there are constant and changing wavelength-service demands from business units or departments. It is clear that current DWDM network architectures and optical component technology have to evolve to be dynamic and efficient to address rapidly increasing bandwidth demands and new types of services such as Gigabit Ethernet, Fibre Channel, and SONET OC-192/SDH STM-64.
Figure 1
Traditional Metro DWDM
CISCO NEXT-GENERATION METRO DWDM ARCHITECTURES
Service providers and large enterprise are poised to evolve their metro DWDM transport infrastructures to accommodate a more dynamic and efficient transport architecture. This is partially influenced by new developments and availability of optical component and intelligent software technologies like reconfigurable OADM (ROADM), Full C-band tunable lasers, standard data-encapsulation protocols like Generic Framing Procedure (GFP), G.709 digital wrapper technology, Forward Error Correction (FEC), and embedded optical component intelligence.
Cisco Systems® offers a carrier-class, next-generation metro DWDM solution with its Cisco® ONS 15454 Multiservice Transport Platform (MSTP). Cisco recently introduced an enhanced 10-Gbps multirate transponder line card and a 4-port 2.5-Gbps muxponder line card-both 10G line cards support Enhanced FEC (EFEC). This next-generation platform (Figure 2) and its enhanced functions help enable service providers to effectively deploy ROADM into their existing networks to achieve greater DWDM multiservice flexibility as well as reduced capital and operating expenses.
Figure 2
Cisco ONS 15454 MSTP with 10-Gbps Multirate Transponder Card and 4-Port, 2.5-Gbps Muxponder Card
The Cisco ROADM solution offers the following benefits:
• Total flexibility-any wavelength, anywhere, anytime
– Does not depend on forecast; service providers can offer flexible wavelengths and services
– The Cisco ONS 15454 MSTP supports up to 32 channels today-no future upgrades required
• Growth-requires only new services interfaces
– The Cisco ONS 15454 MSTP supports wavelength path provisioning
– Simplified provisioning increases service velocity and shortens time to revenue
– Multirate capability is software-controlled
– Never breaks composite line
– Services providers only need to add service interfaces at network endpoints (reduces line card inventory and simplifies network design and maintenance)
The Cisco ROADM solution significantly changes current and future DWDM network designs. ROADMs help enable service providers and enterprises to build network architectures that will be dynamic and efficient while simplifying operations and lowering operating expenses. ROADMs remove the fixed and inflexible features of current DWDM architectures (Figure 3).
Figure 3
ROADM
ROADM allows networks to be built with a single-node type that is capable of adding or dropping a single wavelength or the full available operational optical spectrum (C Band, 1 to 32 wavelengths). This significantly simplifies metro DWDM designs and optical-channel management. Individual wavelengths can be software provisioned and managed. Network cabling is now simplified. ROADM removes the effort of reengineering the network for wavelength-service changes and system growth.
The price curve for ROADMs is decreasing fixed OADMs, especially for system vendors with ROADM technology that can be produced in volume. It is important to note that while ROADM significantly changes the way metro DWDM networks are built, not all ROADMs are equal. The ROADM that provides the ability to add or drop a single or all available wavelengths without a complete system upgrades will provide the most benefit in enabling a truly flexible, dynamic, and operationally efficient architecture. ROADM cards through greater silicon integration using Planar Linear Circuit (PLC) technology provide significant system benefits. First-generation PLCs were simple, often including only a 32-channel demultiplexer or multiplexer without any power measurement capability or use of VOAs. Today's PLCs can demultiplex all 32 channels with power taps for measurement of individual channel powers and offer a per-channel VOA. The ROADM packages can be a very small with the ability to demultiplex, multiplex, attenuate, and switch on a per-channel basis.
To appreciate the benefits of ROADM, it is important to note a challenge with static OADMs in configurations with cascading amplifiers used to boost all the wavelength signal powers. This creates power variation, or gain tilt, in the channels. ROADMs act as channel equalizers and can continually flatten the spectrum automatically. Traditionally, this has been a manual function, requiring manual per-band equalizers to attenuate bands of channels. Because each ROADM node has the ability to manage the full wavelength spectrum, DWDM design rules can be simplified.
The ROADM is one of the primary enablers for the wavelength-transmission network that provides any-to-any connectivity that data networks require for effective bandwidth delivery (Figure 4).
