IEC: On-Line Education: WPF: Introduction to Optical Transmission in a Communications Network

A simple, fixed, point-to-point network does not allow changes to the way the traffic is delivered between the points; it is very inflexible. A flexible network allows changes to the way the points are connected and can respond to new connection requests faster than a fixed network.

A fixed, point-to-point network is used when the end-user connections are not expected to change—hence, no flexibility is required (see Figure 8). The equipment used to implement a point-to-point connection is called in TDM a terminal multiplexer (mux) or line system. In WDM, it is called an optical mux/demux. A mux offers fixed connections between end-user termination points.

Figure 8. Fixed Point to Point

The other type of connection is through a flexible network (see Figure 9). This can be implemented using cross-connects or bus structures. A cross-connect is a piece of equipment that provides flexible connections between its termination points. A bus structure allows, in the same way, flexible connections between the termination points of the elements making up the network. The bus route has some bus stops, and the flexibility of going from one point to any other point comes from being able to jump on, jump off, or stay on at any bus stop by means of the add/drop multiplexer (ADM).

Figure 9. Flexible Network

The dotted line around the bus structure in the Figure 9 shows that the bus can be considered a cross-connect to some extent. In TDM (handling bit streams), the cross-connect is called digital cross-connect system (DCS). In WDM, it is called the photonic cross-connect (PXC). In TDM, the synchronous mux is just an ADM. In WDM, it is an optical add/drop multiplexer (OADM), which handles wavelengths.

Network Survivability

Network survivability is a key issue because it reduces operating costs (the carrier need not send a team out in the middle of the night) and provides the carrier with an opportunity for increased revenue (as a result of offering better quality of service [QoS]).

How can a connection remain up if a node fails or if a link breaks? First, a simple circuit setup must be made more resilient. In a point-to-point scenario, link resilience can be introduced by duplicating the link (see Figure 10). Maximum protection is achieved if these two links are routed separately. In the case of a flexible network implemented with a cross-connect, a mesh with its multiple routes offers survivability options. In a bus structure, resilience is achieved by adding an extra link to the network to make a ring, offering simple alternative routes for the connections should they be needed.

Figure 10. Addressing Resilience

Another network element not yet mentioned is the regenerator. It does not add functionality in terms of traffic handling but is present for transmission purposes.

Attenuation and Dispersion

Two main effects will be discussed here: attenuation and chromatic dispersion (see Figure 11). With distance, pulses get smaller, as a result of attenuation. Pulses also get fatter or more spread out, as a result of chromatic dispersion. The game is to keep the pulses tall and thin so that the receiver at the end can recognize and separate them. Small and fat pulses would indeed blur together after some distance, making the receiver unable to recognize the original signal.

Figure 11. Attenuation and Dispersion

Pulses of light must go as far as possible along a fiber to minimize the cost of the link. Light can typically travel 40 or 80 km before it gets too attenuated or dispersed. The farther it can travel without needing a site with equipment to boost the signal, the cheaper the resulting network will be.

The typical way to get light to go farther has been to terminate the link (i.e., go back to electrical signal) and start again. This process is simple, neat, and unfortunately, expensive at high data rates as a result of the cost of the opto-electronics to receive the optical signal, generate the electrical signal from it, and regenerate an optical signal.

In a network, it means that after a given distance (depending on the source, receiver, and type of fiber—perhaps 40 to 60 miles), a site is needed to house a regenerator. Other less expensive options revolve around the optical layer and the amplifier.

The alternative to regeneration is to simply give more energy to the light signal without terminating it. If one imagines a relay race, regenerating is getting a new runner to run part of a span (passing the relay), whereas amplification is keeping the same runner for the whole race but feeding her at strategic places to keep her going. Regeneration is expensive because it must be done per wavelength, whereas amplification technology is attractive because it boosts the entire optical signal in one try—hence, the numerous wavelengths. The cost is in this way shared between the various channels.

Figure 12 shows types of amplifiers.

post-amplifier—This comes just after the transmitter. A post amp may just give the reach needed to get to the other end without the necessity of an intermediate site.
pre-amplifier—This may be added just before the receiver if a post-amplifier is not enough.
line amplifiers—One or more of these are added in the middle of the span. A combination of post-, pre-, and line amplifiers gives a reach of several hundred kilometers. A number of amplifiers can be cascaded (used one after the other), but there is a limit after which regeneration is needed.

Figure 12. Amplifiers/Repeaters

Economics

Figure 13 shows one of the business cases for WDM. The WDM signal made up of several wavelengths is given extra energy by an optical amplifier. The most common of these are erbium-doped fiber amplifiers (EDFA). The alternative before amplifiers was to use regenerators, and in the case of WDM, this meant that a regenerator was needed for each of the optical signals or colors.

Figure 13. WDM Network Economies

Thus, using amplifiers on a WDM system becomes very economical in comparison to the stack of regenerators that would be needed otherwise. The expense of fiber and equipment needed on the line are eliminated. However, the advent of amplifiers does not mean the end of regenerators in a WDM system. When a span is so long that the maximum number of amplifiers cascaded is reached, then regenerators must be used.

Figure 14 is an example of a link with two spans, each with amplifiers. The maximum number is reached, although not all are represented here. Regenerators work on individual wavelengths and therefore all individual wavelengths of the optical signal must be muxed/demuxed at the regenerator point.

Figure 14. Complete Point-to-Point Optical Link

Overview

The following is an overview of the main elements of a point-to-point WDM system:

The optical transmitter of the transmission equipment sends the correct wavelength; if the equipment does not have a transmitter generating the precise color required by the WDM system, then a wavelength translator can be used to terminate the optical signal and generate the WDM wavelength. A wavelength translator allows any optical traffic to be supported by the WDM system. However, synchronous equipment suppliers supporting the complete transmission portfolio (SONET/SDH–based multiservice platforms and the optical portfolio) can choose to have their transmitter generate the correct wavelength directly and thus eliminate an extra component and create more reliable and economical systems.

What gets in the way of light transmission is attenuation and dispersion. Here is a quick overview of the solutions to address the chromatic dispersion issue:

fibers—These are designed to minimize dispersion. However, special fiber means extra cost.
dispersion compensation module (DCM)—The fiber type in a network determines the amount of dispersion to which the optical signal will be submitted. This can be remedied by the use of DCM. A DCM is a length of fiber that distorts the light pulses in a manner that creates a better pulse shape at the other end of the link.
transmitters—Another way to address dispersion is to design the light source to generate a cleaner pulse.

Lasers can be modulated in two ways: direct modulation or external modulation. Direct modulation means that the laser is switched on and off. The technology to do this is cheap but would not be effective for long distances. External modulation consists of leaving the laser on and masking the signal; this is more advanced and more expensive technology. However, long distances can be achieved with this.

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