Synchronisation and DWDM (Dense Wavelength Division Multiplexing)

Synchronisation and DWDM (Dense Wavelength Division Multiplexing)


The role of synchronisation plan is to determine the distribution of synchronisation in a network and to select the level of clocks and facilities to be used to time the network. This involves the selection and location of master clocks for a network, the distribution of primary and secondary timing through out the network and an analysis of the network to ensure that acceptable performance levels are achieved. Improper synchronisation planning or the lack of planning can cause severe performance problems resulting in excessive slips, long periods of network downtime, elusive maintenance problems or high transmission error rates. Hence, a proper synchronisation plan which optimises the performance is a must for the entire digital network. The status of synchronization in the BSNL network is as follows :

3 nos. of cesium clocks at VSNL Bombay provide the Master National Reference Clock (MNRC). The backup NRC is available at Delhi. The MNRC feeds the reference signal to the VSNL GDS at Mumbai and from the GDS both the new technology TAXs at Mumbai are synchronised. From these two TAXs at Mumbai, all the other TAXs are to be synchronised. Part of this work has already been done. However, all the Level–I TAXs are yet to be synchronised. A direct synchronisation link is also available between GDS Mumbai and Karol Bagh TAX at Delhi.

For synchronisation of the SDH network, it has been decided to use the clock source available through the TAXs at the major stations. The synchronisation plan is based upon provision of Synchronisation Supply Units (SSUs) which will be deployed as an essential component of the synchronisation network which will support the synchronised operation of the SDH network. The architecture employed in the SDH requires that the timing of all the network clocks be traceable to Primary Reference Clock (PRC) specified in accordance with ITU Rec.G.811. The classical method of synchronising network element clocks is the hierarchical method (master–slave synchronisation) which is already adopted in the BSNL network for the TAXs. This master-slave synchronisation uses a hierarchy of clocks in which each level of the hierarchy is synchronised with reference to a higher level, the highest level being the PRC. The hierarchical level of clocks are defined by ITU as follows :

  • R.C.
  • Slave Clock (Transit Node)
  • Slave Clock (Local Node)
  • SDH Network Element Clock.



Each node is associated with a particular hierarchical level of clock prescribed above and is referred to as a nodal clock. The SSU is an important component of this hierarchical master-slave synchronisation network scheme and of a slave clock belonging to the transit node level or the local node level as defined in ITU Rec. G.812.

4.4   The BSNL, therefore, has decided to go in for 10–20 nos. of SSUs to provide a clean reference primary source for other stations. This SSUs are basically high stability filter clocks which eliminate phase transients, jitter and wander and provide the exact sync. signal needed for every network element.

DWDM (Dense Wavelength Division Multiplexing)

  1. Evolution of Transmission Capacity

In the 80’s, it was possible to transmit 140 Mbit/s with optical PDH – systems.  SDH technology in the 90’s has improved this capacity.  SDH can transmit the capacity of 16 times 140 Mbit/s or 155 Mbit/s (16 X STM 1 = STM 16, 2.5 Gbit/s) or up to 64 times 140 Mbit/s or 155 Mbit/s (64 X STM 1 = STM 64, 10 Gbit/s).

Currently, it is possible with WDM wavelength division multiplex systems to transmit between 32 and 96 times 10 Gbit/s (320 Gbit/s) over very large distances.  Soon we will have 160 times 10 Gbit/s, and in the laboratory it is possible to transmit in the terabit range (10 X 1012).

In the case of optical systems the available bandwidth can exceed several Terahertz (1012Hz).  TDM could not be used to take advantage of this tremendous bandwidth due to limitations on electrical technology.  Electrical circuits simply cannot work on these frequencies.

The solution was to use frequency multiplexing at the optical level or Wavelength Division Multiplexing.  The basic idea is to use different optical carriers or colours to transmit different signals in the same fibre.

Consider a highway analogy where one fibre can be thought of as a multi-lane highway.  Traditional TDM systems use a single lane of this highway and increase capacity by moving faster on this single lane.  In optical networking utilizing DWDM are analogues to accessing the unused lanes on the highway (increasing the number of wavelengths on the embedded fibre base) to gain access to an incredible amount of untapped capacity in the fibre.  An additional benefit of optical networking is that the highway is blind to the type of traffic that travels on it.  Consequently, the vehicles on the highway can carry ATM packets, SDH and IP.

A distinction is made between WDM and DWDM (Dense Wavelength Division Multiplexing). With WDM the spacing between channels can be relatively large.

In Dense multiplexing the frequency spacing between channels can be as small as 50 GHz or less, increasing the overall spectral density of the transmitted signal.


2. Transmission Windows

Today,  usually the second transmission window (around 1300 nm) and the third and fourth transmission windows from 1530 to 1565 nm (also called conventional band) and from 1565 to 1620 nm (also called Long Band) are used.  Technological reasons limit DWDM applications at the moment to the third and fourth window. The losses caused by the physical effects on the signal due by the type of materials used to produce fibres limit the usable wavelengths to between 1280 nm and 1650 nm.  Within this usable range the techniques used to produce the fibres can cause particular wavelengths to have more loss so we avoid the use of these wavelengths as  well.


