3Com Switch 4800G 24-Port Configuration Manual page 542

542 C 41: M
HAPTER ULTICAST
O
VERVIEW
In unicast transmission, the traffic over the network is proportional to the number
of hosts that need the information. If a large number of users need the
information, the information source needs to send a copy of the same information
to each of these users. This means a tremendous pressure on the information
source and the network bandwidth.
As we can see from the information transmission process, unicast is not suitable
for batch transmission of information.
Broadcast
In broadcast, the information source sends information to all hosts on the
network, even if some hosts do not need the information, as shown in Figure 163.
Figure 163 Broadcast transmission
Source
Server
Packets for all the network
Assume that only Hosts B, D, and E need the information. If the information
source broadcasts the information, Hosts A and C also receive it. In addition to
information security issues, this also causes traffic flooding on the same network.
Therefore, broadcast is disadvantageous in transmitting data to specific hosts;
moreover, broadcast transmission is a significant usage of network resources.
Multicast
As discussed above, the unicast and broadcast techniques are unable to provide
point-to-multipoint data transmissions with the minimum network consumption.
The multicast technique has solved this problem. When some hosts on the
network need multicast information, the multicast source (Source in the figure)
sends only one copy of the information. Multicast distribution threes are built for
the multicast packets through multicast routing protocols, and the packets are
replicated only on nodes where the trees branch, as shown in Figure 164:
Host A
Receiver
Host B
Host C
Receiver
Host D
Receiver
Host E