Operation Manual – MulticastH3C S3100 Series Ethernet Switches Chapter 1 Multicast Overview1-31.1.3 Information Transmission in the Multicast ModeAs described in the previous sections, unicast is suitable for networks with sparselydistributed users, whereas broadcast is suitable for networks with densely distributedusers. When the number of users requiring information is not certain, unicast andbroadcast deliver a low efficiency.Multicast solves this problem. When some users on a network require specifiedinformation, the multicast information sender (namely, the multicast source) sends theinformation only once. With multicast distribution trees established for multicast datapackets through multicast routing protocols, the packets are duplicated and distributedat the nearest nodes, as shown in Figure 1-3:SourceServerReceiverReceiverReceiverHost AHost BHost CHost DHost EPackets for the multicast groupFigure 1-3 Information transmission in the multicast modeAssume that Hosts B, D and E need the information. To transmit the information to theright users, it is necessary to group Hosts B, D and E into a receiver set. The routers onthe network duplicate and distribute the information based on the distribution of thereceivers in this set. Finally, the information is correctly 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 multicastdata flow on each link.z With the multicast mode used to transmit information, an increase of the number ofusers does not add 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.
