Over the past few years, the concept of end-users being able to send and receive audio and video (known collectively as multimedia) at the desktop has gained considerable attention and acceptance. With high-performance 486, Pentium, and PowerPC CPUs, more than 80 percent of the personal computers sold during 1995 were multimedia capable. Today, it is not uncommon for end-users to run video editing and image processing applications from the desktop.


The proliferation of more and more multimedia-enabled desktop computers has spawned a new class of multimedia applications that operate in networked environments. These network multimedia applications leverage existing network infrastructure to deliver video and audio applications to end users. Most notable are videoconferencing and video server applications. With these applications, video and audio streams are transferred over the network between peers or between clients and servers. There are three types of multimedia applications:


•Unicast—Unicast applications send one copy of each packet to each host that wants to receive the packet. This type of application is easy to implement, but it requires extra bandwidth because the network has to carry the same packet multiple times—even on shared links. Because unicast applications make a copy of each packet, the number of receivers is limited to the number of copies of each packet that can be made by the CPU that runs the unicast application.


•Broadcast—Broadcast applications send each packet to a broadcast address. This type of application is easier to implement than unicast applications, but it can have serious effects on the network. Allowing the broadcast to propagate throughout the network is a significant burden on both the network (in terms of traffic volume) and the hosts connected to the network (in terms of the CPU time that each host that does not want to receive the transmission must spend processing and discarding unwanted broadcast packets). You can configure routers to stop broadcasts at the LAN boundary (a technique that is frequently used to prevent broadcast storms), but this technique limits the receivers according to their physical location.


•Multicast—Multicast applications send each packet to a multicast group address. Hosts that want to receive the packets indicate that they want to be members of the multicast group. This type of application expects that networks with hosts that have joined a multicast group will receive multicast packets. Multicast applications and underlying multicast protocols control multimedia traffic and shield hosts from having to process unnecessary broadcast traffic.


This case study examines multicast protocols that have been developed for the Internet Protocol (IP) and for AppleTalk, as well as Cisco Internetwork Operating System (Cisco IOS) features that can help your network deliver video and audio smoothly.