# Presentation: 279779

## 
Optical Fiber Computer Interconnect:
The Simultaneous Multiprocessor 
Exchange (SOME)-Bus

Rhonda Kay Gaede

The University of Alabama in Huntsville
Department of Electrical and Computer Engineering
Huntsville, AL 35899

UAH

**Optical Fiber Computer Interconnect:**
**The Simultaneous Multiprocessor **
**Exchange (SOME)-Bus**
- Rhonda Kay Gaede
- The University of Alabama in Huntsville
- Department of Electrical and Computer Engineering
- Huntsville, AL 35899
- UAH

- Optical Interconnects for High Performance 	                           System Level Optical Interconnect
- Computing Workshop
- Oak Ridge, Tennessee   			                            November 8 & 9, 1999

## Disciplines

- Optical Engineering
- Electrical Engineering
- Computer Engineering

## Outline

- Computer Networks
- Electronic and Optical Technologies
- SOME-Bus Architecture
- Optics and Electronics Implementations
- Programming Support
- Future Work
- Conclusions

## Computer Networks

- Carries information between processors
- Influences computer system performance
- Many Variations
- Shared Bus
- Point-to-Point

## Shared Bus

- Single set of wire(s) used for data exchange, e.g. Ethernet segment
- Advantages
- Low wiring costs
- Direct connection between processors
- Disadvantages
- Limited communication bandwidth
- Arbitration overhead

## Point to Point

- Dedicated sets of wires used between pairs of processors
- Advantages
- Greater aggregate bandwidth
- Limited number of taps on channel
- Disadvantages
- More wiring
- Processors not directly connected

## Electronics Technology

- Advantages
- Dense digital VLSI circuitry
- Inexpensive fabrication
- Disadvantages
- Low fan-out of signals
- Low bandwidth per channel
- Point-to-point communication channels

## Optics Technology

- Advantages
- High fan-out of signals
- High bandwidth per channel
- Multi-cast communication channels
- Disadvantages
- Single point insertion
- Weak computing facilities

## SOME-Bus Architecture

- Exploits strengths of electronics and optics technologies
- One dedicated unidirectional channel per processor
- Each processor observes all channels
- Performance not limited by communication medium (fiber)

## Network Characteristics

## Optics Implementation

- Ribbon of multiple optical fibers
- Input coupler
- One per fiber
- Multiple wavelengths of light
- Multiple processor channels on single fiber
- Output coupler
- Multiple output couplers per fiber
- Bragg grating in fiber
- Wavelength specific
- Very small output per grating (~1%)
- Aligned with optical detectors

## SOME-Bus Implementation

## Electronics Interface

- Integrated Circuit (IC)
- Array of receivers
- Interface to Processor bus
- 128 1024 byte buffers and associated logic fit in a die size of 1.6 cm x .8 cm
- Receiver Functions
- Convert optical detector’s analog signal into digital signal
- Filters out uninteresting bus traffic
- Buffer data until processor bus available

## Programming Support

- Communication models
- Shared memory
- Message passing
- Simplified model for programming
- Direct connections between processors
- No interference on channel from other processors
- High-speed broadcast
- Fast barrier synchronization mechanism

## Factors Affecting Performance

- Bandwidth is an important concern, but not the only one
- Latency is critical in many computer operations such as
- Cache coherency messages
- Shared memory data exchanges
- Handshaking and synchronization signals
- Network flow control/resource allocation

## Future Work

- Form gratings in fibers and characterize footprint and selectivity
- Examine issues of inter-mixed analog and digital circuitry
- Build silicon v-groove devices to hold fibers for alignment with photodetectors
- Characterize performance for realistic program loads using simulations.
- Investigate whether diffractive optic elements are required, build if necessary
- Build and characterize 32 channel prototype

## Conclusions

- Bandwidth scales with the number of processors
- Network diameter = 1
- Supports shared memory, message passing and synchronization
- As technology matures, the system bandwidth will increase

## Receiver Front End Logic