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A.~.5 Communication Networks and Protocols
Traditionally, communications networks have been based on two general techniques:
1) Circuit switched for voice, video, etc., signals which require controlled delay time from
source to destination and exact ordering of transmission data. These networks are
implemented by He telecommunications industry. Technologies include TI, SONET, and
related multiplexing, cross-connect, etc., products. These networks are isochronous meaning
"equal delay."
2) Packet switched for data (especially E-mail, computer networks, Internet access, etc.) which
can tolerate random (but reasonable) time delays and resulting occasional out-of-order
packet reception. These networks are implemented by the computer, LANJ~AN/WAN,
data services (e.g., Compuserve, AOL, etc.) industries.
Current ITS-related systems have been implemented using equipment and technologies of bow
techniques:
I) Field infrastructures typically employ circuit switched techniques, with simple end-to-end
protocols to support real-tune requirements.
2) TOC computer networks employ packet-switched techniques (e.g., L`ANS) to interconnect
large screen displays, workstations, printers, servers, etc.
A clear industry trend has been an evolution of these techniques to support transport of
multimedia voice, data, and video over He public telecotnmunicabons network and
LAN/MANIWAN packet networks.
This section will discuss the network protocols and systems that are available to support He
emerging ITS multimedia requirements.
L:~ - c2~t N~3-51. P~2F~Re~n A1-196
OCR for page 205
A.~.5.1 Packet Networks, OS! Stack, and Standards
In the 19SOs and 1960s as computers emerged and were commercialized, We need for
interconnected computer networks also emerged to share data In the early 1970s, more formal
computer networks and products began emerging.
Researchers quickly discovered that data networks had different characteristics Ran He widely
deployed c~rcuit-switched voice networks and required different methodologies. The solution
was He packet network which partitioned longer data messages at the source into smaller
packets, transmitted He packets over He communication network, and reassembled the packets
back to the ong~nal data message at He destination. This concept is illustrated in
Figure A.~.S.~-~.
Much effort has been focused on "Store and Forward" (sometimes denoted "Hold") packet
networks. The unique characteristics and factors of the data networks Hat favor packet
technology include:
1) Random interarrival times of source data
2) Random message lengths (pnor to packetization).
3) CaB (or connection) setup for short bursty data messages creates significant inefficiencies in
c~rcu~t-sw~tched networks which are best suited for voice cans urge durations of minutes as
opposed to fractions of seconds. Packet technologies handle this more efficiently.
4) Data networks can accommodate random delays of packet reception at He destination and
can handle out-of-order reception sequences if Hey occur.
5) Modern digital error detection and correction technologies can be efficiently applied as
required at bow He packet and message level.
L::\NCHRP`Phase2. ~NCHRP3-51e Phase2FmalReport A1-197
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6) Network performance, in terms of average delay and throughput, is better in packet
networks.
The performance of a packet network is typ~caRy measured in terms of average throughput and
delay. Models based on "Queuing Theory" are available to simulate the performance of packet
networks. Figure A.~.5.~-2 illustrates how packet links and networks generally perform by
plotting average delay versus percent throughput capacity. As We figure illustrates, if a network
is lightly loaded, Me average delay is largely fixed and a function of propagation time and
processing delays. But, as the load approaches 80-90% of capacity, Me delay detenorates
rapidly. Thus, network managers strive to maintain percent link and network loading wed below
80-90% capacity by:
I) Simulations in network design to predict performance. (Computer s~mulabon tools exist.)
2) Use of network management to collect statistics on actual network and link loadings during
actual operation.
3) Plan rather than react to Increase capacity before problems emerge.
In 1983, the International Standards Organization aso' released the Open Systems Interconnect
fOS ~ Reference Mode! (often caned the OST Stack) Mat is shown in the block diagram in
Figure A.~.5.~-3. The OST reference mode! is for packet networks generally performing Me
functions described In Table A.~.5.~-~. Much like ITS-related systems today, Me private and
public packet networks (e.g., LANIMAN/WAN, X.25, and Intemet data networks) of Me 83 era
were experiencing problems urge multivendor interoperability and modulanty to support
multiple physical media and multiple application layers. The 7-layer OS] reference mode!
provides a methodology to address these problems by applying Me following principles:
I) Each layer should perform well defined functions Mat are reasonably cohesive (i.e., naturally
related).
