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Project P709 Planning of Full Optical Network Deliverable 1 Considerations on Optical Network Architectures: Functionalities, Configurations and Client Signals Suggested readers: Managers, PNO Optical Network planners Experts on Standard Bodies (ITU T SG-13/15 and ETSI TM1 WG2/3) Optical systems and equipment manufacturers For full publication January 1999

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Project P709

Planning of Full Optical NetworkDeliverable 1 Considerations on Optical Network Architectures: Functionalities, Configurations and Client Signals

Suggested readers: Managers, PNO Optical Network planners Experts on Standard Bodies (ITU T SG-13/15 and ETSI TM1 WG2/3) Optical systems and equipment manufacturers

For full publication

January 1999

EURESCOM PARTICIPANTS in Project P709 are:

Finnet Group Swisscom AG Deutsche Telekom AG France Tlcom MATV Hungarian Telecommunications Company TELECOM ITALIA S.p.a. Portugal Telecom S.A. Telefonica S.A. Sonera Ltd.

This document contains material which is the copyright of certain EURESCOM PARTICIPANTS, and may not be reproduced or copied without permission All PARTICIPANTS have agreed to full publication of this document The commercial use of any information contained in this document may require a license from the proprietor of that information. Neither the PARTICIPANTS nor EURESCOM warrant that the information contained in the report is capable of use, or that use of the information is free from risk, and accept no liability for loss or damage suffered by any person using this information. This document has been approved by EURESCOM Board of Governors for distribution to all EURESCOM Shareholders.

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Preface(Prepared by the EURESCOM Permanent Staff) Network traffic is increasing at an unprecedented rate, driven by the dramatic growth of the Internet and corporate data communications. The evolution of photonics makes the development of optical switching and routing structures in the core and metropolitan part of the transport network possible. This brings an increase in capacity and reduces transport costs. The Wavelength Division Multiplexing (WDM) technique jointly with optical crossconnect (OXC), and Optical Add-Drop Multiplexing (OADM) equipment, will permit the realisation of a switched optical layer based on wavelength routing of semipermanent paths and fast protection/restoration mechanisms for the large amount of information flows carried on the optical links. As a consequence, the development of an optical network infrastructure will enable the flexible, reliable and transparent provision of transport services for any type of traditional and innovative services and applications. Taking into consideration the current trends, the objective of network planning is to find the best possible balance between network implementation cost, network flexibility, network availability and survivability, subject to service requirements and topological constraints. The aim of the P709 EURESCOM Project is to investigate a number of alternative strategies for the planning of the optical transport network - with massive deployment of WDM, OADM, and small size OXC - that will be used in a middle term future. This is the first Deliverable (D1) of P709. D1 provides an overview over network architectures, which potentially may be used in the future. It also summarises the requirements on optical networks as well as maturity and availability of optical functionalities. It should be noted that the Deliverable could not include all new functionalities of optical devices since it is an ongoing technology and due to the limited study period, this was not possible. P709 is a logical continuation of the P615 Project (Evolution towards an optical network layer) and some input from this Project was used in D1. D1 is a very useful study for Managers, Optical Network planners, and experts on Standard Bodies of ITU-T SG15 and ETSI TM1 (WG2 & WG3).

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Executive SummaryOptical WDM network is gaining more and more attention and is being implemented in a number of field trials. Several commercial products are appearing on the market with certain maturity. In USA, Europe and Japan, most of PNOs are planning to increase the capacity of their transport network with massive deployment of WDM point-to-point system as well as fixed OADM and small size OXC. The aim of EURESCOM Project P709 Planning of full optical network is to investigate a number of alternative strategies for the planning of optical transport network. This Deliverable D1, the first one of P709 Project, concludes the results of Task 2 Considerations on Optical Network Architectures activities. This document is aimed at those people who work on Network Planning for PNOs, Experts on Standard Bodies related to the optical technologies, systems and networks, or manufacturers building equipment for WDM networks. The fundamental idea of the document is to show how the WDM technique could bring new network architectures through the use of novel optical functions, and how the latter optical network layer could transport multi-client signals. It could provide companies with an overview of the state-of-the-art of optical functions, and useful considerations on optical network architectures impacting the network planning process. The first part of this Deliverable discusses the general characteristics of optical functions as they are available now or will be in the near future. Commercial WDM point-to-point systems are also described and compared. Different classes of network architecture, from the simple topologies to more complex structures are presented in Section 2 in order to select reference network architectures. The possible combinations of basic optical network architectures are collected, in relation with work carried out in P615 Project. The resulting selection of reference two-level network architectures is the following: CS-Ring architecture OMS-SP Ring architecture mesh-ring architecture ring-mesh architecture

The document goes on to discuss important network parameters which characterise the WDM networks in terms of architecture, demand, physical limitation, topology and survivability. In the last Section, the possibility to plan an optical network using a non SDH client signal is proposed. After a brief investigation into ATM and IP client signals performance and functionalities, multi-layer network configurations are proposed using IP, ATM, SDH and WDM network functionalities. A first evaluation of ATM over WDM, IP over SDH, IP over ATM and IP directly over WDM configurations is discussed. Main achievements of the Deliverable are: identification of new optical functions considered necessary in order to enable the migration to WDM future optical networks or desirable to enhance offered network functionality

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selection of optical network architectures contribution to determine the physical limitation and typical values of network characteristic parameters contribution to determine the ability of planning an optical layer carrying non SDH signals.

While addressing considerations on optical network architectures, the elementary optical functionalities, network configurations and the possibility of carrying non SDH client signals are discussed. The selected optical network architectures will be used in Task 3 and Task 4 in comparative studies of planning methodologies.

