APPLICATION OF RADIO OVER FIBER TECHNOLOGY TO ENABLE CONVERGED OPTICAL AND WIRELESS NEXT GENERATION NETWORKING Marian Marciniak National Institute of Telecommunications, Department of Transmission and Optical Technologies 1 Szachowa Street, 04-894 Warsaw, Poland,
[email protected] Abstract-A novel approach to the future hybrid communication network capable to support the traffic of different kind, e.g. real time and packet traffic and wireless signal in a single transparent network is proposed. The proposed model combines different technologies as connection and connectionless networking, optical cable and wireless (microwave/ millimeter wave or optical wireless) and it is suitable for a variety of purposes and services in order to achieve global broadband networking. Keywords: Radio-over-Fiber, Next Generation Networking, transparent networks, converged optical and wireless networks, security, Quality of Service. INTRODUCTION & MOTIVATION Nowadays communications target to transmit a variety of services. Those are classical telephony, facsimile transmission, but also the Internet traffic, data transmission, radio and television broadcasting etc. Consequently, various transmission media are used as metal and fiber cables, and microwave, millimeter wave, and optical free space communication links. However, owing to top performance of contemporary optical fibers there is a tendency to exploit optics as far as possible [1]. Thus fibers are used not only for digital voice or Internet traffic transmission, but also for expanding Radio-over-Fiber transmission applications that exploit the optical carrier wave amplitude modulation with a microwave carrier [2], including analogue cable television transmission. The advent of Erbium Doped Fiber Amplifiers in the previous decade upgraded optical fiber transmission with the transparency of the links and with a possibility of long distance DWDM transmission with hundreds of independent transmission channels within a single fiber. However, while DWDM network application for voice and data transmission is already in a mature and highly sophisticated stage, novel kinds of traffic and services can be allocated to optical systems, and attempts to develop hybrid architectures for circuit and packet switched networks were reported recently [3]. While microwave and millimeter wave links have excellent mobility characteristics impossible to achieve for other transmission media (wireless optical links have rather performance if compared with microwave ones), they still suffer from a number of constraints, most of them resulting from EMC (electromagnetic compatibility) requirements, in order to avoid the interference and the crosstalk resulting from. Also the wireless links suffer from the attenuation of the signal due to air characteristics, weather, smog, and the local shape of terrain or the occurrence of trees and buildings. The line of sight between the transmitter and receives is usually an essential requirement for a reliable transmission. This also means that the microwave spectrum is expensive and limited. Radio-over-fiber transmission can be realized in the core networks even at large distances, with potential of amplification/switching in the optical domain [4]. Radio-over-fiber arrangements can be applied in the access part of the Mobile Broadband Systems (MBS) in 60 GHz band which is the goal frequency band for mobile broadband services allocation. However, the attenuation of the air is as high as 10dB/km at 60GHz, while attenuation of light in fibers is less than 0.2dB/km at 1.55µm wavelength. This is why Radio-over-Fiber technology is so much attractive. This paper discusses incorporation of the Radio-over-Fiber technology into a converged network and to allocate a variety of services with different kinds of traffic and QoS requirements to different wavelengths in a single wavelength-division multiplexed optical network in order do satisfy the requirements of the customer specific to particular service used. The Next Generation Networking (NGN) initiative has been recently adopted by The International Telecommunication Union (ITU) as a goal to be achieved during Study Period 2005-2008 [5]. While a general belief is the Internet will support the majority of services offered by NGN, it should be carefully noted to select and separate the services in the network is a necessary condition to assure the Quality of Service and security for the individual ones. Fixed and mobile communications will continue to converge coming years. Internet traffic and packet services are globally and increasingly used for a variety of services now in parallel to classical point-to-point circuit switched connections. It is generally (but not necessarily truly) accepted that the packet traffic should replace the circuit-switched one everywhere, although the Quality of Service and security issues are considered as urgent problems to be resolved. That is what we will criticize in this paper and propose an optimal hybrid
