3com 4210 PWR 9-Port 3CR17341-91-ME User Manual

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. 

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). 

Server

Host A

Host B

Host C

Host D

Host E

Packets for the multicast group