Multicast Overview 187
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 57:
Figure 57 Information transmission in the multicast mode
Assume that Hosts B, D and E need the information. To transmit the information
to the right users, it is necessary to group Hosts B, D and E into a receiver set. The
routers on the network duplicate and distribute the information based on the
distribution of the receivers in this set. Finally, the information is correctly delivered
to Hosts B, D, and E.
The advantages of multicast over unicast are as follows:
■ No matter how many receivers exist, there is only one copy of the same
multicast data flow on each link.
■ With the multicast mode used to transmit information, an increase of the
number of users does not add to the network burden remarkably.
The advantages of multicast over broadcast are as follows:
■ A multicast data flow can be sent only to the receiver that requires the data.
■ Multicast brings no waste of network resources and makes proper use of
bandwidth.
Roles in Multicast The following roles are involved in multicast transmission:
■ An information sender is referred to as a multicast source ("Source" in
Figure 57).
■ Each receiver is a multicast group member ("Receiver" in Figure 57).
Source
Server
Receiver
Receiver
Receiver
Host A
Host B
Host C
Host D
Host E
Packets for the multicast group
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