Background on Gigabit Ethernet

ECE 4006 C

G3: Karen Cano, Scott Henderson, Di Qian

Dec, 5 2002

Ethernet History (Timeline)

• 1973 – (2.94Mbps) First developed at Xerox’s Palo Alto Lab (Robert Metcalfe and David Boggs)

• 1979 - (10Mbps) Improvement by DEC, Intel and Xerox. The DIX standard. Thick Ethernet System

• 1983 - Formally standardized as IEEE 802.3

Timeline (con’t)

• 1983–1989 – Improvements on bus topology and transmission distance.

• 1990 – version IEEE 802.3i, 10Base-T technology.

• 1995 - (100Mbps) version IEEE 802.3u, also call “Fast Ethernet”.

Timeline (con’t)

• 1998 – (1 Gbps) version IEEE 802.3z, fiber optics; and IEEE 802.3ab, twisted pair. Also know as “Gigabit Ethernet”.

• Present – (10 Gbps) standard completed in 2002.

Project Tasks

• 1. Research on the transmitting and receiving modules.

• 2. Examine the testing board • 3. Search for the components• 4. Testing the evaluation board with

purchased components• 5. Connecting the purchased components

with parts from other groups.

Project Goal

• Duplicate the data transmitting and receiving module functionality of the Gigabit Ethernet technology with purchased components that provide optimum performance at a minimum price.

Possible Solutions• Transmitting module (laser source)

– VCSEL

• Receiving module (Photo-detector) – PIN photodiode

• Other Specs: - SC connectorized (optical)- SMA connectorized (electrical)- 850nm

- Multimode (fiber) - relatively low cost

Laser Basics• What is a Laser?

– Light Amplification by Stimulated Emission of Radiation

• How? 1) Electrons in low-energy levels bumped into high levels by injection of energy

2) When an electron drops to a lower energy level, excess energy is given off as light.

VCSELs

• Vertical Cavity Surface Emitting Lasers

• Physical makeup– Bragg mirrors– Active region

• Fabrication techniques– Molecular beam epitaxy– Vapor phase epitaxy

VCSELs

• In EELs no pre-cleaving tests can be performed, testing VCSELs is much cheaper

• Less current required for VCSELs

• Output beam easier couple into fiber and much less divergent than EELs

• Smaller and faster than EELs

VCSELs vs. EELs

• Edge Emitting Lasers - give out their light from the sides or edges, therefore no pre-cleaving tests can be performed

• Since VCSELs emit light from the top and bottom, they do not have this problem. Testing them is much cheaper

Interesting Facts

• In a typical VCSEL, as many as 60 individual semiconductor layers are stacked within a structure 10 microns thick.

• 20,000 individual laser die can be fabricated on a single 3 inch wafer.

Multimode

• Multimode- light is injected into the core and can travel many paths through the cable (i.e. rattling in a tube).

• Each path is slightly different in length, so the time variance this causes, spreads pulses of data out and limits the bandwidth.

Singlemode

• Fiber has such a narrow core that light takes one path only through the glass.

• Not limited to modal-bandwidth.

• Very small amount of pulse-spreading is consequential only in Gigabit speed applications.

Photodetectors

• Necessary for light pulse detection

• Wide variety of of types – Photoconductors– Avalanche photodiodes– PIN photodiodes– MSM photodiodes

Photoconductors

• Operation based on varying conduction

• Many important factors affecting bandwidth– Transit time– Surface area of photon acceptor region– Noise ratio (Johnson noise)– Quantum efficiency

Avalanche Photodiodes

• Exemplify the “gain-bandwidth” tradeoff

• Use the p-n junction model to operate

• Take advantage of the avalanche effect– Carrier multiplication– Associated gain– Time constant associated with avalanche– Bandwidth penalty

PIN Photodiode

• PIN– Reason for name– Doped region, undoped region, doped region– Unity gain– Functions under reverse bias

• Most important parameter for operation– Transit time

Bandwidth vs. Depletion Width

• Transit time– Time for subatomic

particle to get from one electrode to the other

• Based on quickest, typically electron– e- mobility > h+ mobility

• Capacitance limited

Transit Time (continued)

• Dependence on intrinsic region length

• Minimizing this region

• High bandwidth applications

MSM Photodiode

• Metal-Semiconductor-Metal– Associated work functions– Atomic level metal-semiconductor marriage

• High speed (up to 100GHz)

• Majority carrier devices

• Not developed for Gigabit Ethernet on scale as large as PIN

Concluding, thus far…..

• Obvious choices for devices:– VCSEL@850nm

– PIN photodiode w/ acceptable bandwidth

– Multimode fiber

– SC optical connectors

– SMA electrical connectors

• Gigabit Ethernet is a popular application• If you are buying less than five-million devices

then be prepared to stand at the end of the line.