186 CHAPTER 15: MULTICAST OVERVIEWthe server must send many pieces of information with the same content to theusers. Therefore, the limited bandwidth becomes the bottleneck in informationtransmission. This shows that unicast is not good for the transmission of a greatdeal of information.InformationTransmission in theBroadcast ModeWhen you adopt broadcast, the system transmits information to all users on anetwork. Any user on the network can receive the information, no matter theinformation is needed or not. Figure 56 shows information transmission inbroadcast mode.Figure 56 Information transmission in the broadcast modeAssume that Hosts B, D, and E need the information. The source server broadcaststhis information through routers, and Hosts A and C on the network also receivethis information.As we can see from the information transmission process, the security and legaluse of paid service cannot be guaranteed. In addition, when only a small numberof users on the same network need the information, the utilization ratio of thenetwork resources is very low and the bandwidth resources are greatly wasted.Therefore, broadcast is disadvantageous in transmitting data to specific users;moreover, broadcast occupies large bandwidth.InformationTransmission in theMulticast ModeAs described in the previous sections, unicast is suitable for networks with sparselydistributed users, whereas broadcast is suitable for networks with denselydistributed users. When the number of users requiring information is not certain,unicast and broadcast deliver a low efficiency.Multicast solves this problem. When some users on a network require specifiedinformation, the multicast information sender (namely, the multicast source) sendsSourceServerReceiverReceiverReceiverHost AHost BHost CHost DHost EPackets for all the network
