OCAM7 Flashcards
What is WDM?
Wavelength Division Multiplexing allows for the transport of multiple data streams, each on a different wavelength, on a single fibre
What is DWDM?
Dense WDM - a physical transport technology with dense referring to the closing spacing between wavelengths. DWDM is based on 194THz as the centre frequency, which corresponds to 1550nm (the C-band). There can be unto 100+ wavelengths in a DWDM system and with amplification can span distances up to 1000s of Kms
What is CWDM?
Coarse wave Division Multiplexing - supports up to 18 widely spaced channels at 20nm. Is much less expensive that DWDM as doesn’t require complicated filters and the laser tolerances are less strict. However, can only cover spans of 80km and so not supported in OC transport networks
What is a client card?
An optical interface unit that interfaces customer protocol and customer bit rate over customer-side wavelengths. Client cards will interface the client wavelength onto a DWDM network
Bit rate can range from 100mps-100Gbps
What is a line card?
An optical interface unit with ports for transmitting and recovering signals. Line cards face the DWDM network and have fixed or a flexed protocol and bit rate. Some cards may support a specific wavelength or be tunable.. Other cards ay also have client-side optics known as transponder cards.
Bit rate can range from 1-400Gbps
What are the characteristics of a card?
1) Protocol and bit rate support for the client signal
2) DWDM wavelength supported for the card
3) Distance supported by the transmitter on the card
4) Ability to support wavelength selectivity
What is a band?
A subset of wavelengths, grouped together to take advantage of the fixed filters to support bulk add/drop multiplexing
What is a network element?
A network element or chassis are defined by the requirements of the particular network where they are located and their placement on that network. Their function is defined by the cards they contain. NEs are connected to create an optical transport network
What are the minimum components of a NE?
1) Card cage and or chassis
2) Power supply or input
3) Fans
4) Visual alarm and power indicators
5) Management interface card
6) Client and/or network facing cards (line)
What defines where DWDM network components are placed?
1) Adding or dropping customer signals at desired locations
2) The required number of customer channels
3) The physical distance between two sites
4) Operating over the existing fibre plant between provider sites
5) Future growth requirements
6) Ease of manageability
These components may be serrate drives or combined onto one card
What do DWDM components include?
1) Line cards
2) fliters to add/drop or pass through wavelengths
3) Fibre plant
4) Amplifiers
What is an optical filter?
Optical filters allow for the multiplexing of wavelengths onto a single fibre, allowing for capacity gains. However, the also add optical loss and are power limited.
The pass through traffic of a filter is a band rather than a single channel
What is a fixed filter?
An add/drop module that supports a subset of wavelength grid for flexibility and cost savings. This filter type is passive so no electrical power is needed. They are deployed in pairs to facilitate channels near-end add/drop to far-end add/drop. They pass through all incoming traffic that is not dropped locally.
What is an OMX?
Optical Module is a fixed filter consisting of multiple optical filters packaged together that adds/drops only the wavelengths that the individual filter is tuned to add/drop
Properties of Bragg Grating fixed filter
Bragg gating is an optical component packaged in an optical filter containing:
1) One band add and one band drop filter
2) One channel multiplex and one channel demultiplex filter
Hundreds of gratings are fixed to a specific range of ITU wavelengths.
Supporting wavelength range must match DWDM line optics
Each Grating reflects back some of the specified wavelength and allows the other to pass through
Cheaper filters add more loss and use more real estate
What is an optical circulator?
Can be integrated with Bragg grating to select different wavelengths to be dropped or demultiplexed at different ports. They pass light thought filters that shift the angle of the light, preventing reflection back to the previous input
What are Arrayed Waveguide Grating Fixed Filters?
AWGs do not have a bandpass component, instead all wavelengths are dropped/added. This allows them to provide a very high capacity with a fixed loss. Very well suited to deployments with Wavelength Selective Swithes (WSS) to create flexible Reconfiguration Optical Add/Drop Multiplexer (ROADM) sites
What are star couplers?
