A communications network can provide many types of service. The most basic type of service is known as simplex. This service provides one-way communication. Examples of this type of service are TV distribution, and the transmission of burglar alarm messages.
Most networks transfer data in two directions and are known as duplex communications links. Duplex links are classified as either full duplex or half duplex (also known as two way alternate), depending upon whether both local and remote nodes may simultaneously transmit, or whether one must wait for the other to finish before starting transmission. (A telephone line is capable of full duplex transmission, although most human users choose to adopt a half duplex procedure to listen to what the remote person has to say.)
Some networks are able to transmit a message to all nodes which are connected to the network (this is common in LANs). This is known as broadcast transmission. In addition to broadcast, some networks also allow simultaneous transmission to a specific group of users, this is known as multicast transmission. An example of multicast transmission is the use of the IP network protocol to support packet video allowing remote participation in conferences across the Internet.