Towards Digital Optical Networks - IEEE Xplore

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end solutions. These advanced optical network infrastructures will enable accessibility of new services and applications to the end user offering full access to the ...
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Towards Digital Optical Networks I. Tomkos and A. Tzanakaki Athens Information Technology Center, Athens, Greece Markopoulou Ave., PO. BOX 68, 1902 Peania, Athens, Greece e-mail: {itom, atza}@ait.edu.gr ABSTRACT Technical breakthroughs in optical communications research are expected to further accelerate the realization of transparent optical networks to offer increased transmission bandwidth, integrated transmission and switching capabilities and optical signal processing functionality. This progress is not only expected in the core networks, but also in the metropolitan and access network segments to provide scaleable, transparent and flexible end-toend solutions. These advanced optical network infrastructures will enable accessibility of new services and applications to the end user offering full access to the global information network with improved system performance and reduced cost. The primary objective of the EU COST 291 Action “Towards Digital Optical Networks” is to focus on novel network concepts and architectures exploiting the features and properties of photonic technologies, to enable future telecommunications networks. It is aiming to propose a new generation of optical systems and networks that will accommodate the unpredictable and growing size of traffic demands over global distances requiring an agile Communication Grid supporting quality of services. This advanced photonic infrastructure will employ dense wavelength division multiplexing technologies for signal transmission and routing, optical signal processing and dynamic impairment management to eliminate the limitations of the analogue nature of traditional optical networks, and optical packet and/or burst switching to provide fine bandwidth granularity, network efficiency and flexibility. The presentation will outline resent research advancements towards the realization of Digital Optical Networks and will present some key activities undertaken within the framework of COST 291 project. 1. INTRODUCTION Currently there is a strong desire to migrate from the existing SONET/SDH – based network architecture towards a more dynamic and intelligent, multi-service optical network. This will allow service providers to avoid forklift upgrades or laying more fiber (which is time and cost intensive) and thereby enable cost-effective migration towards a “future – proof” network. The focus of research (and market interest) has also recently shifted towards WDM networks with increased degree of transparency through the use of OADMs and OXCs, with the goal of bringing the benefits (cost and network efficiencies) of optical networking towards the end users. From both technical and economic perspectives, the ability to potentially provide unlimited transmission capacity is the most obvious advantage of WDM technology. However, WDM simply as transmission technology that provides increased system capacity is not as promising for short-haul networks, and needs bring more benefits to operators to gain acceptance. Bandwidth aside, the most compelling technical advantages of WDM networking can be summarized as follows: − Transparency A real catalyst behind the use of WDM in short-haul networks may be its promise of “transparency” in offering new high-end wavelength – based services. Several “shades” of transparency have been envisioned, spanning the spectrum from “full” transparency (format, protocol, bit rate) to some subset. Full transparency at this level of the network can provide the network infrastructure with the capability to accommodate bit rate increase associated with higher bandwidth requirements and new traffic types related with new services and applications available to the end users and reduce the equipment in the signal path, resulting in significant cost advantage. Transparency facilitates the elimination of unecessary in-line opto-electronic (OEOs) conversions. However, such an approach may result in significant engineering problems due to the fact that signal impairments will accumulate in the network and may degrade the system performance. − Dynamic Service Provisioning and Bandwidth Allocation Fast, simple and dynamic provisioning of connections gives service providers the ability to offer broadband services to enterprises/end-users in days rather than months. This feature requires, among others, the ability of network reconfiguration without physical layer constraints (simple network engineering and performance monitoring). Transparent optical networking through the use of OADMs and OXCs can support functionalities such as provisioning, protection, and restoration of high-bandwidth services in the optical layer. However, the minimum use of OEO interfaces introduces a special set of challenges to network designers. The flexibility that WDM systems offer may come at the expense of the scalability (in terms of the number of channels, bit rate, etc.) or the

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geographical extent of the network (i.e. in terms of number of nodes that can be transparently cascaded). The limitations arise mainly from: − The accumulation of transmission and switching node impairments: Several effects, unique in transparent optical transmission and networking systems, limit the maximum physical size that a transparent network can support, since the WDM channels are passing through many optical components and are transmitted over fiber that introduce deterioration of the signal quality. − The difficulty in engineering the network: The network would have to be engineered for the “worst case scenario”. As a consequence, the worst path (e.g. the longer restoration path) will have to be known from the outset, all components must be specified for this worst path (and the specifications will be tighter the larger the network reach). Furthermore, transparency requires that the whole network is engineered before installation of the equipment. Once engineered, the network can not be extended beyond its intended design. 2. IMPAIRMENT MITIGATION TO ENABLE INCREASED TRANSPARENCY In particular at 40 Gb/s transparent networking is very challenging primarily because of two reasons. Firstly the 40 Gb/s networks comprise higher spectral efficiency than 10 Gb/s networks (for example in case of 40 Gb/s – 100GHz channel spacing). Secondly most modulation formats that have been initially proposed for 40 Gb/s systems had wider bandwidth requirements than conventional NRZ modulation used in 10 Gb/s systems. One way to deal with traffic demand increase is to upgrade the transmission systems to 40 Gb/s per channel and of course it would be desirable to be able to do that without having to re-engineer the network. However, this required upgrade to an N x 40 Gb/s infrastructure is not a trivial task, especially in mesh connectivity networks which in most cases are based on mixed fiber types, where various possible connection paths are established. An already optimized 2.5/10 Gb/s infrastructure requires a number of components in every network node (terminal, amplification/regeneration or ADM). These components are optimized to handle transmission impairments and issues for the specific data rates. Therefore, it will be a significant technological breakthrough with important impact on the evolution of optical networks if existing 2.5/10 Gb/s based installed network infrastructures could be upgraded to 40 Gb/s without any modifications in the transmission in-line system. Recently, worldwide a lot of attention and significant effort is put towards the development of 40 Gb/s systems. This effort is focusing on a number of techniques aiming to upgrade to 40 Gbit/s existing infrastructures currently operating at lower rates. These techniques can be summarized as follows: − − − − −

