QoS management with dynamic bearer selection schemes
Introduction
The Terrestrial Trunked Radio system (TETRA) [1] is an ETSI standard for professional mobile radio (PMR) users such as public safety services (police, fire, etc.) and the commercial sector (railways, buses, taxis, etc.). TETRA is a suitable example for illustrating a formal methodology for QoS management due to its feature-rich bearer services that support data and voice with varying levels of error protection. In the proposed scheme, the mapping of an application specific contract is performed onto a hierarchy of intermediary contracts down to TETRA resources units. During the communications phase, the adaptive system maintains the QoS which was initially negotiated at the call admission. In a wireless environment, it is difficult to predict what can be achieved by the system in terms of QoS. This means that maintenance mechanisms have to counter variations in the channel conditions. This paper presents a system based on the TETRA air interface that dynamically adapts the link configuration to maintain a contracted level of QoS. For optimisation, a priority mode is assigned to each contract and enables the resource manager to prioritise one or more of the quality aspects, viz., delay, bit rate or bit error rate (BER), along with the minimisation of the number of resource units required to fulfil the contracts.
This paper develops the notion of QoS contract which specifies connection requirements in terms of performance parameters such as bit rate, delay and BER, an optional degradation allowance and a priority mode. Once a connection is admitted into the system, internal functions such as link adaptation [2] or resource reservation maintain the contracted level of QoS. Associated to the notion of QoS contract is the concept of commitment which is defined as the probability that the associated contract requirements will be fulfilled. A level of commitment can be predicted for each call and is dependent on parameters such as the network topology, user mobility profiles, system load and service requirements. This level of commitment informs the network operator on what can be delivered to the user for a given contract according to the system configuration. Link quality measures can be exploited during the communications phase for link adaptation but also for service adaptation, for instance, by decreasing the colour depth/resolution from the video application when the user is approaching a geographical zone where the channel quality is expected to be low. In this study, focus has been given to the TETRA circuit mode (voice + data) for which three traffic channels (TCH) are available, namely,
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TCH/7.2 for unprotected data at 7.2 kb/s net rate/slot;
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TCH/4.8 for low-protected data at 4.8 kb/s net rate/slot;
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TCH/2.4 for high-protected data at 2.4 kb/s net rate/slot.
Higher bit rates can be achieved by allocating up to four timeslots for a connection. Interleaving can also be used in order to increase the service quality. In this paper, network quality refers to the channel quality before error protection, and service quality refers to the channel quality after error protection has been applied.
The remainder of this paper is composed of three sections. Section 2 presents the proposed adaptive system. Section 3 presents simulation results that show the system performance. Finally, Section 4 concludes with a mention of further work.
Section snippets
The connection contract
In this study, an instance of the TETRA connection contract is specified by setting values over the following seven parameters:
Maximum delay. The maximum delay to be experienced.
Minimum bit rate. The minimum bit rate to be supported.
Maximum BER. The maximum BER to be supported.
Degradation allowance. The proportion of quality measures allowed to be non-compliant with the three first parameters over a sliding monitoring period.
Monitoring period. The period of time to which the degradation
System evaluation
In order to examine the operation of the contract-based resource manager, an event-driven simulator of physical layers of the TETRA system was used [5]. This section first shows what the system can attain with the various bearer configurations when link adaptation is not in operation. Afterwards, the impact of link adaptation on resource use and QoS is evaluated. Finally, the benefits of using the priority scheme and the switching margin are assessed. In the legends of the figures, the
Conclusions
This paper has presented a system that supports QoS contracts. The QoS contract is the means for users/applications to specify the desired level of network performance with an optional degradation tolerance. TETRA has been used in this study to evaluate the proposed system under various scenarios. First, capability of available bearer configurations has been evaluated when link adaptation is not in operation. Furthermore, it has been shown that the level of commitment and resource utilisation
Acknowledgements
The authors acknowledge the support of the UK Virtual Centre of Excellence in Mobile & Personal Communications, MVCE in the funding of this work. More detailed information and software tools associated with this research are available to Industrial Members of MVCE.
John Dunlop is a Professor of Electronic Systems Engineering and Head of the Mobile Communications Group, University of Strathclyde. He has been involved in various RACE and ACTS projects. Professor Dunlop is a Director of the UK Virtual Centre of Excellence in Mobile and Personal Communications (MVCE). He is co-author of Telecommunications Engineering which has been adopted as a standard text in many British Universities and is a co-author of Digital Mobile Communications and the TETRA System
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Cited by (2)
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John Dunlop is a Professor of Electronic Systems Engineering and Head of the Mobile Communications Group, University of Strathclyde. He has been involved in various RACE and ACTS projects. Professor Dunlop is a Director of the UK Virtual Centre of Excellence in Mobile and Personal Communications (MVCE). He is co-author of Telecommunications Engineering which has been adopted as a standard text in many British Universities and is a co-author of Digital Mobile Communications and the TETRA System which was published by John Wiley in August 1999.
Gwenaël Le Bodic obtained his Ph.D from the Department of Electronic and Electrical Engineering, University of Strathclyde in 2001. In 1997, he graduated in Computer Science from the Institut National des Sciences Appliquées (INSA), Rennes, France. He has been involved in research activities of the UK Mobile VCE research programme. His research interests include QoS management, distributed systems and agent technology.
James Irvine, B.Eng., Ph.D., M.IEEE, is a lecturer in the Department of Electronic and Electrical Engineering at the University of Strathclyde in Glasgow, from where he obtained his B.Eng and Ph.D degrees in 1989 and 1994, respectively. Currently, a co-ordinator of the Networks and Services work of the UK Mobile VCE research programme, his research interests include resource allocation and coding theory. Dr. Irvine is a co-author of Digital Mobile Radio and the TETRA System, published by John Wiley in 1999.
Demessie Girma, B.Sc., Ph.D., C.Eng., M.IEE, M.IEEE, is a senior lecturer in the Department of Electronic and Electrical Engineering, University of Strathclyde, with active research interests in mobile communications, RF and microwave engineering, CAD of electronic systems and computer networks. Dr. Girma is a co-author of Digital Mobile Radio and the TETRA System, published by John Wiley in 1999.