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A.2.4 Multimedia Communications Networks
Multimedia networks refer to networks which integrate voice and/or video web data on a
common network infrastructure. This integration requires special consideration to account for
different characteristics of traditional data and voice~v~deo networks. Basically, data networks
are most efficient when implement as packet networks (e.g., LANS) while voice and video are
most efficiently implemented using TI, SONET (i.e., circuit-switched, synchronous technologies)
networks. These requirements have been discussed in Section A.~.5 and, in particular, in Table
I.5.13-~. Thus, ITS multimedia communication network design must address several issues.
Multimedia networks require the conversion of voice and video to digital information so that it
can be combined win other data through multiplexing techniques for transfer between points
within He network. Voice and modon video have timing constraints to achieve real-time fidelity
criteria for listeners/viewers. Networks which are synchronous, such as TI and SONET,
guarantee timing and are capable of supporting multimedia Asynchronous network technologies
(e.g. LANs, etc.) must have a means of guaranteeing timing for voice and video signals. One
method is to give He video and voice data a higher network transfer priority Han over data.
Examples include the emerging Asynchronous Transfer Mode (ATE standard and FDDI-D
draft standard. Another method is to provide separate channels for isochronous versus
asynchronous data as is provided in the recently adopted ~ ~ HE 802.9 isochronous ETHERNET
standard. Traditional LAN standards (e.g., 802.3) do not support these methods; however, a
clear mend is to provide these capabilities in emerging standards.
When a network is not inherently synchronous the term "isochronous" ("iso for equal" and
"chronous for delay) is used to indicate Hat He network design will accommodate voice and
video. The major advantages of a multimedia network are presented in Table A.2.41.
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l
Table A.2.4~1
Advantage of Multimedia Networks
_ .
Feature Benefit
.
Single communications technology rather Lower life cycle cost
than mixed technologies
All communications needs accomplished Optimum use of medium's bandwidth and
over a single medium for segments on the thus lower installation cost; Single fiber pair
total network does alla
Networ < distribution addressing added to Facilitates interoperability with distributed
voice and video users and thus, lower information
distribution cost
. _
Voice, data, and video in digital form Computer fnendly~ format simplifies
interfaces with information processing,
storage, and retrieval environment
Network management technology supports Built-in test and status reporting supports
voice and video, as well as data use of open standard protocols; minimizes
maintenance cost
_
Fault tolerant features of networks also High availability for all information,
applied to video and voice enhancing safely and reducing maintenance
cost
Unlike point-to-po~nt or analog overlay link video distribution, multimedia networks allow video
and voice to be "addressed" to users. Thus, single users, work groups, or broadcast distribution
techniques may be used. Voice is digitized supporting "voice maid' features of systems.
Information is accessible from any compatible terrn~nal or the network assuming need and
authority to use We ~nfonnabon. Access control is accomplished by a variety of techniques from
addressing gateway locks (channel access "code key" locks and information
encryption/descnption).
There is a significant distinction between single frame video and Fill motion video. S~ngle frame
video has no frame-to-frame synchronizad0n requirement. It is essentially a "stand alone"
picture which is digitized. Compression aIgonthms that handle frame video (such as JPEGS) do
not accommodate full-motion video. Full-motion video requires an algorithm that
accommodates changes from frame-to-frame. MPEGS ~ is the modern algorithm handling fuB-
motion video.
1
.,
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hnage quality in teens of horizontal and vertical resolution maintained through Me
compression/decompression process and equivalent frames per second, defines needed data rate
for compressed video. Generally as data rate is decreased, image quality is decreased. In fun
motion video, not only is resolution decreased but a point is reached where equivalent frame rate
causes image "jumping" and a rapidly changing image, especially in terms of percent of pixels
consumed by the image from flame to frame (such as wig a rapidly approaching truck) will
result in '~blocking." Blocking is when He aIgori~m cannot convert the compressed image back
to pixels and Bus small colored blocks (perhaps "c x c") are displayed. These blocks are in fact
the cells used to process pixels in Be compression algorithm.
Thus, in Me design of a muldme~a network, one must define Be acceptable image quality. For
full-motion, color video, two TIs (3.08 Mbps) is Be limit of acceptable quality condor
surveillance video MPEG ~ provides. As data rates for MPEG ~ approach four, TIs
(6.6 Mbps), picture quality approaches Cat of a nonnal analog television broadcast image of a
high motion scene (such as a ball game or action movie). Older algorithms, such as Be 11 U
(CC11-1) 261 standard, require higher data rates for equivalent image quality. ~stoncaDy,
45 Mbps have been used for video CODEC. Currently, 6 to ~ video channels can be supported
at T3 data rates using MPEGS B.
There are a number of voice CODECS which reduce the histor~caBy used DS-0 (64 kbps) data
rate to as low as 4.5 kbps. Digital cellular telephones are deploying 7.5 kbps voice CODECS
which provide high quality voice, where pitch is maintained, Be speaker's voice is identifiable
and words are easily recognizable.
Wig selection of video and voice CODECS, data loading on Be multimedia network can be
defined. Digital video and voice data are added to Be computer-generatec] data to provide total
multimedia Ott load for network segments.
One important consideration: If video from surveillance CCTV cameras is required continually
by someone on Be network, nothing is gained by using ATM over SONET. It is more cost
effective to use Be synchronous channels of SONET for video. Continuous video placed over
ATM, will continuously have highest priorly and continuously consume Me bandwidth, Bus
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Representative terms from entire chapter:
data rates
