1-1
1 Multicast Overview
Multicast Overview
With development of networks on the Internet, more and more interaction services such as data, voice,
and video services are running on the networks. In addition, highly bandwidth- and time-critical services,
such as e-commerce, Web conference, online auction, video on demand (VoD), and tele-education
have come into being. These services have higher requirements for information security, legal use of
paid services, and network bandwidth.
In the network, packets are sent in three modes: unicast, broadcast and multicast. The following
sections describe and compare data interaction processes in unicast, broadcast, and multicast.
Information Transmission in the Unicast Mode
In unicast, the system establishes a separate data transmission channel for each user requiring this
information, and sends a separate copy of the information to the user, as shown in
Figure 1-1:
Figure 1-1 Information transmission in the unicast mode
Source
Server
Receiver
Receiver
Receiver
Host A
Host B
Host C
Host D
Host E
Packets for Host B
Packets for Host D
Packets for Host E
Assume that Hosts B, D and E need this information. The source server establishes transmission
channels for the devices of these users respectively. As the transmitted traffic over the network is in
direct proportion to the number of users that receive this information, when a large number of users
need this information, 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.
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