advanced modulation formats: Duobinary, DPSK, DQPSK, OSSB, AP-RZ, PSBT, FEC coding, electrical processing, optical pre-equalization (OPE), all-optical regeneration.

The combination of these novel techniques should enable long unregenerated/uncompensated reach (by avoiding dispersion and nonlinearity problems) and reduce switching related impairments (such as crosstalk and filter concatenation effects). The techniques proposed are expected not only to reduce/eliminate a number of components associated to the network nodes but to offer increased performance as well. These techniques will lead to a layer-1 evolution towards simpler and transparent network infrastructures. The proposed techniques introduce more advanced transceivers that remove the complexity from in-line systems. 3. ELECTRONIC AND OPTICAL TECHNIQUES IMPLEMENTED AT THE TRANSPONDERS In the past some techniques that allow information recovery (e.g. from a disk drive) or optimize performance of older generation telecommunication systems (e.g. wireless, DSL) have been developed. Similar and modified methods have been used in modern optical systems as well. Based on these techniques totally distorted signals/data can be fully restored. In principle these techniques comprise electronic processing at the transmitter, receiver, or both sides in order to alleviate the impact from the majority of effects introducing performance degradation. The specific process is called channel equalization (CE). Electronic CE (ECE) is the standard way in older generation telecom systems and can be also used in optical systems following the optoelectronic conversion at the receiver photodiode. Forward error correction (FEC) is another electronic method use to improve system performance at the cost of bandwidth efficiency. Except for these purely electronic methods, optical systems employ all-optical methods that perform processing in the optical domain (e.g. chirped fiber grating, etc.) which can be assumed as OPE techniques. Finally another method is to use an alternative modulation format with respect to the traditional NRZ. A number of electrical and/or optical components/circuits are required in order to implement these formats. These advanced technologies that enable 40 Gb/s networks are described in brief in the following paragraphs.

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4. OPTICAL REGENERATION Another proposed solution is aiming to design and develop optical regenerators that will offer single or multiwavelength operation i.e. regeneration of multiple wavelengths through a single device. In order to significantly improve the cost viability of the proposed system it is important to handle a number of channels using a single regeneration element, rather than having to demultiplex the individual wavelength channels and apply regeneration discretely to each one of them. In addition, the optical regeneration technique should be able to support operation at high data rates (160 Gbit/s). The current commercial status of regeneration technology is based mostly on optoelectronic regeneration techniques, whereby the optical signal is received, regenerated in the electronic domain and converted to optical. This is a costly solution particularly for systems supporting a large number of channels at high data-rates. An improvement to this approach was the integration of a number of these o/e/o regenerators on a single chip, which is a technology currently available by equipment vendors such as Infinera. Regarding all-optical regeneration most of the techniques that have been reported in the literature or are available as product offerings support single wavelength regeneration and are not suitable for very high data rates beyond 40 Gbit/s. In terms of optical multi-wavelength regeneration a technology approach that has been reported in the literature suitable to support high data-rates is based on quantum-dot (QD) semiconductor optical amplifiers (SOAs). An alternative technology that has the potential to also support optical multi-wavelength regeneration at high data-rates is crossphase modulation in highly nonlinear fibers. It should be noted that none of the two technologies has demonstrated multi-wavelength operation so far. It is clear that multi-wavelength operation has the potential to follow the paradigm of optical amplification and its application in WDM systems which has introduced a major revolution in optical communications making optical networking a powerful and economically viable solution. As discussed above all-optical networks are analog in nature and the physical impairments degrading the performance of optical signals accumulate across the optical paths. This makes the network engineering more complex, more sensitive to noise, dispersion, nonlinear effects, crosstalk, filtering effects etc, less flexible to new customers or services, and more challenging to engineer, deploy, manage, and upgrade. The ideal requirements that future network infrastructures should meet include the service flexibility associated with optical networking (e.g. optical add/drop and cross-connection of any optical service at any network node) with simple network engineering and operations. This can be provided through a digital optical network where appropriate systems will perform regular “cleanup” of analog optical impairments, grooming and multiplexing. However, this can be practically feasible only through suitable network solutions with affordable cost that can compare favorably to the cost of the existing solutions. Regeneration (and all-optical regeneration) has different flavors. Typically one classifies devices that provide Reamplification and Reshaping as 2R regenerative devices and devices that provide additional Retiming as 3R regenerative devices. While simple and efficient 2R regeneration at 10 Gb/s has been successfully tested in loop experiments, 3R regeneration is seems to be needed for bit-rates at 40 Gb/s and higher. From a cost point-of view 2R regeneration is more desirable and should be pushed to 40 Gb/s and beyond. Another cost advantage might result from the introduction of multi-wavelength regenerators. So far research has mostly focused on single-carrier operation, and multi-wavelength regeneration has only recently been observed. 2R multiwavelength regenerators might significantly change today’s wavelength-division-multiplexed (WDM) systems. Multi-wavelength regeneration functionality alone does not solve all problems related with optical networking engineer. It should be complemented by other functionality like optical switching and grooming. For example, some may claim that multi-wavelength regeneration might not be that useful in a scenario where there is a mix of 40 and 10 Gbit/s signals in the same network, and you may need to regenerate 40 Gbit/s more often. However, if the multi-wavelength regenerator is located at an OXC with capability to selectively route wavelengths carrying e.g. 40 Gbit/s signals on demand to the regenerator (depending on the architecture of the node), then we have the ultimate solution. It is clear that multi-wavelength operation has the potential to follow the paradigm of optical amplification and its application in WDM systems which has introduced a major revolution in optical communications making optical networking a powerful and economically viable solution. In addition, the proposed optical regeneration technique should be able to support operation at high data rates (up to 160 Gbit/s). At a channel line rate of 160 Gbit/s, the jitter might become the most pressing degradation mechanism, and so it might become necessary to add multi wavelength retiming modules to the 2R regenerators deployed for lower line rates. Figure 1 attempts to give a schematic representation of the cost comparison between the different regeneration technologies. As can be seen in Figure 1a the cost of the systems based on existing o/e/o regenerators supporting single channel operation increases as expected when the capacity of the systems

