Terminal equipment can require a specific transmission quality – Quality of Service (QoS). Individual types of transmission are different and frequently have contradictory requirements. Real-time voice and video transmission require a defined delay with minimum dispersion, which will not do without the warranty of a certain minimum agreed width of the transmission band and, at the same time, it excludes the use of large buffer memories. On the other hand, data transmissions do not have strict requirements for minimum transmission band, cell delay and its dispersion, and require large buffer memories for efficient transmission. These different requirements for the quality parameters of the transmission of ATM connection were solved through the introduction and definition of several classes of services. At present, four types of classes of services are defined; they are labeled with the letters A, B, C and D, or with names expressing the relationship between a given class of services and their transmission speeds.