Optical switches benefit ATM network capacities, efficiencies and costs

Installed in asynchronous transfer mode networks, optical switches and crossconnect systems confer enhancements in network disaster recovery, testing and management

ken garrett

astarte fiber networks inc.

Important issues in the physical management of fiber-optic networks in asynchronous transfer mode environments involve the use of optical switches and optical crossconnect systems and how they can provide improved network utilization and reliability in the areas of disaster recovery, testing and network management.

As the preferred ATM transmission medium, fiber-optic networks must follow the same physical management routines common to copper-based networks. Network survivability, route diversity, multiple local area network interconnect and test/monitoring access abilities must be preplanned by network providers.

Most high-speed protocol algorithms address these issues from a logical perspective, but are limited by the properties of the transmission medium. For example, if a cable is cut, no logical network recovery plan will work if no physical recovery path exists to reroute transmissions. If an alternate route exists, then such issues as throughput testing, loop backs and fault isolation must be addressed to restore the primary path.

Fiber-optic transmission equipment can be used to optimize and, in some cases, enhance ATM networking capabilities. Equipment such as optical switches, optical crossconnect systems, wavelength-division multiplexers and fiber-optic test equipment can augment electronic switching and monitoring routines to provide physical layer network management. These systems can compress multiple wavelength signals to allow bidirectional and duplex transmission abilities over a single fiber. The results include increased network capacities, reduced costs and improved network efficiency.

Optical switches

Optical switches (such as 1ٴ, 2ٴ or 1¥n, where n is the number of ports) and optical crossconnect systems do not electronically convert fiber-optic transmissions into electronic signals and then reconvert them back to the optical domain. Instead, they provide multiple wavelength transmissions, protocol transparency, unlimited bandwidth and bidirectional communications. These characteristics result in optimal solutions for managing high-speed optical networks carrying ATM, fiber distributed data interface, enterprise systems connection or Fibre Channel transmissions.

Optical crossconnect systems provide both route diversity, as well as flexibility in network recovery. An optical crossconnect system offers all the features of 1ٴ, 2ٴ or 1¥n optical switches with additional features such as test access and remote control abilities. Switching is accomplished by directing the optical transmissions from any input fiber to any output fiber within the system. A servo-control algorithm is employed to ensure connectivity. This active switching control eliminates environmental issues such as temperature and vibration effects on fiber alignment.

An optical crossconnect system can switch single, dual or multiple fibers at the same time. This capability enables the system to operate in ATM and other data communications environments where transmit and receive fibers must be switched simultaneously.

Disaster recovery

Optical switches used in ATM networks are able to store and implement recovery routines in the event of a network failure. These routines normally involve reprogramming transmissions to standby ports within the switch that are dedicated for disaster recovery. The number of dedicated ports varies with the type and sensitivity of information being transmitted. The problem facing network managers concerns the costs involved in providing this service.

Disaster recovery routines normally utilize a path level recovery plan where a dedicated path and port of the ATM switch is reserved in the event of a network or equipment failure. Optical switches can be employed to offer this type of recovery while minimizing port utilization to the ATM switches. For example, 1ٴ optical switches can be used to route optical transmissions over a redundant path in the event of a network failure.

Some 1ٴ or 2ٴ switches use electromagnetic positioning via motors or solenoids to perform switching. They function similar to an electronic relay switch where only one path is active at any time. This operation offers an alternative transmission route without having to dedicate an additional ATM port for recovery.

If path level recovery methods are used on a one-to-one basis, the cost for recovery can be high. Currently, high-speed ATM port costs range from $2500 to $5000. ATM path level recovery without optical switches requires a dedicated port, fiber and transmission equipment on the alternative route. If optical switches are used, the port costs are reduced, but the redundant route must still be in place. If the transmission path covers a long distance, then both routes must have dedicated optical signal regenerators and terminating equipment, which adds to the network expense. The result in both scenarios is under-utilized ATM switch capacity as well as increased costs in providing network recovery routines.

Some 1ٴ or 2ٴ optical switches protect against network failures but may not offer a redundant route in the event of an equipment or port failure on the ATM switch. These types of optical switches are also limited in their ability to be remotely controlled and monitored either by an operator or from a network management center.

Some manufacturers offer systems that control a group of 1ٴ or 2ٴ switches. These systems address the needs of networks comprising multiple fibers that require remote control. However, they are limited in their ability to provide test access and other fiber-optic cable management capabilities.

Optical crossconnect systems can utilize preconfigured switching routines based upon the type of network recovery required. These systems are passive to network transmissions and enable a 1+n recovery capability. This type of configuration minimizes transmission equipment costs and protects against both equipment and network failures. The optical crossconnect systems can also be monitored and managed via standard network management protocols or network control interfaces.

Network testing

Remote test and monitoring of network performance can also be accomplished through the use of optical switches, network analyzers and optical time-domain reflectometers. A common testing routine involves the use of a 1¥n optical switch combined with an OTDR to form a test and measurement system. Some 1¥n switches operate similarly to 1ٴ/2ٴ switches using electromagnetic positioning of a single fiber that can be connected to an array of possible target fibers. As with 1ٴ/2ٴ switching, only one possible communications path is active at any time. In addition, these systems offer remote and local control access into the physical network while maintaining ATM connections.

Some 1¥n switches can be employed to perform testing on fibers that are not being used for transmission within a fiber bundle to monitor the integrity of the cable. A limitation of this implementation is how to provide access to the fiber carrying an ATM signal without blocking the transmission.

Several new test systems have been developed that incorporate wavelength-division multiplexing with fiber-optic test equipment. These systems generate different wavelengths through a fiber to collect performance characteristics without affecting data throughput. In this scenario, test fibers are tapped into the transmission fiber to insert test and monitoring equipment. A wavelength-division multiplexer is then employed to couple the test signal from the OTDR access onto the fiber. This setup provides the capability to test the integrity of the fiber by using a different wavelength from the one used for ATM transmissions.

These measurement applications provide test access to monitor the status of the physical path. They cannot, however, be used to check ATM data integrity on an end-to-end basis or furnish both test access and disaster recovery routines. To provide these capabilities, 1¥n optical switches can be interconnected to develop a matrix switch capability. This configuration offers improved network management capabilities, but might be limited in its ability to expand to meet network growth. For example, an 8ٺ matrix would consist of eight (1ٺ) optical switches on one side of the matrix all physically spliced or attached to eight (1ٺ) switches on the other side of the matrix. If growth beyond 8ٺ is required, replacement of existing switches or additional splicing of new fibers and switches into the existing matrix may be required. u

Ken Garrett is vice president for sales and marketing at Astarte Fiber Networks Inc. in Boulder, CO.