One of the following network devices that works at the physical layer is（）
A. Hub
Bridge
C. Router
D. Ethernet Switch
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Baud rate means（）
A. Number of transmitted signal symbols per second
Bits transmitted per second
C. Number of possible signal changes per second
D. Bytes transferred per second
In the OSI architecture, what directly serves the Application Layer is（）
A. Presentation Layer
B. Conversation Layer
C. Transport Layer
D. Network Layer
In the OSI reference model, which of the basic unit of transmission is frame（）
A. Data Link Layer
B. Physical Layer
C. Network Layer
D. Transport Layer
The semantics of the three elements of network protocol stipulate（）
A. What should communication parties “say” when communicating
B. “How to say” in communication
C. Structure or format of data and control information
D. Execution sequence of communication events
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