Introduction to Multicast 1087Figure 318 Multicast transmissionAssume that Hosts B, D and E need the information. To receive the informationcorrectly, these hosts need to join a receiver set, which is known as a multicastgroup. The routers on the network duplicate and forward the information basedon the distribution of the receivers in this set. Finally, the information is correctlydelivered to Hosts B, D, and E.To sum up, multicast has the following advantages:■ Over unicast: As multicast traffic flows to the node the farthest possible fromthe source before it is replicated and distributed, an increase of the number ofhosts will not remarkably add to the network load.■ Over broadcast: As multicast data is sent only to the receivers that need it,multicast uses the network bandwidth reasonably and brings no waste ofnetwork resources, and enhances network security.Roles in Multicast The following roles are involved in multicast transmission:■ An information sender is referred to as a Multicast Source (“Source” inFigure 318).■ Each receiver is a Multicast Group Member (“Receiver” in Figure 318).■ All receivers interested in the same information form a Multicast Group.Multicast groups are not subject to geographic restrictions.■ A router that supports Layer 3 multicast is called multicast router or Layer 3multicast device. In addition to providing the multicast routing function, amulticast router can also manage multicast group members.For a better understanding of the multicast concept, you can assimilate multicasttransmission to the transmission of TV programs, as shown in Table 43.SourceServerReceiverReceiverReceiverHost AHost BHost CHost DHost EPackets for the multicast group