They are placed at the input and output of AWG filters to split and combine light. Each guide is a different length so applied differing phase shift in such a way that the output channel receives only the light of a certain wavelength
Properties of a DWDM Colourless Fliter
1) Splitter with an amplifer
2) Optical splitters/couplers packages with Erbium Doped Fibre Amplifiers (EDFAs)
3) Require tunable transmitters and receivers on specialised colourless optics
4) Number of channels supported is less than in a standard DWDM deployment
What are optical transceivers?
Pluggable devices that both transmit and receive while supporting different rates
What are the types of transceivers?
1) Small Form Factor Pluggable (SFP) - 4.25Gbps
2) Small Form Factor Pluggable Plus (SFP+) - 16Gbps
3) 10Gbps Form Factor Pluggable (XFP)
4) 100Gbps Form Factor Pluggable (CFP) - 40 or 100Gbps
5) CFP2 - half the size of CFP
6) CFP4
What is a Wavelength Selective Switch?
The heart of the the ROADM as it enables an operator to direct a wavelength or band to any outbound fibre on any port. Can be deployed as a card
What is an optical amplifier?
Amplifies are optical components that can provide signal gain. They do not regenerate the signal so no optical-electrical-optical conversion is required. They have the ability to amplify signals across a defined range of the EMF spectrum. One amplifier can provide gain for many/all the operational client signals
When is optical amplification deployed?
Amplification is only but not limited to deployment in a DWDM baed oc network due to the cost and capacity benefits DWDM provides. Amplification is considered mandatory
What are the types of amplification?
1) Erbium Doped Fibre Amplifier (EDFA)
2) Raman Amplifier
Properties of EDFA
1) Use laser diode and stimulate emission within a 30-40. foot segment of SMF doped with erbium ions
2) A pump laser injects high powered optical light into the doped fibre to stimulate the erbium ions to a higher state
3) Requires electrical power but amplification is optical
4) Couplers are integrated on the amplifier to distribute the signal onto multiple fibre paths or combine onto a single path
What are the amplification windows of EDFAs?
1) C Band amplification - 1515-1565nm range
2) L Band amplification - 1570-1610nm range
What are the types of EDFA laser pumps?
1) 980nm pump - low to modest gain (10-15db) and produce little Amplified Spontaneous Emission noise (ASE). Suitable where noise levels are a concern
2) 1480nm pump - high gain (25-30db) but twice as much ASE. Suitable for boost or inline amplifier applications
How are EDFAs constructed?
Can be single, dual or multi stage designs. A dual stage will first use a 980nm pump to amplify with reduce noise and the second stage will use a 1480nm pump for high output gain. Mid stages might have dispersion compensation and/or a gain flattening filter. A multi stage design allows for predictable, consistent gain across operation wavelength range
Why is it important to equalise power before entering the amplifier?
1) Unequalised power levels leads to issues with non-linear effects
2) Can cause unpredictable performance such as cross talk or channel traffic loss
How are amplifiers packaged?
1) Fixed gain amplifiers (FGA)
2) Variale gain amplifiers (VGA)
Properties of a FGA
1) provide consistent gain, +23db
2) External power balancing of active wavelengths is required
3) Require higher level of human intervention
Properties of VGA
1) Controlled manual or by auto power software
2) reduced time to react to optical span changes
3) Support automatic power control of span
4) Reduced operation expense and increases network reliability
5) Photodetectors monitor input and output levels and use the information to adjust the gain of the amplifier
What is RAMAN Amplification?
Makes use of Stimulated Raman Scattering. When SRS occurs the higher wavelength absorbs the energy of the 100nm lower wavelength, causing amplification. In practice, a high powered pump generates a low wavelength is coupled to the high data carrying wavelength. Gain can be 7-11db.
A drawback is the laser pump requires an output of 500mW to 1 W of optical power to induce SRS amplification
