Abstract:
This thesis examines two areas of research that are relevant to the design of an interim Broadband Integrated Services Digital Network CB-ISDN) that is based on a combination of circuit and packet transfer modes. These are (i) the integration of voice and data using packet switching, and (ii) the access control of heterogeneous traffic to a common transmission resource. With the integrated packet switching of voice and data, the main difficulty lies in the need to meet simultaneously the conflicting transmission requirements of these two types of traffic. Of particular significance are the implications of network delays and delay variability, inherent in the packet switching technique, on voice communication. Two methods are investigated for minimising the effects of these delays. The first method involves modifying the packet switching protocol architecture to suit the transmission requirements of voice better. A new protocol architecture based on the widely used CCITT Recommendation X.25 protocol architecture is proposed and demonstrated by computer simulation to improve the delay performance of voice. The second method employs priority strategies to handle voice packets within an integrated voice/data packet multiplexer. Two ways in which priority can be accorded to voice are studied. For each of these two ways, priority transmission, and data flow control, a strategy that not only improves the delay performance of voice, but also reduces the resultant degradation in the performance of data, is proposed and analysed. It is observed that the method of data flow control is more effective, and this is attributed to the effectiveness of the flow control principle in general, in reducing packet congestion within the queueing buffers of the multiplexer. The access control of heterogeneous traffic to a common transmission resource is important for multiplexing the traffic of the various services onto the digital channel (pipe) that connects a user's premises to the nearest B-ISDN node. In general, such access control must strike a balance between two conflicting performance criteria pertaining to the network operator and the user. These are respectively the operator's desire to maximise the utilisation of the digital channel, and the user's wish to have the best grade of service out of his usage of the network. In the absence of an efficient technique to optimise simultaneously these two conflicting performance criteria, eight heuristic access control policies that are intuitively appealing, implementable, and analytically tractable are studied for a population of wideband and narrowband traffic types. Of these, four employ the technique of bit rate adaptation on the wideband traffic to improve channel utilisation. By means of a combination of mathematical analysis and computer simulation, the performance of these access control strategies are analysed and compared. It is observed that, among those strategies that do not use bit rate adaptation, one scheme that allows the complete sharing of the available channel capacity between the wideband and narrowband traffic, and accords pre-emptive priority to the wideband traffic only, yields the best results in terms of a combination of wideband and narrowband performance. Also, as expected, with bit rate adaptation, channel utilisation is improved significantly. Although it is realised that these improvements are clearly achieved at the expense of wideband service quality, it is demonstrated that the proposed bit rate adaptation access control strategies will merit consideration under certain situations. These include systems where (i) the normal bit rate requirement of a wideband service is to support an enhanced quality of service which can be degraded without significant objection, (ii) at the lowest level of transmission bit rate, an acceptable quality of service is still maintained for a wideband service, and (iii) the mix of narrowband and wideband traffic loads is such that wideband arrivals are infrequent compared to narrowband arrivals, so that changes to wideband transmission bit rates are also infrequent.