ensuring fast data-transfer
offers many advantages over traditional point-to-point
request/reply methods. Here is an overview of multicasting
and the benefits it can provide.
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.
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.
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
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
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
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
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
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
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
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 firstname.lastname@example.org