Implementing Wavelength Services on the ONS 15454 MSTP

From an industry perspective, wavelength services are implemented either as a managed service offering or by resale of DWDM facilities. The managed service approach supports customer ownership of the DWDM network facilities; however, the service provider handles day-to-day maintenance and operations. This implementation is usually more attractive to customers who need the bandwidth flexibility of DWDM wavelengths but have little or no operational experience with DWDM design and implementation. Conversely, customers experienced with DWDM network installation, maintenance, and operations typically choose a resale option, in which the DWDM service provider installs a dedicated DWDM network and resells it to the customer.

Regardless of the deployment methodology, wavelength services are typically differentiated by three key characteristics:

• Channel protection

• Variety of services offered

• Speed of implementation

The previous sections covered the first two characteristics. It is clear from these sections that the ONS 15454 MSTP provides the operator with a variety of bit rates and services capability for DWDM wavelength deployment. Additionally, flexible transponder/muxponder and network-protection options can be implemented to provide tiered levels of service resiliency.

Many factors affect the time required to implement DWDM wavelength services. First, the physical network must be engineered and constructed to provide the DWDM services infrastructure. For the ONS 15454 MSTP, the DWDM network can be engineered manually, for simple network topologies, or it can be engineered with the MetroPlanner design tool. The manual approach provides several challenges, in that each interdependent design constraint creates the need for network redesign or reoptimization. For example, if a DWDM node is planned as a pass-through initially, but engineering reveals that the site requires optical amplification, that node and others must be re-engineered to accommodate the high optical power levels for amplified channels, versus low-powered, nonamplified signals. Additionally, changes among any of the factors discussed earlier regarding DWDM designattenuation, optical signal-to-noise ratio (OSNR), fiber nonlinear effects, and so onrequire manual redesign/reoptimization. Thus, the time required to complete and finalize a design for ONS 15454 MSTP DWDM wavelength infrastructure can vary from as little as a few hours to as much as a few weeks.

Conversely, the MetroPlanner design tool can be used to mitigate the time gap associated with DWDM design so that both simple and complex designs can be finalized within, essentially, the same span of time. Using the MetroPlanner tool for wavelength services infrastructure design enables the carrier to achieve economies of scale for DWDM deployments, thereby improving the service provider's profit margins and/or resulting in lower-priced services to the end user.

When the physical DWDM network is in place, DWDM channel-provisioning time becomes a critical factor in the total amount of time required to implement wavelength services. Optimization of the channel-provisioning time greatly depends upon the type of DWDM optical add/drop multiplexing (OADM) channel technology selected during the physical design phase. Essentially, two choices exist for optical add/drop design: fixed-channel add/drop and reconfigurable optical add/drop multiplexing (ROADM).

Fixed-Channel Optical Add/Drop

The fixed-channel optical add/drop scenario allows DWDM wavelength channels to be dropped either singularly or as groups (bands) of channels. With this approach, DWDM channel assignments are preplanned for each node on the DWDM network. Therefore, where bands of channels are allocated to a DWDM node, the possibility of "stranded" bandwidth exists if the site channel requirements never reach the planned capacity.

For example, traditional DWDM networks used four- or eight-channel band fixed OADMs. Deployment of these band filters implies that either four or eight DWDM channels are ultimately required at the DWDM node. If the actual channel capacity required is less than the number of channels dropped on the band filter, DWDM wavelengths are dropped at the services node but are not used. Because these channels are dedicated to the dropped node, they cannot be reused elsewhere in the network without re-engineering the DWDM infrastructure. Thus, the possibility exists that a 32-channel-capacity DWDM wavelength system cannot be fully used because of channel bandwidth stranding. Moreover, to implement the re-engineered network, the DWDM network most often requires partial disassembly, resulting in temporary, protracted network outages.

The fixed-channel approach also lends itself to operational inefficiencies for DWDM channel growth or modifications. To successfully deliver an end-to-end wavelength service, OADM and transponder/muxponder equipment must be installed and provisioned at nearly every node on the DWDM network to accommodate pass-through and add/drop channels. For intermediate sites, DWDM channel pass-through equipment/fiber must be installed to provide wavelength continuity across the network. Optical amplifiers and variable optical attenuators (VOAs) within the network also require manual adjustments to accommodate the change in DWDM wavelength channel assignments. From an operations perspective, these equipment additions/adjustments result in time, capital, and personnel expenditures associated with the dispatch of qualified DWDM technicians to administer the network.

ROADM

Alternatively, the DWDM wavelength services network can be engineered with ROADMs at each services drop site. The ROADM approach allows for the drop/insertion of individual DWDM wavelength channels within the 32-channel system spectrum. Each of the channels operates independently of one another, allowing for separate protection schemes and channel-routing characteristics for each DWDM wavelength service. Because the system is initially designed to accommodate 32-channels add/drop at each node site, changes in wavelength assignments do not necessitate a corresponding change in design. Additionally, the ONS 15454 DWDM/ROADM system is self-optimizing. When the initial equipment settings are in place (for example, amplifier power and VOA settings), each affected equipment component self-adjusts for the addition/subtraction of wavelength channels. Thus, to provision an end-to-end wavelength service, only the DWDM channel endpoints require a site visit.

By providing both operational and capital expenditure efficiencies, the ROADM-based network tends to be the most effective deployment for wavelength services delivery. Cisco Systems, Inc., recently conducted a study to determine the economic savings of using ROADM-based DWDM networks versus the traditional point-to-point DWDM approach. The results indicate that networks requiring redundant DWDM wavelength channel assignments and carrier-class resiliency are more economically suitable for ROADM deployment. This is illustrated in Figure 11-5. The graphs represent the normalized capital cost to deploy five typical DWDM networks using ROADM versus traditional fixed-channel add/drop.

Figure 11-5. OADM Technology Deployment Comparison