Constraint based routing due to physical impairments in automatically switched transport networks

Diploma Thesis
Universität Dortmund Fakultät für Elektrotechnik und Informationstechnik Lehrstuhl Hochfrequenztechnik

Constraint based routing due to physical impairments in automatically switched transport networks

by Stephan Pachnicke
2002

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ... 5

ERKLÄRUNG ... 6

EINWILLIGUNG ... 6

ABSTRACT ... 7

SYMBOLS AND ABBREVIATIONS ... 8

1 INTRODUCTION ... 11
1.1 Introduction ... 11
1.2 Optical communications systems ... 11
1.3 Aims and objectives ... 14
1.4 Structure of this thesis ... 15

2 THEORY ... 16
2.1 Chapter overview ... 16
2.2 Maxwell’s equations ... 16
2.3 Reflection and refraction ... 17
2.4 Dispersion and loss ... 18
2.5 Effective length and area ... 21
2.6 Nonlinear Schrödinger equation (NSE) ... 21

3 COMPONENTS ... 23
3.1 Chapter overview ... 23
3.2 Modulation format ... 23
3.3 Filter ... 25
3.4 Assessment of the signal quality ... 27
3.4.1 Eye opening penalty ... 27
3.4.2 Q-factor ... 28

4 LINEAR DEGRADATION EFFECTS ... 31
4.1 Chapter overview ... 31
4.2 Inter-channel crosstalk ... 31
4.2.1 Continuous-wave (CW) case ... 31
4.2.2 Non-return to zero (NRZ) case ... 33
4.2.3 Return to zero (RZ) case ... 36
4.3 Narrow-band spectral filtering ... 40
4.4 Optical demux filter optimization ... 42
4.5 Dispersion ... 45
4.5.1 Group velocity dispersion (GVD) ... 45
4.5.2 Third-order dispersion (TOD) ... 50

5 NONLINEAR DEGRADATION EFFECTS ... 53
5.1 Chapter overview ... 53
5.2 Four-wave mixing (FWM) ... 53
5.2.1 Approximation of the signal-to-crosstalk ratio ... 53
5.2.2 Simulations of the NRZ case ... 57
5.2.3 Simulations of the RZ case ... 63
5.3 Self-phase modulation (SPM) ... 66
5.4 Stimulated Raman Scattering (SRS) ... 72
5.4.1 Theoretical considerations ... 72
5.4.2 Continuous-wave (CW) case ... 78
5.4.3 Non-return to zero (NRZ) case ... 78
5.4.4 Effects of group-velocity dispersion (GVD) ... 81
5.4.5 Simulations of multi-span systems with GVD ... 84

6 EXAMPLES OF NETWORK PLANNING ... 85
6.1 Chapter overview ... 85
6.2 Variation of the channel input power ... 85
6.3 Variation of the number of WDM channels ... 86
6.4 Variation of the channel spacing ... 88

7 CONCLUSION AND OUTLOOK ... 90

8 REFERENCES ... 93

ABSTRACT

Next generation optical communication systems will be characterized by increasing data rates, high signal powers and dense wavelength division multiplexing (DWDM). In future-optical networks channels will be routed through complex, meshed networks (ASTN, Automatically Switched Transport Networks, ITU-T Recommendation G.808 0/Y.1304). These networks will be able to setup transparent optical paths without converting the optical signals to electrical signals. In all-optical networks the physical impairments and degradation effects play an important role. There is a multitude of degradation effects like dispersion, noise, crosstalk, fiber nonlinearities, polarization dependent loss, etc. To enable a fast setup and the best choice of one of the available paths, the signal quality along the whole transmission distance has to be evaluated very fast.
Ideally, only a single figure of merit (FOM), e.g. the bit error rate (BER), will be computed, which incorporates all degradation effects. Therefore it is important to characterize the different physical impairments analytically. Signal distortions can be measured by an eye opening penalty (EOP) and degradation effects due to noise by the optical signal-to-noise ratio (OSNR). The goal is to find and calculate these impairments from the signal parameters (modulation format, data rate, duty cycle, channel spacing, etc.) as well as the route parameters (fiber lengths and parameters, EDFA powers, etc.). Due to the need of fast routing algorithms, time-consuming numerical methods or a complete system simulation are not practical. In addition, it is not possible to linearly accumulate the different degradation effects.
The focus of this work will be to find analytical or heuristic formulas for each degradation effect. These approximation formulas will be compared to the results obtained from a complete simulation of a reference system with the help of PHOTOSS.

