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Switched Ethernet is being used for real time transmissions in industrial automation more and and more. Most modern industrial switches are equipped with mechanisms to deal with time predictability. However, real-time transmission not only requires these mechanisms, but also the proper policies for managing network resources. This paper proposes the use of contracts. A contract is a set of transmission specifications which are negotiated between the applications and the run-time support. They define the application workload and the required performance guarantees. We implement contracts for real-time streaming as an extension of FRESCOR (framework for real-time embedded systems based on contracts). This framework was initially thought for providing deterministic performance guarantees to strictly periodic workloads. This work extends it by using the concept of classes of service (CoS) to deal with a wider range of workloads and guarantees and, particularly, with the transmission of highly variable bit rate (VBR) streams, like video. CoS enables, for example, joint transmission of real-time periodic workloads and VBR streams. CoS are implemented using a combination of resource reservation and resource preallocation techniques. The packet scheduling facilities of managed switches and Linux are shown to be key for managing network resources. Evaluations about the effectiveness of the extended FRESCOR framework and the feasibility of using switched Ethernet in real-time industrial environments are also presented.