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List of AuthorsJamil CHAWKI France Tlcom BD-CNET Task 2 & PIR 2.4 Leader

Antnio Jaime Ramos: Portugal Telecom/ CPRM-Marconi Hlder Gaspar: Eduardo Sampaio: Reinald Ries: Ralf Herber: Paulette Gavignet: Andr Hamel: Franois Tillerot: Gza Paksy: Teresa Almeida: Portugal Telecom/ CPRM-Marconi Portugal Telecom/ CPRM-Marconi Deutsche Telekom AG Deutsche Telekom AG France Tlcom BD-CNET France Tlcom BD-CNET France Tlcom BD-CNET Hungarian Telecom MATAV Portugal Talcum

PIR 2.3 Leader

PIR 2.2 leader

Antnio Jaime Ramos: Dag Roar Hjelme (Sintef, Norway): Martjin Luyten (NL):

Internal Reviewer External Reviewer External Reviewer

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Table of ContentsPreface .............................................................................................................................i Executive Summary...................................................................................................... iii List of Authors................................................................................................................v Table of Contents ..........................................................................................................vi Abbreviations ............................................................................................................. viii Introduction ....................................................................................................................1 1 Assessment of optical functionalities and WDM point-to-point systems ...................3 1.1 Description of available functions....................................................................3 1.1.1 Signal Transport (Single mode fibre)..................................................3 1.1.2 Transmitter ..........................................................................................3 1.1.3 Receiver...............................................................................................4 1.1.4 Transponder.........................................................................................4 1.1.5 Dispersion compensation ....................................................................4 1.1.6 Optical Amplifier OA, 1R (EDFA) .....................................................5 1.1.7 Filters...................................................................................................5 1.1.8 Optical Add Drop Multiplexer OADM ...............................................6 1.1.9 Space switch (matrix) ..........................................................................6 1.2 WDM point-to-point Systems...........................................................................7 1.2.1 Description of a WDM point-to-point link..........................................7 1.2.2 N x 2.5Gbit/s systems..........................................................................8 1.3 Identification of new/desirable optical functions ..........................................10 1.3.1 Wavelength conversion .....................................................................11 1.3.2 Optical signal monitoring functions (QoS, optical spectrum, and Failure detection) ....................................................................11 1.3.3 Optical 3R regeneration.....................................................................11 1.3.4 Network survivability (protection, restoration).................................11 1.3.5 Management functions ......................................................................12 1.3.6 Optical time domain multiplexing OTDM (Long term function) ........................................................................................12 1.3.7 Optical packet switching (Long term function) ................................12 1.4 Conclusion ......................................................................................................12 2 Assessment of optical network architectures ............................................................13 2.1 Complex topologies ........................................................................................13 2.1.1 Connected rings .................................................................................13 2.1.2 Meshed domains interconnected by a ring trunk...............................13 2.1.3 Ring domains interconnected by a meshed trunk..............................13 2.2 Characteristic parameters ...............................................................................13 2.2.1 Specific characteristics of optical network .......................................14 2.2.2 Parameters related to topology ..........................................................15 2.2.3 Parameters related to physical limitations.........................................16 2.2.4 Parameters related to demands ..........................................................17 2.2.5 Parameters related to architecture .....................................................18 2.2.6 Parameters related to the survivability approach ..............................19 2.3 Selection of reference network architectures .................................................19 2.3.1 Two-level CS-Ring architecture........................................................20

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2.3.2 Two-level OMS-SP Ring architecture .............................................. 20 2.3.3 Two-level mesh-ring architecture ..................................................... 21 2.3.4 Two-level ring-mesh architecture ..................................................... 22 2.3.5 Characteristics of the selected optical network architectures ........... 22 2.4 Identification of physical network parameters limitation .............................. 23 2.4.1 Identification of mechanisms originating limitations [4, 5].............. 23 2.4.2 Identification of systems/components which introduce limitations...................................................................................... 24 2.4.3 Processes to overcome limitations at present and solve them in the future ....................................................................................... 25 2.5 Identification of Ranges of values.................................................................. 25 2.5.1 Functional layer characteristics ........................................................ 26 2.5.2 Ranges of values ............................................................................... 26 2.6 Conclusions .................................................................................................... 28 3 Potential of WDM routing for different client signals.............................................. 29 3.1 ATM client signal........................................................................................... 29 3.1.1 ATM Network functionalities and physical layer............................. 29 3.1.2 ATM Services ................................................................................... 29 3.1.3 ATM Performance Parameters.......................................................... 29 3.2 IP client signal ................................................................................................ 30 3.2.1 Internet network layers and services ................................................. 30 3.2.2 IP protocols : IP v4/v6, RTP and RSVP ........................................... 31 3.3 Network configurations required by ATM/IP client signals.......................... 32 3.4 Impact of non SDH client signal on the planning of optical network ........... 33 3.4.1 ATM over SDH over WDM : SDH protection vs. WDM protection....................................................................................... 33 3.4.2 Configuration ATM over WDM ....................................................... 35 3.4.3 Configuration IP over ATM [9] ........................................................ 36 3.4.4 Configuration IP over SDH [10, 11, 12] ........................................... 37 3.4.5 Configuration IP over WDM ............................................................ 37 3.5 Conclusion...................................................................................................... 38 4 Conclusion ................................................................................................................ 39 References.................................................................................................................... 40 Appendix 1: Recent Progress in the Performance of Optical Transmission System Components ............................................................................................. 41

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AbbreviationsAAL ABR ACK APS ATM AWG BER CBR CBFG CDV CER CLR CMR CS Ring DA DCF DFF DSF EDFA FTP FWM HDLC IP IPv4 / v6 LAN LLC MAPOS MCTD MS MSP MS-SP Ring OA OADM ATM Adaptation Layer Available Bit Rate ACKnowledgement Automatic Protection Switching Asynchronous Transfer Mode Arrayed Waveguide Grating Bit Error Rate Constant Bit rate Chirped Bragg Fibre Grating Cell Delay Variation Cell Error Ratio Cell Loss Ratio Cell Miss-insertion Rate Coloured Section Ring Dispersion Accommodation Dispersion Compensating Fibre Dispersion Flattened Fibre Dispersion Shifted Fibre Erbium Doped Fibre Amplifier File Transfer Protocol Four Wave Mixing High level Data Link Control Internet Protocol Internet Protocol version 4 / version 6 Local Area network Logical Link Control Multiple Access Protocol Over SDH Mean Cell Transfer Delay Multiplex Section Multiplex Section Protection Multiplex Section Shared Protection Ring Optical Amplifier Optical Add Drop Multiplexer

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OC or OCH OC-DP Ring O/E OMS OM-SDP Ring OMS-SP Ring OPS OSC OTDM OTS OXC PDU POH PPP QoS RSVP RTP SDH SDXC SHR SMF SNAP STM TCP UBR UDP VC VP WDM WWW

