1-3
Figure 1-2 Information transmission in the broadcast mode
Source
Server
Receiver
Receiver
Receiver
Host A
Host B
Host C
Host D
Host E
Packets for all the network
Assume that Hosts B, D, and E need the information. The source server broadcasts this information
through routers, and Hosts A and C on the network also receive this information.
As we can see from the information transmission process, the security and legal use of paid service
cannot be guaranteed. In addition, when only a small number of users on the same network need the
information, the utilization ratio of the network 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.
Information Transmission in the Multicast Mode As 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 users
requiring information is not certain, unicast and broadcast deliver a low efficiency.
Multicast solves this problem. When some users on a network require specified information, the
multicast information sender (namely, the multicast source) sends the information only once. With
multicast 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: