Friday 15 December, 2017 | RSS Feed

Passive Cwdm vs Dwdm – Which to choose?

by www.fiber-mart.com

It is quite common when we receive questions from our customers as this example “Hey – we have available one dark fiber between our two network nodes and we need to organize 6x10G bi-directional links over that fiber and distance is about 30km. Future plans not clear, but we think we may need some additional capacity next year. We heard about passive WDM technologies – but it is not clear for us which to choose? What is difference CWDM vs DWDM and what You recommend?”
In such case, our responsible sales consultant would reply something like this: “First selecting technology depends on planned capacity – CWDM has 18 wavelengths available (9 pairs for bidirectional links) and DWDM with 100GHz spacing has up to 40 (20 pairs for bidirectional links). If You don’t see much higher capacity as 9x10G, then we would suggest You to go with passive CWDM, as it is cheaper.”
Probably two, three years ago such answer would be totally right, but industry of optical communications changing rapidly, component prices dropping, new applications and standards becoming commercial, so now in 2017 this answer could be wrong.
Passive CWDM vs DWDM – let us compare technologies from today’s perspective. Both are wavelength-division multiplexing which combine number of optical links on same physical fiber using different colors (or wavelengths) of laser light. Both technologies, standardized in 2002 by respective ITU-T G.694.1 and G.694.2 recommendations, have a major difference – spectral width of channels. In case of CWDM channel width is 20nm, accordingly DWDM 100GHz version has 0.8nm channel spacing.
CWDM-DWDM Channel Plan
From channel spectral width aspect, DWDM is much more efficient – however it requires much higher laser precision, as narrow channels are much more demanding regarding drift of wavelength. This high precision requirement were main reason leading to need higher cost of DWDM transceiver laser components. Second aspect was that initial DWDM lasers were causing higher power and heat dissipation – this is why DWDM transceivers for a long period were available only in more bulky form factors – such as XFP, XENPAK and X2. But lately with industry advancement DWDM transceiver components has same power and heat dissociation as CWDM ones and are available in industry’s mainstream SFP+ form factor. As DWDM becoming increasingly popular, prices of DWDM transceivers are about 20-25% less then CWDM. However – for building passive CWDM or DWDM connections, beside transceivers You need also passive components such as Mux and OADM’s – there price is lower for CWDM components. So overall total investment in passive WDM network based on CWDM on DWDM technology will be very close – almost the same, but on most occasions, DWDM will be more economical, as most of investment usually are transceivers. As today price as one of main competitive advantages for CWDM is not valid any more, lets compare passive CWDM vs DWDM from pure technical application viewpoint:
CWDM vs DWDM – Channel Uniformity:
As CWDM spectrum for 18 channels spans from 1260nm up to 1620nm compared to DWDM C-band 1530 – 1565 nm, CWDM has weakness from channel uniformity aspect. Attenuation in wide spectrum is different based on wavelength – for example, typical attenuation of G.652.C optical fiber is 0.38 dB/km at 1310nm wavelength and 0.22 dB/km at 1550nm. So in CWDM system You can get quite great disparity of channel optical performance using different CWDM wavelength. Uniformity of optical channels across whole 1260-1620nm spectrum depends on fiber cable specification. – we suggest checking carefully if You plan using passive CWDM. Especially it is very important for old G.652 specification fiber – it has so called “water-peak” phenomena in range of 1390 and 1490 nm that are not usable for CWDM connections at all. DWDM is clear winner here – due it’s narrow spectrum channel properties on same fiber will be almost identical.
CWDM vs DWDM – Capacity:
It’s clear winner here – while maximum capacity of CWDM system is 18 wavelengths all spectrum, DWDM using traditional C-Band 1530 – 1565 nm allow to have 45 100GHz spaced DWDM channels, but with introduction of 50GHz spaced transceivers we can double number of channels up to 90. In future, we can expect to have 25 GHz and even 12.5 GHz frequency offset even multiplying number of possible channels to 180 or 360. If that is not enough – there is S-band (1460-1530 nm) and L-band (1565-1625 nm) which can be used with DWDM as well, just is not mainstream yet.
CWDM vs DWDM – Distance:
Maximum distance of xWDM connection depends on two main factors – maximum budget of optical transceivers and attenuation of all passive elements – fiber itself, number of joints and splices, attenuation of passive filters (Chromatic dispersion as well, but we don’t consider it much a factor up to 80km). If looking on 10G connection data rate, with both, CWDM and DWDM, passive technologies You can have up to 23 dB guaranteed budget using popular SFP+ transceivers (With XFP You can have 26dB budget), what is enough to have 80km xWDM link with both technologies. But big advantage of DWDM is, that due it’s narrow spectral width it’s possible to use cost efficient and widely available EDFA (Erbium Doped Fiber Amplifier) boosters, which is one very cost efficient way allowing extension of DWDM reach.
CWDM vs DWDM – Spare Parts:
Even optical transceivers are mature elements and failure-rates are very uncommon, introducing xWDM technology You would like to have backup stock of all active elements. If You are planning to have just small scale deployment and connect just two or few network nodes, it could mean that You basically need to back up everything – resulting on doubling up of your investment. DWDM is a winner here as well, due availability of Tunable DWDM transceivers, with can replace all Your different wavelength DWDM transceivers with one or two units. You can read more in our article Tunable Optical Transceivers – When To Use?
Our Conclusion – with current industry advancements trend that has equalized costs of optical elements such as transceivers, in technical battle of CWDM vs DWDM more advancements are in DWDM’s cup. It leaves us with question – what is use case of CWDM..? CWDM still has price advantages for connection rates below 10G and for short distances with low data rates it’s currently most feasible technology.

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