Optical Channel Optical Channel Dedicated Protection Ring Opto-Electronic Optical Multiplex Section Optical Multiplex Section Dedicated Protection Ring Optical Multiplex Section Shared Protection Ring Optical Protection Switching Optical Supervision Channel Optical Time Division Multiplexing Optical Transmission Section Optical Cross Connect Protocol Data Unit Path Over Head Point-to-point Protocol Quality of Signal / Service Resource Reservation Protocol Real Time Protocol Synchronous Digital Hierarchy Digital Cross Connect Self-Healing Ring Single Mode Fibre Sub Network Attachment Point Synchronous Transport Module Transfer Control Protocol Unspecified Bit Rate User Datagram Protocol Virtual Circuit of ATM or Virtual Container of SDH Virtual Path of ATM Wavelength Division Multiplexing World Wide Web

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IntroductionIn order to cope with the increasing network traffic driven by the dramatic growth of the Internet and corporate data communications, the evolution of the optical technologies makes the development of optical switching and routing structures in the core and metropolitan part of the transport networks possible. The Wavelength Division Multiplexing (WDM) technique jointly with optical nodes will permit the wavelength routing of semi-permanent paths and fast protection/restoration mechanisms in the optical layer. The purpose of this document is to show how the WDM technique could bring new network architectures through the use of available and forthcoming optical functions, and how the latter optical network layer could be compatible with the transport of signals with various formats. As a logical continuation of the P615 Project (Evolution towards an optical network layer), some input from this Project was used in this document. Beyond the scope of the P615 Project, this document provides results from the investigation of the commercially available and the desirable optical functionalities especially concerning optical amplifiers, advanced functions such as wavelength conversion and optical nodes. It also provides more complex network configurations, based on network interconnections, and identifies characteristic parameters. The first part of this Deliverable discusses the general characteristics of optical functions as they are available now or will be in the near future. Commercial WDM point-to-point systems are described and compared. The optical functions needed for the future WDM networks are progressing rapidly. A set of new optical functions considered necessary in order to enable the migration to WDM future optical networks or desirable to enhance offered network functionality, is also identified. From these possible network functionalities described in Section 1, different classes of network architecture, from the simple topologies to more complex structures, are presented in Section 2 in order to select reference network architectures. The resulting selection of reference two-level network architectures is the following : CS-Ring architecture OMS-SP Ring architecture mesh-ring architecture ring-mesh architecture

The selected optical network architectures will be used in Task 3 and Task 4 in comparative studies of planning methodologies. In Section 2, the most important network parameters which characterise the WDM networks from the point of view of architecture, demand, physical limitation, topology and survivability, are summarised. Physical limitation and typical values of network characteristic parameters are presented in the last part of this section. Finally, in Section 3, the possibility to plan an optical network using a non SDH client signal is proposed. After a brief investigation into ATM and IP client signals performance and functionalities, multi-layer network configurations are proposed using IP, ATM, SDH and WDM network functionalities. A first evaluation of ATM

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over WDM, IP over SDH, IP over ATM and IP directly over WDM configurations is discussed.

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1

Assessment of optical functionalities and WDM pointto-point systemsOptical network planning activities have to reflect a variety of physical as well as practical conditions and constraints in order to produce useful results. Among these is the set of the available optical functions which is to be used for the construction of the network under consideration. In this first section of the Deliverable, the state-of-the-art of optical functions, as they are available now or will be in the near future, will be presented. The performance of available WDM systems (point-to-point) will be compared. In this part such results are already available from EURESCOM P615 Project. The state-of-the-art of optical components and of realised functions which can be used in WDM optical networks, however, is progressing very rapidly. Thus it is necessary to update the information on the presently available optical functions. Finally, a list of desirable optical functions will be discussed in the last paragraph.

1.11.1.1

Description of available functionsSignal Transport (Single mode fibre)All-optical networks are based on a passive fibre infrastructure which serves as the physical transport medium between the network nodes. The most relevant properties of transmission fibres are attenuation, dispersion and non-linearity. Standard single mode fibres (SMF) as well as dispersion shifted or flattened fibres (DSF, DFF) are commercially available with standardised properties according to ITU-T recommendations G.652 ...655. TrueWave fibre also is a kind of dispersion manipulated fibre. With attenuation values close to that of SMF its dispersion, however, is kept non zero at a value optimised in order to produce minimum distortions due to the combined effects of non-linearity and dispersion. In table 1 the characteristic data of various fibre types are summarised [1], [2] .Fibre type Zero dispersion wavelength (m) Dispersion coefficient at 1.55m (ps/(nm.km) SMF 1.312 17 DFF 1.535 1.565 ? 1.56 0.1 3.5 True wave 1.518 -1 5.5

Table 1: Characteristic data for some types of single mode fibres

1.1.2

TransmitterLaser diode with direct and integrated modulation The standard optical transmitter element in WDM systems is a laser diode. Integrated laser modulation, ILM, offers a high dispersion tolerance through the use of electroabsorption modulator on the same laser chip. Key features of these devices are : optical output power modulation bandwidth dispersion tolerance 0 to +10 dBm 10 GHz 1000 to 10000 ps @ STM-16

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stability & accuracy of optical frequency 0.05 nm

Laser diode with external modulation External modulation is used in order to reduce the chirp of the optical transmitter and thus increase its dispersion tolerance. Transmitter modules are available for 10 Gbit/s transmission and in laboratory systems modulation bandwidth of 100 GHz has been demonstrated. Apart from reduced width of the optical spectrum the other properties are the same as for directly modulated laser diodes.

1.1.3

ReceiverOptical receivers are found in optical line terminations and in transponders where they convert the signal from the optical into the electrical domain. State-of-the-art optical receivers reach sensitivities close to 30 dBm for 2.5 Gbit/s and around 20 dBm for 10 Gbit/s (BER 10E-10).

