The network layer provides services to the transport layer．It call be based on either vir

Networks can be interconnected by different devices．In the physical layer，networks can be

Together with the network layer,the transport layer is the heart of the protocol（）The net

（)是NS（GB Network Service Layer)的功能 .A.LLC PDU 的传输B.对NSE中多个NS－VCs 的上下链路

● Networks can be interconnected by different devices. In the physical layer，networks can

●WWW is a large network of Internet servers providing （75) and other services to terminals

● WWW is a large network of internet servers providing （75)and other services to terminals