1-4Figure 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).z All receivers interested in the same information form a multicast group. Multicast groups are notsubject to geographic restrictions.z A router that supports Layer 3 multicast is called multicast router or Layer 3 multicast device. Inaddition to providing multicast routing, a multicast router can also manage multicast groupmembers.For a better understanding of the multicast concept, you can use the analogy of a transmission of TVprograms, as shown in Table 1-1.
