544 CHAPTER 41: M ULTICAST O VERVIEWn ■ A multicast source does not necessarily belong to a multicast group. Namely, amulticast source is not necessarily a multicast data receiver.■ A multicast source can send data to multiple multicast groups at the sametime, and multiple multicast sources can send data to the same multicast groupat the same time.Advantages andApplications ofMulticastAdvantages of multicastAdvantages of the multicast technique include:■ Enhanced efficiency: reduces the CPU load of information source servers andnetwork devices.■ Optimal performance: reduces redundant traffic.■ Distributive application: Enables point-to-multiple-point applications at theprice of the minimum network resources.Applications of multicastApplications of the multicast technique include:■ Multimedia and streaming applications, such as Web TV, Web radio, andreal-time video/audio conferencing.■ Communication for training and cooperative operations, such as distancelearning and telemedicine.■ Data warehouse and financial applications (stock quotes).■ Any other point-to-multiple-point data distribution application.Multicast Models Based on how the receivers treat the multicast sources, there are two multicastmodels:ASM modelIn the ASM model, any sender can send information to a multicast group as amulticast source, and numbers of receivers can join a multicast group identified bya group address and obtain multicast information addressed to that multicastgroup. In this model, receivers are not aware of the position of multicast sources inadvance. However, they can join or leave the multicast group at any time.Table 51 An analogy between TV transmission and multicast transmissionStep TV transmission Multicast transmission1 A TV station transmits a TVprogram through a channel.A multicast source sends multicast datato a multicast group.2 A user tunes the TV set to thechannel.A receiver joins the multicast group.3 The user starts to watch the TVprogram transmitted by the TVstation via the channel.The receiver starts to receive themulticast data that the source sends tothe multicast group.4 The user turns off the TV set ortunes to another channel.The receiver leaves the multicast group orjoins another group.