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increases. It should be noted that the capacity increase assumes the addition of extra wavelength channels into the system. If the single channel o/e/o solution is replaced by an integrated o/e/o device which incorporates a number of regenerators on a single chip then although the overall cost of the system increases with capacity the slope of increase is lower compared to the single device per chip case. This is due to that the cost of each device is now shared between a larger number of channels, while the same demultiplexing/multiplexing equipment is required in both cases. Comparing single channel optical regeneration (particularly 3R) using existing commercially available technology one will discover that it will be more expensive than its o/e/o counterpart. In the case of the multi-wavelength optical regeneration approach having a single device supporting the regeneration of several wavelength channels it can be observed that it introduces the highest cost among all technology options for low capacity levels. However, it starts costing in as soon as the capacity of the system exceeds a certain level and will provide the most cost-effective solutions compared to all alternatives for high capacity levels (large number of wavelength channels). The implementation of a multi-wavelength regenerator is expected to require more sophisticated technology compared to a single-wavelength regenerator introducing higher cost levels, however it gives significant cost benefits as it requires a smaller number of regenerators and eliminates the need for demultiplexing and multiplexing equipment. This benefit will increase for higher channel counts and data-rates Fig. 1b.

Single optical regeneration Multi-wavelength optical regeneration

Cost/bit

Regenerator Cost

Single o/e/o regen M integrated o/e/o regens Single optical regen Multi-wavelength optical regen

Capacity (Nx10Gbit/s)

(a)

Capacity/channel

(b)

Figure 1. Schematic representations of the cost efficiency provided by the optical multi-wavelength regeneration. 5. TRAFFIC AGGREGATION/GROOMING In next generation wavelength-routed networks, switching will be performed through reconfigurable optical add/drop multiplexes (ROADMs) and optical cross-connects (OXCs) supporting provisioning, protection and restoration at the optical layer. In OXCs using electrical switching, sub-wavelength switching granularity offering traffic grooming can be supported as well as inherent regeneration, wavelength conversion and bit-level monitoring. However, in transparent OXCs the finest switching granularity is at the wavelength level with no regeneration impacting significantly the network scalability. Transparent mechanisms for traffic aggregation and grooming i.e. sub-wavelength switching granularity should be somehow provide by new systems. 6. RELEVANT COST 291 ACTIVITIES COST 291 WG1 Optical and electronic processing for digital network performance. This workgroup focuses on physical layer and implementation related issues of transparent optical networks and covers advanced topics such as: − Dynamic management of physical impairments. This activity aims to optimize the system performance and minimize the margins required to accommodate penalties associated with e.g. dispersion, unequal amplifier gain spectrum, etc and includes techniques such as electronic dispersion compensation (EDC), forward error correction (FEC), optical channel equalization as well as the study of various modulation formats. − Per bit optical signal processing. This includes functions such as logical gates, ultra short pulse generation and manipulation, header recognition and label swapping, optical buffering, per bit signal monitoring, wavelength conversion, optical 3R regeneration, multiwavelength regeneration and optical grooming.