Imagine a telecom network where an optical network can be set up and torn down in an instant without any human intervention. An optical burst switching (OBS) protocol at work at the Microsystems Computer North Carolina Research Development Institute (MCNC-RDI) in Research Triangle Park, North Carolina, does just that.

OBS combines the best features of optical circuit switching and optical packet switching. An OBS network can switch variable-sized data bursts instead of individual data packets. In an OBS network, transmission of data bursts can begin even before those bursts are completely formed. These features of the OBS networks are similar to an optical circuit-switched network. Like an optical packet-switching network, an OBS network can dynamically control system resources, assigning wavelengths of optical fiber to individual data bursts only when that user needs to transmit data. Unlike some optical packetswitched networks, an OBS network does not require optical buffers.

The MCNC-RDI has developed a NASA-funded OBS protocol implementation, called JIT (Just-In-Time), which recently achieved successful testing in an ATDnet (Advanced Technology Demonstration network) testbed. Established by the Defense Advanced Research Projects Agency (DARPA) for demonstrating advanced networking technology, the all-optical ATDnet runs at 2.5 Gbps through six sites using eight wavelengths and wavelength division multiplexing (WDM) switches. The testbed included applications in multiple areas like optical networking, network security, and networked information systems.

Technology Overview
WDM is a method of transmitting data from different sources over the same fiber-optic link at the same time; each data channel is carried on its own unique wavelength. The result is a link with an aggregate bandwidth that increases with the number of wavelengths employed. In this way, WDM technology can maximize the use of the available fiber-optic infrastructure – what would normally require two or more fiber links will now require only one.

WDM technologies primarily differ in the number of available channels. Coarse wave division multiplexing (CWDM) combines as many as 16 wavelengths onto a single fiber; dense wave division multiplexing (DWDM) combines as many as 64 wavelengths onto a single fiber.

With DWDM technology, the wavelengths are closer together than CWDM, meaning that transponders are generally more complex and expensive than CWDM. However, with DWDM, the advantage is a much higher density of wavelengths, and also longer distance. DWDM is emerging as a preferred solution for providing scalable and efficient optical networking technologies of the future.

The key objective of the hardware-based OBS protocol implementation is to dynamically manage commercially available WDM switches. An OBS network comprises OBS network controllers and clients with OBS network interface cards (NICs). OBS network controllers direct the optical data bursts received from a source-client OBS NIC to a destination-client OBS NIC.

Advances in Xilinx FPGA technology have made it possible for the MCNC-RDI to build a NIC that implements the JIT signaling protocol for an OBS network. The OBS NIC uses DWDM technology to transmit and receive data optically on specific wavelengths and is capable of handling data rates as high as 1.25 Gbps. The NIC card can be tuned dynamically to as many as eight different DWDM wavelengths.

In the JIT protocol, a control packet reserves a wavelength channel in the network for a period of time L equal to the burst length, starting at the expected arrival time R (this can be adjusted by the number of hops that a burst needs to travel and the processing time at each intermediate node).

If the reservation is successful, the control packet adjusts the offset time for the next hop and forwards it on. If the reservation is not successful, the burst will be blocked and the packet will be discarded. Because JIT is a one-way reservation protocol, buffering does not occur at the node level, thus reducing any latency. Implementation of JIT with an efficient scheduling algorithm can further decrease the probability of burst loss.

The JIT protocol uses a SETUP message to announce a burst in the OBS network. Each optical burst of data, comprising some number of contiguous packets destined for a specific destination, is sent immediately after the node receives a SETUP ACK from the ingress OBS node. An out-of-band SETUP message is sent across all switches before this step to prepare all path switches for the burst data. OBS does not use any optical buffering or packet parsing. For a long burst, a KEEPALIVE message may be required to keep all switches in active state. The JIT signaling scheme is shown in Figure 1.

The Role of the FPGA
The development of the OBS NIC was enabled by the availability of integrated high-speed multi-gigabit RocketIO™ transceivers in the Virtex-II Pro™ FPGA, allowing high-speed data streams (1-10 Gbps) to directly reach the core of the FPGA for processing. Dense FPGA logic available in the Virtex-II Pro FPGA facilitates implementation of complex state machines of the JIT protocol. The availability of embedded IBM™ PowerPC™ 405 processors in the Virtex-II Pro FPGA allows implementation of complex scheduling algorithms and timers associated with the JIT protocol.

The OBS NIC contains a Virtex-II Pro XC2VP20 FPGA. Three Gigabit Ethernet channels are used in this implementation of the OBS NIC. The first channel on the OBS NIC connects to an off-the-shelf Gigabit Ethernet card plugged into the host. This channel carries data and host messages between the OBS NIC and the host. The second channel is for signaling and connects to the OBS network controller; it carries the JIT OBS signaling messages. The third channel is used as the data channel and is connected to the optical front-end card.

The optical front-end card consists of an optical tunable transmitter and receiver. The OBS NIC generates the tuning commands for the laser and optical receivers on the optical front-end card. Figure 2 illustrates the architecture of the OBS NIC.

The Virtex-II Pro FPGA on the OBS NIC uses a PCS/PMA core and a MAC layer to connect the external gigabit channels to the JIT engine.

The JIT engine implements the JIT OBS protocol in the OBS NIC. Functionalities for both the source and destination state machines of the JIT OBS client are implemented in the JIT engine. The JIT engine processes three kinds of messages – messages from the host, signaling messages from the network, and internally generated timing messages.

The JIT engine uses two functional state machines (FSM): the scheduling FSM, using a round-robin scheme, picks up a message from one of the three message queues (for different types of messages) and dispatches them for further processing, while the processing FSM is responsible for taking a message and processing that message. Several processing sub-modules can be activated by processing FSM as needed, such as a hashing module or a state machine module. Figure 3 diagrams the processing of messages in the JIT engine.

Conclusion
We believe that communications will be bi-modal within the next 25 years. All land lines will be optically based, with optical access to the user or device that is a client of the network. All backbone connections will be across optical trunks.

Networking will be predominantly implemented in the “optical layer,” with little or no additional layering above it. Optical networks will be mostly a transparent transport media for applications. To meet the increasing demands of bandwidth and cost reduction, several technologies in the optical communications paradigm have been under intensive research.

Just-In-Time signaling applied to the optical burst switching paradigm has the promise of being able to provide either circuit- or packet-switched services. JIT OBS implements the best of optical circuit switching and optical packet switching but avoids their shortcomings. JIT signaling aims to better utilize the variable parameters that can exist within both an optical and a wireless network, such as frequency availability and data-rate differences.

For more information on the research conducted by MCNC-RDI in the field of optical networks, visit www.mcnc-rdi.org.

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