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fiber101
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##Fiber
- Waveguide for light
- most commonly made of glass/silica, but can be plastic
- glass is low cost to produce and install
- very light, Flexible and high density
- can carry latge numbers of independant signals over the same strand, without interfaces, no regeneration required may require amplification.
- continue to evolve on existing fiber infra
- Parts of Fiber cable
- core - this is where light stays
- cladding - used to create reflection so light stays inside.
- Acceptance cone - angle for light to coming into core so it maintain reflection.
- Duplex vs Simplex
- Duplex
- one pair of fiber is used for each point to point link.
- one strand is used to transmit and other to receive.
- requires less expansive optical components to do so.
- Simplex
- requires expansive optical components to send/recieve signals in both directions.
- done where Fiber is expensive or constrained (eq riser capacity or long-haul etc)
- MultiMode
- wide core, allows for cheaper, less precisely focused and aimed light sources.
- Wide core causes Modal Dispersion that typically limit distances to ten to hunderd of meters.
- OM1/OM2 aka FDDI grade (orange fiber jackets), 62.5 micron for OM1 and 50 micron (um) core for OM2. starts to fail at 10 G Speeds
- OM3/OM4 aka Laser Optimized (aqua fiber jackets). Designed for 850 nm short reach laser, Supports 10g at 300-550m. 40/100G at 100-150M
- SingleMode
- Much smaller core, 8-10 micros, no distance limitation caused by modal dispersion
- supports upto 80KM without Amplification
- singlemode OS1 = tight buffered packaging for indoor use.
- singlemode OS2 = Losse buffered packaging for ducts.
- SMF-28 has the lowest attenuation.
#Basic Optical Terms and Concepts
- Optical power and Attenuation
- Light travels through fiber, some of its energy is lost due microscopic imprefections in the fiber or absorbed by dopants and converted to heat.
- Loss of intensity is called Attenuation
- typicall measured in db (decibels) which is 1/10th of a Bel
- db = logarithmic ratio between two values
- -10 db == 1/10th the signal, -20db is 1/100th the signal
- Easy to remember: +3bd is double and -3 db is half
- dBm : This is decibel relative to 1 milliwatt of power
- 0 dBm is 1 mW, 3 dBm is 2mW, -3 dBm is 0.5mW etc
#Wave Division Multiplexing (WDM)
- CWDM (anything not DWDM)
- 8 channels with 20 nm spacing, with low water peak fiber another 10 channels are possibles.
- can also be used to refer to a simple 1310/1550 nm mux.
- DWDM (Dense wave division multiplexing)
- from 100 GHz to 25 GHz giving you 40/48 channels in C-band to 160-192 channels in C-band
- 25 GHz is defunt
- 50GHz with 0.4nm spacing gives 80-96 Channels. Common for long haul and 100G systems.
- 100 GHz MUX = channel spacing [This is very simplied description since there are multiple other factors like passband etc]
NOTE: C Band has far lower attenuation and simple Erbium Doped Amplifiers (EDFAs) work.
- 10 GE had 4x 2.5 G channels
- 40 GE has LR4 (4x 10G)
- 100 GE has LR4 (4x 25G)
- WDM Networking
- Mux/DeMux : simple, passive (unpowered) device which combines/splits multiple colors of light to/from a single common fiber.
- Use Filter or Prism is used to filter specific colors.
- A complete system requires both Mux and a deMux for the TX and RX operations
- Fixed Optical Add/Drop Multiplexer (FOADM) : Passive device
- Selectively Adds and Drops certain WDM channels
- While muxes at major endpoint to break out all channels, OADMs are often used at mid-points within rings.
- ROADM aka Reconfigurable Optical Add/Drop Multiplexer
- This allows individual or multiple wavelengths carrying data channels to be added and/or dropped from a transport fiber without the need to convert the signals on all of the WDM channels to electronic signals and back again to optical signals.
- ROADM allows for remote configuration and reconfiguration.
- Optical Amplification
- Purely optical way to extend signal reach, no regenration.
- Booster Amplifler, Inline Amplifler and Pre-Amplifier
- EDFA (Erbium Dopen Fiber Amplifier): In this a piece of fiber is doped with Erbium ions.
- EDFA in addition to the intended boosting of signal, also generate noise.
- Raman Amplification: Plasma wave is sent in opposite direction of probe, enery is tranfered from Plasma wave to Probe which depletes pump and amplify probe. Pump is filtered out after aplify.
- EDFA and Raman can be used togather to reduce the Noice.
#OTN optical transport network (G.709)
- replacement for SONET/SDH
- implemented as a wrapper around other protcols
- Used for pure optical channels for high speed protocols. ex a single 100 GE service, delivered over a 100G wave.
- OTN can allow you to MUX many different single types.
#What we transmit over fiber
- Modulation
- Sending Analog waves, Amplitude shift keying = bright for a 1, dim for a 0
- requires direct detech system used for RX
- 10G and below are in Non-return to Zero (NRZ) based.
- Baud
- Rate at which you modulate a signal
- 10 Gbps means flashing bright/dim, 10 billion times/sec AKA 10GigaBaud
- Two states(bright/dim) means we represent 1 bit per symbol.
- Worked well for 10gigabaud rate, more parallel 10G links were build using DWDM.
- Coherent Optical Tech or Phase Shift Keying (PSK)
- Able to track phase chnages in optical signals
- Uses a concept called "local oscillator" AKA uses Laser on the recieive side of the signal.
- Gave true 100G signal, enabled high bandwidth for lang hauls.
#Will Laser burn my Eye
- Every single channel laser that can be connected to router is Class1 or 1m (even the long reach 200km+ optics)
- Class 1 or 1m is no issues
- Class 3 or class 4 will burn so avoid, normally found in optical applifies, raman amplifiers and dwdm equipment specially if you have 40 channels muxed into 1 - dont look at directly when though signal quickly disperses.
- use your phone camera if you really have to use it.
#Notes
- Clean fiber before plugging as the connector is pressed with great force and dust spec can damage optics .
- Speed at which light traffic inside fiber == speed of light/ refractive index
- ex: on SMF-28 with RI of 1.4679, 1ms per 62.5 miles of RTT, you will see more latency due to fiber uses scenic routes over long distance.