asic M/G/R-PS model assumes ideal capacity sharing among active flows. But in practice the TCP flows are not always able to utilize their fair share of the available bandwidth. TCP's effectiveness of capacity sharing is determined by the TCP flow control mechanism (see section 6.3.1). The TCP slow start mechanism leads to the increase of transfer time as a result of not completely utilizing the available bandwidth at the beginning of each transmission process. For small transactions and longer round trip times, the impact of the TCP slow start becomes more significant. Because TCP connections spend a considerable amount of their overall transmission time in the first slow start phase. Short transactions might not even get through the slow start phase at all. As a consequence, the achieved throughput will be lower than the theoretically computed one derived from the basic M/G/R- PS model. For network dimensioning, it means that the calculated capacities would not be sufficient to guarantee the desired QoS. To solve this problem, A. Riedl and T. Bauschert propose extending the basic M/G/R-PS model in [RPBP00], which considers the impact of the slow start algorithm. The following gives a detailed introduction of this extended M/G/R-PS model. Figure 6.19 shows that the data transfer over TCP can be in principle divided into two parts: the first part consists of necessary RTTs which are mostly spent waiting for acknowledgements as a result of the slow start. RTT is the time period between sending out a datagram and receiving the corresponding acknowledgment (ACK). In the second part, the source starts sending the rest of the data with the available bandwidth, i.e. during the second part the TCP flow can fully utilize the available capacity given by the network. In order to derive an approximation for the expected transaction time, it is assumed that the total amount of data x is transferred in TCP packets with maximum TCP packet size (Maximum Segment Size), moreover there are no packet losses during the TCP slow start