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Network coding (NC) has been an attractive research topic in recent years as a means of offering a throughput improvement, especially in multicast scenarios. The throughput gain is achieved by introducing an algebraic method for combining multiple input streams of packets which are addressing one output port at an intermediate node. We present a practical implementation of network coding in conjunction with error control schemes, namely the Stop-and-Wait (SW) and Selective Repeat (SR) protocols. We propose a modified NC scheme and apply it at an intermediate SW ARQ-based link to reduce ARQ control\ud signals at each transmission. We further extend this work to investigate the usefulness of NC in the Butterfly multicast network which adopts the SR ARQ protocol as an error control scheme. We validate our throughput analysis using a relatively recent discrete-event simulator, SimEvents®. The results show that the proposed scheme offers a throughput advantage of at least 50% over traditional SW ARQ, and that this is particularly noticeable in the presence of high error rates. In the multicast network, however, simulation results show\ud that when compared with the traditional scheme, NC-SR ARQ can achieve a throughput gain of between 2% and 96% in a low bandwidth channel and up to 19% in a high bandwidth channel with errors.

The passive optical network1 has been shown to be a cost effective method of introducing fibre to the access network. However, the expected increase in demand for broadband services will inevitably mean that upgrades in capacity will be needed. Wavelength division multiplexing offers an alternative to increasing the bit rate. Two broad strategies exist for the implementation of multiple wavelengths on passive optical networks (PON), fixed wavelength schemes and dynamic wavelength assessment. Wavelength reallocation at call setup allows a flexible response to changes in network demand. It does not, however, take full advantage of statistical multiplexing and the ATM approach. In order to do so, new protocols need to be developed which combine the allocation of wavelengths and time slots. The approach here shows a method based on wavelength allocation at the frame level. A scheme to reduce cell delay variation recently proposed for single wavelength systems is adopted for WDM and an improvement to it demonstrated.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A new protocol mechanism is described for carrying broadband traffic in passive optical networks. Simulation results show an improvement compared with an existing protocol for bursty traffic and a proposal for a hybrid scheme for allocating bandwidth to multiple traffic classes is outlined. (6 pages)

The introduction of wavelength division multiplexing into the access network is a potentially attractive alternative upgrade strategy to increasing the bit rate of passive optical networks (PON). A number of different methods have been proposed to implement a multi-wavelength PON and hence a careful assignment needs to be made of the most cost effective, both in the short term and for future increases in capacity. One key issue is the use of dynamic allocation of wavelengths, which allows the reconfiguration of the network in response to network demand. A model has been produced to simulate demand patterns on a PON, in order to quantify the benefits of dynamic assignment.

A comparison is made between wavelength routing and dynamic wavelength assignment in passive optical networks (PON), to determine quantitatively the benefits of the latter. A Gaussian approximation to the demand spread across a fibre to the cabinet PON is used, firstly to examine an ideal system with perfect reallocation and then to simulate a possible algorithm for wavelength allocation.

In this paper the integration of a filter with the photodiode is considered by analysing resonant cavity enhanced (RCE) photodiode devices for use in optically preamplified receivers. The analysis assumes conditions that the optical amplifier is quantum limited with 3 dB noise figure and gain sufficient to make shot noise negligible. Under these conditions, both amplified signal and spontaneous emission power are proportional to gain. All non-amplifier noise is neglected as are signal imperfections caused by nonideal extinction ratios.

A novel proposal to integrate the filter necessary in optically preamplified systems with a photodiode is described. The concepts involved in accurate analysis of optically preamplified receivers are introduced. This is followed by a description and analysis of resonant photodiodes which have been demonstrated in recent years. Such diodes possess the filtering capabilities required for spontaneous noise reduction and so eliminate the need for an external filter device. Preliminary analysis of the performance of resonant diodes with optical amplifiers is presented and an indication of the design parameters concerned given. The conditions for optimum bit error rates are discussed and indications given of substantial further work both in the optimization of device characteristics and the improvement of calculation methods.

