Trong ý nghĩa đó, các tham số xác định lưu lượng truy cập thời gian thực sự chậm trễ. Lớp đàm thoại được định nghĩa cho giao thông rất chậm trễ nhạy cảm, trong khi giao thông chậm trễ không nhạy cảm nhất là nền giao thông lớp. Hai lớp học đầu tiên, | Third Generation Wireless Mobile Communications and Beyond 23 QoS Concept in UMTS UMTS is planned to include variety of services each with different QoS characteristics. Hence four QoS classes are defined for UMTS 16 as follows Conversational class Streaming class Interactive class Background class. When defining UMTS QoS classes which are referred to as traffic classes one should take into account the characteristics of the air interface . bandwidth limitations and error characteristics . The main distinguishing factor between the QoS classes is the requirement for real-time service. In that sense the parameter that defines real-time traffic is delay. Conversational class is defined for very delay-sensitive traffic while the most delay-insensitive traffic is background traffic class. The first two classes conversational and streaming are specified to carry real-time traffic. The others interactive and background classes are mainly defined for nonreal-time applications. A typical example of services in conversational class are circuit-switched telephony . GSM-like but IP telephony and videoconferencing belong to this traffic class as well. Also some other real-time communication that includes live end users may be added to the conversational class. Streaming class is created for one-way real-time transport when a user is looking at or hearing a real-time video or audio stream. By the term stream we denote one-way communication flow to a live human destination. This class is also delay sensitive but without strict delay requirements. Low delay variations may be neutralized by the receiving end. For real-time services retransmission of lost or corrupted traffic packets is not desirable due to delay sensitivity. This is not the case with control packets for this type of application which usually use some transport control mechanism . TCP . Interactive class is defined for applications where the end user either a machine or a human is requesting data .
