Transcript Background on Gigabit Ethernet

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 precleaving 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
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Photoconductors
Avalanche photodiodes
PIN photodiodes
MSM photodiodes
Photoconductors
• Operation based on varying conduction
• Many important factors affecting bandwidth
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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
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Carrier multiplication
Associated gain
Time constant associated with avalanche
Bandwidth penalty
PIN Photodiode
• PIN
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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:
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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.