L::`NCHRP`Phasc2rp ~NCHRP3-51 · Phace2FmalReport Al-199
1
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2) The functionality of We layers should be sufficiently self contained so that information flow
across the interface to other layers is reasonably minimized.
3) The functions of each layer should be conceived to support development of international
standards for the layer. These standards define services provided by each layer as well as
protocols for the Interfaces between layers.
4) The resulting standards for each layer should define services, functions, and interfaces to
adjacent layers but not dictate how they must be implemented in hardware or software.
5) The stack should be modular so each layer may be changed without affecting adjacent layers
and interfaces. Thus, new operational requirements can be accommodated as weD as
advances in architecture, communication, hardware, software, and related technologies.
6) It should be possible to bypass (or consolidate) a layer when not needed. As examples,
many implemented protocol stacks consolidate layers 5 Trough 7 as done in NEMA's
NTCIP ITS protocol stack.
The OS] stack is not a communication protocol standard, but a reference model standard to
facilitate consistent, modular, and flexible protocol standards development. Although each layer
is often referred to as a protocol, application protocol stacks, or profiles, such as NEMA's
NICIP Signal System protocol, require definitions of Be individual layer protocols, services,
and interfaces.
With the OST reference model, Me assignment of funchonality to individual layers has achieved
significantly better standardization. However, of necessity, some variability still exists for a
variety of reasons. These include:
I) Technology improvements (e.g., x.25 to frame relay, TCP/IP., etch
2) Different applications requirements (e.g., LAN versus MAN versus WAN).
L:~h~.~t NC~ 3-51 · Pee 2 Fee Re"n A1-203
OCR for page 212
3) Implementation requirements (e.g., high speed LAN versus lower speed Internet protocols).
4) Complexity of (even supple) protocols makes nerd standardization bow undesirable and
virtually impossible.
5) Different preferences, interpretations, arid priorities of the individual standards defining
organizations.
Thus, a protocol stack requires Me integration of public standard protocols (e.g., TCP/IP) and
application-specific protocols (e.g., NTCIP) and the definition/ selection of many required
~mplementabon profiles and parameters.
In Figure A.~.5.~-3, a communication infrastructure is identified Nat includes (I) the physical
layer, (2) He link layer, and (3) We network layer. This cormnunicabon infrastructure is He
distnbuted communication public and/or private infrastructure of a packet network. End
users/equipment are physically located at He required source or destination of data and are
provided end-to-end communication services by the communications infrastructure consisting of
mediums, modes, hubs, terminals, etc. Commercial public communication services (and private
networks) provide protocols for the lower Free layers only. Layers ~ Croup 7 are provided in
end user equipment such as field equipment terminals and network/TOC communication servers
and related equipment. ITS-related field communication infrastructure has not traditionalRy
employed OSI-based standard protocols; however, NTCIP initiates an evolution within ITS
toward this goal.
Many public protocol standards are suitable for 11 S-related applications. Figure A.~.5.~-4
presents an overview of currently deployed or anticipated LAN/WAN standards with potential
AS application. This figure illustrates the various OSI layers addressed by He standards. Users
implementing a complete application stack must define and implement undefined higher and
lower layers. ITS systems have many options and choices that include:
:`NCHRP`Phasc2~pr NCHRP3-51e Phase2FmalRepoIt
A1-204
OCR for page 213
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I) The LAN 802.x standards only define the physical and link layers as LANs have
tradidonaBy provided only shared bus, point-to-point, links of a few hundred meters. Higher
layers have traditionally been implemented in Network Operating Systems (NOS), TCP/IP,
etc.
2) TCP/IP are intemetworking protocols for MAN/WAN applications and only address He
transport (TCP or 4~) layer and network (IT' or 3rd) layer. Typically, the employed lower
layers are HDLC win dialup modems or, more recently, ISDN circuits. These are integrated
in LAN systems (e.g., 802.x).
3) ElA-232, 422, 485, X.2l, etc., are physical (~) layer only protocols and multivendor
interoperability requires specification of at least link (2) and often network (3) and higher
layer protocols.
