Tech Breakfast: Fibre Optic Multiplexing

CWDM & DWDM

phil.crawley@jigsaw24.com @IsItBroke on Twitter http://www.root6.com/author/phil

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

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

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.

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.

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.

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

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

How does the technology work?

A combination of tuned SFP modules and optical engineering

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…!

CWDM SFPs

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.

CWDM SFPs

Typical distribution of wavelengths – so go up the fibre, some come back.

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)

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.

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.

Outside Broadcast camera – optical triax

• SMPTE camera cables are expensive; typ. >£1k

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

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!)

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.)

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…!

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.

Thank-You!

phil.crawley@jigsaw24.com @IsItBroke on Twitter http://www.root6.com/author/phil