1 INTRODUCTION

1.1 Introduction

The significance of fiber optics has grown rapidly in the last decades. In 1980, the first fiber networks were installed in the US and not earlier than 1988, the first transatlantic optical fiber cable (TAT-8) was installed. It operated at mere 140 Mbit/s and used electrical regenerators [22].
Optical fibers are used in different fields of science and technology. First of all, fiber optical devices are used in modern optical telecommunications. The main arguments for the use of fiber optics in this field are the extremely low loss and the very high bandwidth. However, there are also other fields, which make use of fibers extensively. In medicine – for example – the requirements are quite different, and the small size and high flexibility of the fiber are the main arguments in favour.
Therefore it is understandable that this area of science is a field that needs a lot of research to satisfy not only the demands of future telecommunications but also of many other fields.

1.2 Optical communications systems

The invention of the telephone by Graham Bell allowed global communication via telephone networks. At first twisted pair wires were used and were later replaced by coaxial cables with higher data rates and lower loss. As the amount of the transmitted data increased continuously, this technology also reached its limit very fast.
The invention of optical fibers was a revolution for long distance communications, because it allowed transmitting signals of very high bandwidths. At first the losses in the fiber were very high, but already in 1966, Kao predicted an attenuation of
3 dB/km, and in 1968, a fiber with 20 dB/km was realized by Maurer and Corning. Today fibers with an attenuation as low as 0.2 dB/km are used. Another important step was the invention of the laser by Maiman in 1960. With the laser it was possible to use a coherent light source, which is needed to couple a light beam into the fiber. Further inventions like the GaAs laser, EDFAs, low-loss fibers and sophisticated multiplexing techniques made it possible to transmit several data streams simultaneously over a single optical fiber (WDM). Due to the exponential growth of the world-wide-web, the demand for very fast broadband transmission media to exchange large amounts of data all over the world has grown rapidly. Optical fibers, which are now commonly used, are the best choice for this application. Modern optical fiber amplifiers offer the possibility to compensate the loss occurring during the transmission.
Two of the main problems in the transmission of light signals are the occurring dispersion and nonlinear effects. These effects limit the maximal bandwidth or the length of the fiber. Being able to compensate these degradation effects is becoming even more important, since erbium doped fiber amplifiers (EDFA) are available and there may be very long distances of up to several thousand kilometres without (electrical) regeneration of the signal. The idea of the erbium doped fiber was developed at Southampton University. EDFAs make it possible to amplify a weak optical signal (in the C-band around 1545-1560 nm) without converting it into an electrical signal. Recent developments employ differently doped materials, like thulium doped fibers, to amplify different band regions (e.g. S-band at 1470-1510 nm) [15]. Instead of opening up new bandwidth regions to increase the amount of data to be transmitted, also the spectral efficiency of a signal could be enhanced [17]. This could be done, on the one hand, by utilizing polarization multiplexing, which means that two signals with orthogonal polarization states are launched simultaneously. On the other hand, the channel spacing could be reduced, which requires precise wavelength stabilization of the laser sources. Currently wavelength tolerances can be stabilized within about 150 MHz.
For transmitting optical signals over a long distance, normally NRZ-modulation is employed (non-return to zero). NRZ allows minimizing the signal distortions due to fiber nonlinearities and chromatic dispersion. The energy of a single bit is distributed uniformly over the whole bit period, thus both the peak power as well as the optical bandwidth are relatively small. Unfortunately, the use of NRZ modulation is limited to low signal powers and bit rates. If a higher optical signal-to-noise value (OSNR) is required and as a consequence the signal power is increased, signal distortions will occur due to the nonlinear optical effect of self-phase modulation (SPM), which cannot be compensated.
A new approach is the use of solitons in optical fibers. This technique allows compensating the occurring dispersion by the interaction of the nonlinear self-phase modulation of the pulse and the nonlinearities of the optical fiber. The soliton is RZ-modulated with a hyperbolic secant pulse shape. The utilization of soliton pulses (or “sech”-pulses) makes it unnecessary to employ dispersion compensating fibers (DCF).
Today most of the existing optical networks are circuit switched. An optical path (wavelength) is setup permanently for a specific user. Changes can only be made by the network operator, therefore the net is called “static”. In the near future all-optical networks will be in use. In these networks an optical path can be setup automatically over several nodes without the need of converting the optical signal into an electrical one. Such a system is also referred to as an “automatically switched transport network” (ASTN). One important advantage of these systems is that the data – that is passed through a node – remains in the optical domain, and only the data intended for that node has to be processed electronically. Hence, the burden on the underlying electronics at the node for very high data rates will be reduced significantly [3].
The key network elements needed for all-optical networks are optical add/drop multiplexers (OADMs) and optical crossconnects (OXCs). An OADM takes in a WDM signal at multiple wavelengths and selectively drops some of these wavelengths. It also selectively adds wavelengths to the composite outbound signal. OXCs have a large number of input and output ports. They are able to switch wavelengths from one input port to another. Both of these devices may incorporate wavelength conversion capabilities [7]. In today’s optical transport networks (OTN) an optical path consists of a cascade of optical transmission lines (fibers) and optical switching centres (OXCs). The switching fabric of an OXC can be realized with mirrors. The whole system is controlled by a centralized network management system (NMS). A great disadvantage of these systems is that the path setup has to be done manually. Thus, such networks are also referred to as “semi-static”. In the future the path setup will be performed automatically by the network management. A subscriber may request a certain transmission bandwidth over the user network interface (UNI). This end-to-end signalling functionality also enables new services like bandwidth-on-demand or optical virtual private networks (OVPN) [5]. Another point important to mention is the one of diversity [6]. Two light paths are said to be diverse, if they have no single point of failure. In the event of system failure (e.g. through cut fiber cables), diversity allows to route the data streams simply through other routes to the desired destination. It should also be stated that transparent networks not only offer advantages. A transparent network must standardize technology choices at the outset, and thus cannot exploit new developments such as broader amplifier bandwidths or narrower channel spacing as they become available [21].
Also optical packet switching could become reality in the near future. Optical packet switching will not only route a certain wavelength from one point to the other, but allows routing single data packets. These packets consist of a specific header, which contains all-important routing information, and a payload, which can be flexible and may change with the amount of data to be transmitted. In contrast to conventional packet switching, the payload is remaining completely in the optical domain, and no electrical detection of the signal is needed. Ideally, also the routing of these packets would be performed in the optical layer, but in practice, certain functions, such as processing the header and controlling the switch, will be relegated to the electronic domain [23]. Optical packet switching can also be subdivided in conventional packet switching and optical burst switching (OBS). In OBS, a certain number of packets with the same destination are combined to a burst. The header of a burst is transmitted in a separated channel from the payload, and the path is setup before the payload will be sent. This may be advantageous because no optical delay line is needed during the setup of the path, and the time needed for the reconfiguration of the path may be much larger than in conventional optical packet switching. In today’s first optical packet switched networks OBS is already utilized.
A few words should also be said about the routing algorithms. In general, routing algorithms can be ranked according to their blocking probability, connection setup time and bandwidth requirements for control messages. Examples for such algorithms are the alternate shortest-path routing (ASPR) or multi-path routing (MPR) [14].

