SAN - Decomplexing Multiplexers

Published 27 September 2002

Authors: Jon Tate


This tip discusses multiplexers.


Multiplexing is the process of simultaneously transmitting multiple signals over the same physical connection. There are three common types of multiplexing used for fiber optic connections:

  • Time Division Multiplexing (TDM)
  • Wavelength Division Multiplexing (WDM)
  • Dense Wavelength Division Multiplexing (DWDM)

Time-Division Multiplexing
Time-division multiplexing (TDM) was created by the telecommunication industry to maximize the amount of effective traffic that could be carried over an existing fibre network. Previously in the telephone industry, every phone call needed its own discrete physical link. This system was obviously very costly to operate and limited for growth. The multiplexing facility enabled many phone circuits to be sent over a single link. In summary TDM enables many signals to be concentrated into a single fibre and all within the same wavelength. TDM can be thought of as highway traffic. Traffic that needs to traverse from one city to another will start out on minor routes, it will then make it to a main (single lane) highway where it will join into a slot between other vehicles. The traffic will be controlled so that it is fair to all minor routes. Once the vehicles arrive at their destination they will be taken off to minor routes again. This method is used in synchronous TDM devices. It increases the capacity of the fibre by reducing time slots into smaller intervals. This enables more bits in the same fibre which increases the effective bandwidth of the fibre. Within the TDM the input sources are multiplexed in a fair time share manner. Each signal will therefore have a packet space allocated to it whether it has signal input to send or not. This can be inefficient because there can be unused packets within the frame. Some protocols will reduce the effect of this by keeping data flowing into the channel. This is more likely in an asynchronous architecture.

The figure below shows a TDM and its method of combining several slower speed data streams into a single high speed data stream. Data from multiple sources is broken into portions (bits or bit groups) and these portions are transmitted in a defined sequence. Each of the input data streams then becomes a “time slice” in the output stream. The transmission order must be maintained so that the input streams can be reassembled at the destination.
An image of a TDM combining data streams

Wavelength Division Multiplexing
Wavelength Division Multiplexing (WDM) differs from TDM in that it does not use time to multiplex on — it uses the many wavelengths of light to multiplex on. WDM receives incoming optical signals from many sources (devices) which it converts to electrical signals, it then assigns them a specific wavelength (or lambdas or ë) of light and retransmits them on that wavelength. This method relies on the large number of wavelengths available within the light spectrum. You can think about WDM as though each channel is a different color of light; several channels then make up a “rainbow.” In summary WDM enables many signals to be concentrated into a single fibre all being sent at different wavelengths. WDM allows the simultaneous transmission of a small number of data streams over the same physical fiber, each using a different optical wavelength. The advantages of WDM over TDM are that transmission order does not need to be maintained and that the information streams can use different protocols and bit rates. The WDM is sometimes described as the coarse wave division multiplexer.

The figure below shows each of the input signals coming into the WDM being multiplexed at different wavelengths on the output.
An image of WDM being multiplexed at different wavelengths

This output is all multiplexed into the fiber. At the other end of the fiber, the signals are demultiplexed by the receiving WDM. This differs from the TDM method where time is displaced. Within the WDM environment each signal has its own wavelength. This means that they can all coexist within the same fibre at the same time. This results in all channels having full bandwidth.

Dense Wave Division Multiplexer
Dense Wave Division Multiplexer (DWDM) uses the same design principles as the WDM, it can simply handle a much larger number of wavelengths. By reducing the spacing of these wavelengths more channels can be accommodated. As each channel maps to its own individual wavelength, the more wavelengths we have, the greater the total capacity or bandwidth of the device. We currently see DWDM devices that are capable of driving 256 discrete wavelengths along one single mode fibre. DWDM is an approach to opening up the conventional optical fiber bandwidth by breaking it up into many channels, each at a different optical wavelength (a different color of light). Each wavelength can carry a signal at any bit rate less than an upper limit defined by the electronics, typically up to several gigabits per second. DWDM has all the advantages of WDM but with the added benefit of supporting far more independent transmissions over the same fiber. Due to the nature of these boxes, they are often considered transparent to protocol and bit rate.

The business drivers for DWDM are clear and deliver:
  • Bandwidth: If you currently have a pair of fibre installed that you are using for a single channel, then by employing DWDMs, that could bring you 255 extra channels per fibre.
  • Protocol independence: The DWDM is deployed as part of the physical layer. It is therefore independent of protocol, simply passing signal information in the format it is received. Examples of the protocols it can support are ATM, Gigabit Ethernet, ESCON, and Fibre Channel.
  • Growth on demand: A DWDM solution can preserve investment in an already deployed fibre infrastructure, and easily can be expanded to meet growing capacity.
  • Speed to market: Once a DWDM solution is deployed it can be used quickly and efficiently for a new application, transparent of application and protocol. This enhances your ability to react to platform changes at an enterprise level. This can never be understated in the new e-business world.

DWDM summary
The figure below shows a generic overview of the components within a DWDM. The incoming signals from inputs are varied, and we have simply shown SAN, ESCON and other protocols. Other Protocols could include, Gigabit Ethernet, SONET (OC-3, OC-12, OC-48), SDH (STM-1, STM-4, STM-16), Fibre Channel (1 Gbps), ESCON, FICON, and more.

These are often vendor specific as to what is supported and should be checked at the product level. We show the translation within the Optical to Electrical to Optical converter or Transponder. This takes the input signal on a specific wavelength and converts it to electrical signal, which is then re-modulated on the new frequency that it will use during transit within the dark fibre media. These new wavelengths should adhere to the ITU-T grid, however different vendors will often use different channels from this grid and may even skip a channel. Note: Some DWDM solutions use TDM within a channel (wavelength) for some low data rate protocols. This allows more efficient use of the available bandwidth.

An image of the components within a DWDM

Some of the differentiations within the DWDM market place are:
  • Optical filters
    • Thin-film
    • Laser-welded
    • Fibre Bragg Grating
    • Planar-waveguide
  • Channel expansion methods
    • Interleaving
    • Reduced channel sizes greater bandwidth
  • Modulation techniques
    • Direct
    • Indirect
Many of these features have price and performance trade offs.

Special Notices

This material has not been submitted to any formal IBM test and is published AS IS. It has not been the subject of rigorous review. IBM assumes no responsibility for its accuracy or completeness. The use of this information or the implementation of any of these techniques is a client responsibility and depends upon the client's ability to evaluate and integrate them into the client's operational environment.

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