3Com WX3000 operation manual Roles in Multicast, 3Information transmission in the multicast mode

Figure 1-3Information transmission in the multicast mode

Host A

Receiver

Host B

Source

Server

Host C

Receiver

Host D

	Receiver
Packets for the multicast group	Host E

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

z

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:

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z

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:

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z

An information sender is referred to as a multicast source (“Source” in Figure 1-3). Each receiver is a multicast group member (“Receiver” in Figure 1-3).

All receivers interested in the same information form a multicast group. Multicast groups are not subject to geographic restrictions.

A router that supports Layer 3 multicast is called multicast router or Layer 3 multicast device. In addition to providing multicast routing, a multicast router can also manage multicast group members.

For a better understanding of the multicast concept, you can assimilate multicast transmission to the transmission of TV programs, as shown in Table 1-1.

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