Frequency chirp in modulator devices is of increasing importance at higher bit rates and increased transmission distances. The chirping behavior of reflective, resonant electroabsorptive optical modulators is analyzed and compared to the non-resonant case for a Framz-Keldysh effect based device. The chirp is found to display oscillatory behavior which is explained by the interaction of the electric field induced changes in complex refractive index with the cavity. The effect of a front face reflectivity lower than that for matching is shown to be poorer modulation and chirping performance. If the front face reflectivity is greater than that needed to match then negative chirp is possible but the insertion loss is greater than 8 dB. By fine tuning the length of the device it is shown that this can be reduced to 3 dB producing a contrast ratio of 4.5 dB and -500 MHz chirp for 400 ps, 15V pulses.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

An enhanced multistage optically amplified passive optical network (PON) known as SuperPON is presently being investigated by the ACTS-PLANET consortium as a possible future solution for broadband access networks. As it is extremely probable that user bandwidth requirements will continue to increase substantially in the short to medium term it is highly pertinent to consider the upgradeability of SuperPON topologies. In this paper capacity upgrades in the wavelength, space and time domains for both the downstream and upstream directions are discussed with reference to a particular basic SuperPON.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper is concerned with the development of a long range, wide splitting passive optical network (PON) for deployment in the access network, termed SuperPON. Such a network requires careful attention at the medium access control (MAC) layer in order to provide for the high round trip delay. In particular the paper deals development and testing of a software simulation platform which may be used to investigate the performance of potential SuperPON MAC layer protocols. Furthermore, a new MAC protocol developed for operation on a SuperPON is simulated and its performance characteristics are discussed. In addition for purposes of comparison a previously utilized PON protocol is also modeled on the SuperPON and hence the benefits of the new SuperPON protocol are demonstrated.

Combining asynchronous transfer mode over a passive optical network (APON) can provide broadband services as defined by the International Telecommunications Union (ITU). The medium access control (MAC) layer is of primary importance to the access scheme as it controls the flow of traffic in the access network. It is therefore beneficial to model MACs for APONs to provide a performance evaluation without the expensive and time consuming job of building prototypes. This paper describes the simulation and performance comparison of a selection of proposed MAC schemes for APON, using the block oriented network simulator (BONeS) simulation software. The protocols simulated are all based on the TDM/TDMA technique, each with a different polling scheme, namely; demand, cyclic and semi-static. Both the development of a simulation strategy using a common simulation platform and the implementation of the permit distribution algorithm for the MAC protocols are described. The simulations show that the performance characteristics of the MACs are to a large degree comparable. There is, however, a superior performance in the parameters measured by the protocol based on demand polling. (6 pages)

The chirping behaviour of reflective, resonant electroabsorptive optical modulators is analysed for the first time and an equivalent to the chirp parameter for these devices derived. Using the example of a Franz-Keldysh Effect based device the frequency chirp behaviour of a matched resonator is investigated and compared to the non resonant case. The chirp is found to display oscillatory behaviour which is explained by the interaction of the electric field induced changes in complex refractive index with the cavity. The effect of a front face reflectivity lower than that for matching is shown to be poorer modulation and chirping performance. If the front face reflectivity is greater than that needed to match then negative chirp is possible but the insertion loss is greater than 8 dB. By fine tuning the length of the device it is shown that this can be reduced to 3 dB producing a contrast ratio of 4.5 dB and −500 MHz chirp for 400 ps, 15 V pulses.

A novel class of optical signature codes based on combinatorial designs is proposed for optical spectral-amplitude code-division multiple-access (CDMA) systems. It is applicable to both synchronous and asynchronous incoherent optical CDMA and is compatible with both frequency-encoded and time-spreading schemes. Simplicity of construction, larger code cardinality and larger flexibility in cross-correlation (CC) control make the proposed code family an interesting candidate for future optical CDMA applications that require a large number of simultaneous users. It has been shown that the system performance can be significantly improved by using the proposed codes with ideal in-phase CC in preference to Hadamard codes.

Performance of optical wireless communication (OWC) systems can be optimised using constant optical power transmission in conjunction with forward error control coding. A new family of constant power error correcting codes using multiple pulse position modulation optical transmission are introduced. This doubly featured technique may extend the capability of many difficult wireless optical links beyond their existing barriers and may enable a higher error performance for OWC networks. The potential and design of look-up table codes are investigated. The Monte Carlo methods provide supportive data for the performance and code word search.

The important role of quality of service (QoS) in deployment of a resilient dense wavelength division multiplexing (DWDM) backbone for global networks requires critical design-phase planning optimisation. The design issues of resilient DWDM networks for bandwidth and delay sensitive applications of dedicated path protection are addressed. A genetic algorithm (GA) model has been developed to solve the routing and wavelength assignment problem using binary variable-length chromosome encoding under two different schemes of bandwidth optimisation (BOS) and delay optimisation (DOS).The performance of the new GA-based resiliency model has been evaluated for four benchmark networks: PAN EUROPEAN, COST239, NSFNET and ARPA2. Simulation results show a superior capability and efficiency for the model to solve this complex, multi-constraint and nondeterministic polynomial-hard problem for BOS and DOS. The nonlinear nature of this process reveals a significant sensitivity for optical layer network topology on the optimum-design QoS. The results also demonstrate that the PAN EUROPEAN network shows the highest flexibility for primary path design, NSFNET for the secondary path and ARPA2 comes with the lowest design flexibility for both primary and secondary paths.