1.1.4

TransponderTransponders are opto-electronic frequency converters which basically consist of an optical receiver and transmitter. The receiver converts the optical input signal into the electrical domain where it is amplified and sometimes even reshaped and re-timed. This signal is used to modulate a laser diode optical transmitter which produces the required optical carrier frequency. Most WDM system manufacturers rely on transponders as input interface into the WDM system. They are available now for bit rates up to 10 Gbit/s. The input sensitivity varies considerably (-5 dBm ... 20 dBm) for devices from different manufacturers. Transponders accept 1.3 m- as well as 1.55 m input signals and their output powers are around 0dBm. Some manufacturers still offer devices with output frequencies not matching the ITU-T recommendations concerning the WDM channel frequencies. Transponders which do not regenerate the input signal will work with any type of intensity modulated digital (binary) client signal independent of signal format (e.g. SDH, ATM...) and bit rate (155 Mbit/s, 622 Mbit/s, 2.5 Gbit/s, 10 Gbit/s) within the limits of the specifications. Frequency- or phase modulated optical input signals cannot be used with transponders.

1.1.5

Dispersion compensationBesides fibre attenuation it is the effect of fibre chromatic dispersion which mainly limits the achievable repeater spacing in optical links. The origin of the latter effect is the variation of the group delay as a function of the optical frequency. In fibre optical transmission lines the dispersion effect increases linearly with fibre length and width of the optical spectrum and causes pulse distortion and bit interference. As chromatic dispersion is a linear effect it can be compensated by inserting additional appropriate optical elements into the transmission link. Dispersion compensating fibre Dispersion compensating fibre (DCF) is a special type of fibre which for light in the 1.55 m wavelength region has negative dispersion coefficient in the order of 80 ps/(nm.km). Thus 1km of DCF is needed to compensate the dispersion of about 5 km of SMF as the corresponding value is 17 ps/(nm.km) in SMF.

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The value of the dispersion coefficient varies as a function of optical frequency in DCF as well as in SMF. Therefore it is not possible to achieve perfect dispersion compensation in a large frequency range. The attenuation of DCF with typical values around 0.6dB/km is still considerably larger than that of SMF. Chirped Bragg fibre grating Using chirped Bragg fibre gratings (CBFG) is another option for dispersion compensation. These devices provide a low loss solution. However, they work in reflection mode and therefore optical circulators or fibre couplers are necessary to separate input and output signal. Presently compensation bandwidth of only some hundreds of GHz achievable with one CBFG is more limited compared to DCF. A wider bandwidth can be achieved through the use of longer gratings or cascaded gratings. But the requirement of additional circulators or couplers may be regarded as a drawback. On the other hand, their non-linearity is practically zero which may be particularly important in very high bit rate systems with 10 Gbit/s and more.

1.1.6

Optical Amplifier OA, 1R (EDFA)Erbium doped fibre amplifiers (EDFAs) are one of the key building blocks of WDM systems. They allow the economical power amplification of all the signals in the different WDM channels. The system relevant optical properties of EDFAs are: power gain, saturated output power, noise figure, optical bandwidth and polarisation mode dispersion. The power gain is calculated as the ratio of output to input signal power of the amplifier. This value directly determines the maximum link segment attenuation between consecutive EDFAs. It depends on the number of channels and on total link length. In practical links this value varies from below 20 dB to 30 dB. The saturated output power is the upper limit of the total output power from the amplifier for high input power. Typical values range from 13 dBm to 17 dBm while EDFAs with output powers of up to 30 dBm are commercially available.

1.1.7

FiltersFibre Bragg Grating Filters These filters are based on photosensitivity in Ge-doped core optical fibres; reflection gratings are written by illuminating the fibre with a standing wave interference pattern. In recent years, fibre Bragg Gratings have proven successful as in line filters. This device has the advantages of being low loss, with a narrow pass-band characteristic (0.5 nm) and potentially low cost. This is a very promising technology for fixed filtering with a channel spacing in the nm range. Tuneable filtering is obtained by stretching the fibre where the Bragg filter is deposited. They are commonly used with optical circulators to obtain the OADM functionality. Diffraction Gratings and Phased Arrayed Gratings Grating devices are suited to address several wavelengths simultaneously because they pass a discrete set of predefined wavelengths. Two types are available; the first one is a micro-optic diffraction grating. The typical insertion loss per channel of a diffraction grating device is 3 dB with a -30 dB adjacent crosstalk level. The other type is an integrated optic device (SiO2 or InP) called Arrayed Waveguide Grating; for an AWG, the typical insertion loss is 5 dB and the crosstalk is 25 dB (figure 1). Components are commercially available for 8, 16 or 32 channels with 100

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or 200 Ghz channel spacing and the insertion losses for a transit wavelength are 6 to 10 dB.

Figure 1: Spectral response of a wide pass-band AWG

1.1.8

Optical Add Drop Multiplexer OADMConcerning optical add and drop facilities, two main suppliers are offering them : Ciena and Pirelli. Indeed 8 channels can be added or dropped at each amplifier site in the Ciena Multiwave 4000 and 12 in the Pirelli Wavemux. Figure 2 indicates where this function can be introduced in a WDM point-to-point system. It shows the example of insertion and extraction of channels at an amplifier site. The available add/drop functions are fixed but selectable add/drop facilities have already been announced as well as reconfigurable OADMs and OXCs. This is planned for the next years but to our knowledge no exact dates have been given.W DM te rm in al W DM te rm in al

a m p lifie rs tra n sit

d ro p a d d

Figure 2: Possible position of an add/drop function in a WDM point-to-point system

1.1.9

Space switch (matrix)Switching matrices are available which are suited for realising of flexible OADMs and OXCs. Various approaches have been followed to perform the switching function. Devices relying on mechanical operation contain actuators, e.g. motors, electro-statically or piezo-electrically deflected micro mirrors for the switching of the optical signal. Due to the required mechanical movements of part the switching times

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achieved so far range from 30ms to 500ms. Wave-guide devices which make use of thermal or electro-optic effects are considerably faster as can be seen in table 2 which contains data of various switching matrices. With respect to insertion losses and channel crosstalk wave-guide devices do not perform as well as mechanical switching matrices.Technology actuator size insertion loss (dB) switching time (ms) channel isolation (dB) mechanical motor mirror Electro-static thermal polymer thermal 8 x 8 8x8 27 x 27 8 2 50 8 2 40 8x8 8 5 years

1.3.7

Optical packet switching (Long term function)Definition: performs dynamic routing (switching) of optical packets Application: enable very high-speed digital optical packet networks. Needs improvement of optical signal processing techniques like optical addressing, optical header procession... and components such as fast optical switches, optical memories... Availability: 5 years

1.4

ConclusionAfter a presentation of available optical functions and WDM point-to-point systems, a set of new optical functions considered either strictly necessary in order to enable the introduction/ migration to WDM future optical networks or desirable to enhance offered network functionality, was identified. A possible definition and the application context were analysed. Most of these functions have already been developed and tested either in laboratory or in field trials. The use of these functions can however not be considered independently of the architectural network context. The next Section proposes reference network architectures for the optical layer, where the above described optical functions could find their place.

