3Information Transmission in the Multicast ModeAs described in the previous sections, unicast is suitable for networks with sparsely distributed users,whereas broadcast is suitable for networks with densely distributed users. When the number of usersrequiring information is not certain, unicast and broadcast not efficient.Multicast solves this problem. When some users on a network require specified information, themulticast information sender (namely, the multicast source) sends the information only once. Withmulticast distribution trees established for multicast data packets through multicast routing protocols,the packets are duplicated and distributed at the nearest nodes, as shown in Figure 1-3:Figure 1-3 Information transmission in the multicast modeSourceServerReceiverReceiverReceiverHost AHost BHost CHost DHost EPackets for the multicast groupAssume that Hosts B, D and E need the information. To transmit the information to the right users, it isnecessary to group Hosts B, D and E into a receiver set. The routers on the network duplicate anddistribute the information based on the distribution of the receivers in this set. Finally, the information iscorrectly delivered to Hosts B, D, and E.The advantages of multicast over unicast are as follows:z No matter how many receivers exist, there is only one copy of the same multicast data flow on eachlink.z With the multicast mode used to transmit information, an increase of the number of users does notadd to the network burden remarkably.The advantages of multicast over broadcast are as follows:z A multicast data flow can be sent only to the receiver that requires the data.z Multicast brings no waste of network resources and makes proper use of bandwidth.Roles in MulticastThe following roles are involved in multicast transmission:z An information sender is referred to as a multicast source (“Source” in Figure 1-3).z Each receiver is a multicast group member (“Receiver” in Figure 1-3).
