1-3
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 not efficient.
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:
Figure 1-3 Information transmission in the multicast mode
Source
Server
Receiver
Receiver
Receiver
Host A
Host B
Host C
Host D
Host E
Packets for the multicast group
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:
z No matter how many receivers exist, there is only one copy of the same multicast data flow on each
link.
z 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:
z A multicast data flow can be sent only to the receiver that requires the data.
z Multicast brings no waste of network resources and makes proper use of bandwidth.
Roles in Multicast The following roles are involved in multicast transmission:
z An information sender is referred to as a multicast source (“Source” in Figure 1-3).
z Each receiver is a multicast group member (“Receiver” in Figure 1-3).