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2

Assessment of optical network architecturesIn this section we provide an overview of classes of network architectures, from the simple topologies to more complex structures which potentially may be used in future optical networks. It points out most important network parameters, which characterise the WDM networks in terms of architecture, topology and survivability. The aim of this section is to select and propose complex optical network structures for further study of Task3 and Task4. Typical values of parameters, related to topology, physical limitations, are listed at the end of this section.

2.1

Complex topologiesA network characterised by a complex topology is composed of sub-networks which are directly interconnected by sharing nodes. In such a network topology the aim is to optimise the capacity of the network by mixing the different types of traffic on as few network elements as possible. The sub-networks will have basic topologies.

2.1.1

Connected ringsThe combined advantages of good protection performance, low cabling costs and efficient use of the network elements can be achieved by a network structure composed of connected rings.

2.1.2

Meshed domains interconnected by a ring trunkThe ring trunk network gives excellent protection capabilities with a minimum of interconnections. However, the requirements of transport capacity between neighbouring nodes are high compared to the mesh, and therefore the trunk ring is mainly advantageous in areas where the cost of installing cables is high.

2.1.3

Ring domains interconnected by a meshed trunkThe meshed trunk network has the advantage of providing excellent node-to-node physical connectivity and, thereby, provides many alternative routes for traffic. The traffic on the cables in a meshed network can, to a large extent, be considered pointto-point and, therefore, the requirements to the transmission capacity on the links are easy to predict.

2.2

Characteristic parametersThe purpose of this section is to present and discuss various parameters to be taken into account to allow the future routing and dimensioning of the optical networks. Techniques and principles commonly applicable to existing networks do not necessarily apply to the dimensioning of networks based on wavelength routing, and this justifies the need for a specific analysis of network requirements.

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2.2.1

Specific characteristics of optical network High capacity: the core network has to deal with many applications and services envisaged for the future, which will probably have different bandwidths. The overall traffic volume is expected to be large and to increase as applications and services become cheaper and easier to use. Hence, the network has to have a large capacity and to be able to handle the granularity of optical channels. Transparency: in order to take into account most of the assets of optical functions and to reduce the complexity of equipment, the signal should not be converted to the electrical domain wherever it is possible. Several levels of transparency could be specified such as signal format, bit-rate, transfer mode and service. Full transparency usually does not exist, as physical constraints always cause transparency limitations. Flexibility : refers to the ability of the network to accommodate changes in traffic patterns. This could be easier in optical networks since the granularity of handled signals is higher. Connectivity is the network ability to establish connections independently of the actual state of the network. Full connectivity means that any connection between any two points of the network can be established at any time. In optical networks the availability of wavelengths, OXC blocking or limited number of OADM wavelengths are the main barriers to full optical connectivity. Scaleability: is the possibility of capacity or functionality upgrade of a network by adding new facilities in uniform steps. In optical networks the gradual increase of available wavelengths without changing the whole WDM terminal is a key scaleability feature.

The relationship between characteristic parameters of different network layers and other network related requirements is shown in figure 4. During a network planning process topological, architectural and general requirements are to be considered and fulfilled.

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Input data

Traffic demands Network planning

Geographical topology

Topological characterisics

Basic network structures Service availability requirements

Architectural characteristics Network characteristics Functional layer characteristics

Transmission performances

Functional layer characteristics Network implementation

Capacity Transparency Connectivity Fexibility Granularity Scalability

Available equipment hw,sw

Network engineering Operation and Maintenance

Figure 4: relationships of network characteristics and network requirements

2.2.2

Parameters related to topologyThe definition of network design rules for WDM optical networks, the evaluation of topology optimisation algorithms and network planning tools have to be done using numerical simulations. However, the demonstration of general results can be achieved in some cases, which provide simple design rules that could be used for guidelines in network planning. Some of the most important network parameters are: The number of nodes (N), a node being either a source of traffic (optical channel) or a pure transit node The node degree (D), defined as the mean number of nodes directly (i.e. without any transit) connected to a node via one or more fibres. The link length (LF) normalised to the node spacing. The number of fibre per link (F) The shape of the network (to be defined) The network density

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From those parameters, it has been demonstrated [3] that a good estimate of W, number of wavelengths required to meet the traffic demand (T being the number of channels per connection) can be expressed as :

N 3 2T W FDLF 2 1 2 N 1However, those parameters may not be fully sufficient for describing a network, since topological particularities can be noticed. The network density d is defined by the formula: d = N 1 . This parameter reflects the depth of the mesh in the network. Given a fixed number of nodes in the network, extreme values are obtained with full mesh (d=1) and ring (d=2/(n-1)). The figure below illustrates various situations for network topologies with a given link density.

D

Full mesh D = 4; N = 5 d=1

Mesh D = 3; N = 6 d = 0.6

Ring D = 2; N = 6 d = 0.4

Mesh D = 2.5; N = 8 d = 0.357

Figure 5: Examples of various graph densities

2.2.3

Parameters related to physical limitationsThe quality of the signal across the network dictates engineering rules for network planning. In an all-optical network, the transmitted data remain as optical signals all along the path in the optical layer. However, each path cannot be fully considered as a point-to-point WDM link because : signals on different paths may travel through a different number of optical devices, the number of wavelengths and signal characteristics on fibres can differ with links. The physical limitations lead to degradation of signal quality through cross-talk, signal distortion and noise accumulation. The design of optical cross-connects and the definition of the architecture of the nodes should ensure the nodes provide the necessary functionality. However, there is the issue of how many nodes can be cascaded along one optical path while keeping the signal quality to an acceptable level. As long as the regeneration of the signal has to be performed by the electrical layer, a limitation on the optical path length should be addressed as well. Another issue is related to the limited number of wavelengths per fibre due to cross-talk problems and limited amplifier bandwidth. The problem can be smoothed by wavelength re-use, which complicates the wavelength allocation and routing problem. In summary, the following parameters should be considered, which are closely related to the signal quality: the average and maximum link length in the network