1.3 Aims and objectives

The aim of this project is to characterize the different physical impairments on an optical signal, which is transmitted in an optical network, analytically. In contrast to small optical networks, in transparent optical networks it is mandatory to consider the physical impairments. It cannot be assumed that for all optical paths the needed BER can be reached. These physical impairments, as well as economical factors, are considered in “constraint based routing”. It has been shown that the total network cost can be decreased drastically with the help of constraint based routing [12]. In future-systems not only a single dominant degradation effect (e.g. PMD) needs to be considered. Due to the high signal power a multitude of nonlinear effects, which accumulate differently, will be deteriorating the signal. Beyond their importance for constraint based routing, the knowledge of the physical impairments is also important for network planning to find easy design rules.
In this work the different degradation effects will be investigated individually, and for each effect an analytical approximation formula will be given. The results obtained from these approximation formulas will be compared to the results obtained from a full-scale simulation with PHOTOSS. Furthermore, it will be important to show how these effects change, if multiple fiber spans are considered. Finally, a single figure of merit (e.g. EOP or BER) will be calculated to evaluate the signal degradation.

1.4 Structure of this thesis

After a brief introduction into optical communications has been given in this chapter, essential parts of the theory needed for understanding the propagation of light will be explained in the next chapter. The theory is based on Maxwell’s equations as well as on the nonlinear Schrödinger equation (NSE).
Chapter 3 will introduce the different parameters needed for the characterization of a signal (modulation format, data rate, duty cycle, etc.) and the different values used for the assessment of the signal quality (BER, OSNR, Q-factor, etc.).
In Chapter 4 and Chapter 5 the different degradation effects, first the linear ones and afterwards the nonlinear ones, will be examined. For each effect an analytical approximation will be given, and the results will be assessed and compared to the results of the full-scale simulation.
Chapter 6 will be built upon the results of the previous chapters and will present some easy network planning considerations.
In the last chapter a conclusion will be drawn, and the whole project will be critically reviewed. Some suggestions for further work in this area will be proposed.

[...]