Flow control is of particular importance in TSN networks, where timeliness of messages is guaranteed through several types of traffic shaping. This has given rise to a number of papers discussing the worst-case response time analysis of, in particular, the credit-based traffic shaping approach. In this work-in-progress paper, we concern ourselves with the possibility that traffic shaping can be implemented in different layers of the protocol stack, in particular the PHY and MAC layers. We argue that the analysis performed in literature is only applicable for a PHY layer interpretation, but cannot be readily used in the context of higher layer implementations. In particular, in a MAC layer implementation of credit-based shaping, some of the basic properties of credit that are crucial for the analysis, are no longer valid after overhead is added by the PHY layer. As an ongoing effort, we plan to revisit the eligible interval analysis of \cite{Cao} with an adaptation to the MAC layer.