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solution satisfying the needs and constraints of both real-time and packet services. Indeed, the Internet as being based on a ‘best-effort’ principle and carrying a traffic of a statistic nature in inherently vulnerable as Quality of Service is concerned. Moreover, the Golden Age of the Internet when it was a network connecting exclusive scientific community has passed for ever. Now everybody can access the Internet, and obviously not honest people only. In contrary, a mass attacks towards the global Internet network or towards dedicated important targets seems to be inevitable in not a distant future. Here we propose a novel approach to the future hybrid next generation network capable to support the traffic of different kind, e.g. real time and packet traffic and wireless signal in a single transparent network. While microwave and millimeter wave links themselves have excellent mobility characteristics impossible to achieve with other transmission media, they still suffer from a number of constraints, most of them resulting from EMC (electromagnetic compatibility) in order to minimize the interchannel interference and the crosstalk resulting from, and also from a strong attenuation at higher frequencies (60 GHz or above) in the air. The paper is organized as follows. It discussed the optical transparency advantages and limitations. Then it reviews the International Telecommunication Union (ITU) policy for introducing Next Generation Networks (NGN) in the ITU Study Period 2005-2008. The guidelines for the actual network evolution in order to efficiently combine real-time delaysensitive services and non-real time services including mobile services in a single optical transport network with a maximized use of the available bandwidth are outlined. Finally, the Quality of Service guarantees for real-time and non-real time services are discussed in the concluding part. TRANSPARENT OPTICAL TRANSMISSION Here we discuss the optical transparency and its fundamental limitations due to physical constraints as dispersion, polarization mode dispersion, and fiber nonlinearities, and we evaluate the achievable network performance. The notion of transparency has already been applied also for metallic cable based electrical links: those links are so called transparent if the output signal is proportional to the signal at the input. Transparency in optical domain has also its common sense: the medium is transparent if the light goes through. The advent of Erbium Doped Fiber Amplifiers resulted in transparency of optical link, thus in a possibility of WDM transmission. Wavelength-Division Multiplexing (WDM) technology is one of the most promising and cost effective ways to increase optical link total throughput. In a WDM system many information channels are transmitted through one fiber using different optical wavelengths modulated by independent data streams. This method is analogous to Frequency Division Multiplexing (FDM) which is widely exploited in other communication systems, especially in radio broadcasting. Using WDM we can easily increase the capacity of already existing fiber links that is particularly significant in the areas where placing new cables is impossible or too expensive. WDM is a technique compatible with the idea of all-optical networks, where one can create transparent optical paths connecting successive network nodes by switching optical channels organized at the different wavelengths. Unfortunately, in real systems one is faced to the lack of the ideal transparency rather than to the transparency itself. Namely, the signal quality suffers from physical limitations of the fiber, which are the attenuation, chromatic dispersion, and nonlinear distortion. An ideal transparency is not realizable in an optical network, since even an ideal glass fiber exhibits attenuation, chromatic dispersion of the first and higher orders, and glass optical nonlinearities. Moreover in real fibers Polarization Mode Dispersion (PMD) results from random local lack of circular symmetry of the fiber due to technology imperfections and local stresses caused by cable layout. Those analogue features of a fiber result in distortion, crosstalk, and noise of the transmitted optical signal. The term "PMD" is used both in the general sense of two polarization modes having different group velocities, and in the specific sense of the expected value of differential group delay between two orthogonally polarized modes. PMD causes the spreading of a pulse in the time domain and it is actually the main transmission distance-limiting factor in 40 Gbit/s systems and above, and as such it became recently a subject of intense research both for fiber optimization and characterization as well [6]. Chromatic dispersion is an inherent feature of an optical link that severely limits the transmission distance of high bit-rate data streams. Although dispersion-compensating fibers (DCF) are commonly used in order to cope with the chromatic dispersion, they have a substantial drawback as they introduce additional power losses. Another way to combat dispersion effects is to use chirped fiber Bragg gratings as dispersion compensators. The transmission performance of a system with chirped Bragg gratings has been proven to be significantly superior to that of an equivalent DCF module [7]. Nonlinear impairments results directly from the optical nonlinearity of silica glass used as the row material for communication fibers. Modern high-speed DWDM systems are typically built of several transmission spans, each consisting of an erbium-doped fiber amplifier, a single-mode fiber transmission section, and the dispersion compensation