4) X.25 is a legacy protocol (70s, 80s) stack for layers 1, 2, and 3 that was conceived for
networks interconnected by error-proof lower quality links. Thus, each layer of He protocol
stack has significant overhead for error protection/control which makes X.25 unsuitable for
higher speed links. TCP/IP addressed this problem by minimizing layers I, 2, and 3 error
checking overheads and performing much of these functions in layer 4 ~CP) which is
resident only in end user equipment and not the network infrastructure. Modern
communication links are less error prone and can operate efficiency win these overhead
error control functions on He periphery of the communications infrastructure. Lower
overhead in layers I, 2, and 3 of He infrastructure is essential for higher link bit rates.
5) SONET and the T} hierarchy are physical (~) layer protocols that were developed to
accommodate high speed transmission for the telephone industry. Bow public arid private
Implementations have been extensively used for MAN/WAN intemetworkir~g of LAN
networks. These win be discussed in more detail in Section A.1.2.3.
6) ATM is a telephone switching technology to support "On Demand" multimedia voice, data
(LAN), and video. Standards are embryonic (1995) win many gaps and emerging end user
products. Eventually, as standards and products evolve, ATM should prove cost-effective
and valuable for lids applications.
:\NCHR~Phasc:.'p ~NCHRP 3-51 · Phase 2 Anal Report
A1-206
Point to Point Protocol which as the name implies, assumes a "one-on-one" sort
of exchange. This class is targeted toward major "center to center'' exchanges of
information, supporting interactive exchanges (Telnet) and file transfer ~P).
Both of these application layer protocols provide a degree of security through user
authentication procedures.
Conformance Statements and Testing
As even common protocols are complex, implementation developed solely from a common
specification by multiple suppliers would not guarantee interoperability. To maximize We
potential of interoperability, standards organ~zabons, including NEMA's NTCIP, require
manufacturers (for certified ~mplementions) to complete a Protocol Implementation
Conformance Statement (PICS) Cat answers detailed questions about mandatory and optional
features provided. A PICS provides:
1) Verification of features implemented;
2) Checklist for conformance testing by independent test organizations; and
3) Detailed listens) of requirements for procurers.
Even PICS do not provide 100% guarantee of interoperability on multivendor implementations,
especially early releases; however, vendors conforming to standards will usually accept
contractual statements requiring minor warrantee modifications to achieve interoperability.
A list of Dublished standards and documents providing more details on NTCIP are contained in
Table A.1.5.2-7.
A.1.5.3 Circuit-switched Technology
Packet networks are based on the concept of disassembly of messages into smaller packets for
transmission store-and-forward at intermediate nodes in a network. No direct connection
between origin and destination is established although virtual connection may be employed. This
~;\NCH~Phase2.rp: NCHRP 3-51 · Phase 2 Final Report A1-227
pre-establishes addressing/routing parameters so Mat actual transmissions are more efficient and
can support higher throughput.
Table A.~.5.2~7
NTCIP Standards and Supporting Documents
Standards
1 ) NTCIP Steering Group - Point to Mulfi-point Protocol (PMPPJ
2) NEMA Standards Publication TS3.1 - 1996 National Transportation Communication for ITS
Protocol- Overview
3) NEMA Standards Publication TS3.2 - 1996 National Transportation Communication for ITS
Protocol - Simple Transportation Management Framework
4) NEMA Standards Publication TS3.3 - 1996 National Transportation Communication for ITS
Protocol - Class B Profile
5) NEMA Standards Publication TS3.5 - 1996 National Transportation Communication for ITS
Protocol Object Definition for actuated Traffic Signal Controller Units
6) National Electrical Manufacturers Association, Draft Standard - NTCIP Object Definitions for
Ramp Meter Controllers
7) National Electrical Manufacturers Association, Draft Standard - NTCIP Object Definitions for
Variable Message Signs
8) National Electrical Manufacturers Association, Draft Standard - NTCIP Object Definitions for
Camera Controllers
Supporting Documents (Ge! Complete Titles)
1 ) NEMA Standards Publication, How to Use NTCIP
2) NEMA Standards Publication, NTCIP Systems Developers Guide
3) NEMA Standards Publication, Hows and Whys of NTCIP
4) NTCIP Steering Group 1996 Draft - Class A Profile
5) NTCIP Steering Group 1996 Draft- C/ass B Profile
6) NTCIP Steering Group 1996 Draft- C/ass C Profile
7)
NTCIP Steering Group 1996 Draft - Class E Profile
Circuit switching establishes a permanent connection between origin and destination. It is We
memos used by me telephone industry for switching voice. In its infancy, c~rcuit-sw~tch
technology was analog win stepper relays. Since the 19SOs, digital switching has been replacing
analog switching, due to me availability of low cost digital components aIld computer technology
to control and manage circuit-switched networks. Key requirements for voice and video
L:\NCHRP\Phase2.rpt NCHRP3-51 e Phase2FmalReport Al-228
networks are controlled time delay of the digitized signals and an intolerance to random ordering
of data at the destination.
