John Barthel and Tom Chuh
Using available matrix-based optical switches, equipment manufacturers are constructing wavelength-selective crossconnects (WSXCs) for a future generation of all-optical networks. The WSXC is vital to the implementation of wavelength routing and will enable service providers to maintain services dynamically and to migrate to transparent network architectures. Such networks will be constructed with transparent optical crossconnects, where the applied signal will remain in the optical domain and not undergo optical-to-electrical-to-optical conversion.
WSXC fabric using matrix optical switches
A transparent optical-crossconnect switch can be fabricated today using the WSXC architecture for the switching fabric (see Fig. 1). The WSXC takes a layered approach because each wavelength is treated as a complete network path and is provided with a unique wavelength-specific switch. Each switch, at a minimum, needs to have as many ports as there are fibers at the node. Additional ports are often provided for further functionality, such as adding and dropping channels for local traffic and routing to signal-monitoring and conditioning circuits.
FIGURE 1. A transparent optical-crossconnect switch can be fabricated with available technology by using the WSXC architecture for the switching fabric.
Recent technological innovations have made matrix optical switches readily available in small-port-count configurations with the flexibility needed for WSXC design. True matrix switches, also referred to as N x N switches, allow any input channel to be switched to any output channel. The N x N switches typically use an N2 type of switching fabric. For example, in an 8 x 8 MEMS-based switch, the core is an array of 64 mirrors that are moved into and out of the beam path. An optical connection is established when a mirror moves into the beam path, coupling the signal between any of the two fibers. Each optical switch is a miniature transparent optical crossconnect and each individual wavelength is treated as a complete network path. The WSXC architecture, however, reduces the need for extremely complex "any-to-any" software-routing algorithms and, with sufficient system capacity, may avoid the need for wavelength conversions.
Nevertheless, this design does give up some flexibility and overall network efficiency. Routing and protection requirements for a dedicated wavelength limit the capacity that can be provisioned onto a specific wavelength. But this limitation can also be viewed as a benefit, because the signal stays within the specified wavelength from point to point and all potential routes and protection paths can be tested prior to going live with traffic.
The WSXC architecture is also very scalable and can be deployed with only a partial number of the potential wavelengths active. System capacity can then be extended as wavelengths are incrementally turned on. Adding capacity only requires adding an optical switching card to handle the new wavelength and updating the software configuration to acknowledge the new pathway. Therefore, carriers can initially invest in the basic network, and own a "pay as you grow" system.
Once the fabric is in place, the wavelength-division multiplexing (WDM) system can also be extended as new technology emerges. As higher-density channel-spacing WDM components are introduced, the original switches can be reused and new matrix optical switches are added to accommodate the wavelengths. Optical-switch fabrics enable a system to grow from a 40-channel system today to an 80- or 160-channel system in the future. In addition, since most matrix switches are wavelength- and protocol-independent, the switching fabric can also be reused as the protocol changes and the data rate increases. This WSXC design results in an optical-switch fabric that is long-lived, avoids "fork-lift" upgrades, and minimizes the number of switching cards required for redundancy and spares.
Matrix optical-switch configuration
At the wavelength level, the system designer can dynamically configure the channels to be routed to any destination using a matrix optical switch. The flexible design configuration of the matrix switch reveals several interesting facts. First, because the switch is bidirectional, any of the ports to the switch can be specified for signal inputs. Therefore, for four-fiber networks, the channels can be grouped by destination or by direction, whichever works best for the system fiber management. Second, most matrix optical switches are also nonblocking, meaning that the signals on one channel do not interfere with any other signal, channel configuration, or protocol. Third and perhaps most important, multiple switching-fabric configurations can be used as the network grows to full capacity. This enables the equipment designer to control the cost of the initial system hardware and efficiently use all the available switch paths. For example, in a four-fiber ring network, a single 8 x 8 switch can be used as the switching fabric for two separate wavelengths, reducing the overall number of switches required. The switch is simply configured with a matrix command that activates the mirrors required to connect the input ports to the desired output ports (see Fig. 2).
As the network expands, there will be additional fiber drops and new connections within each node. The equipment designer can then reconfigure the optical switches as single-wavelength crossconnects for 6 or 8 channels, and the same switches are efficiently reused. The future generations can be built using the same switching core. This flexibility of this optical-layer configuration facilitates creativity among network equipment engineers. It allows them to build effective and efficient system hardware, while enabling unique solutions for their company and dynamic networks for their customers.
Implementing these dynamic next-generation crossconnects to gain the network benefits today is a challenge, however, because the technology is not 100% available. The challenge to the equipment designers and carriers is to provide a migration to the future while using existing hardware. The WSXC architecture is one way to achieve this; matrix optical switches are the core enabling technology and are available from multiple sources.
John Barthel and Tom Chuh are product marketing managers at Onix Microsystems Inc., 4138 Lakeside Dr., Richmond, CA 94308. Tel: 510- 669-2020; e-mai: [email protected].