Figure 1-2Information transmission in the broadcast mode

Host A

Receiver

Host B

Source

Host C

Server

Receiver

Host D

 

Receiver

Packets for all the network

Host E

 

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:

1-3

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3Com WX3000 Information Transmission in the Multicast Mode, 2Information transmission in the broadcast mode