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OCR for page 366
The composite data load is a reasonable estimate of He data load that must be accommodated at
the TOCs and on backbone links. Table A.2.5-3 summarizes anticipated ITS link loads.
It should be noted that the CCTV camera at 3 Mbps represents Be dominant data load. CCTV
is an emerging ITS requirement that significantly increases We llS communication infrastructure
requirements.
Table A.2.5~3
Anticipated AS Link Loads
I Link I Data Rate I
Local Links
Data 2400- 9600 bps
Digital Video 3 - 8 Mbps
Digital Voice ~
Backbone Links ~ ~ ~ 000 Mbps
(Node-to-Node)
Center-to-Center Links 155.2 Mbps
District-to-Region Links 15~
Region-to-National Links 9~ eel
A.2.6 ITS Network Architecture and Topology
Architecture refers to technology, fault tolerant redundancy, TOC backup, security, and standard
requirements. Topology refers to geographic placement of field equipment, communication
nodes and hubs, and interconnecting communication links (i.e., mediums) to implement He
communication infrastructure.
The Communications Handboolfor Traffic Control Systems, April 1993, presented several
representative legacy traffic control communication architecture classifications (Figure 5.2 of
Handbook). These are represented in Figure A.2.6-~. These communication architectures were
conceived for a single application (traffic control) and for data (not multimedia). Additionally,
they have typically been implement why He proprietary non-open standards of He controller
manufacturers.
L;`NCHRE~\ NCHRP3-51 · Phase2FmalReport A2-22
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OCR for page 369
As video requirements emerged, traditional ITS-related communication systems have employed
an overlay of analog video circuits and digital data circuits, as depicted in Figure A.2.6-2. It is
necessary to note ~at:
I. The legacy signal systems are on a data network with 1200/2400 bps and mulddrop
modems, usually on a private TWO cable plant.
2. Later emerging CCTV video requirements have often been addressed by parapet analog
po~nt-to-point (CCTV camera to TOC) fiber circuits or analog fiber frequency division
multiplexed 0:DM) network.
a. In some larger system deployments, fiercer analog FDM of data has been deployed in hub-
to-TOC links to achieve cost-effec~ve multiplexing of data where longer fiber-based links
are required.
It should also be noted that these FDM systems have been mostly non-standard proprietary
systems from a particular multiplexer vendor. This overlay approach has been cost effective as
He data network was already installed. Fortunately, legacy lTS-related systems have not
required significant integration and sharing of data across applications and jurisdictions, which
would be difficult on these overlay networks because Hey are usually implemented using
incompatible propnetary multi-vendor products.
Several impor~t considerations will create a need in future ITS systems for integrated ITS
multimedia communication (sub) systems:
1. The ITS goal of integrated ITS services consisting of multiple applications (or services),
data sharing by multiple jurisdictions, and the possibility of shared communication
infrastructures. This requires open standards Hat are most efficiently supported on a
multimedia network.
t;`NCH~Phase~p'` NCHRP3-51 · Phase2F'nalReport
A2-2S
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Representative terms from entire chapter:
communication infrastructure
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2. Many jurisdictions are upgrading or adding systems Nat include new video and data
infrastructure. Usually, except for small systems or where existing facilities are expanded,
the most cost-effec~ve commun~cadons options are integrated voice, data, and video
networks.
Video signals should maintain end-to-end 45-60 dB SIN (signal-to-noise) for acceptable
video quality. Analog video signal gracefully degrades SIN over distance which cannot be
restored at amplifiers. Digital video signals are completely restored at repeaters (assuming
proper link designs) and suffer essentially no degradation over distance. ITS has a
requirement for video shanug with multiple, often distant, ITS service providers and
junsdictions.
4. Standard interfaces are required for rRs data sharing and Me emerging MPEG-2 digital
video standard web defined interfaces to Me T] digital hierarchy and SONET are wed suited
for this purpose.
5.
The emerging trend toward greater distributed intelligence in field controllers due to Me
deployment of modern 16/32 bit microprocessor technology, such as the 2070 controller.
Local T~me-of-Day (TOD) plans convert real hme second-by-second control commun~cadon
to less demanding communication consisting of monitoring, TOD plan activation, and plan
downloading. These microprocessors have Me processing power to support modem high
speed communication links and associated protocols as weD as required transportation
functions.
These ITS requirements and communication industry trends favor the communication
architecture of Figure A.2.6~03) as expanded in Figure A.2.6-3 to illustrate Me concept of a
modem integrated multimedia digital communications network. In this integrated network,
Distribution Links (DL) connect field devices to communication nodes. At nodes, the various
digitized voice, data, and video signals are multiplexed for communication over Me higher bit
rate Backbone Links (BL) for communication to Me TOC. ~ large networks, a hierarchy of
nodes and hubs may be employed with hubs providing more multiplexing and still higher bit
rates. These nodes, hubs, backbone links, and distribution links are referred to as Be
communication infrastructure.
L;wCHRP\Phase2~p'\ NCHRP3-51 · Phase2FmalReport
A2-28
A cost trade-off always exists when multiplexing is employed. Single Mode Fiber Optic
(SMFO) cable and installation costs are marginally more expensive with additional fiber per
cable. SMFO has repeaterless distances of 50 to 100 miles and bit rate capacities over 10 Gbps.
