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Chapter 9
Multicast
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Multicast Overview
In the network, packets are sent in three modes: unicast, broadcast and multicast. In unicast,
the source server sends separate copy information to each receiver. When a large number of
users require this information, the server must send many pieces of information with the same
content to the users. Therefore, large bandwidth will be occupied. In broadcast, the system
transmits information to all users in a network. Any user in the network can receive the
information, no matter the information is needed or not.
Point-to-multipoint multimedia business, such as video conferences and VoD
(video-on-demand), plays an important part in the information transmission field. Suppose a
point to multi-point service is required, unicast is suitable for networks with sparsely users,
whereas broadcast is suitable for networks with densely distributed users. When the number of
users requiring this information is not certain, unicast and broadcast deliver a low efficiency.
Multicast solves this problem. It can deliver a high efficiency to send data in the point to
multi-point service, which can save large bandwidth and reduce the network load. In multicast,
the packets are transmitted in the following way as shown in the following figure.
Figure 9-1 Information transmission in the multicast mode
Features of multicast:
1. The number of receivers is not certain. Usually point-to-multipoint transmission is
needed;
2. Multiple users receiving the same information form a multicast group. The multicast
information sender just need to send the information to the network device once;
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