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section (typically a piece of dispersion compensating fiber or a chirped Fiber-Bragg Grating). Such cascaded configuration leads to an accumulation of the products of nonlinear optical interactions. That in turn results in an increase of the optical interchannel crosstalk and degrades the temporal and spectral characteristics of the signal, including the decrease of signal to noise ratio. Consequently, in real transmission links strong limitations for number of channels, channel spacing, bit rate and distance occur due to nonlinear interactions[8]. The most characteristic and essential problem for multichannel optical systems is inter-channel crosstalk [9]. In WDM systems the interchannel crosstalk is caused by non-linear interplay between many different spectral components of the aggregate optical signal. The non-linear optical phenomena involved are self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM), stimulated Raman scattering (SRS), and stimulated Brillouin scattering (SBS). In spite of the intrinsically small values of the nonlinearity coefficients in fused silica, the nonlinear effects in silica glass fibers can be observed even at low power levels because of very large interaction distances. This is possible because of important characteristics of single-mode fibers, a very small optical beam spot size, and extremely low attenuation. Major problem of the network upgrade is to know to what extent the already existing infrastructure can be modernized. As a consequence, the network designers should know the limitations for number of channels, maximum transmission speed, as well as the distance between neighboring optical amplifiers. Those system parameters are determined by fiber attenuation, dispersion, and the optical noise level which results from the nonlinear optical phenomena in the silica fiber itself. The transmission system working on higher average optical power is more susceptible on signal distortion caused by nonlinear optical phenomena. Similarly, that problem occurs in multichannel systems because more channels mean higher total optical power in fiber. Signals co-propagating in neighboring channels strongly interact producing unpredictable noises and decreasing signal-to-noise ratio (SNR) for signals in different channels. Those phenomena are to be carefully investigated, especially in the case of utilizing new fiber types with decreased dispersion. The transparent analogue nature of modern fiber communication systems provides a potential to modulate and detect the optical wave power with microwave or millimeter wave envelope. Broadband wireless signal might be transmitted as an optical wave properly modulated in an analogue way. This works very well in a DWDM network with Erbium-Doped Fiber Amplifiers (EDFA). In modern DWDM optical networks, one has to distinguish the physical network infrastructure (fibers and cables) from the virtual infrastructure (wavelengths). A question arises; do we really need separate networks for different services? Or separate fibers in a single network? Why do not use separate wavelengths for that? NEXT GENERATION NETWORKING The expansion of Internet traffic worldwide forces the global communication community to shift from classical circuit switched, connection oriented networks to modern packet switched, connectionless transmission of data, with a strong interest in guarantees of the network reliability and availability as well as the security of the information and of the infrastructure, generalized mobility etc. This revolutionary change is reflected in the International Telecommunication Union (ITU) policy on the Next Generation Networks [5]. Consequently, NGN are expected to be deployed widely starting from the ITU Study Period 2005-2008. NGN is a packet-based network capable to provide a variety of services including conventional telecommunication services and new NGN services. It should assure an unrestricted access by users to different service providers, supports generalized mobility, and it allows consistent and ubiquitous provision of services to users. In fact the changes to the NGN service provision infrastructures have already started in the industry. The specific objectives of the ITU Next Generation Networks project are to facilitate convergence of networks and services, to coordinate all ITU-T activities related to the establishment of implementation guidelines and standards for the realization of the Next Generation Network, to ensure that all elements required for interoperability and network capabilities to support applications globally across the NGN are addressed by ITU-T standardization activities. NGN target is a generalized mobility allowing a consistent provision of services. The user is considered as a single person even if s/he temporarily changes the access point whatever it is. NGN will connect both existing customer terminals such as analogue telephone sets, fax machines, ISDN sets, cellular mobile phones, GPRS terminal devices, Ethernet phones through PCs, cable modems, etc., and emerging ‘NGN aware’ devices. Till now, similar services are offered to users both on fixed accesses and on mobile networks. However, they are still considered as different customer classes, with their specific service configurations. No efficient bridging occurs between fixed and mobile services. The user may be able to roam between similar public wireless accesses only. Very limited nomadic displacements are allowed between some fixed accesses.
