REAL-TIME SYSTEMS DESIGN AND ANALYSIS phần 5 doc

Table 1 Major intersection control system hardware components 1.9 Vehicle Presence Detector Interface Unit not shown in Figure 2 4 1.10 Pedestrian Request Detector Interface Unit not sho

188 4 SOFTWARE REQUIREMENTS ENGINEERING

Table 4.6 Imperatives found in requirements specifications and their purpose [Wilson97]

an addition to the requirements being specified

defined

environment are to provide to the capability being specified

ž Weak phrases

ž Imperatives

Imperatives are given in Table 4.6

Continuances follow an imperative and introduce the specification of ments at a lower level Continuances include:

Readability statistics, similar to those used to measure writing level, can beused as a quality measure for SRS Readability statistics include:

ž Flesch Reading Ease Index Number of syllables/word and words/sentence.

ž Flesch-Kincaid Grade Level Index Flesch score converted to a grade level

(standard writing is about seventh or eighth grade)

Table 4.7 Certain ratios derived from software requirements specifications and their purpose

Number of imperatives found at each

document levels

Counts the number of lower-level items that are introduced at a higher level by an imperative followed by a continuance Specification depth to total lines of

text

Indicates conciseness of the SRS

190 4 SOFTWARE REQUIREMENTS ENGINEERING

ž Coleman-Liau Grade Level Index Uses word length in characters and

sen-tence length in words to determine grade level

ž Bormuth Grade Level Index Same as Coleman-Liau.

Any of these requirements metrics can be incorporated into a metrics-managementdiscipline, and if used consistently and intelligently, will improve the real-timesystem in the long run

4.10 APPENDIX: CASE STUDY IN SOFTWARE REQUIREMENTS SPECIFICATION FOR FOUR-WAY TRAFFIC INTERSECTION TRAFFIC LIGHT CONTROLLER SYSTEM

The following is an excerpt from the SRS for the traffic intersection controlsystem introduced in Chapter 1 It embodies many of the elements discussed inthis chapter in more detail and provides a fully developed example of an object-oriented approach to requirements specification of a complex real-time system

1 INTRODUCTION

Traffic controllers currently in use comprise simple timers that follow a fixed cycle to allow vehicle/pedestrian passage for a pre-determined amount of time regardless of demand, actuated traffic controllers that allow passage by means

of vehicle/pedestrian detection, and adaptive traffic controllers that determine

traf-fic conditions in real-time by means of vehicle/pedestrian detection and respond accordingly in order to maintain the highest reasonable level of efficiency under varying conditions The traffic controller described in this specification is capable of operating in all three of these modes.

1.1 Purpose

This specification defines the software design requirements for an intersection control system for simple, four-way pedestrian/vehicular traffic intersections The specification is intended for use by end users as well as software developers.

1.2 Scope

This software package is part of a control system for pedestrian/vehicular traffic intersections that allows for (1) a fixed cycle mode, (2) an actuated mode, (3) a fully adaptive automatic mode, (4) a locally controlled manual mode, (5) a remotely controlled manual mode and (6) an emergency preempt mode In the fully adaptive automatic mode, a volume detection feature has been included so that the system

is aware of changes in traffic patterns Pushbutton fixtures are also included so the system can account for and respond to pedestrian traffic The cycle is controlled by an adaptive algorithm that uses data from many inputs to achieve maximum throughput and acceptable wait-times for both pedestrians and motorists A preempting feature allows emergency vehicles to pass through the intersection in a safe and timely manner by altering the state of the signals and the cycle time.

1.3 Definitions, Acronyms, Abbreviations

The following is a list of terms and their definitions as used in this document.

192 4 SOFTWARE REQUIREMENTS ENGINEERING

The time difference between cycle start times at adjacent intersections Applies only

to coordinated intersection control, which is not covered by this specification.

1.3.19 Orthogonal Route

A route through an intersection that requires a vehicle to turn.

1.3.20 Pedestrian Presence Detector

A button console located on the corner of an intersection which gives pedestrians who wish to cross a street the ability to alert the intersection control system to their presence.

1.3.21 Pedestrian Traffic Standard

Signals facing in the direction of pedestrian cross walks which have lighted indicators marked ‘‘Walk’’ and ‘‘Don’t Walk.’’

1.3.22 Phase

The state of an intersection A particular period of the regulatory traffic pattern.

1.3.23 Remote Override

A computer host that includes a software interface allowing a remote administrator

to control the intersection remotely.

1.3.26 SNMP (Simple Network Management Protocol)

The de facto standard for inter-network management, defined by RFC 1157.

1.3.27 Split

The duty cycle for a given phase, expressed as a decimal or percentage.

1.3.28 Vehicle Traffic Standard

A traditional traffic signal with red, yellow, and green indicators.

1.3.29 Upstream

Direction opposite to the normal direction of vehicle travel.

1.3.30 Vehicle Presence Detector

See Occupancy Loop.

SPEED

LIMIT

55

OCCUPANCY LOOP

194 4 SOFTWARE REQUIREMENTS ENGINEERING

The target class of intersection has the following characteristics:

1 Four-way crossing.

2 Roadway gradients and curvatures are small enough to be neglected.

3 No right-turn or left-turn lanes or right-turn and left-turn signals (note, however, that the intersection is wide enough to allow vehicles passing directly through

to pass to the right of vehicles turning left).

4 Intersecting roads of different priorities (e.g., one road may be an arterial while the other may be a secondary road) or of equal priority.

