David Harle

Dynamic bandwidth allocation algorithm for differentiated sservices over WDM EPONs

A variety of passive optical network (PON) systems has been proposed to target the bottleneck problem in the local access. The medium access controls (MACs) of the PON systems are either based on cyclic rotation or polling mechanism. However, these MACs have inherited a potential problem; the cycle time (or polling cycle in the polling MACs) increases linearly as the number of attached ONUs scale up, which can be caused by increasing customer numbers. The longer cycle time means that the ONUs have to wait longer before the next transmission window arrives thus contributing to longer packet delays and poorer QoS. In this paper, a new WDM PON system is proposed where multiple wavelength channels are established in both upstream and downstream directions. The MAC applied is based on an adaptive polling mechanism, and can be used as an upgrade solution for PON systems

Performance evaluation of shortest multipath source routing scheme

Multipath routing in mobile ad-hoc networks allows the establishment of multiple paths for routing between a source-destination pair. It exploits the resource redundancy and diversity in the underlying network to provide benefits such as fault tolerance, load balancing, bandwidth aggregation and the improvement in quality-of-service metrics such as delay. Previous work shows that on-demand multipath routing schemes achieve better performance under certain scenarios with respect to a number of key performance metrics when compared with traditional single-path routing mechanisms. A multipath routing scheme, referred to as shortest multipath source (SMS) routing based on dynamic source routing (DSR) is proposed here. The mechanism has two novel aspects compared with other on-demand multipath routing schemes: it achieves shorter multiple partial-disjoint paths and allows more rapid recovery from route breaks. The performance differentials are investigated using NS-2 under conditions of varying mobility, offered load and network size. Results reveal that SMS provides a better solution than existing source-based approaches in a truly mobile ad-hoc environment.

Mitigation of Beat Noise in Time–Wavelength Optical Code-Division Multiple-Access Systems

This paper presents an analysis of two methods for enhancing the performance of two-dimensional time-wavelength Optical code-division multiple-access systems by mitigating the effects of beat noise. The first methodology makes use of an optical hard limiter (OHL) in the receiver prior to the optical correlator; a general formula for the error probability as a function of crosstalk level for systems adopting OHLs is given, and the implications of the OHLs nonideal transfer characteristics are then examined. The second approach adopts pulse position modulation, and system performance is estimated and compared to that associated with on-off keying

On the Experimental Characterization of Beat Noise in 2-D Time-Spreading Wavelength-Hopping OCDMA Systems

The results of a preliminary experimental characterization of beat noise for the case of a particular two-dimensional time-spreading wavelength-hopping optical code family are presented. Beat noise is evaluated at a data rate of 2.5 Gb/s with and without time gating. Results indicate that this form of noise introduces significant system power penalties for both cases, with the former approach more robust in this respect

Performance analysis of 2-D time-wavelength OCDMA systems with coherent light sources: code design considerations

The performance of two-dimensional (2-D) time-wavelength (TW) optical code-division multiple access systems at both physical and data link layers is considered in the presence of a multitude of noise sources. Particular consideration is given however to both beat noise (BN) and multiple-access-interference effects. A general procedure for calculating BN is presented. Comparisons between several 2-D TW codes comprising single-pulse-per-column/row and multiple-pulse-per-column/row strategies are performed. Results reveal that BN is strongly dependent on the code properties and the distribution of wavelengths within a code. Several aspects for 2-D TW code design are presented that mitigate the effect of BN

Code Flexibility of 2-D Time-Spreading Wavelength-Hopping In OCDMA Systems

The potential performance and successful experimental demonstrations of two-dimensional time-spreading wavelength-hopping (TW) optical code-division multiple access systems have motivated a large volume of research into the development of new TW codes. The choice of the code is crucial in network design since the code not only affects the overall system performance but also sets limits on the physical implementation of the encoding-decoding process. In this paper, ldquoflexibilityrdquo of TW codes is investigated, providing clarity on the tradeoff between key code factors, specifically the number of available wavelengths and time chips. Network performance is evaluated by taking into consideration multiple access interference and beat noise at both the physical and link layers. Furthermore, a general formula describing the impact of the extinction ratio (ER) on the bit-error probability is explored. The results indicate the importance of code flexibility and ER in system performance optimization.

An Advanced SOM Algorithm Applied to Handover Management Within LTE

A novel approach to handover management for Long-Term Evolution (LTE) femtocells is presented. Within LTE, the use of self-organizing networks (SONs) is included as standard, and handover management is one of its use cases. Base stations can autonomously decide whether handover should take place and assign the values of relevant parameters. Due to the limited range of femtocells, handover requires more delicate attention in an indoor scenario to allow for efficient and seamless handover from indoor femtocells to outdoor macrocells. As a result of the complexity of the indoor radio environment, frequent ping-pong handovers between the femtocell and macrocell layers can occur. A novel approach requiring a small amount of additional processing using neural networks is presented. A modified self-organizing map (SOM) is used to allow a femtocell to learn the locations of the indoor environment from where handover requests have occurred and, based on previous experience, decide whether to permit or prohibit these handovers. Once the regions that coincide with unnecessary handovers have been detected, the algorithm can reduce the total number of handovers that occur by up to 70% while still permitting any necessary handover requests to proceed. By reducing the number of handovers, the overall efficiency of the system will improve as the consequence of a reduction in associated but unnecessary signaling. Using machine learning for this task complies with the plug-and-play functionality required from SONs in LTE systems.

Pattern discovery and specification techniques for alarm correlation

Ever increasing amounts of alarm data threaten the stability of management systems in high speed telecommunications networks. As networks continue to develop, becoming larger, using substantially higher bandwidth links and using more complex equipment, so the danger of alarm inundation is increased. Alarm correlation systems have been seen to play a vital role in dealing with the problem. However, the question of what to correlate and how to recognise and specify related alarms has either been left largely unanswered or distinct from the physical correlation process. In this presentation, we describe a unifying framework which uses a purpose-designed language to specify alarm patterns and then use the results in a real-time correlation engine. In order to test the effectiveness of the solution, the language was translated onto an existing proprietary correlation system and fed with alarm data from an SDH network test-bed. Preliminary evaluation has indicated the system to be extremely fast, potentially robust to network dynamics and, importantly, resilient to a degree of input space error. Furthermore, it is easy to use, intuitive and may be extended through the incorporation of artificial intelligence modules.

Effect of beat noise on the performance of two-dimensional time-spreading/wavelength-hopping optical code-division multiple-access systems

The effect of beat noise on optical code-division multiple-access (OCDMA) systems using a range of two-dimensional (2-D) time-spreading/wavelength-hopping (TW) code families is presented. A derivation of a general formula for the error probability of the system is given. The properties of the 2-D codes--namely, the structure, length, and cross-correlation characteristics--are found to have a great influence on system performance. Improved performance can be obtained by use of real-time dynamic thresholding.

Keeping Order: Determining the Effect of TCP Packet Reordering

Packet reordering over TCP/IP networks is a phenomenon which is becoming increasingly important in network performance analysis. Reordering is a consequence of network equipment manufacturers increasing switch and link level parallelism on the Internet, seeking performance, reliability and economical improvements. This paper presents a methodology for simulating and measuring TCP reordering, providing an insight into the behaviours of the congestion and retransmission algorithms, and demonstrating that reordering has a measurable effect on performance. These measurements illustrate that there is a maximum reordering delay threshold that should be applied to packets, regardless of percentage reordering, below which reordering has negligible effects. Determination of this threshold, on a specific path, is key to ensuring that a specific switch or router does not introduce reordering to such an extent that it causes unnecessary retransmissions and an associated reduction in throughput.