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the maximum number of optical nodes that can be crossed without regeneration the maximum optical path length allowed without any regeneration the number of wavelength conversions along the path the number of wavelengths, optical channels, per fibre being used BER Degradation and system bandwidth optical channel individual integrity (Optical power level, Wavelength stability...) reconfiguration time (re-routing and switching time)

2.2.4

Parameters related to demandsThe demand matrix (or matrices) has an important role in the planning process, because it can affect the selection of the architecture, the grooming and routing policy, the transport characteristics of the optical layer, and so on. In principle the demand matrix for the optical layer should be expressed in wavelengths, but often it is expressed in the typical unit of the client layer that leaves to the optical layer planner a higher degree of freedom in the optical layer design . Demand distribution The distribution of demands among optical nodes is an important characteristic of the network. Different distributions can lead the planner towards different network architectures and allow him to use different methodologies and algorithms in the planning process. Both single wavelength and multi-wavelength demands should be addressed. Optical channel granularity The design of present transport networks is based on static traffic conditions. A large set of objectives can be taken into account in the optimisation. Entries, generally, are given in Mbit/s. The granularity, or channel capacity, defines the correspondence between the demand matrix in the optical layer and the wavelengths matrix. The choice of a granularity iss of courses dictated by technology. Most of multiwavelength systems recently available use STM-16 granularity. However, announcements were made towards STM-64 granularity, allowing mixed granularity on the same fibre. Capacity per link The physical limitations lead to a limit in the number of optical channels per fibre. The transmitted power per wavelength must be large enough to provide an acceptable Signal to Noise Ratio at the receiver. However, it is not possible to increase signal power since optical amplifier gain may saturate and non-linear effects like four-wave mixing will degrade signal transmission performance. Thus, there is a compromise between the capacity per link, the distance between amplifiers and the number of cascaded amplifiers (link length). Demand grooming and consolidation Depending on the network services offered, various kinds of traffic demands can be carried in the optical layer : voice, video signals, data, leased lines... The bit-rate allocated to each kind of signal may be different. The planning process also includes the grooming and consolidation of those signals towards the optical layer. Different

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grooming alternatives can be taken into account, mainly based on the demand distribution, their requirements and the target value of utilisation of the optical layer. For that purpose, grooming optimisation should also take into account the impact in the optical layer.O p tic a l la y e r G r o o m in g D em ands (lo w e r le v e l)

Figure 6: Grooming process

2.2.5

Parameters related to architectureFlat or hierarchical networks A single layer network can be structured in two different ways: flat or hierarchical. A flat network does not impose any constraints on the demand routing, while a hierarchical one allows, in principle, a communication between two peer hierarchical nodes through one or more nodes of a highest hierarchy. Very often traffic in large telecommunication networks is allocated to hierarchical levels or tiers. Two, three or, in some special cases, four level networks are designed. The main advantages of traffic hierarchisation are the clear routing rules and easy manageability. However, sometimes this results in longer paths for individual demands. In the near future, alloptical networks with two hierarchical levels will probably be common for core network applications. Many European projects considered a two-level all-optical network, which included an upper level meshed configuration based on OXCs, and a lower level based on WDM rings. Number of sub-networks on each hierarchical level Both a flat and a hierarchical network can be split into several sub-networks. Generally a sub-network is defined by the node connectivities and by the independence of its own survivability mechanism (that is the reason why these subnetworks are often called Survivable Sub-Networks). Generally, in a hierarchical network, interconnections of sub-networks in the same network level are allowed only via the next higher hierarchical level. The number of sub-networks on a network level is not limited, and it depends only on the actual network size. Types of sub-networks A sub-network can be any network partition, but usually consists of some kind of basic network topology, like rings or small meshes. Candidates for sub-networks are in optical networks WDM rings and optical meshes. Number of transiting nodes (hubs) per sub-network Unless the demand distribution does not need to cross more than one sub-network, each sub-network must have at least one special node where the outgoing traffic is transited. These nodes (often called hub nodes) have special functionalities for interworking with other sub-networks.

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2.2.6

Parameters related to the survivability approachA protection or a restoration mechanism (or both) is generally applied to a network in order to increase the demand survivability against failures. The recovery mechanism often adds some new constraints to the network planning activity such as restoration time, length of paths, routing ... These constraint will be addressed in the Deliverable D2.

2.3

Selection of reference network architecturesHaving studied network topologies we now have to select the reference network architectures that will be implemented in those topologies. In practice the PNOs core network is divided into hierarchical levels from the traffic routing point-of-view. This hierarchisation can be naturally translated to appropriately interconnected topological domains, resulting in complex network topologies. The different domains can then be implemented using several network architectures. In the following we investigate those kinds of hierarchical optical architectures which consist of the basic optical network domains like: CS-Rings, OMS-SP Rings OXC-based optical mesh

The reasons for choosing this particular set of network architectures were the following: the CS-Ring was considered by the previous EURESCOM Project P615 as a good first step in introducing optical functionalities in SDH networks since this architecture combines SDH functionalities of existing equipment (routing and linear MS protection) with optical routing for logical node ordering. the OMS-SP Ring is a very advanced full-optical architecture, where both routing and protection are implemented optically. Therefore, it is expected that the OMSSP Ring will bring specific problems that must be taken into account in the planning of optical networks. the OXC-based mesh is seen as an advanced optical architecture, where the introduction of optical routing and restoration might contribute to simplify the complexity of the electronic equipment in high-capacity mesh networks.

-

-

The hierarchical network configurations selected are seen as interesting possibilities for the partitioning of real networks in more manageable domains. These configurations are expected to be useful in different network scenarios and will, possibly, be deployed at different stages in time, in the evolution from existing networks to networks based on the new optical network architectures. In Table 5 possible combinations of these domains are collected. The investigation is limited to two-level architectures only because these architectures can be considered to be a realistic solution for core networks. Complex optical network architectures consisting of these domains will have a performance dependent on the different protection schemes and re-routing strategies applied over individual sub-networks. Only the dual node interconnecting architectures are selected because in large capacity core networks the disjoint alternative routing is an essential requirement. In

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the following paragraph, we will provide a qualitative comparison since more precise results on network planning for the various architectures proposed here should be provided in Deliverable D3.Upper level Lower level Selected architecture Study CS-Ring CS-Ring 1 P615 OMS-SP Ring OMS-SP Ring 2 P709 OMS-SP Ring Optical Mesh 4 P709 Optical mesh OMS-SP Ring 3 P709

Table 5: Selected reference architectures for comparison of two-level optical networks

2.3.1

Two-level CS-Ring architectureA two or more level (tier) CS-Ring architecture consists of hierarchically interconnected CS-rings. Interconnection and cross-connections are carried out at the SDH client layer. The main advantages of this architecture are: Wavelength allocation can be planned for each ring independently. Equipment is available now.