3. Application Advantages

Optical networks are opening up new horizons for telecommunication operators. Technologies such as wavelength division multiplexing (WDM) and optical amplification are giving them a multitude of ways to satisfy the exploding demand for capacity.  New architectures will increase network reliability and decrease the cost of bit rates and distance, therefore, creating economic benefits for network operators and users alike.  Based on existing fibre optic backbone networks, the idea of an all optical network (AON) is revolutionizing the structures of our communication networks.  In short, optical networks are the future of the information super highway. The biggest advantages of such an optical network would be :

Properties Applications
Multiple use of fibres Ideal in cases of fibre shortage
Extremely high transport capacity at low cost Multiple use of opt. amplifiers yielding decreased investments & maintenance costs.
Format and bitrate transparency Data, video and voice over a common N/w

4. Transponder Applications

A Transponder Terminal can be used to transmit a wide variety of signal types, like SDH, ATM or PDH signals.

The Transponder adapts to the arbitrary bit rate of the incoming optical signal, and maps its wavelength to the chosen WDM channel.  Its  main function is OEO.  It converts wavelength (say 1550 nm) coming from user equipment to electrical signal and electrical signal is converted into optical signal of a specific wavelength, which forms an optical channel for particular user.

Optical transparency yields a multitude of new application options and enables network operators to utilize existing network resources in a far more flexible manner.  It provides major advantages such as :

  • Greatly enhanced transmission capacity.
  • New services offered.
  • Transmission of restructured signals.
  • Use of devices and interfaces from other vendors.

The semitransparent transponder keeps one of the major advantages of the DWDM i.e.  Protocols are transmitted transparently,  providing a very high flexibility.


5. Optical NE Types

(a) Optical Multiplexer/Demultiplexer

Multiplexing and Demultiplexing of different wavelength  signals.

(b) Optical Amplifiers

Pure optical 1R regeneration (just amplification) of all transmitted signals.

(c) Transponders

Wavelength “change” and 2R regeneration (reshaping and amplification) or 3 R regeneration (reshaping retiming and amplification).

(d) Regenerators

Real 3 R regeneration (reshaping, retiming and amplification) of the signal. Therefore, the signals have to be demultiplexed, electrically regenerated and multiplexed again.  They are necessary if the length to be bridged is too long to be covered only by optical amplifiers, as these only perform reshaping and retiming.

(e) Optical Add/Drop Multiplexer

Adding and Dropping only specific wavelengths from the joint optical signal.  This may use complete de-multiplexing or other techniques.

(f) Optical cross-connects

To cater for the huge amount of data expected in an optical network even the cross-connects have to work on a purely optical level.

  1. Future Trends
  • Use of Optical Amplifier – The best developed optical amplifiers are Erbium doped fibre amplifier (EDFA) which operate at 1550 nm and praseodymium doped fibre amplifiers operating at 1300 nm.
  • Use of non-zero dispersion shifted fibre (NZ – DSF).
  • Use of passive optical components (PON).
  • Wave Division Multiplexing of Optical Signal (WDM).
  1. Description of Optical Multiplexer and Demultiplexer :

An optical demultiplexer can be built as an association of optical filters or as a single stand device.  The purpose is to extract the original channels from a DWDM signal.  The requested properties of this device are the same as for the optical filter : isolation and signal distortion.  However channel number and spacing must be considered now because demultiplexers can  impose limitations on the number of channels or the total available bandwidth.  Most demultiplexers are symmetrical devices and can also be used as multiplexers.

(a) By using Prism

The easiest and best-known optical demultiplexer is the prism.

Using the effect of dispersion (different speed of light for different wavelengths),  light is split into its spectral components.

(b) By using Diffraction Grating

The function of a diffraction is very similar to that of a prism, only here interference is the important factor.  A mixture of light is also split into its contributing wavelengths.

With such a grating sometimes also called a bulk grating channel spacings of done to 50 GHz can be achieved.


8. Optical Amplifiers

(a) Introduction

Fiber loss and dispersion limit the transmission distance of any fibre-optic communication system.  For long-haul WDM systems this limitation is overcome by periodic regeneration of the optical signal at repeaters, where the optical signal is converted into electric domain by using a receiver and then regenerated by using a transmitter.   Such regenerators become quite complex and expensive for  multichannel lightwave systems.  Although regeneration of the optical signal is  necessary for dispersion-limited systems, loss limited systems benefit considerably if electronic repeaters were replaced by much simpler and potentially less expensive, optical amplifiers which amplify the optical signal directly. Several kinds of optical amplifiers were studied and developed during the 1980 s.  The technology has matured enought that the use of optical amplifiers in fiber-optic communication systems has now become widespread.

(b) Optical Amplifier Applications

  • In-line amplifiers
    • Booster amplifiers
    • Pre-amplifiers

In-line amplifiers are used to directly replace optical regenerators.  Booster amplifiers are used immediately after the transmitter or multiplexer to increase the output power.  Pre-amplifiers are used before the receiver or demultiplexer to increase the received power and extend distance.

The use of each configuration as advantages and disadvantages that must be considered by the systems designer.

The problems come when considering non-linear effects in the transmission fiber and also generated by the amplifiers.


Some of the requirements for optical amplifiers for DWDM purpose are :

  • high gain
  • low noise
  • flat amplification profile

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