theory of processor sharing on a common link is illustrated in Figure 6.15, which shows two cases with a single shared link. Each TCP connection can transmit data with the peak data rate (rpeak). For the capacity C of the shared link, assuming here that 2 If only two TCP connections are active (Figure 6.15a), each connection can transmit with its peak data rate. However, when a third flow becomes active (Figure 6.15b), the shared link becomes a bottleneck as the link capacity C is now less than the sum of 6 Dimensioning for Single ATM-Based lub all peak rates. Thus, instead of sending with peak data rate, each flow lowers its transmission rate to C/3. In general, if there are n elastic flows sharing a common link with capacity C, the data rate r of each flow equals to r = min(C/n, peak) [Rie04]. flow rate flow rate C/3 < file transfer 1 file transfer 2 Node B Node B file transfer 3 lub Link lub Link a) no additional bandwidth limitation b) bandwidth limitation Figure 6.15: Illustration of rate sharing For link dimensioning, the theory of processor sharing can be applied to derive the dependency between the expected transmission time (the sojourn time) and the amount of data, given the peak data rate of the individual connections and the capacity of the shared link. However, it needs to be noticed that the basic processor sharing model only considers the bandwidth sharing property of TCP, which it assumes to be ideal. But in practice TCP flows are not necessarily able to fully utilize their fair share of available bandwidth as a consequence of TCP slow start, packet losses, etc. The basic processor sharing model does not take into account the time spent for TCP connection establishment and termination, and disregards the impact of TCP slow start algorithm and network characteristics such as round trip times, possible packet losses and resultant TCP retransmissions. All these aspects can have considerable impact on TCP's effectiveness to utilize its fair share of available bandwidth. Therefore, to consider these im