Figure 4
ROADM and Intelligent DWDM Networking
The Cisco ROADM solution not only increases flexibility in service delivery, it also gives DWDM system providers the ability to explore new wavelength-protection options like Optical Channel Shared Protection, which is akin to SONET bidirectional line switched ring (BLSR) or SDH multiplex section shared protection ring (MSSPR) shared-channel protection mechanisms. This opens the door for new wavelength-service delivery and quality of service (QoS) options.
Also of significance in enabling a truly dynamic transport architecture is the availability of tunable wavelength sources or lasers. Tunable lasers simplify inventory management, provide easy wavelength reconfiguration, help enable end-to-end wavelength provisioning, and facilitate a dynamic and intelligent DWDM network at the wavelength service layer. Laser tenability will help to realize the network architecture depicted in Figure 5.
Figure 5
Tunable Lasers and Intelligent DWDM Networking
The new metro DWDM architecture depicted in Figure 5 shows a flexible, dynamic DWDM architecture with any-to-any wavelength connectivity that takes advantage of the wavelength-transport capability of the DWDM ROADM-based optical transport network. A network architecture that employs full C-band tunable transponders and router, switch, or add/drop multiplexer (ADM) devices with integrated, reconfigurable ITU lasers provides the required service-layer flexibility that facilitates dynamic bandwidth provisioning and wavelength-service-layer flexibility.
Open DWDM Architectures
The need for next-generation DWDM architectures to be "open" is important. New metro DWDM networks must support "alien" wavelength sources. This requirement is influenced by the need to reduce costs by eliminating transponders where possible and by the wide availability of ITU gigabit interface converters (GBICs), Small Form-Factor Pluggable (SFP) optics, 10 Gigabit Ethernet SFP (XFP) ITU optics, and Xenpaks. Large router and switch vendors like Cisco are providing integrated ITU interfaces on their routers and switches. Next-generation SONET/SDH Multiservice Provisioning Platforms (MSPPs) are also equipped with ITU trunks for system efficiency. The trend for optics integration is continuing and customers are demanding that metro DWDM networks support these new service interfaces.
INTELLIGENT OPTICAL TRANSMISSION AND NETWORK PLANNING
An important component of the new flexible, dynamic DWDM architecture is the need for intelligence at the optical-transmission layer to enable automation and operational efficiency. Optical-layer intelligence helps enable network topology autodiscovery, link optical connection setup, point-and-click node regulation, automatic power control (APC) to manage channel variation (system upgrade, channel failures), and automatic power control to manage aging effects on lasers, fibers, and changing operating conditions (Figure 6).
Figure 6
Automatic Power Control
Intelligence at the transmission layer facilitates automation in networking provisioning and control. Intelligent DWDM systems tune and monitor themselves relative to overall system-design goals.
Essential to the building and operational feasibility of dynamic DWDM architectures is the need for network-planning tools to optimize system design and account for transmission impairments and network elements, lack of span uniformity, OADM-insertion loss variation, OADM-node variation, OADM-loss reuse, and end-of-life system margins. Planning tools are essential to help ensure rapid network analysis and determine network parameters prior to provisioning.
OPTICAL CONTROL PLANE FOR DYNAMIC DWDM NETWORKS
As more and more next-generation DWDM networks are built with transmission-layer architecture flexibility, coupled with the any-to-any (full tunable laser) wavelength service-layer provisioning flexibility, control-plane protocols such as Generalized Multiprotocol Label Switching (GMPLS) and User Control Point (UCP) could be applied for even more flexible architecture with an IP control plane that offers integrated control and recovery, integrated network management, unified procedures for deploying facilities, faster deployment, and improved management for Layer 1 technologies.
CONCLUSION
The Cisco ROADM solution combined with tunable lasers, transport-layer intelligence, and automation facilitate the creation of data-optimized, intelligent DWDM architectures for efficient and flexible delivery of dynamic wavelength services (Figure 7).
Figure 7
Data-Optimized Intelligent DWDM
This new DWDM architecture provides operational efficiency through automated network setup and tuning and in-service capacity expansion with hitless service upgrade. DWDM transport networks are able to truly meet the demand for service and provisioning flexibility required by overlay bandwidth-intensive storage and data networks.
The Supercomm 2005 demonstration network - an optical network configuration based on Cisco's ROADM technology - is shown below. (Figure 8).
Figure 8
Supercomm 2005 ROADM Demo Configuration
