Background on Gigabit Ethernet

1
Background on Gigabit Ethernet

• ECE 4006 C
• G3 Karen Cano, Scott Henderson, Di Qian
• Dec, 5 2002

2
Ethernet History (Timeline)

• 1973 (2.94Mbps) First developed at Xeroxs 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

3
Timeline (cont)

• 19831989 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.

4
Timeline (cont)

• 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.

5
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.

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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.

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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

8
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.

9
VCSELs

• Vertical Cavity Surface Emitting Lasers
• Physical makeup
• Bragg mirrors
• Active region
• Fabrication techniques
• Molecular beam epitaxy
• Vapor phase epitaxy

10
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

11
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

12
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.

13
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.

14
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.

15
Photodetectors

• Necessary for light pulse detection
• Wide variety of of types
• Photoconductors
• Avalanche photodiodes
• PIN photodiodes
• MSM photodiodes

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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

17
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

18
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

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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 gt h mobility
• Capacitance limited

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Transit Time (continued)

• Dependence on intrinsic region length
• Minimizing this region
• High bandwidth applications

21
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

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Concluding, thus far..

• Obvious choices for devices
• VCSEL_at_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.