186 CHAPTER 15: MULTICAST OVERVIEW
the server must send many pieces of information with the same content to the
users. Therefore, the limited bandwidth becomes the bottleneck in information
transmission. This shows that unicast is not good for the transmission of a great
deal of information.
Information
Transmission in the
Broadcast Mode
When you adopt broadcast, the system transmits information to all users on a
network. Any user on the network can receive the information, no matter the
information is needed or not.
Figure 56 shows information transmission in
broadcast mode.
Figure 56 Information transmission in the broadcast mode
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
Source
Server
Receiver
Receiver
Receiver
Host A
Host B
Host C
Host D
Host E
Packets for all the network
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