What is multicast?

What is multicast?

Broadcast is the term which is oldest and has a traditional meaning associated which TV and radio. It generally meant an indiscriminate transmission which can be received by anyone having the correct equipment. Certain media are by their nature broadcast: satellite channels, CATV coax, Ethernet, ... In other words any transmission into the medium is "heard" by all attachments. Of course higher level protocols can be used to direct data to a particular destination even though all destinations get it at the lower level. In the world of IP a broadcast is usually designated by an all ones address. If an Ethernet packet has a MAC address of all one's then all stations attached are expected to pass it up to the next higher level. The RIP protocol is one such protocol which makes use of this capability. So then what is multicast? As far as I know, multicast is an Internet Protocol designation only. It is a mechanism for designating groups of arbitrary size such that any transmission from a member of the group into the net is received by all other members of the group. It is somewhat like a large party line in which all members can "hear" all transmissions. It is generally used for more restricted cases such as an audio channel which originates from one site and goes to many others. In the Internet there are two types of addresses: unicast and multicast. One confusion is that a host or terminal generally has only one unicast address but can be a member of many multicast groups. Now the Internet address space is based on 32 bit addresses. There are 4,294,967,296 unique address. The addresses are divided into classes. Class D is the multicast class. It is composed of all addresses having 1110 in the first four bits. This means that there are over 250,000,000 class D addresses. If all the Internet had multicast capability then there could be many groups to join. Unfortunately multicasting as a feature has only limited actual applicability. One of its major drawbacks is that there is no built in mechanism for determining who received a burst of data. There are various initiatives in progress to rectify this shortcoming. I hope this has helped. There is a lot more to this story - but then that is another story. --

-------------------------------------------
 Bob Reid
 Protos, Inc.
 4708 Old Village Lane
 Dunwoody,  GA  30338-5124
 e-mail: breid@ix.netcom.com
 Voice and fax: (404) 713-0721
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Ref: c:\scada\041997\msg00020.xml

Multicast - Broadcast and VSAT

The following information on VSAT (Very Small Aperture terminal) technology may throw some more light on this subject. VSAT's typically are used when sites are distributed over large geographical areas. They also are employed for point to multipoint communications. Nodes in a VSAT network communicate with each other by sending a radio signal to a geosynchronous satellite which shifts the frequency and retransmits the signal. In a two way VSAT network all nodes are set up to transmit and receive. There are essentially two kinds of 2 way VSAT networks - mesh and star. Star networks are used for point to multipoint communications - a large hub antennae transmits to many small inexpensive VSAT antennaes. In a mesh or hubless network the VSAT's communicate with one another directly. A control center may be incorporated into the mesh network to assign channels and perform other housekeeping duties. Once the channel between points is set up, the stations the stations themselves control the transmission. Mesh networks usually carry voice, although some carry digital video. A star network consists of four primary components

1. Hub earth station. (Center of the network).

2. Remote sites.

3. Network management center - the VSAT installation can be managed from the hub or remotely.

4. The satellite transponder. The cost varies with the configuration. A hub costs from $US 900k to $2m. Most users will lease hub capacity from a service provider. Hubs and remote sites are connected by 2 radio links - inroutes and outroutes. On star networks, outroute bandwidth is shared by a number of remote sites through time division multiplexing.(TDM) Inroute transmissions use a combination of TDMA and frequency division multiple access. FDMA divides the frequency allocation into smaller segments. Frequency selective filters at the transmitting and receiving stations isolate these segments enabling them to be used independently. TDMA makes it possible for a star network to handle different types of data. There are a number of TDMA techniques used - slotted Aloha, transaction reservation, stream TDMA etc. The following communications protocols (at least) are supported over the VSAT systems. - Appletalk, Decnet, Banyon Vines, LLC/Netbios, LLC/SNA, Novell IPX, and TCP/IP. A previous response to this thread discussed the issue of TCP/IP and broadcast and multicast.

Ref: c:\scada\041997\msg00022.xml

Re: Multicast

In a crowded place like Singapore with restrictive radio frequencies you also find mainly (only?) multi-cast radio - where the vendors are assigned the radio frequency and customers can only buy a radio modem which will be "sharing" the freqency with their other customers. The vendors down play the security issue and say because of the addressing design you cannot see anyone else's data and they cannot see yours. But I suppose their technicians are perfectly capable of setting up duplicate receivers so someone could see others data. Best Regards;

 Lynn August Linse, linsela@robustdc.com
 Robust DataComm Pte Ltd, 221 Henderson Road #04-10
 Singapore 159557, Ph(65)272-2340  Fx(65)272-0582
 http://www.robustdc.com
 http://www.singnet.com.sg/~linsela

Re: Multicast??

Srinivas: The difference between Broadcast and Multicast is in the eyes of the receiver. This is a link layer concept. In both cases a message is sent out on a media that is shared (all potential recipients will see the message). In the case of a Broadcast (a particular address is reserved for this purpose, and is found in the destination address field of the message), ALL recipients will act upon the received message. In the case of a Multicast, only those recipients that have been configured to respond to the destination address in the message will act upon the message. The destination address in the message will be one of a reserved set of multicast addresses. With Broadcast, a single address is typically reserved; for Multicast, a range of addresses are usually reserved. Thus, each unique multicast address can be used for defining a different group of respondents - typically on a functional basis. Ethernet is probably the best known implementation of these concepts, but they are applicable in some HDLC-like implementations as well. As to whether these techniques are applicable to VSAT, I really don't know personally. Satelite is a shared media, so it could apply (with respect to messages from the 'host' outward at least). Whether it can be used will depend on the link layer protocol employed, and the accessibility of those characteristics to the application layer. Somebody more knowledgable on this topic will have to address this aspect for you.

                             ////////////////////////////////////////
                             ///  R. Murray Reid   (403) 541-4787 ///
                            ///       murray.reid@pipe.nova.ca   ///
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Ref: c:\scada\041997\msg00010.xml