NCast Telepresenter M4 - Manual (Page 45)

NCast Telepresenter Reference Manual
The passing of “floor control” triggers two important events for the participants in the conference. First, the
media stream being generated by the Coordinator is shut down, and a new media stream being generated
by one of the participants starts up. Only one graphics media stream is present on the network, leading to
constant network utilization in terms of traffic load or bandwidth used. The total bandwidth utilized remains
flat, independent of how many participants are in the conference.
The second event occurring when the floor is passed is a signal that is sent to all units which tells them to
look for and display the new media stream being generated by a participant. The content being displayed to
conference viewers automatically switches from one source to another, that is, from one site to another site.
When the Coordinator retakes control, another signal is sent and the units relock on the Coordinators media
stream.
In Collaboration mode, audio from all sites is mixed and presented as one combined stream. Everyone can
hear everyone else. Echo-canceling microphones or equipment are required for this type of usage. The
bandwidth used for audio increases linearly as the number of units in the conference increases.
For a two-unit conference there is a subtle difference between Full-Duplex streaming and Collaboration
Mode. In Full-Duplex streaming two graphics streams are occupying the network bandwidth, so the total
bandwidth used is 2B+2A where B is the graphics bandwidth used and A is the audio bandwidth used. In
Collaboration Mode, the total bandwidth used is 1B+2A, that is, roughly half the utilization of Full-Duplex.
4.2.3. Video Format
Select the type of compression encoding desired. The two choices:
▪ MPEG-4 (MPEG-4 Part 2 encoding, original MPEG standard)
▪ H.264 (MPEG-4 Part 10 encoding, latest industry standard)
4.2.4. Video Address
An address entered for Video (the Video or Graphics media stream) will be either a valid multicast address,
or the numeric host IP address of the remote unit if a point-to-point connection is being established.
A full discussion of multicast addressing is beyond the scope of this document, but briefly summarized, there
are three classes of multicast addresses typically used by multicast applications:
• Global Addresses – The range of multicast addresses 224.0.1.0-238.255.255.255 are used for
global communications. These addresses are dynamically allocated and not statically reserved.
These addresses are unsuited for static assignment in the Channel Table.
• GLOP Addresses – A document, IETF RFC 3180, describes a mechanism for statically assigned
multicast addresses in the address space 233/8 based on a formula that incorporates the
Autonomous System (AS) number in the middle two octets. The AS number is owned by the ISP
providing service to the account and use of a GLOP address must be coordinated with that ISP.
• Administratively Scoped Addresses – The multicast address range of 239.0.0.0 to 239.255.255.255
has been defined to be a range of administratively scoped multicast addresses in IETF RFC 2365.
These addresses may be statically assigned by the administrator of an organization’s network, and
there will be no conflict with other organization’s use of these addresses because border routers on
the edge of an organization’s network enforce policies to stop multicast traffic flow for addresses
within this range. Further, these same border routers can enforce policies so that subsets of these
addresses are contained within administrative boundaries, such as a local LAN, a building, a
campus or a region. These are safe addresses to use in setting up an organization’s multicast
network and ideal entries for use in the Channel Table. Consult with the Network Administrator for
the particular address ranges in use on the network hosting the Telepresenter.
All units in one session use the same multicast address. The Video, Audio and Collaboration multicast
addresses for a session can be the same if the Port numbers are different. Units engaged in different
sessions should use different multicast addresses to minimize traffic loading on the network (the multicast
routers distribute all traffic to all points subscribed on a particular multicast address, independent of the port
number).
NCast Corporation Revision 2.2
Page 45