(第1空)With circuit switching, a ( )path is established between two stations for communication . Switching and transmission resources within the network are ( )for the exclusive use of the circuit for the duration of the connection . The connection is( )：Once it is established ，it appears to attached devices as if there were a direct connection. Packet switching was designed to provide a more efficient facility than circuit switching for ( ) data traffic. Each packet contains some portion of the user data plus control information needed for proper functioning of the network. A key distinguishing element of packet-switching networks is whether the internal operation is datagram or virtual circuit . With internal virtual circuits,a route is defined between two endpoints and all packets for that virtual circuit follow the( )route, With internal datagrams,each packet is treated independently,and packets intended for the same destination may follow different routes.