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T. Holynski:
"Analytical Performance Modelling and Evaluation of the HDLC Protocol Based on Queueing Theory";
Supervisor: H.R. van As, S. Krawczyk; Institut für Breitbandkommunikation E-388, 2009; final examination: 04-23-2009.

The HDLC (High-level Data Link Control) protocol is the most fundamental error- and flow control procedure used in data communications for a single physical channel. Apart from the classical HDLC standard of ISO, the generic recovery mechanisms of this protocol constitute the basis for a whole family of data link protocols designed for wired and wireless systems. Due to wide deployment, the importance of performance studies on HDLC and its derivatives is unquestionable. The following thesis presents a series of detailed analytic studies on performance modelling and evaluation of an HDLC-controlled link wherein various retransmission options and traffic scenarios are considered. The goal of the analysis is mean link throughput and mean frame flow time computation with respect to a set of link and protocol parameters. In practise, it may happen that even a large link capacity provided by the physical layer is utilised only to a fractional extent due to poor understanding of the procedure controlling this layer. Presented results may enable more awareness on the exploitation of row bit rate and may enable a system designer or a field engineer to adjust optimal operation point in terms of frame length, window size, duration of time-outs for a given bit-error probability, channel load and distance between communicating stations. The chosen method of analysis is based on the M/G/1 queueing model with a so-called virtual server. This random service time spans the entire protocol's activity devoted to a successful frame transfer over the link and that is why it is called a virtual transmission. With help of the Laplace transform the service time can be decomposed into explicitly computable time phases that reflect nearly all details of stochastic actions of HDLC. In such a way, it is possible to study a truly real protocol in contrast to numerous studies that either simplify the protocol mechanism or the link scenario and usually have little practical application. This powerful approach, which was proposed in the past and yielded very good exactness with simulations, has been rarely used in the literature. From the academic point of view, this work is a revision of the underlying methodology and a preparation stage for further analysis on more complex point-to-point error and/or flow control procedures at various communication layers. Beyond the strictly analytical part, the thesis includes a systematic introduction to retransmission techniques and a necessary background on queueing theory.

HDLC, queueing theory, performance analysis, error control, flow control, data link layer protocol, network protocols, stochastic processes