Case Study

When deploying a fiber optic network, SONET and DWDM each offer advantages to the network service provider. Based on bit rate alone, DWDM has a fourfold advantage even over TDM (STM 64/OC 192). To compare the two technologies, however, it is necessary to review and outline the ideal technological solution for expanding network capacity. This review and comparison must be performed in a broad sense, recognizing that there are instances in which TDM may offer a better solution than DWDM.

When deciding to deploy SONET or DWDM within a network service provider backbone, the following items need to be considered:

• Compatibility with Fiber Plant The majority of the legacy fiber plant cannot support high bit-rate TDM; it is expensive and can limit the ability of network service providers to migrate to the greater bandwidth available through DWDM at STM 16/OC 48 rates.

  DWDM implementations can use existing fiber, with necessary upgrades being accomplished by the swapping out of the LTE on both ends of the fiber optic cable.

• Transparency and Interoperability The solution must be vendor independent, vendor interoperable, and conform to industry standards. The solution also must be capable of supporting mixed protocols and signal formats.

  DWDM systems provide this transparency and can be used with any SONET/SDH bit rates, as well as with asynchronous/PDH protocols; in essence, DWDM can carry SONET/SDH traffic.

• Migration and Provisioning Strategy The solution must meet the following requirements:

  - Offer the network service provider the capability to expand the network with minimal cost

  - Capable of supporting differing bit rates and have channel upgrade capability

  TDM systems are nearing their technological barriers with the deployment of STM 64/OC 192. These TDM systems can represent a practical limit, exceeded only by DWDM.

• Network Management The solution must meet international network management standards, interface with the network service provider's existing operating system, and provide direct connection for all of the network elements for performance monitoring, fault identification, fault isolation, and remedial action.

  SONET implementations provide automatic failover and protection against facilities (fiber) based network outages. The only instance SONET will fail during a network outage is if the working and protecting rings have both failed and/or are damaged in an outage (such as that caused by multiple fiber cuts).

• Technical Constraints The systems deployed must be able to resolve some of the technical issues present in current lightwave systems: signal dispersion compensation, filtering and channel cross talk, nonlinear four-wave mixing, and physical equipment density.

  There is no clear technical advantage to either a SONET or DWDM implementation. These constraints can be improved only through continued research and development (R&D) by various vendors and technology labs.