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An important NGN issue is the security of information, of the transport functions and infrastructure, and the customer’s privacy and rights. This problem stretches far beyond telecommunications as computing and networking have begun to touch almost every aspect of our life. The move to a complete IP-based infrastructure will lead to even greater challenges. Security in NGN is inherent but nevertheless crucial and is touching many areas as: network architecture, QoS, network management, mobility, billing, and payment. Thus NGN security mechanisms should be able to protect its infrastructure, to fight against the fraudulent use of the services, and to protect the own infrastructure from outside attacks. The NGN networks are no longer conceived as a monolithic systems with clear interfaces. The ITU NGN work on security concentrates on the development of a compound security architecture for NGNs, and on the development of NGN specific security protocols. The lack of adequate security in particular in the Internet is very serious and fast becoming worse. Without proper security, the Internet may become unusable in a few years’ time. Moreover, mobile phones are more and more replicating the functionality of PCs, thus mobile networks are increasingly susceptible to malicious attack. INCLUSION OF RADIO OVER FIBER TECHNOLOGY IN A CONVERGED NETWORK Here we propose a novel non-conventional approach to the future optical and wireless hybrid transport network that is capable to support the dominating kinds of traffic, i.e. real time voice, wireless, and packet traffic in a single transport network. From the networking point of view the novelty of the approach consists of an upgrade of real-time traffic with the microwave modulated optical wave (Radio-over-Fiber), in order to transmit the conventional mobile wireless via optical fibers through comparatively long distances and without a significant distortion. The model assumes specific DWDM channels are dedicated to real time and non-real time service traffic. The network intelligence has to be located at IP routers [10] and it has to provide the real-time subnetwork including microwave radio-over-fiber with a sufficient number of wavelengths according to the instantaneous demand. All remaining available wavelengths are dedicated to packet traffic. Table 1 reports the differences between real-time and non-real time traffic within a hybrid converged network. Table 1 – Comparison of main characteristics of real-time and Internet traffic. Characteristics
Voice, real-time, incl. wireless signal Internet, VoIP, data (Radio-over-Fiber)
Basic principle
Circuit-switched
Packet-switched
cell/packet length
Constant length cells
Variable
Latency (delay)
Unnoticeable
Allowed
If some data are lost
No retransmission
Retransmitted
Quality of Service
Guaranteed by overprovisioning
Best-effort
Traffic
Deterministic
Statistic
Other
Instantaneous bandwidth (# of λs) controlled logically in IP routers
Intelligence
Transparent
Allows all-optical opacity
Bandwidth
Dedicated on demand
As wide as available
Security
Inherent
To be improved permanently
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The above hybrid network concept assumes that the voice and broadband wireless signals are transmitted via circuitswitched subnetwork with digital (voice) or analogue (wireless) coding, while IP is transmitted as packet-switched connectionless traffic. Voice/wireless is carried on dynamically allocated wavelengths, according to instantaneous demand for real-time services. All remaining wavelengths are for the IP traffic. The two kinds of traffic are separated and interleaved in frequency (wavelength) domain, not in time domain. The conventional mobile microwave/millimeter wave signal transmission can be included in the transparent real-time part of the network by the means of modulating the optical carrier wavelength with the mobile signal, i.e. in the ‘Radio over Fiber’ fashion. Radio over Fiber technology is especially well suited to transmit the 60 GHz frequency that otherwise is highly attenuated in the air [11]. Then it can be transmitted at long distances via fibers before being detected at an optical receiver and proceeded further. This approach allows to profit fully from both SDH/ATM technology best suited for real time-circuit switched services, and from IP protocol developed uniquely for packet-switched traffic [12]. The novelty of the above approach consists of not to put the real-time traffic on top of packet network as opposed to classical networking but to position them in parallel in the network rather, to provide an effective space for inclusion of microwave signal within the converged network, and finally to optimise conditions for quality of service and security requirements specific to different kinds of traffic. QUALITY OF SERVICE & SECURITY As the user perceives the Quality of Service level necessary for his/her satisfaction (and payment), two classes of services are commonly distinguished: real time services which are transmission delay sensitive (voice calls, videophone), and non real time services which allow short transmission delays as