running on the remote computer, open to connection requests from clients. A client program runs only when it is needed while a server program runs all the time because it does not know when its services will be needed. Clients can be run on a machine either iteratively or concurrently. When the clients are run iteratively, the machine allows one client to start, run, and Client Client Internet Internet Server Figure 26.2 Client server model. Fundamentals of EMS, NMS and OSS/BSS terminate before another client is started by the machine. When the clients are run concurrently, the machine allows running two or more clients at the same time. Similarly the servers can be run on a machine either iteratively or concur- rently. An iterate server accepts, processes, and sends the response to the client before accepting another request; that is, it processes a single request at a time. A concurrent server processes a large number of requests at the same time. It shares its time between a large numbers of requests. The transport layer protocol and the service method facilitate the server operation. UDP-connectionless transport layer protocol or TCP-connection oriented transport layer protocols are used commonly. Based on the number of requests the server can handle and the type of connec- tion, servers can be classified into four types: Connectionless iterative: The connectionless iterative server processes a sin- gle request at a time and uses a connectionless transport layer protocol like Connectionless concurrent: The connectionless concurrent server processes many requests at the same time and uses a connectionless transport layer protocol like UDP. Connection oriented iterative: The connection oriented iterative server pro- cesses a single request at a time and uses a connection oriented transport layer protocol like TCP. Connection oriented concurrent: The connection oriented concurrent server processes many requests at the same time and uses a connection oriented transport layer protocol like TCP. 26.5 Socket Interfa