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Message Addressing and Transmission Methods: Unicast, Broadcast and Multicast Messages
(Page 1 of 2)
In a networking technology that uses
messages to send data, there are a number of tasks that must be undertaken
in order to successfully transmit the data from one place to another.
One is simply the addressing of the messageputting an address
on it so that the system knows where it is supposed to go. Another is
transmitting the message, which is of course sending it to its
There are several different ways
of addressing and transmitting a message over a network, One way in
which messages are differentiated is in how they are addressed, and
to how many recipients. Which method is used depends on what the function
of the message is, and also on whether or not the sender knows specifically
whom they are trying to contact, or only generally.
Message Transmission Methods
To help explain these different methods,
I will use a real-world analogy. Consider a social function with 300
people that is being held in a large hall. These people are mingling
and are having different conversations. There are different kinds of
messages that may need to be sent in this setting, much as is the case
Bearing this analogy in mind, consider
these three kinds of message transmissions, which are illustrated in
- Unicast Messages: These are messages that
are sent from one device to another device; they are not intended for
others. If you have a friend at this social event, this is the equivalent
of pulling him or her aside for a private conversation. Of course, there
is still the possibility of someone else at the event overhearing your
conversationor even eavesdropping on it. The same is true in networking
as welladdressing a message to a particular computer doesn't guarantee
that others won't also read it, just that they normally will not do
- Broadcast Messages: As the name suggests,
these messages are sent to every device on a network. They are used
when a piece of information actually needs communicating to everyone
on the network, or used when the sending station needs to send to just
one recipient, but doesn't know its address.
For example, suppose a new arrival at the social gathering saw a blue
sedan with New Hampshire plates in the parking lot that had its lights
left on. He of course does not know whose car this is. The best way
to communicate this information is to broadcast it by having the host
make an announcement that will be heard by all, including the vehicles
owner. In networks, broadcast messages are used for a variety of purposes,
including finding the locations of particular stations or the devices
that manage different services.
- Multicast Messages: These are a compromise
between the previous two types: they are sent to a group of stations
that meet a particular set of criteria. These stations are usually related
to each other in some way, such as serving a common function, or being
set up into a particular multicast group. (Note that one can
also consider broadcast messages to be a special case of multicast,
where the group is everyone.
Back to our analogy: this would be somewhat like a group of friends
who go to this large social hall and then stay together in a small discussion
groupor perhaps use radios to talk to each other from a distance.
Multicasting requires special techniques that make clear who is in the
intended group of recipients.
Figure 4: Unicast, Multicast and Broadcast Message Addressing and Transmission
The three basic type of addressing and message delivery in networking are illustrated in this simplified local area network. Device #6 is sending a unicast message to #2 shown in purple. Device #4 is sending a multicast message to multicast group X. In this case, that group includes devices #1 and #3, shown in green. Finally, Device #5 is sending a broadcast message, which goes to all other devices on the LAN.
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Version 3.0 - Version Date: September 20, 2005
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