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Multicasting: ensuring fast data-transfer

Multicasting offers many advantages over traditional point-to-point request/reply methods. Here is an overview of multicasting and the benefits it can provide.

If every TV program required a dedicated one-to-one transmission from the studio to the set, there would be very few programs, and only the wealthy would own TVs. This is a ridiculous scenario, but it's similar to one faced by most network guys. Since, file transfers require a one-to-one transmission from the source to the client, companies have to limit the number of applications that require large file transfers and control the number of users that have access to those applications.

By increasing network bandwidth and capacity, IS managers may be able to add new users and increase the number of large file transmissions. But this solution is expensive and usually results in only incremental benefits perhaps a few dozen extra users.

This is where multicasting has an edge over traditional point-to-point request/ reply methods. Lets take the example of radio (invented in 1896 by Macron) and telephone (invented in 1876 by Graham Bell) to explain this. It quite obvious why radio became so popular and spread its nets all over the world in such a short time where as we are still working to connect entire India through telephones. A cheaper, further-reaching solution is IP multicasting. In contrast with the point-to-point or request/reply communications methods, which require a separate file be sent to each user, multicasting transmits the file once, letting an unlimited number of users access it. As a result, it uses much less bandwidth. Routers play the most important role in multicasting.

Still evolving
IP multicasting isn't that new, but its components are still evolving. Some early adopters report it to be an efficient way to improve network performance.

Multicasting works through hierarchical network architecture. The IP multicast stream is captured by routers only if any computers connected to those routers have subscribed to the stream. The routers then send the stream only to those switches and hubs that have clients that are "listening." With unicasting, even large companies with state-of-the-art networks could offer to transmit multimedia to only a few hundred people at a time. With multicasting, one can stream several multimedia-training programs simultaneously to several thousand people.

Much of the interest in multicasting has centered on multimedia applications. Organizations are considering it for implementing applications such as teleconferencing, distance learning, and newscasts to the desktop and virtual corporate meetings. The entertainment industry can use multicasting to provide multimedia clips to Web surfers.

Moving data files
Multicasting is also being used to help companies transmit large data files, such as software upgrades. Instead of using CD-ROMs to update software to say over 1,000 clients, which is time-consuming and unreliable, one can move to multicasting.

Moving to satellite-based IP
multicasting over a VSAT satellite network meant the properties could get software updates concurrently, and IS could be sure that all clients would use the same version of its property-management application.

IP multicasting can also help
companies that need to transmit small data files to many users. For e.g. a company that broadcasts personalized news and information to business customers, can use software like TIB/Rendezvous, IP multicasting software from TIBCO Inc. (www.tibco.com), into its central broadcast facility. This can be easily done because with multicasting, bandwidth has become virtually a non-issue. IP multicasting is also bound to reduce the number of geographically placed servers, as there is no need for point-to-point communication and a single broadcast will reach clients across the Net.

Industry approach
There are basically two types of IP multicasting software applications. Products from companies like StarBurst (www.starburstcom.com) and TIBCO are used to transmit data. StarBurst was in news recently for its Multicast File Transfer Protocol used by European Space Agency for Satellite Multicasting. Other noteworthy products such as Microsoft "NetShow" are geared toward transmitting video.

The difference between the two types of products results from the fact that with multimedia, it's not absolutely necessary for users to get every bit as it is with database or file transfers. Negative point is that IP multicasting does not provide the level of reliability offered by TCP, which requests a resend if a packet does not arrive. IP multicasting sends data over UDP, which discards the packet if a transmission error occurs.

So far, IP multicasting products intended to send data files use proprietary methods to increase reliability. Soon, however, a standard means of improving multicast reliability may be available. The standard, which TIBCO supports, is called PGM (Pragmatic General Multicast).

The challenge is to improving multicast reliability, doing what TCP does would increase network usage. If every time a packet does not arrive, the client has to send a negative acknowledgment and the system has to respond to that individual client, then benefits of multicasting are defeated.

PGM pretty much solves that problem in two ways. The best part is that it takes the help of the router to accumulate negative acknowledgments and deals with them all at once rather than one at a time. Second, rather than requesting a resend from the source, the missing packets is recovered from the router nearest to the client that sent the acknowledgment.

As the Internet, Intranets, and the Web take on more services that were initially done by client/server systems, multicasting may provide a means of expanding services without completely redesigning a network. Although some standards issues remain, the technology is relatively mature. There is need for more companies to come forward and implement multicast solutions. Till then we will have to work on improving the proprietary methods of data retrieval if we want to implement IP multicasting.

Ajay Barvey is a senior programmer with Plexus Technologies and can be reached at ajay_barvey@yahoo.com


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