Table A.~.5.3-1 summarizes and contrasts packet-switched and c~rcuit-switched technologies.
More information on digital c~rcuit-switched technologies is in Section A.~.5.3 which discusses
T} and SONET multiplex~ng/transmission hierarchies.
Modern technology advances have blurred He use of packet technology for data and circuit
switched technology for voice and video. V~rtual connections in packet networks and
Asynchronous Transfer Mode (ATM) are defining packet capabilities for circuit-switched
telephone service providers.
A.~.5.4 LANs
Local Area Networks ~ANs) are being deployed in TOCs in ITS-related systems within TOCs
to link operator workstations with venous servers, and to integrate field data.
AS presented in Figure A.~.5.~, L`AN protocol stacks typically define Be physical (1) layer
and link (2) layer protocols. The physical layer specifies a medium (e.g., TWP, coaxial cable,
fiber, etc.~.
The hardware elements of an LAN consist of:
I) Network Interface Cards Tics) Mat interface PCs, pnnters, workstations, etc., to the
medium;
2) Servers which are computers, such as file servers, communication servers, or application
servers; and
3) Communication devices such as hubs, repeaters, bndges, routers/brouters, and gateways.
:\NCHRP\Phase~p ~NCHRP 3-51 · Phase 2 Fmal Report A1-229
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Figure A.~.5.4-l presents an overview of a typical LAN environment in the popular star
topology.
LANs employ several network topologies: bus, star, ring. The topologies are presented as
Figure A.15.4-2. The most widely deployed Ethernet (' ME 802.3) LAN ong~naDy used a
10 Mbps bit rate on a shared coaxial cable bus with ad devices employing Catner Sense
Multiple Access wad Collision Detect (CSMA/CD). This essentially says, before access: listen,
Den talk, and if two or more messages collide, then (each Ones wait random and try again. This
worked well in small fixed systems, but users encountered He following problems:
~ Reconfigurations for moves, addidons, and deletions were difficult; and
2) Network throughput was detenruned poorly as numbers of devices and network traffic
increased.
The star topology provides an alternative that substantially reduces these problems. Each device
is provided win a dedicated TWP connection from its NIC to a hub at Be full lO Mbps data
rate. The hub implements Be bus and experiences the collision. Like telephone circuits to Be
desk, LANs also employ TWP wide parallel installation. Premise TWP standards for design,
installation, configuration, and maintenance are defined by EIA/IIA, NEMA, and Bellcore.
Additionally, this star arrangement permitted more natural segmentation of LAN networks to
nary workgroups using bridges or routers. By proper segmentation of workgroups, collision
only occur within shared segments undess a remote segment device is addressed. Thus, ovemll
network capacity is increased. Many additional extensions to He E~emet standards are
emerging Hat include 100 Mbps operations and fiber medium. These are summarized in Table
A.1.5.4-1.
A competing, but less popular, LAN configuration is the Token Ring ~ FIEF. Standard
~ lid it 802.5. Access to the shared ring is permitted only when an authorizing token is received
by a device. This eliminates He collision problem of Ethernet and allows the network to operate
at a throughput capacity close to He link bit rates of 4 and 16 Mbps, regardless of total system
c:~t New 3-51 · Pie 2 Fig Rein
A1-23 1
