单项选择题 Data transfer is slow between the source and destination. The quality of service requested by the transport layer in the OSI reference model is not being maintained. To fix this issue, at which layer should the troubleshooting process begin?（）

单项选择题 Which layer in the OSI reference model is responsible for determining the availability of the receiving program and checking to see if enough resources exist for that communication? （）

单项选择题 A company implements video conferencing over IP on their Ethernet LAN. The users notice that the network slows down, and the video either stutters or fails completely. What is the most likely reason for this?（）