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Digital communications
Digital communications

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4.2 Fibre in the core network

All new trunk transmission – that is, transmission between telephone exchanges – is now over optical fibre. Mostly it uses either PDH or SDH links. Year by year the data rates have increased, so that at the time of writing STM-64 products are available commercially.

SAQ 12

What, approximately, is the bit-rate of an STM-64? What is the bit-rate of the next stage to be developed?

Although STM-256 systems are expected to be available soon, there are difficulties with operating them on currently installed fibre, and there are doubts that the trend of ever-higher time-division multiplexed rates will continue indefinitely.


An STM-64 will run at approximately 64 × 155.52 = 9953 Mbit/s, which is about 10 Gbit/s.

The SDH hierarchy increases by factors of 4, so the next stage is STM-256, at approximately 40 Gbit/s.

Activity 15

What are the problems with operating at ever higher data rates over single-mode fibre?


As the data rates increase the tolerance to pulse-spreading decreases. At 40 Gbit/s, on single-mode fibre, dispersion and polarization-mode distortion significantly limit the transmission distance. (Multimode fibre could only be used for very short distance because of multimode distortion.)

The solution being used for increasing further the capacity of fibre is therefore wavelength division multiplexing, WDM.

In Recommendation G.671 the ITU-T identified three categories of WDM, defined by the spacing between the channels as follows:

You will see later that wide WDM, WWDM, is used to mean something rather different in the standard for 10 Gigabit Ethernet. Coarse WDM (CWDM) device

A class of WDM devices having a channel wavelength spacing less than 50 nm but greater than 1000 GHz (about [X] nm at 1550 nm and [Y] nm at 1310 nm). Devices within this class can cover several spectral bands. Dense WDM (DWDM) device

A class of WDM devices having a channel spacing less than or equal to 1000 GHz. Devices within this class can cover one or more spectral bands. Wide WDM (WWDM) device

A class of WDM devices having a channel wavelength spacing greater than or equal to 50 nm. This device class typically separates a channel in one conventional transmission window (e.g. 1310 nm) from another (e.g. 1550 nm).

The mixtures of wavelengths and frequencies used in these quotes from the ITU standard is typical of the literature of optical-fibre communications: wavelengths used to describe the windows (e.g. 1310 nm and 1550 nm) and large channel spacings (e.g. 50 nm), but frequencies used for small channel spacings (e.g. 1000 GHz). Notice, though, that in the definition for CWDM it is technically incorrect to say ‘… channel wavelength spacing less than 50 nm but greater than 1000 GHz …’ because it is using the units of frequency (GHz) for a wavelength spacing. What it could say instead is ‘…channel wavelength spacing less than 50 nm but greater than a frequency spacing of 1000 GHz…’.

SAQ 13

The ITU specification for CWDM quoted above included the wavelength equivalents (in nm) of 1000 GHz at 1550 nm and 1310 nm, but I have replaced them by X and Y respectively. Use the appropriate formula to calculate X and Y.


The formula you need is:

The frequency spacing, Δf, is 1000 GHz (1012 Hz) and c, the speed of light (to three significant figures) is 3.00 × 108 m s−1. For X, λ, is 1550 nm, so that the wavelength spacing is:

So X, the wavelength spacing in the 1550 nm window, is 8 nm.

For Y, λ is 1310 nm, so that the wavelength spacing is:

So Y, the wavelength spacing in the 1310 nm window, is 5.7 nm.

Early applications of WDM were of the WWDM type, used with directional couplers for the multiplexing and demultiplexing (with λ1 = 1300 nm and λ2 = 1550 nm).

The characteristics and applications of CWDM are summarised in ITU-T Recommendation G.694.2:

Coarse Wavelength Division Multiplexing (CWDM), a WDM technology, is characterised by wider channel spacing than Dense WDM (DWDM) as defined in G.671. CWDM systems can realise cost-effective applications, through a combination of uncooled lasers, relaxed laser wavelength selection tolerances and wide pass-band filters.

CWDM systems can be used in transport networks in metropolitan areas for a variety of clients, services, and protocols.

and those of DWDM in Recommendation G.694.1:

Dense Wavelength Division Multiplexing (DWDM), a WDM technology, is characterised by narrower channel spacing than Coarse WDM (CWDM) as defined in G.671. In general the transmitters employed in DWDM applications require a control mechanism to enable them to meet the application's frequency stability requirements, in contrast to CWDM transmitters which are generally uncontrolled in this respect.

The significance of the reference to ‘uncooled lasers’ is that in general the wavelength of light from a laser varies as the temperature changes. DWDM needs very stable wavelength sources, so the temperature of the lasers must be controlled. This is not necessary for CWDM.

Systems using DWDM generally also use optical amplifiers. Although DWDM can be used in links without amplifiers, it is the availability of amplifiers that has made DWDM so attractive. For one thing, wavelength multiplexers and demultiplexers tend to be quite lossy, so that without amplifiers the power budget available for the transmission path would be quite small. If a regenerative repeater were needed to extend the transmission distance it would have to demultiplex all the wavelength channels, detect each channel with a photodiode, regenerate each channel and then recreate the multiplexed optical signal. An optical amplifier, which amplifies all channels simultaneously, is much simpler.

ITU-T Recommendations G.694.1 and G.694.2 contain wavelength grids – tables of the wavelengths that should be used. The table for CWDM from G.694.2 uses 20 nm channel spacing and is reproduced in Table 5. For DWDM, G.694.1 recommends channels based on a grid spaced by 12.5 GHz (approximately 0.1 nm), as shown in Table 6. For systems using channel spacings of 25 GHz (approximately 0.2 nm) every alternate wavelength on the 12.5 GHz grid is used; for spacings of 50 GHz (approximately 0.4 nm) every fourth wavelength on the 12.5 GHz grid is used; and similarly for wider channel spacing.

Table 5: Nominal central wavelengths for CWDM
Nominalcentral wavelengths (nm) for spacing of 20 nm
Source: ITU-T (2002) G.694.2, Table 1

Note: The end-points of this table are illustrative only.

Table 6: Nominal central wavelengths for DWDM
Nominal central frequencies (THz) for spacings of:Approximate nominal central wavelengths (nm)
12.5 GHz25 GHz50 GHz100 GHz and above
Source: ITU-T (2002) G.694.1, Extract from Table 1

SAQ 14

In what band are the frequencies of Table 6? If the whole of this band could be filled with channels at 25 GHz spacing, how many channels in total would there be?

You will need to convert from wavelengths to frequency, using f = c/λ, where fis the frequency, λ (lambda) the free-space wavelength and c the speed of light, which, to six significant figures, is 2.99792 × 108 ms−1.

If each channel carries an STM-64, what will be the total data rate carried by the fibre?

At the time of writing, commercial products are available that can support up to 160 STM-64 channels.


The wavelengths of Table 6 are all in the C-band.

The C-band extends from 1530 nm to 1565 nm, which is, in terms of frequency, from


This is a range of 195.94 − 191.56 = 4.38 THz, and at a spacing of 25 GHz (0.025 THz) per channel that would allow 4.38/0.025 = 175.2 channels. Rounding down, that would in practice be 175 channels.

If each channel carries an STM-64 this is a total capacity of approximately 175610 Gbit/s = 1.75 Tbit/s.