Digital Signal Processing Reference
In-Depth Information
a large burden on the router however, and this should be
considered, given the router is generally supporting a lot of other
traffic. The router is also responsible for detecting and processing
requests to add new ports or drop ports as users either request to
watch, or drop off, the program.
Unlike unicasting, the user has no control over the delivery of
a multi-cast. When connecting they will join at whatever point the
multicast program happens to be, and each viewer sees the same
content simultaneously, similar to broadcast television. These
restrictions have to be set against the significant advantages of
network traffic. Just as the video bandwidth from the video server
is equal to the unicast video bandwidth, so the video bandwidth is
equal between various routers. If there are multiple viewers
watching from a downstream router, then only the unicast
bandwidth is required. Only if viewers are connected to different
routers does the video stream need to be replicated at different
parts of the network.
Multicast addressing is set up using session announcement
protocol (SAP). The SAP informs all multicast-enabled receivers
of the programs being multicast broadcast on the network. The
details of either connecting or disconnecting from a multicast are
covered in the Internet Group Management Protocol (IGMP).
Using IGMP, the routers need to keep track of all users in their
downstream path, to know whether or not the multicast program
is to continue to be broadcast on a given port. If a new user
requests to join, the appropriate router must replicate the
program in that portion of the network if it is not already being
broadcast. The routers will also broadcast the SAP messages.
18.9 Video Conferencing
Video conferencing that feels like everyone is in the same
room has been a business goal for a long time. Video confer-
encing can be done over IP, using private networks, but latency
must be carefully controlled so the interactions and conversa-
tions between people are not delayed.
The H.320 standard defines video conferencing over switched
telephone lines (ISDN), and is used in much early corporate
video-conferencing equipment, usually in dedicated conference
rooms at different sites within a company. H.323 evolved from
H.320, allowing video (and audio) conferencing to be packet
based. It is a full-featured system, with considerable maturity.
A number of vendors offer systems based upon H.323, and
generally equipment from different vendors will interoperate.
Most systems support only audio, as audio or voice conferencing