Internet and data transmission. On the other hand, two entirely different basic traffic principles are dominating today. Those are: classical circuit switched, connection oriented traffic, and packet-switched, connectionless traffic that has emerged with the advent of the Internet. Consequently, the two kinds of traffic differ fundamentally not only in their performance, but also in the types of service requirements they are specifically suited for. For example, the Quality of Service is automatically guaranteed for circuit switched networks by the oveprovisioning of the network resources (that means the network parameters are much better than actually needed), while Internet is based on the ‘best-effort’ principle (a packet network does its best, but no more). The statistic nature of the packet traffic does not allow to achieve any arbitrary level of performance in all circumstances. This poses serious challenges to ensure a good network performance and satisfactory Quality of Service level for as far as possible. Moreover, Internet is inherently vulnerable to a dishonest use and malicious attacks. One has to recall that while the origins of the Internet were to cope with a large network infrastructure destruction, however the actual threats are of different nature and a malicious code inserted to the network might affect and even potentially block the network. The important issues outlined above imply that to adopt a single global Internet network is not a mostly wise direction. Alternatively, our hybrid convergent network model proposed above assumes keeping the circuit-switched connections dedicated to secure transmission with guaranteed Quality of Service mostly for real-time services, in parallel to packet transfer for non-real-time services that may accept short transmission delays or even data retransmission, but keeping in mind they are suspected to be a subject of an attack. We have shown how the transparent fiber-optic networking with Dense Wavelength Division Multiplexing (DWDM) capabilities is well suited to accommodate both connection and connectionless traffic. Moreover we have shown how to insert the wireless traffic to that hybrid network especially at higher frequencies (e.g. 60 GHz) via the Radio-over-Fiber technology, i.e. via modulating the optical carrier wave with microwave carrier wave with the use of fast tunable lasers [13]. The Quality of Service may be inherently guaranteed for real-time connection traffic via over-provisioning the network resources, i.e. via assuring the network transmission parameters much better then actually needed. Also security constraints are respected in the real-time subnetwork as only the sender and the recipient are connected via a dedicated connection. Thus security is inherent in real-time connection subnetwork, while it has to face permanently the evolving threats in the IP subnetwork. It has to be noted that transparency fundamental limitations due to physical constraints as dispersion, polarization mode dispersion and fiber nonlinearities impose ultimate limits to networks performance. In particular, a tradeoff between decreasing the DWDM channel spacing and increasing the single channel modulation speed has to be conserved. However, it has to be noted that Radio-over-Fiber transmission of high frequency wireless signals being of an analogue nature is especially challenging, as the impairments due to limited transparency accumulate along the transmission link [14]. CONCLUSIONS AND FUTURE DIRECTIONS
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We have proposed a novel model of a future hybrid communication network capable to support the traffic of different kind, e.g. real time and packet traffic and wireless signal in a single transparent optical network. Whole available bandwidth can be fully exploited with that approach. We distinguish and combine in a single DWDM network real-time traffic and non-real time traffic. Real-time traffic includes Radio-over-Fiber transmission of wireless signals realised via dynamically allocated wavelengths as circuit-switched transparent traffic. The number of wavelengths is dynamically allocated according to the instantaneous demand for real-time traffic. Non-real time packet switched traffic uses all remaining wavelengths. It is free of real-time restrictions, with potential of: variable-packet length, no idle bits, best-effort scheme. Quality of Service can be differentiated for IP subnetwork. The novelty of the proposed approach consists of not to put the real-time traffic on top of packet network as opposed to classical networking but to position them in parallel in the network rather, to provide an effective space for inclusion of microwave signals, and finally to reach the optimal conditions for quality of service and security requirements specific to different kinds of traffic. The hybrid network guarantees the Quality of Service as well as security for real-time traffic. We strongly believe the hybrid network model presented here is an optimized approach to the future network design, assuring maximal use of the available bandwidth and optimizing the network availability for various kinds of traffic and services. The