Switching at Physical Layer ? Switches ?

What are Switches ?

Sitches are devices capable of creating temporary connections between two or more devices linked to the switch.

Switching at Physical Layer :

At the physical layer, we can have only circuit switching. There are no packets exchanged at the physical layer. The switches at the physical layer allow signals to travel in one path or another.

Switching at Data-Link Layer

At the data-link layer, we can have packet switching. However, the term packet in this case means frames or cells. 

Packet switching at the data-link layer is normally done using a virtual-circuit approach. 

Switching at Network Layer

At the network layer, we can have packet switching. In this case, either a virtual-circuit approach or a datagram approach can be used. Currently the Internet uses a datagram approach, as we see in Chapter 18, but the tendency is to move to a virtual-circuit approach. 

What is network? Connected Devices? what is network criteria in data communication?