Adaptation of QoS in DWDM optical networks routing and the wavelength assignment (RWA) problem, known as an NP-hard problem, are addressed. The QoS constraint based routing RWA computes lightpaths for the original-destination node pair, (O, D), to meet the QoS propagation delay requirements. A heuristic algorithm based on a genetic algorithm (GA) is then developed to solve this problem. A simulation program is developed to demonstrate the efficiency of the GA for the COST 239 network. The trade-off between the capacity and the propagation delay has been analysed. Results show that the trade-off is 8.7% of the average capacity, increasing to 10.6% decrease for the average delay.

The bit error rate performance of on-off keyed spectrally sliced burst mode receivers in the presence of dispersion is analysed for the first time. The treatment includes realistic excess noise statistics plus the effects of pulse distortion and intersymbol interference resulting from the dispersion. Numerical answers are obtained using the saddlepoint approximation, known to be both computationally efficient and accurate. The variation in power penalty with fibre path length, using the optimum optical filter bandwidth, is determined in addition to the impact of burst mode preamble lengths. The optimum ratio between the fibre path length and the distance for the root mean square width of spectrally sliced pulse to double is found to be between 0.75 and 0.76. Although the optical power penalty is found to be greater than in the case of a laser-based burst mode system, operation close to this optimum ratio reduces it to

Significant weaknesses in a recently proposed chaotic based communication technique are reported. It is shown that radial basis functions may be used to obtain a fit to a chaotic signal which masks user information and this may then be used to extract the hidden signal for the purpose of eavesdropping. The results obtained are extremely accurate and obtained with minimal signal processing showing that considerable refinement is necessary before chaotic schemes are usable in the secure environment for which they have been proposed. >

The error rate perormance of an on-off keyed optically preamplified burst mode receiver using a Fabry-Perot filter is analysed including the effects of pulse distortion, ISI resulting from the filter characteristic and realistic noise statistics. The optical power penalty is 1.38 dB for a 1 bit preamble, decreasing to 0.19 dB for 10 bits.

The bit error rate performance of an OOK optically preamplified receiver using a resonant photodiode is considered. The effect of the relationship between the normalised absorption coefficient and the device end face reflectivities upon error rates is quantified. Using integrate and dump detection, a sensitivity comparable to an external filter is achievable (89 photons/bit for a 10–9 error rate). To obtain this performance the photodiode must conform to a matching condition and the normalised absorption coefficient should be between 0.001 and 0.0013.

An approximation technique that is useful for error rate calculations in ON-OFF keyed lightwave receivers with optical preamplification, Fabry-Perot filtering and post integration, is presented. The method is justified mathematically and is then shown to produce a four-fold speed increase with a very small loss in accuracy. Such an improvement enables optically preamplified receivers to be analysed far more quickly for non-ideal filtering.

In this paper, we evaluate the use of an electronic nose (EN) containing 13 conducting polymer gas sensors to discriminate between patterns of volatile organic compounds (VOCs) emitted by plants. The VOC patterns examined were produced by tomato, cucumber and pepper plants under both healthy and infected or infested conditions. Leaves from the plants were subjected to mechanical damage or pest and disease attacks (i.e. spider mites infested or mildew infected) and others were judged against undamaged healthy leaves. Support vector machines (SVMs) with linear, polynomial and Gaussian radial basis function (RBF) kernels were used to process and classify the raw data collected. The SVM illustrated an ability to discriminate between different VOC patterns and hence was able to classify correctly the infected leaves using the EN data. The results indicate that the array of 13 EN gas sensors can discriminate among VOC patterns from undamaged and artificially damaged leaves of the three plant species. This study demonstrates the potential application of such an EN technology coupled with suitable pattern recognition and signal processing methods to be used as a real time pest and disease detection system in the greenhouse environment.

Infrared wireless communication possesses two main attractive advantages over its radio frequency counterpart, namely the abundance of unregulated spectrum in the 700 nm 1500 nm region and the ease with which the IR radiation can be confined. Integrating microwave electronics and optics, it is possible to provide wideband communication services but it is well known that the signal level in an optical wireless receiver is weakest at the front end. Therefore, this paper identifies the technical obstacles and limitations in indoor infrared optical wireless front-ends, in addition to techniques for mitigating these effects, showing that infrared is a viable alternative to radio for certain applications.

Describes the design, fabrication and performance of a planar, resonant Franz-Keldysh modular which has produced a 38% transmission change for 20 V applied bias. The device consists of a reverse-biased GaAs pin diode grown by MOVPE and from which the substrate has been removed.