model overcomes the serious drawbacks of an all-IP network, and it accommodates the real-time circuitswitched transmission of a high reliability and security necessary for a number of services with the more flexible but also highly vulnerable packet traffic. The security of information and the reliability and survivability of the network can be categorized for different services. However, Radio-over-Fiber transmission of microwaves, being of an analogue nature, is especially challenging both for the photonic and the electronic layers, especially when the 60 GHz band is concerned. ACKNOWLEDGEMENTS The author acknowledges highly profitable input from the European COST (European Co-operation in the field of Scientific and Technical Research) projects: COST Action 291 Towards Digital Optical Networks (TDON), COST Action 293 Graphs and Algorithms in Communication Networks (GRAAL), and COST Action 2100 Pervasive Mobile & Ambient Wireless Communications, and with The International Telecommunication Union - Study Group 15 ‘Optical and Other Transport Networks’. In addition, interactions with The International Electrotechnical Commission - Technical Committee 86 ‘Fiber Optics’, and The International Union of Radio Science - Commission D ‘Electronics and Photonics’ are highly appreciated. This research is being supported by the State Committee for Scientific Research under COST/51/2006 national grant. REFERENCES [1] M. Marciniak, Optical Fibres – Almost Ideal Transparent Propagation Medium?, 11th International Conference on Mathematical Methods in Electromagnetic Theory MMET*06 - Kharkiv Electromagnetics & Photonics Week 2006, Invited Paper at Session 27-2-2 Propagation 2, Conference Proceedings pp. 358-362, V.N. Karazin Kharkiv National University, Kharkov, Ukraine, June 26 – 29, 2006 [2] A. Kaszubowska-Anandarajah, L.P. Barry,” Remote Downconversion Scheme for Uplink Configuration in Radio/Fiber Systems”, 7th International Conference on Transparent Optical Networks / 2nd Global Optical & Wireless Networking GOWN seminar, Contributed Paper We.C2.5, Conference Proceedings Vol. 2, pp. 161-134, Barcelona, Catalonia, Spain, July 3-7, 2005 [3] Ch.M. Gauger, P.J. Kühn, E. Van Breusegem, M. Pickavet, P. Demeester, Hybrid Optical Network Architectures: Bringing Packets and Circuits Together, IEEE Communications Magazine, Vol. 44, No.8, pp. 36-42, August 2006 [4] M. Marciniak, “Towards Broadband Global Optical and Wireless Networking”, 11th Management Committee Meeting of COST Action 273 “Towards Mobile Broadband Multimedia Networks”, Document TD(04) 164, Duisburg, Germany, September 20-22, 2004 [5] The World Telecommunication Standardization Assembly, Florianópolis, Brazil 5-14 October 2004, http://www.itu.int/ITU-T/
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[6] K. Borzycki, M. Jaworski, M. Marciniak, Temperature dependence of PMD in tight buffered G.652 and G.655 singlemode fibers, 11th European Conference on Networks & Optical Communications, Workshop on Optical Cabling and Infrastructure, Fraunhofer Institute for Telecommunications HHI, Berlin, Germany July 10-13, 2006 [7] H. Rourke, B. Pugh, S. Kanellopoulos, V. Baker, B. Napier, P. Greene, D. Goodchild, J. Fells, Epworth, and R.A. Collar, A., “Fabrication and system performance of dispersion compensating gratings”, European Conference on Optical Communications ECOC’99, Nice, France, September 1999 [8] M. Marciniak and A. Sedlin, "Numerical analysis of optical nonlinearities in multispan DWDM fibre transmission systems", COST P2 Workshop on "Nonlinear Optics for the Information Society NOIS 2000, University of Twente, Enschede, the Netherlands, October 26-27, 2000 [9] R. Sabella, and P. Lugli, High Speed Optical Communications, Kluwer Academic Publishers, Dordrecht/Boston/London, 1999, pp. 239-245 [10] N. Geary, A. Antonopoulos, and J. O’Reilly, “Analysis of the potential benefits of OXC-based intelligent optical networks”, Optical Networks Magazine, Special Focus on Towards Intelligent Multiwavelength Optical Core Networks, Volume 4 Number 2, pp. 20-31, March/April 2003 [11] L. Smoczynski, M. Marciniak, “Radio-Over-Fibre 60 GHz Broadband Access”, International Topical Meeting on Microwave Photonics MWP 2003, Nefertiti Workshop on Broadband Optical/Wireless Access, Budapest, Hungary, 9 September 2003 [12]M. Marciniak, Transparent optical fibre communications for real-time, packet, and wireless traffic, Invited Paper, 3rd International Conference on Advanced Optoelectronics and Lasers CAOL 2006, co-located with 1st Multiconference on Electronics and Photonics, Conference Proceedings pp. 1-3, University of Guanajuato, Mexico, November 7-11, 2006 [13] A. Kaszubowska-Anandarajah, E. Connolly, L.P. Barry, D. McDonald, Fast tunable lasers in radio-over-fiber access networks, Invited Paper We.D1.2, Proceedings of the 8th International Conference on Transparent Optical Networks, Nottingham, United Kingdom, June 18-22, 2006 [14] M. Marciniak, Converged Optical and Wireless Networking – Challenges for Photonics and Electronics, The General Assembly of the International Union of Radio Science URSI-GA 2005, New Delhi, India, October 2005
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