PDF of this tutorialAll-optical switches are made possible by a number of technologies that allow the managing and switching photonic signals without converting them into electronic signals. Only a couple of technologies appear ready to make the transition from the laboratory to the network, where they must support the basic feature set of a carrier-grade, scalable optical switches. Arguably, the leading technology for developing an economically viable, scalable all-optical, O-O-O switch is the 3D micro-electromechanical system (MEMS). 3D MEMS uses control mechanisms to tilt mirrors in multiple direction (3 dimensional).
Figure 3. Optical-Switch Technologies
An optical switch adds manageability to a DWDM node that could potentially grow to hundreds of channels. An O-O-O switch holds the promise of managing those light signals without converting the signals to electrical and then back again. This is especially attractive to those carriers operating large offices where up to 80% of the traffic is expected to pass through the office on its way to locations around the globe. MEMS currently affords the best chance of providing an all-optical switch matrix that can scale to the size needed to support a global communications network node with multiple fibers, each carrying hundreds of wavelengths.
Figure 4. 3D MEMS
The increased level of control enabled by MEMS technology can direct light to a higher number of ports with minimal impact on insertion loss. This is the key to supporting thousands of ports with a single stage device. The 3D MEMS–based O-O-O switches will be introduced in sizes ranging from 256 x 256 to 1000 x 1000 bi-directional port machines. In addition, encouraging research seems to show that 8000 x 8000 ports will be practical within the foreseeable future. The port count, however, is only one dimension to the scalability of an O-O-O switch. An O-O-O switch is also scalable in terms of throughput. A truly all-optical switch is bit-rate and protocol independent. The combination of thousands of ports and bit-rate independence results in a theoretically future-proof switch with unlimited scalability.
Some argue that a bit-rate and protocol independent switch encourages rapid deployment of new technologies such as 40 gigabits per second (Gbps) transport equipment. After all, a carrier does not have to worry about shortening the life span of an O-O-O switch by implementing new technology as subtending equipment.
In addition to aiding the scalability of an O-O-O switch, a bit-rate and protocol-independent switch theoretically improves the flexibility of a network. Flexibility can be improved because a carrier can offer a wavelength service and empower its customer to change the bit rate of the wavelength "at will" and without carrier intervention. While this type of service is already being offered in its simplest form—wavelength leasing—it has the future value of supporting optical virtual private networks (O—VPN) and managed- or shared-protection wavelength services.
In theory, a future-proof, scalable, flexible and manageable O-O-O switch meets the requirements for a new-generation optical switch. In the real world, however, a carrier must evaluate the pros and the cons of all possible options and then select the most economically viable solution.
All-Optical Challenges
While the benefits of O-O-O switches are clear, carriers must understand and consider the challenges/implications that may limit the adoption of all-optical switches in a long-haul core optical network. These challenges have hindered mass production of all-optical switches and limited deployment to less than a handful. A more in-depth look at some of these challenges will show why some experts don't expect wide-scale deployment of all-optical switches for several years.
Optical Fabric Insertion Loss
Optical switching fabrics can have losses ranging from 6 to 15 decibels (dB), depending on the size of the fabric, the switching architecture (single stage versus multi-stage), and the technology used to implement the switching function. A multi-stage fabric compounds the insertion loss challenge, because additional loss is encountered each time the stages are coupled together. The 3D MEMS–based switches can be implemented in a single-stage architecture to minimize insertion loss. However, even at the low end—6 dB—a carrier must be aware of the output level of the devices interfacing with the all-optical switch. Subtended equipment, such as DWDM or data routers, must have enough power to ensure that a signal is able to transverse an optical switch matrix. This could lead to the need for higher-power lasers on these devices, thereby increasing the cost burden of the surrounding equipment.
Network-Level Challenges of the All-Optical Switch
The problem of loss is compounded when an O-O-O switch is implemented in an all-optical network. An all-optical network is defined as one that does not use O-E-O conversion in the path of the traffic-bearing signal. Thus, a system consisting of DWDM and all-optical switches will not use transponders or 3R regenerators to mitigate the affects of optical impairments. Optical budget is only one of the considerations, which must be studied carefully before implementing an all-optical switch.
Prior to implementation, carriers must consider the many implications of an O-O-O switch, including physical impairments such as chromatic dispersion, polarization mode dispersion, non-linearities, polarization dependent degradations, wavelength division multiplexing (WDM) filter passband narrowing, component crosstalk, and amplifier noise accumulation.
As stated earlier, the next-generation network must not only be scaleable and flexible, but it also must be dynamic. A dynamic network will generally consist of optical switches deployed in a mesh architecture to support a flexible number of services, restoration paths, and fast point-and-click provisioning. A dynamic network with multiple restoration paths is not conducive to end-to-end optical-path engineering. It is just not practical at this time to engineer an all-optical system to handle all the possible network degradations for all possible provisioning or restoration paths.
In addition to mitigating the effects of physical impairments, carriers require multivendor interoperability and wavelength conversion. They are also unwilling to compromise on network-management functions that are available to them today. These include the following:
- Automatic topology discovery
- SONET–keep-alive generation
- Performance monitoring
- Connection verification
- Intra-office fault localization
- Bridging
