Providing Video Services
It is atypical to include the PBX as part of a video solution; however, some
advanced PBX systems do provide video services.These connections can either
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Figure 1.5 The Digital Waveform
Time
Amplitude
Old World Technologies • Chapter 1 19
be provided over broadband technologies or by way of Ethernet, but it is more
common in many systems to use the PBX as a termination point for multiple
ISDN Basic Rate Interface (BRI) channels.The BRI can transfer 128 Kbps of
user data, and these connections can be combined, or multiplexed, to provide
higher levels of bandwidth. Many video conferencing systems work well with
384 Kbps.
In later chapters, we will discuss the technical specifics of the various protocols
in use for these connections, including the H.320 specifications, which
govern the basic concepts regarding video transmission, including audio and
video processing, and are focused on lower-bandwidth media—ISDN and 56
Kbps specifically.This protocol supports point-to-point and multipoint sessions,
and provisioning for multicast or multipoint connections is an important consideration
in the video environment.
One of the first reactions many users have to compressed video is that it isn’t
like a television picture.The image is smaller and rougher, and, while it does not
have to be so degraded, most vendors haven’t forced the additional bandwidth or
processing requirements on end users. Adaptation, it is hoped, is to be driven by
function, which, in turn, may lead to faster networks and components.This will
likely be a slow process, as evidenced by the migration to high definition television
(HDTV).
In the United States, the analog video standard is called NTSC, or National
Television System Committee. Some in the industry claim that the acronym
should stand for Never Twice Same Color, being that, compared to the European
and Asian standards, the color information is poorly interpreted from set to set.
The NTSC standard specifies a frame rate, or screen refresh rate, of 30 framesper-
second (29.97). Users of these sets are quite accustomed to the grainy picture
provided and poor color resolution, and, while HDTV has been available in various
forms for years, the FCC and other authorities are already concerned in later
2001 that their 2006 mandate for HDTV conversion will fail.Video conferencing
may fail to generate sufficient drivers to make users upgrade their systems, and
may exist in degraded form for some time. Or it may also become the next
killer-application.This conundrum is a common theme in AVVID, and will be
interesting to watch as the old world meets the new.
Audio and video systems require common protocols to define the communications
stream, and these standards can be referred to as the H.300s, G.700s, and
the T.120s, in homage to the base numbering associated with each standard.This
is in addition to the transport protocols of ISDN, Digital Subscriber Line (DSL),
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20 Chapter 1 • Old World Technologies
Plain Old Telephone System (POTS), and others.The H, G, and T standards are
administered by the International Telecommunications Union (ITU).
The most universal of these video protocols is H.320, which defines a
number of parameters including picture size and bandwidth requirements, and
will operate within point-to-point and multipoint applications.
It would be unfair to only note H.320 in a discussion of video conferencing
protocols, however. H.261, for example, specifies the compression of real-time
audio and video data, and defines a screen size of 176 x 144 pixels (Quarter
Common Intermediate Format [QCIF]) to 352 x 288 (CIF). Most of these will
fit into the bandwidth availed by ISDN. H323 is most often referred to today, and
is commonly found in many applications, including the conferencing software
provided with Microsoft Windows.
The technicalities of all of these protocols is not important at this point in a
discussion of AVVID, and subsequent chapters will elaborate on the standards
used by Cisco’s CallManager and other resources, such as the IP phones.You will
find that many of the protocols used in AVVID telephony are the same as those
used in traditional video conferencing, and, because of this, there is integration
between the voice applications of the IP phone and the more traditional video
conferencing systems such as Microsoft’s NetMeeting. For example, one can call a
NetMeeting user from a Cisco IP phone.
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