Thus, a system architecture could employ short links to many multiplexers or longer links to
fewer multiplexers. This is a cost trade-off involving system geographic topology, number of
field dences, interconnecting link distances, tnstaBed fiber costs and multiplexer terminal costs.
Small systems may be most cost effective win no field multiplexing. Multiplexing is often used
to provide interfaces for data sharing among ITS services, which should be considered in design.
Similar tracie-offs exist for revere and wireless ~mplementabons, but with less flexibility due to
lower bit rates and shorter repeaterless distances. This trade-off should be considered in system
design.
Because MPEG-2 Video encoders are still expensive, near-term video should perhaps be
maintained as analog from camera equipment cabinet to a communication node where it is
digitized and switched for transmission to Me TOC and/or over ITS jurisdictions or services. It
should be noted Mat uncompressed video requires approximately 45 Mbps bit rates (DS-3 rate),
while Me MPEG-2 requires 3-6 Mbps. As technology is evolving rapidly, ITS communication
system designs should accommodate an anticipated rapid cost-effec~ve evolution to Me 3-6
Mbps, MPEG-2, encoders In Me camera equipment cabinets or Me camera itself.
A.2.6.1 Recommended ITS Communication Infrastructure Standards
As discussed in Section A.0.2, ITS will be an evolutionary process. Thus, no nerd set of
communication architectures or standards will be suitable for all implementations. Flexibility
must be prowded to accommodate:
1. A multitude of ITS services as described in A.0.2 that Will have evolutionary deployment.
2. Local agency/jur~sdicdonal operational requirements that are unique In terms of services
deployed and exact feature sets.
L;`NCHR~rp'` NCHRP 3-51 · Phase 2 Fmal Report
A2-29
3. A growing complement of field sensor/equipment types that include We traditional signal
systems (loops), VMS, ramp meters, weather, HAR, etc. Each has unique message sets to
support unique data content.
4. Diverse geographical areas ranging from sparse rural area, to small cities, to dense urban
metropolitan areas wad system scope scaled appropriately.
5. Need for multimodal operations and shared data. Often multiple TOCs are required at
multiple jurisdiction locations, but often why emergency and after hour back-up capabilities.
6. Different jurisdictional budget resources and procurement procedures.
7. Legacy ITS-related system that must be integrated.
The deployed ITS communication architectures must be based on standards Cat are modular,
scalable, and extensible. Fortunately, as depicted in Figure A.2.6-4, He communication industry
supports a set of physical layer standards based on hierarchial multiplexed data rate capabilities:
Multiplexed RS-232 (below 64 kbps);
2. Subrate Multiplexing (64 kbps and below);
3. The digital hierarchy (i.e., DS-0, DS-l, DS-3, etch; and
4. The SONET Standards (i.e., OC-l, OC-3, etc.; above 51.84 Mbps).
These standards provide a wed conceived and tested framework for commun~cadng data by
multiplexing for transmission and switching, routing, or bridging at nodes. The key capability is
to identify cost-effective opportunities for multiplexing using techniques, equipment, and
standards employed for years in He LAN/MAN/WAN and telecommunication industries. The
hierarchy has data rates from the traditional 1200 bps deployable over IMP up to 10 Gbps at
SONET OC-192 rate.
L;\Nc}~Ph~pr\ NCHRP3-51 · Phase2FmalReport A2-30
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4876 Mbps
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INTERFACE OC 1 2 L _
622.08 Mbps
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2400 bps
4800 bps
9 600 bps
1 9,200 bps
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21
STANDARDS BASED DIGITAL MULTIPLEXED
I DATA RATE HIERARCHY
~FIGUREA.2.6-4 -~n
~ reality, communication systems are designed to standard interfaces, as depicted on Figure
A.2.6-S, and We commun~cabon infrastructure is designed and implemented as necessary to cost
effectively support current and anticipated future composite multiplexed system data loads as
geographically distributed. Since industry standard interfaces are employed, private networks
and/or commercial communication service networks can be employed, based on cost or over
factors. This hierarchy of data rates and interfaces supports the required interfaces for current
and anticipated ITS field devices and related interfaces:
I. Full-motion video is supported by ~ to 4 DS-Is for MPEG-2 compressed digital video, while
DS-3 is available for uncompressed digital video
2. DS-O for voice;
3. Subrate multiplexing for ElA-232/4227485 for controller network interfaces;
4. Wireline modem technology supportable win TWP or T1 networks (via DS-0 channels and
channel banks);
5. T} and SONET interfaces for TOC and intequr~dictional communications; and
6. T! and SONET interfaces for developing a modular, scalable, and extensible
communication infrastructure that is well suited for integrates} services goals of rRs.
Up to the limit of bit rate capacity, these interfaces can be implemented using wire, wireless, or
fiber media These interfaces are, in fact, the most common and available interfaces provided by
equipment vendors.
u~NCHRPPh~.rpr\ NCHRP3-51 · Phase2F~nalReport A2-32
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