Optical Interconnects: Out of the Box Forever?

Dawei Huang, IEEE Journal of Selected Topics in Quantum Electronics, March/April 2003

Optical Interconnects: Out of the Box Forever?

Jeong-Min Lee([email protected])

High-Speed Circuits and Systems LAB.

2011-1 Special Topics in Optical Communications

mailto:[email protected]

High-Speed Circuits and Systems LAB. 2

Contents

1. Introduction

2. Computer Design (A ~ D)

3. Electrical and Optical Interconnects (A ~ F)

4. Scaling into the Future



Introduction

Recent research: Optical interconnects are a viable alternative electri-cal interconnects for board-to-board, chip-to-chip, and on-chip application

Overview of the performance scaling that has driven current computer design focus on architectural design and effects of these designs on in-terconnect implementation

Optical interconnects outperform electrical interconnects for long-distance applications

Advantages of optical interconnects: time of flight & BW density

Characterize what would be required for optical interconnects to displace wires at the backplane, board, and chip level.



A. Scaling Overview There are three key empirical-power law scaling relationships for computer

systems:1) You can have twice as many transistors for your next design (Moore’s law).2) A processor should have at least enough memory that it takes one second

to touch every word (Amdahl’s law).3) There are many more short wires than long wires (Rent’s rule).

Clock cycle: 1 us 333 ps DRAM density: 1 kb/chip 1 Gb/chip DRAM access time: 1.2 us 50 ns



B. The reason DRAM is slow

DRAM: single 1-T cell (an analog sample and hold), a bit is represented by charge on a capacitor

DRAM is slow the physics of the DRAM storage mechanism, the commod-ity nature of DRAM technology, and the complexity of the memory hierarchy Optical interconnect do not address these fundamental issues.



B. The reason DRAM is slow

The maximum number of bits per bit line is limited by the ratio Capaci-tance: storage node, bit line, input stage of the sense amplifier

When the charge in the storage capacitor is gated onto the bit line, the charge spreads by RC-limited diffusion, which results in access time de-pending on (the number of bits per bit line)2.

In summary, DRAM latency is a significant design constraint, limited by the device design optimization. Optical interconnects cannot solve this DRAM access problem.



C. Simple Machines Optical interconnects has focused on tradeoffs between bandwidth, power,

circuit area, and signal integrity. A large fraction of the work done by a computer merely produces memory

addresses and moves bits back and forth. 4 memory accesses: one to fetch the instruction, two to fetch the operands,

and one to write back the result. Cache: a small fast memory between the process & main memory A cache retains copies of recently fetched memory locations. If the proces-

sor should request a value already present in the cache, the local copy is quickly delivered and latency is reduced.



D. Architectural issues in multiprocessor sys-tems

To increase the size of a, switches are inserted in the path to accommodate additional point-to-point connections. Drawback: significant latency (additional switch stages)



D. Architectural issues in multiprocessor sys-tems Optical interconnects at the backplane level higher density optical

input/output (I/O) can be provided to a switch chip reduction in the number of switch stages reduce latency in large servers

Cache coherence protocol: significant factor for memory latency When large caches are placed close to each processor significant fraction of memory accesses can be satisfied with data from other processors’ caches, rather than from main memory



Modern multiprocessor server

SunFire 12 K–15K family



A. Circuit board wiring density

Two factors control the wiring density on a circuit board the pitch of the signal lines & via construction

The via holes may block half the available space for wiring on every layer of the board Additional wiring layers is required to make routing possible.

Alternative methods for forming vias ① Laminate boards may incorporate buried or blind vias (more expensive than

conventional boards, because of lower yield)② One alternative to laminated circuit boards is multilayer ceramic circuit

boards available with alumina and low temperature glass–ceramic bodies.



B. Electrical wiring limitations

To maintain signal integrity, the signal speed is limited by circuit board material losses, and wiring density on a circuit board is limited by noise control.

Dielectric loss in FR-4 lami-nate rises rapidly above 1 GHz larger than skin effect

At high data rate Reduce SNR & introduce timing er-ror timing margin ↓



B. Electrical wiring limitations

Roger 4000 higher BW density than FR4 (expensive) Signal processing techniques can be used to increase bandwidth preem-

phasis, equalization techniques, and multilevel signaling and coding Crosstalk: major source of noise that can limit wiring density



C. Bandwidth density to memory

While the use of optical interconnects will not help with memory ac-cess latency significantly help to relieve wiring congestion to memory.

DIMM socket produce a difficult routing situation under the sock-ets to route each memory connector contact to the memory con-troller spread out on multiple layers

Using optics instead of a circuit board free-space optical system would not have wiring congestion problems. (require adding optical I/O to each DIMM)



D. Backplanes

1) Electrical backplanes:① Conventional bus topologies: Use a traditional multidrop bus topology② Point-to-point connections: electronic switching to manage traffic

2) Optical backplane technologies and limitations:

① Reducing loss in wave-guides

② Assume reliable optical transmitters and receivers

③ Optical connectors is denser than electrical ones



D. Backplanes

3) Optical backplane technology disruption:– Freedom from wiring congestion In exchange for the optical

components part of the backplane multiple routing chips and some protocol overhead could be eliminated.

– An example of a unique optical solution is global clock distribu-tion in backplane design.



E. Chip-to-chip connection

Processor chip: 5000 connections power (2/3) + high speed signal① The processor is mounted face down in a package transforms the 0.25-mm typical

pitch of the solder ball connections on the chip② Several processors, switches, and cache memory will be located on a single multi-

layer ceramic circuit board

Alignment of the ICs to the optical system remains an open issue. ① Multimode fiber passive alignment② Single-mode fiber active alignment

The materials used for supporting an array of chips that are optically intercon-nected must be selected very carefully Thermal expansion & residual stress

Important consideration is the operating temperature of the optical devices (VCSEL: temperature sensitivity)



F. On-chip wiring

1) Local vs. Global Local wire: connect TR to gates, lithography resolution Global wire: connect functions across the chip, edge length of the chip Thickness of both wire & dielectric layers

controlled by deposition technology (~1um)

2) RC or resistance-inductance-capacitance propagation mode RC time improved with the (lithographic scaling)2

C: conductor dimension, dielectric constant, short wire cap ↓ R: ratio of length to cross section (W, t lithography dimension) Al Cu: less resistance

3) Optical interconnect: wiring, clock distribution Potential of carrying very high-frequency signal



Scaling into the future

In order for the pricing of optical backplanes to become competitive with electrical alternatives standard would need to be established.

Memory: latency is due to the speed of the DRAM component itself, and not due to time of flight limitations Little that optics can do to speed up access to memory

Chip-to-chip optical communication is the easiest of the short dis-tance applications

Recent work on photonic bandgap materials has renewed interest in hybrid optical-electrical interconnects at this scale,


Optical Interconnects: Out of the Box Forever?

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