Figure 7: Two-level CS-Ring architecture

2.3.2

Two-level OMS-SP Ring architectureThe main advantage of this architecture is the optical connectivity between the rings. In the hubs the optical OCH level flexibility depends on the optical cross-connect capability of the applied OADMs. First generation OADMs only have fixed add/drop capability. In this case wavelength conversion could be necessary for the interconnections.

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Figure 8: Two-level OMS-SP Ring architecture

2.3.3

Two-level mesh-ring architectureThis is probably the most promising architecture for the future. Traffic in lower capacity rings is collected and transported by the very high capacity upper level, like in a traditional SDH network. In case of large network sizes OXCs probably have to have wavelength conversion functionality in order to establish large numbers of optical paths. Further study is necessary in order to develop suitable optical protection and restoration mechanisms for this complex architecture. Analogously to similar SDH network architectures, the Optical Sub-network Connection Protection (OSNCP) (1+1 optical path protection) can be a candidate for this purpose.

Figure 9: Two-level Optical Mesh-Optical Ring architecture

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2.3.4

Two-level ring-mesh architectureIn some special cases this architecture could be an optimal solution. In this case a very high capacity optical ring is necessary for interconnection of meshed sub-networks on the lower level. The protection and restoration problems are similar to the mesh-ring architecture.

Figure 10: Two-level Optical Ring-Optical Mesh architecture

2.3.5

Characteristics of the selected optical network architecturesThe following table characterises the reference networks in terms of some of the parameters discussed above.Reference network architectures Characteristic Parameters Selected architecture General transparency connectivity restoration on optical layer flexibility granularity scaleability Architectural Hub equipment No. Of hierarchical levels Flexibility on optical layer Types of subnetworks CS-Ring CS-Ring 1 no full SDH (VC-4/3/12) no SDX DXC VC-4 low SDXC 2(3...) no rings OMS-SPRingOMS-SPRing 2 limited limited optical (OC) yes (limited) limited wavelength low OADM 2(3...) limited rings by 2000 OMS Mesh - OMSSPRing-Mesh SPRing 4 yes optical (OC) yes (limited) good wavelength good OXC 2 limited rings/OXCs/ links by 2000 3 yes optical (OC) (VC-4-3-12) yes good wavelength

OXC (SDXC) 2 yes rings/OXC/ links after 2000

Equipment availability now

Table 6: Some characteristic parameters of the reference network architectures

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2.4

Identification of physical network parameters limitationIn this part of section 2, an attempt is made to identify physical network parameters limitation. By this, we mean the limitations of real existing optical components and subsystems, on realising complete aimed functionality, and its impact on optical networking. These limitations can be grouped into two sets, regarding its origin: intrinsic limitations and technological immaturity limitations. The first set comprises the limitations imposed by the optical nature of used technology, which in spite of the degree of perfection of used components or systems, cannot be eliminated (theoretical limits). The second set groups the limitations mainly due technological immaturity, which result in a non-ideal behaviour or the components; these limitations are expected to be greatly reduced with the improvement of technological aspects.

2.4.1

Identification of mechanisms originating limitations [4, 5]Some effects, their respective causes and originated limitations are listed below:

2.4.1.1

Optical channel individual integrity Crosstalk Crosstalk in WDM systems arises due to filter/Demux imperfections and due to fibre non-linear effects. Both can be kept small enough by appropriate system design so that no significant penalties are expected. The following types of induced crosstalk were identified: Non-linear crosstalk: due to non-linear effects in the fibre; Four Wave Mixing FWM is the most relevant non-linear mechanism in the networks considered. FWM generated cross-talk: depending on the optical power, the wavelength spacing of optical channels, on the fibre dispersion values, and on the transmission distance, it is a limitation on the number of channels for HD-WDM, as it becomes higher for channels more closely spaced and for lower dispersion values. SRS generated crosstalk: depending on the optical power, the wavelength spacing of optical channels, and on the transmission distance, it is a limitation for the number of optical channels and the range of the system. Linear crosstalk: due to non-perfection of components such as filters and switches; can limit cascadability. Two types of linear crosstalk: inter-band crosstalk and intraband crosstalk. Inter-band crosstalk is induced by other wavelength optical channels due to imperfect filtering. Intra-band crosstalk is due to the presence of residual levels of optical power in other wavelengths of the used comb (non perfect filtering), which will add to the signal in those wavelengths. Thus it is a consequence of switching. Crosstalk induced by this process cannot afterwards be removed as it has the same wavelength of the optical signal to which it was added. This constitutes therefore a limitation to cascadability of nodes with crossconnect/ switching functionality. BER degradation: Physical mechanisms that produce pulse broadening with travelled distance, lead to BER degradation, and limit system bandwidth and range. The higher the transmitted bit rate the higher importance these effects have.

