The wires and signals that form the transport layer are an independent thing from TCP/IP.
TCP/IP is a network protocol that defines how computers on the network will talk to one another in a logical manner. It doesn’t define the signals and wires and whatnot that get the packets from point A to point B.
You can think of it sort of like a web api. The api defines how you can interact with the services provided by the server, but it says nothing about what the server actually does to provide that service.
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