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Transcript
Communications in UTMC systems
Introduction
Traffic management and control systems require pieces of equipment located at
several geographically-dispersed locations. These pieces of equipment usually need
to exchange data with other equipment at other locations – often these will include a
central data management computer.
In the UK, suppliers of traffic management and control systems traditionally used
proprietary technical standards for communicating between these various system
elements.
Problems Arising
As a result, the following arose:
•
It became impossible to exchange equipment supplied by one supplier with
equipment from another
•
Local authority customers became ‘locked-in’ to one supplier – if they chose to
procure a system from company X then they could not buy (or more
importantly, use) functionally-identical equipment from company Y
•
As Suppliers therefore faced limited competition there was little incentive to
start new development or to implement new technology. One consequence of
this was that centrally-controlled Traffic Signal Controllers would only work
with a specific communications circuit from BT. When BT announced the
withdrawal of these circuits the traffic management industry panicked as it
meant that traffic lights might stop working.
UTMC to the rescue (?)
The UTMC initiative was conceived to address this specific problem. It has,
however, tackled it in rather a roundabout way.
The core problem was defined in terms of two key issues: ‘interoperability’ (the
ability for two pieces of equipment to exchange data) and ‘interchangeabilty’ (the
ability to replace one piece of equipment with a functionally identical piece of
equipment from another).
Both of these issues were addressed by specifying open industry standards for both
software interfaces and physical interfaces.
As computer networks are designed to exchange data in much the same way as traffic
management systems, the decision was made to use a computer network standard as
the basis for data exchange in UTMC.
TCP/IP is the dominant network standard at present. It contains a number of data
exchange protocols which can be used for different tasks and applications.
Layers
It is important to understand how these protocols are related and how they relate to the
communications medium being used. The key word here is ‘layers’.
To exchange any information between two computers there must be an agreed set of
commands and data formats, this is a protocol. So, for example, the commands and
data sent between a World Wide Web browser and a remote server are a protocol
Different systems may run the same computer programs but different means of
interconnection. For example, suppose each of the two systems above run Microsoft
Word on one computer to retrieve a file from the other computer.
If in one system, the computers are linked by an optical fibre local area network and
by a dial-up telephone line in the other then the means of interconnection are
different. However, Microsoft Word has no knowledge of this interconnection and
runs independent of any specific network connection.
This is achieved by defining a clear interface between "layers" within the PC. At the
top is the layer in which Microsoft Word resides (the Application Layer); lower down
is the layer which decides that the dial-up or optical fibre link is to be used (the
Physical Layer). The protocol explicitly defines the interface between the Application
Layer and the layers below it so that Microsoft Word will run over any network.
Each layer will pass information up and down to the next subsequent layer as data is
processed.
The main benefit of this is that it enables changes in technology to be made. For
example, an organisation can change the network medium in its systems from copper
to fibre to GPRS without having to change the email programs everyone uses.
TCP/IP
In UTMC systems (as well as most other computer networks) these layers are
combined into a ‘stack’ or ‘suite’ known as TCP/IP (Transmission Control
Protocol/Internet Protocol). TCP/IP consists of four layers (Application, Transport,
Network and Physical).
For a network at the core of a UTMC system to operate it needs to support TCP/IP. In
addition, at the Application layer it needs to support CORBA (for instation to
instation applications) and SNMP (for instation to outstation applications).
Finally, within the CORBA and SNMP ‘sub-layers’, UTMC systems need to support
the openly agreed Data Objects and MIBs which are contained in the Data Objects
Registry.
Thus, in a UTMC system the layers are:
Layer
Application Layer
Transport Layer
Network Layer
Physical Layer
Protocol used in UTMC
UTMC Data Objects
UTMC MIBs
CORBA
SNMP
TCP
IP
Eg. LAN, ISDN. GPRS, Wi-Fi etc
Data Objects
The current version of the Data Objects Registry is Version 2. The registry contains
all of the definitions necessary to establish an interface between the identified pieces
of equipment (for example, car park counters, variable message signs, common
databases) and between management applications at the instation.