has an additional effect in a packet switching network: When the queues fill up, excess packets are simply discarded. See Figure 7.10. Mobile Environment Among degradation sources for IP/wireless QoS are: Strongly varying transmission conditions - Fading, shadowing, interference, packet loss, etc. From bandwidth to power dependence. Understanding Changing Telecommunications - Building a Successful Telecom Business Limited unpredictable bandwidth. Handover - e.g. from GPRS to UMTS or between different capability cells in UMTS. Hierarchical cells - high bandwidth in small cells, low bandwidth in large cells. Varying terminal capabilities with respect to computational power and display size - PDA, mobile laptop, PC in wireless LAN. 7.4.5 CONNECTIONLESS TRANSFER In connectionless transfer packets can arrive at the receiver in the wrong order, for example when the path has changed. This and other displacements in time are reasons for buffering, adaptation and 'dejitterization'. See Figure 7.12. 7.4.6 TCP EFFECTS Slow Start Mechanism TCP has a major responsibility for the QoS received by the application on top of TCP/IP, with an error-free transfer as the main goal, at least for best-effort data traffic. To achieve this goal the transmitting end must adapt send speed and possible necessary retransmission to information at suitable speed limits from the receiver and congestion information from the network. Congestion control is a distributed algorithm that is used to share network resources among competing users. It consists of two components: a network algorithm that updates and feeds back, implicitly or explicitly, congestion information to sources; and a source algorithm that dynamically adjusts rate (or window size) in response to congestion in its path. In the current Internet, the source algorithm is carried out by TCP. In case of congestion TCP reduces the send speed. This speed reduction and the subsequent speed recovery has been a subject for much interest during the last few years. Why? Let us first look at Fi