The proper capture of desired system properties is a pivotal step in providing high quality systems. The formal specification of these properties is necessary to provide unambiguous documentation as well as automated transformation of system requirements during all stages of the life cycle. The standardized Formal Description Techniques (FDTs) Estelle and SDL have proved useful for the specification of traditional protocols and distributed systems. With the availability of high-speed networks new applications with additional requirements and characteristics are becoming reality. These requirements are often referred to as Quality of Service (QoS) requirements. We show that the above-mentioned FDTs do not possess the expressiveness to capture important classes of QoS requirements, namely quantitative deterministic real-time-related properties. It is the purpose of this paper to exemplify steps that need to be taken in order to overcome this deficit. We first discuss choices that need to be made when designing a suitable real-time execution model for SDL and Estelle and proceed to present two remedies to the inexpressiveness problem: First, we introduce the concept of complementary real-time specification by reconciling the semantic models of Metric Temporal Logic and SDL and showing how both languages can be used in a complementary fashion. Second, we suggest a language extension and the corresponding semantic interpretation for Estelle. While we present examples from the domain of multimedia and broadband systems, the applicability of our specification methods extends to hard real-time systems. Finally, we discuss extensions of our techniques to capture QoS stochastic properties, and we allude to formal requirements verification and automatic implementation based on our techniques.