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Dispersion: the first cause of pulse broadening and consequent BER penalty, is fibre chromatic dispersion coefficient. This is due to the dependence of propagating velocity on the wavelength. For SMF, in the 3rd window, even for narrow linewidth optical sources this effect becomes important. PMD: Polarisation Mode Dispersion, results from the fact that due to non-perfect geometry of the fibre and induced mechanical stress, the fibre presents birefringence and therefore the two polarisation states have slightly different propagating velocities. This Birefringence varies randomly along the fibre. Special fibres as DCF and EDF (used in EDFAs) present higher values for PMD than SMF. So for long links using DCF and EDFAs the total PMD can result in significant BER degradation. SPM: this is a non-linear mechanism, due to the dependence of fibre refractive index on the optical propagating field intensity. Intensity modulation results into refractive index modulation and therefore phase (and frequency) modulation of the optical signal occurs. This causes linewidth broadening and thus BER degradation. 2.4.1.2 Network element cascadability Optical Amplifier OA Optical amplifier cascadability limitation due to optical amplifier generated noise; this noise accumulates with the number of transverse optical amplifiers therefore limiting cascadability of OAs. Optical amplifier cascadability limitation due to non-flatness of optical amplifier gains, which causes different amplification of individual optical channel power. In case of a change in the number of channels passing through the OA, power transients may cause degradation. OXCs, optical Switches Cascadability limited due to optical power losses, intra-band linear crosstalk and bandwidth reduction by filtering. Optical Mux/Demux Cascadability limited due to optical power losses and bandwidth reduction by filtering

2.4.2

Identification of systems/components which introduce limitationsVarious elements contribute to the physical limitations mentioned above. A relation between the physical nature of the introduced degradation, depending on the component, and the impact on the optical transmission is established in the following list. Optical Fibre Crosstalk: Non-linear effects, in particular FWM System bandwidth limitation: Dispersion BER degradation: Attenuation, Non-linear effects, in particular FWM OXC, Optical Switches BER degradation: Insertion losses Linear crosstalk: optical filtering, optical switching System bandwidth narrowing: optical filtering; filter misalignment; wavelength instability

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Optical Filters , Optical Mux/demux BER degradation: Insertion losses Linear crosstalk: non-ideal optical filtering System bandwidth narrowing: optical filtering; wavelength instability Optical Sources System bandwidth narrowing: Line width, Wavelength accuracy, Wavelength stability, Wavelength tuning accuracy (if tuneable sources) BER degradation: optical power efficiency coupling Optical inline amplifiers BER degradation: amplified spontaneous emission noise BER degradation of less amplified channels: Different channel power amplification due to gain curve (nonflat) Limited cascadability: build-up of optical signal degradation as a result of above mentioned mechanisms, with number of transverse elements.

2.4.3

Processes to overcome limitations at present and solve them in the futureMost of these limiting mechanisms arise from the use of high optical power levels and WDM signals; i.e., already known and existing, their influence only became important in the present context. Some of them cannot be eliminated (however they can be compensated) such as dispersion effect, attenuation, non-linear effects. Others, with techological improvements on the manufacturing processes of the components, can be greatly reduced, almost eliminated, such as linear crosstalk due to filtering and switching, maintaining system bandwidth, by improving wavelength accuracy, stability, tuning, filtering and misalignment etc.

2.5

Identification of Ranges of valuesIn this point, the number of parameters related to the layers of the optical channel, the optical multiplex section and the optical transport section in WDM networks are specified. These values are necessary in the planning process of such networks. Optical layer performance of todays commercially available WDM systems for terrestrial optical networks, however, still keeps improving. In order to take into account the expected technical progress of optical WDM transmission systems as much as possible there are up to three values specified for various parameters. The first of these values represents presently available commercial WDM systems. The second parameter value has been derived from manufacturers publications or advertisements regarding product development in the near future. Whenever possible a third value is presented that reflects results from laboratory experiments or theoretical calculations and which may indicate what will be achieved in future systems.

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2.5.1

Functional layer characteristicsAccording to Rec. G.otn an optical network will consist of the following three optical layers: Optical Channel (OCH) layer Optical Multiplex Section (OMS) layer Optical Transmission Section (OTS) layer

These functional layers can be characterised in terms of the following parameters: Layer OCH Characteristics Transparency Maximum number of cascaded optical nodes Optical cross-connect capacity Optical protection OMS Number of available wavelengths Wavelength conversion Optical protection OTS Power budget Dispersion budget Accumulated noise Cross-talk Number of cascaded optical amplifiers Table 7: Characteristics of optical layers

2.5.2

Ranges of valuesValues reflecting limitations of present implementation of optical functionality and its impact on optical networking, either obtained by calculation, simulation or experimentation, will be presented in this section. Power budget The achievable power budget depends mainly on the optical output power of the EDFAs and on the number of wavelengths used in the network as well as on the maximum number of EDFAs cascaded in any transparent link, which may happen to come into existence due to optical switching, configuration, or restoration processes within the network. Power budget of present systems is limited to a value of 30 dB of attenuation between EDFAs. This value is optimised with respect to a dispersion limited system (no dispersion accommodation) at a bit rate of 2.5 Gbit/s on standard single mode fibre resulting in a total link length of 500 km to 600 km which corresponds to 150 dB of optical power budget. If a larger transparent total link length is desired the power budget between neighbouring EDFAs must be reduced. Dispersion accommodation necessary in this case introduces additional optical attenuation. Dispersion budget If standard single mode fibres (G.652) are to be used without dispersion accommodation (DA) techniques the dispersion budget is limited to values of about 12000 ps/nm for 2.5Gbit/s transmission. 10 Gbit/s transmission without DA is too

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severely limited in length to be attractive. With DA methods the limitations in dispersion budget are relaxed considerably. A values of 75000 ps/nm is experimentally estimated. Accumulated noise The amount of accumulated noise depends on the system design. With a transmission link design, using a shorter span length, that takes into account the network requirements the accumulated noise effects can be kept small enough so that no significant penalties are to be expected Number of cascaded OAs Available WDM point-to-point systems are designed to use a maximum number of up to 6 EDFAs in cascade as outlined above. It is possible to increase this number considerably without running into ASE noise accumulation problems if the optical budget between succeeding EDFAs is reduced. Transmission of 8x5 Gbit/s over a total distance of 4500 km of standard single mode fibre using dispersion accommodation technique has been demonstrated. With dispersion shifted fibre 20x5 Gbit/s over 9100 km have been achieved. With soliton techniques transmission of 8x10 Gbit/s over 10000 km was shown using more than 300 cascaded EDFAs. Parameters values for OCH, OMS and OTS sections are summarised in table 8.parameter available performance announced performance limits

Optical channel (OCH) layer characteristics transparency number of optical nodes in cascade number of wavelength conversions Optical Multiplex Section (OMS) layer number of available wavelengths optical cross-connect capacity protection methods supervision channel Optical Transmission Section (OTS) layer power budget (for link between consecutive EDFAs) in EDFA cascade number of cascaded EDFAs (2.5Gbit/s client)

6-7 1-2

5-10

16-32 ? ?