Content Overview Multimedia applications integrate sound, graphics, animation, text, and video. These types of applications have become an effective means of corporate communication. However, sending combined media over a campus data network requires a lot of bandwidth. IP Multicast is an efficient way of delivering media to many hosts over a single IP flow. IP multicast includes an addressing standard, methodologies for multicast users to become members of groups, source and shared trees, and multicast routing protocols. You can use Cisco IOS command line interface (CLI) configurations to implement IP multicast on Cisco devices. This module provides a moderately detailed overview of IP multicast. It explains the applications that utilize multicast technology and the benefit multicast provides to the user of the applications.
Content 7.1 Explaining Multicast 7.1.1 Explaining the Multicast Group Concept Multicast may be used to send the same data packets to multiple receivers. A multimedia server sends one copy of each packet to a single destination IP address that can be received by many end stations if they choose to “listen” to that address. In Figure , the video server transmits a single video stream to a set of host devices listening to a specific multicast address. Only 1.5 Mbps of server-to-network bandwidth is utilized, regardless of the number of receiving hosts. By sending the data packets to multiple receivers, the packets are not duplicated for every receiver but are sent in a single stream, where downstream routers perform packet multiplication over receiving links when necessary. Routers process fewer packets because they receive only a single copy of the packet. Because downstream routers perform packet multiplication and delivery to receivers, the sender, or source of multicast traffic, does not have to know the unicast addresses of the receiver. Simulcast—simultaneous delivery for a group of receivers—may be used for several purposes, including audio or video streaming, news and similar data delivery, and deploying software upgrades. To send data to multiple destinations using unicast, the sender has to send the same data flow to each receiver separately. The sender has to make copies of the same packet and send them once for each receiver. Some web technologies (for example, webcasting) use a “push” method to deliver the same data to multiple users. Instead of users clicking a link to get the data, the data is delivered automatically. Users first have to subscribe to a channel to receive the data; after that, the data is periodically pushed to the user. The problem with the webcast is that the transport is still done using unicast. Web Links IP Multicast Technology
http://www.cisco.com/en/US/products/
ps6552/products_ios_technology_home.html Internet Protocol IP Multicast Technology
http://cisco.com/en/US/tech/tk828/
tech_brief09186a00800a4415.html
Content 7.1 Explaining Multicast 7.1.2 Unicast versus Multicast Unicast transmission sends multiple copies of data, one copy for each receiver. The unicast example in Figure shows a host transmitting three copies of data and a network forwarding each packet to three separate receivers. The host may send to only one receiver at a time, because it has to create a different packet destination address for each receiver. Multicast transmission sends a single copy of data to multiple receivers. The data is sent to the multicast receivers because they have previously subscribed to receive it. The multicast example Figure shows a host transmitting one copy of the data and a network replicating the packet at the last possible hop for each receiver. Each packet exists only in a single copy on any given network. The host may send to multiple receivers simultaneously because it is sending only one packet. Downstream multicast routers replicate and forward the data packet to all the branches where there may be receivers.
Content 7.1 Explaining Multicast 7.1.3 Multicast Advantages and Disadvantages Multicast transmission provides many advantages over unicast transmission in a one-to-many or many-to-many environment: There are other multicast advantages: Multicast enables a whole range of new applications that were not possible on unicast (for example, video on demand [VoD]). There are also some disadvantages of multicast that need to be considered. Most multicast applications are User Datagram Protocol (UDP)-based. This foundation results in some undesirable consequences compared to similar unicast TCP applications. For example:
Content 7.1 Explaining Multicast 7.1.4 Multicast Applications There are various types of multicast applications. Here are three of the most common models: