A unicast packet is the complete opposite: one machine is
talking to only one other machine. All TCP connections are
unicast, since they can only have one destination host for
each source host. UDP packets are almost always unicast too,
though they can be sent to the broadcast address so that
they reach every single machine in some cases.
A multicast packet is from one machine to one or more. The
difference between a multicast packet and a broadcast packet
is that hosts receiving multicast packets can be on
different LANs, and that each multicast data-stream is only
transmitted between networks once, not once per machine on
the remote network. Rather than each machine connecting to a
video server, the multicast data is streamed per-network,
and multiple machines just listen-in on the multicast data
once it's on the network.
Using Multicast addressing to limit WAN traffic
The DACS and Model Builder can be used to limit the amount
of traffic that would be put out over a wide area network.
This is useful if you have a facility with several systems
and not all of the voice traffic needs to go out over the
WAN. Voice traffic can consume quite a bit of bandwidth (64
Kbits/sec per mulaw or 16 Kbits/sec per CVSD voice stream)
and unecessary voice streams on a WAN could cause potential
The idea is really quite simple, voice streams passed
through the router onto the WAN will have DIS exercise
numbers corresponding to multicast groups that are in use
all across the WAN. For local voice traffic multicast groups
are formed locally but no IGMP reports are submitted to the
router for these groups, therefore they never get passed
through. The DACS on the local network however can process
these packets and permit voice communications.
The DIS exercise number is used as a discriminator to
determine if voice is passed over the WAN. For radios having
DIS exercise numbers below the cutoff point, IGMP reports
are submitted, for radios having DIS exercise numbers above
the cutoff point no IGMP reports are published.
Unicast is a one-to one connection between the client and the server. Unicast uses IP delivery methods such as Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), which are session-based protocols. When a Windows Media Player client connects using unicast to a Windows Media server, that client has a direct relationship to the server. Each unicast client that connects to the server takes up additional bandwidth. For example, if you have 10 clients all playing 100-kilobits per second (Kbps) streams, those clients as a group are taking up 1,000 Kbps. If you have only one client playing the 100 Kbps stream, only 100 Kbps is being used.
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Multicast is a true broadcast. The multicast source relies on multicast-enabled routers to forward the packets to all client subnets that have clients listening. There is no direct relationship between the clients and Windows Media server. The Windows Media server generates an .nsc (NetShow channel) file when the multicast station is first created. Typically, the .nsc file is delivered to the client from a Web server. This file contains information that the Windows Media Player needs to listen for the multicast. This is similar to tuning into a station on a radio. Each client that listens to the multicast adds no additional overhead on the server. In fact, the server sends out only one stream per multicast station. The same load is experienced on the server whether only one client or 1,000 clients are listening
Important: Multicast on the Internet is generally not practical because only small sections of the Internet are multicast-enabled. Multicast in corporate environments where all routers are multicast-enabled can save quite a bit of bandwidth.