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 specificmulticast 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 receiveonly 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 deployingsoftware 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 isdelivered 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. Unicast versus Multicast
Unicast transmission sends multiple copies of data, one copy for each receiver. The unicast example in Figure shows a host transmitting threecopies 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 multicastexample 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 theremay be receivers. Multicast Advantages and Disadvantages
Multicast transmission provides many advantages over unicast transmission in a one-to-many or many-to-many environment: Enhanced efficiency: Network bandwidth is utilized more efficiently because multiple streams of data are replaced with a single transmission.
Optimized performance: Fewer copies of the data require forwardingand processing. Distributed applications: Multipoint applications will not be possible with unicast as demand and usage grows, because unicast transmission does not scale (traffic level and clients increase at a 1:1 rate with unicast transmission).
There are other multicast advantages: For the equivalent amount of multicast traffic, the sender uses less processing power and bandwidth.Multicast packets do not impose as high a rate of bandwidth utilization as unicast packets, so there is a greater possibility that they arrive at their destinations almost simultaneously.
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...
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