tech breakfast: fibre optic multiplexing · proper money - dwdm • cwdm has a channel spacing of...
TRANSCRIPT
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Tech Breakfast: Fibre Optic Multiplexing
CWDM & DWDM
[email protected] @IsItBroke on Twitter http://www.root6.com/author/phil
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Long range fibre optic cabling
• Single mode vs. Multi mode – fundamentally different. • CWDM vs DWDM • Campus-wide connectivity • Facility – DC connectivity • Outside Broadcast – SMPTE optical triax • Facility – Facility connectivity; the Dolby 4k project
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Single mode vs. multi mode cable• Single mode cable (“OS1” / “OS2” or “9µ/125µ”) is the original style of fibre
optic cable developed by BT and Corning in the 1970s • With a nine micron transmissive core the cable is able to contain the 1270 –
1690nm wavelengths efficiently.
V = number of modes k0 = wave number a = core’s radius (9µ vs. 50µ) n1 & n2 = refractive indices
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The cheap ‘n’ cheerful young pretender – Multi mode
• Whereas single-mode contains the light wave near-perfectly multi-mode cable relies on total internal reflection; the wavefront bounces down the fibre core as it hits the core/cladding interface.
• By clever use of interference modes many distinct waves can be launched down the cable and by using an interference detector laser-diode the modes can be recovered. All at 850nm typically.
• Limitations are length – 500 – 1,500m typically • Graded index fibre OM4 is the newest standard.
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Limitations of multi-mode optics
• Distance – due to “modal-dispersion” • Distances typically range from 150 – 650m (depending on
application) and can be calculated using the bandwidth-loss product. 10Gbits-1; typical max data rate in 2017.
• Graded Index OM4 fibre uses a transmissive core that varies it’s refractive-index across the diameter of the core so that the wavefront is better guided down the core.
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The better technology – single mode fibre.
• In many ways very similar to multi-mode technology, same connectors, style of patch panel, cable construction etc.
• Works in an entirely different way – light is optimally constrained in a 9µ transmissive core so no need for total-internal-reflection with problems of modal dispersion etc.
• MUCH higher data rates and distances (80km without amplification)
• MUCH better signal-to-noise ratio; 28dBs with best optics. • Wider application in video and data. • Multiplexing is possible – more of that in the next session.
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What happens if we don’t have enough fibre?
• Single mode fibre is by definition a long range technology and so it may be hard to put in more.
• Internal IT dept may have control over fibre network • Metro networks may cross local authority boundaries • Wouldn’t it be great to be able to stack many signals onto a fibre? • The two technologies are CWDM and DWDM
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CWDM – Course Wave Division Multiplexing• 8/16/18 optical channels • Centre wavelengths according to ITU-T G.694.2 • Channel centre wavelength 1270~1610nm • Channel clear passband ITU+/-7 nm • Insertion loss 8ch-2.5dBm, 16ch-3.5dBm (max) • Passband ripple 0.5dBm (max) • Adjacent channel isolation 30dBm (min) • Non-adjacent channel isolation 45dBm (min) • Return loss 45dBm (min) • Operating temperature range 0 to +70C • Maximum power handling 300mW
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How does the technology work?
A combination of tuned SFP modules and optical engineering
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And which engineer doesn’t take the lid off?!•All the filtered wavelengths go via dichroic elements that filter around that particular band. •The common optic is the other side of a combining element. •A good way to think of this is a prism.Well, nearly…!
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CWDM SFPs
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CWDM SFPs
• Most SFPs are transceivers – they send and receive data
• Their outputs are set for a tuned wavelength • They are colour-blind on their inputs • For passive CWDM systems the multiplexer does
not care what is an input and what is an output; Ethernet is the example.
• The optics are the only limit on each λ’s bandwidth.
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CWDM SFPs
Typical distribution of wavelengths – so go up the fibre, some come back.
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Show & Tell
• A quick demo of BarnFind’s CWDM system • Standard MSA-compliant SFP & SFP-ports • 3G video going up and down a fibre (twice!) • BarnStudio – Barnfind’s management software • Integrates with numerous router panels (BM etc)
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Proper money - DWDM• CWDM has a channel spacing of 20nm • DWDM by contrast uses 0.8nm (100Ghz grid) or 0.4nm
(50Ghz grid) • This means you can achieve 96/192 channel/fibre • 1,000km without amplification (vs. 80km for CWDM) • However, signals are single direction. • Erbium Doped Fibre Amplifier uses a pump laser to bring up
electrons to a higher energy level, amplification is achieved by emitted photons of the same signal wavelength with the help of stimulated emission.
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Expensive
• Unlike passive CWDM parts everything relies on calibrated lasers
• Transatlantic fibres (first TAT-8 in 1988) had limited bandwidth, but in 1996 TAT-12 used EDFA along it’s route to carry DWDM traffic.
• Between buildings in London it’s almost always cheaper to buy more cores and continue with CWDM
• You can exploit the 2nd core that ISPs always provide.
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Outside Broadcast camera – optical triax
• SMPTE camera cables are expensive; typ. >£1k
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Dolby 4k remote grading network
• To allow numerous Soho facilities to control different grading machines from Dolby grading rooms.
• No compression • No latency • 4K and HDR (of whatever flavour) • Remote management interface to allow control of the end-
points • As ever expense is an issue from an equipment and ongoing
cost POV
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A variety of applications• Baselight• Resolve• AN Other grading machine
Each have their own control panel requirements which aren’t easily extended over distance.
UHD/4K – late 2016 (and probably still the case a year later) demands quad-link 3G SDi
(If anyone mentions “doing it over IP” they can leave now!)
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Fibre around Soho
There is now sufficient fibre under W1 that the cost of going point-point (or via Volta etc.) is not onerous.Being IT-based EU-Networks etc. provide a duplex pair of single-mode fibres which serves us well for backup as we really only need one core for;
•4 x 3G SDi •3 x Gigabit Ethernet•3 x KVM signal type (when using a non-Baselight config.)
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Specific configuration
Three networks are necessary;
1.Baselight host control – PXE boot from remote Baselight machine 2.Dolby CMU control – for PQ HDR workflows 3.Barnfind management console so Ian & Ady can keep an eye on things…!
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Further reading
https://www.barnfind.no/downloads
Barnguide v 2.8 – everything you need to know!
http://www.root6.com/blog
All things relating to facilities engineering.