As Enterprises look for increased bandwidth and ways of reducing the overall cost of cabling, the market is quickly adopting Fibre Passive Devices or “FPDs” into their network architecture.

FPD’s are generally characterised as passive optical components that split, combine, filter or modify optical signals. These are used to control and/or modify optical signals as they travel through an optical network.

FPD’s can be used to create a “virtual fibre network” by adding capacity to existing infrastructure without the need to install new cabling.

Here are some examples of commonly used FPDs that help maximise fibre utilisation by creating a “Virtual Fibre Network” on existing single mode optical fibre.

A single wavelength OC or dual wavelength OCW FPD is one of the most cost effective ways of reducing fibre capacity by half or quarter.  Utilising “standard” wavelengths of 1310nm or 1550nm, the OC or OCW FPD allows bi-directional communication of 1310nm, 1550nm or 1310nm + 1550nm over the one fibre.

The OC FPD therefore allows 1310nm or 1550nm traffic that would normally require two fibres for full duplex operation to occur over the one fibre, reducing required fibres by a ratio of 2:1.  OCW FPD’s combine the 1310nm and 1550nm traffic, bi-directionally.  This creates a reduction of 4:1 by allowing both wavelengths to travel bi-directionally on one fibre.

See Figures 1 and 2.

Coarse Wave Division Multiplexer (CWDM) & Dense Wave Division Multiplexer (DWDM) combine (multiplex) optical wavelengths onto one fibre. A CWDM or DWDM Mux combines the wavelengths onto one fibre while a De-Mux splits the wavelengths back out onto individual fibres.

See Figure 3.

The International Telecommunication Union (ITU) defines the channel spacing and wavelengths used in CWDM (ITU G.694.2) and DWDM (ITU G.694.1)

CWDM’s operate between 1270nm & 1610nm with a channel spacing of 20nm and are typically used in short haul transmission where no amplification or re-generation of signal is required.

When designing these networks, users must also take into consideration single mode water peak attenuation.  This is an area at 1383nm +/-3nm which has a higher attenuation caused by absorption during manufacture.  This attenuation zone has been overcome in newer fibre types referred to generally as ZWP (Zero Water Peak) or LWP (Low Water Peak) however users of older fibre must take this attenuation zone into account when selecting wavelengths.

See Figure 4.

DWDM wavelengths are more closely packed, typically down to 0.8nm in spacing and normally operate in the C-Band.  As the wavelengths can be more closely combined, this allows better utilisation of fibre capacity.

See Figure 5 and 6.

Over time, more and more of these FPD’s will appear in networks to make better use of existing cabling infrastructure. This is one of the advantage fibre optic cabling provides over traditional copper networks.

Craig Buckingham RCDD has been involved with the ICT industry since beginning his career as Telecommunications Mechanic in the British Army 15 years ago. He has worked on all facets of the cabling industry including network configuration, cabling installation, design and consultation. Craig has been with AFC for 4 months now as their Technical Development Manager delivering technical Learning Sessions to Consultants and End Users alike.

Visit www.afcgroup.com.au.