Collective Communications on a Reconfigurable Optical Interconnect

In this section, we introduce some techniques for implementing the beam routing, and collision avoidance in the recievers. Each node has a beam router, such as a computer generated hologram or a deformable mirror, which is capable of redirecting k (or 1 in the single-port model) optical beams concurrently. The reconfiguration (programming the computer generated hologram, or deforming the mirror) takes d time steps (d is an integer) while the actual transmission takes only one step. By current technology, programming a computer generated hologram takes a few microseconds [2]. Each node has a transmitter for sending an optical beam toward the beam router to be directed or distributed to the receivers at the other nodes. Also, each node has a number of receivers at its input ports. Some of these input ports may be used for collective communications while others can be used for other pair-wise communications. The number of receivers can also be degenerated to 1 if, for example, more than one operation (either collective or pair-wise communication) is supported. Path-setup phase can be done by sending an encoded light beam to the beam router to reprogram the computer generated hologram, or to deform the mirror so that the actual message can be delivered to the destination(s) directly. We propose two alternative ways for path setup phase in the circuit switching between any two nodes. A node that wants to reconfigure its optical interconnect may include the destination address in the header of its message. The router at the computer generated hologram or the deformable mirror upon receiving the message stores it in a buffer and then configures its output links so that it can forward the message to the destination node(s). This approach needs a buffer for the entire message at each beam router which is of high cost. The better approach may be to send a single beam including only the destination address to the beam router for path setup phase. Then, d steps later, the second beam containing the actual message can be sent to the already configured beam router. Several beams may arrive at a destination node at the same time. If we want to assign only one fixed wavelength to these beams, resembling the address of the receiving node (that is the transmitters are tunable, and the receivers are fixed and tuned to the wavelengths resembling their node address), then there would be a …