5 Two vehicle traffic standards per approach: one suspended by overhead cable, the other mounted on a pedestal.

6 One pedestrian crosswalk per approach.

7 Pedestrian traffic standards, pedestal mounted, on each side of each crosswalk.

8 Pedestrian presence detectors (pushbuttons) on each side of each crosswalk.

9 Stop-line vehicle presence detectors (loop detectors) in all approaches (one per approach) for detecting vehicle presence and for counting vehicles passing through the intersection.

Traffic regulatory personnel will remove the manual override device from the control box and press buttons to control the intersection manually.

Remote operator uses a software control panel either to control the state of the intersection directly or to observe and manipulate the parameters and state of a specific intersection control system.

Maintainer accesses system through Ethernet port to perform maintenance.

2.2.3 Hardware Interfaces

The Intersection Control System hardware interfaces are summarized in Figure 2 on the following page.

Table 1 Major intersection control system hardware components

1.9 Vehicle Presence Detector Interface Unit (not shown in Figure 2) 4 1.10 Pedestrian Request Detector Interface Unit (not shown in Figure 2) 8

2.2.3.2 Wired Interfaces – Internal

Hard-wired connections between the intersection controller and the following ware components within the intersection controller enclosure are provided:

hard-1 Traffic Standard Lamp Drivers (20)

2 Traffic Standard Lamp Current Sensors (40)

3 Vehicle Presence Detector Interface Units (4)

4 Pedestrian Presence Detector Interface Units (4)

5 Green Signal Safety Relay (1)

6 Manual Override Console (1)

7 Maintenance Connector (2; 10-base T twisted pair)

Hard-wired connections between the intersection control enclosure and the following external hardware components are provided:

1 Pedestrian Presence Detector

2 Pedestrian Traffic Standard

3 Vehicle Presence Detector

4 Vehicle Traffic Standard

5 Emergency Vehicle Transponder

6 DOT Wide-Area Network (WAN)

The emergency vehicle transponder is a radio frequency link between the intersection control system and the emergency vehicle override controller.

Interaction between the software system and the remote operator console is ducted over a standard 10 base-T local area network Each intersection control system is identified with a unique, statically assigned IP address.

The intersection control system must be able to interact with the software control panel to allow remote user access This interface provides a remote user the ability

to modify system parameters, perform maintenance functions, or assume manual control of the intersection The standard protocol for this communication will be SNMP version 1.

198 4 SOFTWARE REQUIREMENTS ENGINEERING

RAM will be used for application execution The system shall not require more than

32 MB of RAM Upon boot, the RTOS, application program and static data needed for execution will be copied from flash into the RAM.

2.2.7 Operations

1 Automatic, unattended operation (normal operation)

2 Local manual operation (through override console)

3 Remote manual operation (through WAN port)

4 Local observed operation (through maintenance port)

5 Remote observed operation (through WAN port)

6 Remote coordinated operation (option; through WAN port)

2.2.8 Site Adaptation Requirements

This is summarized in Section 2.1, above.

2.3 Product Functions

The Intersection Control System provides the following functions:

1 Control of the intersection vehicle traffic standards.

2 Control of the intersection pedestrian traffic standards.

3 Collection and processing of traffic history from all approaches.

4 Adaptive control of intersection timings in response to traffic flow.

5 Actuated control of intersection in response to vehicle presence.

6 Timed control of intersection in response to a fixed scheme.

7 Handling of pedestrian crossing requests.

8 Handling of emergency vehicle pre-emption.

9 Intersection control in response to manual override commands.

10 Intersection control in response to remote override commands.

11 Management of traffic history and incident log databases.

12 Handling of maintenance access requests from the maintenance port.

13 Handling of maintenance access requests from the DOT WAN.

2.4 User Characteristics

2.4.1 Pedestrians

General population, including persons with disabilities.

2.4.2 Motor Vehicle

Automobiles and trucks, depending on roadway use limitations.

2.4.3 Traffic Regulatory Personnel

Authorized DOT, police, or other personnel trained in use of the Manual Override console Must have key to the system enclosure.

2.4.4 System Administrators

Authorized DOT personnel with training in the use of this system.

2.5 Constraints

System Constraints include the following:

1 Regulatory policy (e.g., ADA).

2 DOT specifications.

3 Local ordinances.

4 Hardware limitations.

5 Minimum time for pedestrian to cross.

6 Minimum stopping distance for vehicles.

7 Momentary power droops/outages.

8 Interfaces to other applications.

2.6 Assumptions and Dependencies

1 SI units are used for all physical quantities.

2 Commercially available RTOS is used.

3 Hardware interfaces have resource managers (drivers) already developed and available for integration with the software system specified here.

4 DOT WAN will use SNMP to communicate with intersection control system.

5 Watchdog circuitry forces safe default intersection state through hardware.

3 SPECIFIC REQUIREMENTS

This section describes the basic functional elements of the intersection control system In particular, the software object model is described in detail, with attributes and methods enumerated External interfaces to users, hardware, and other software elements are described, and background on the adaptive algorithm to be used

is provided.

200 4 SOFTWARE REQUIREMENTS ENGINEERING

3.1 External Interface Requirements

3.1.1 User Interfaces

Normal Vehicular Traffic Under Signal Control

Pedestrian Preempted Crossing

Emergency Vehicle Preempted Traffic Maintenance

Access

Remote Parameter Update Remote Override

Manual Override Intersection Controller

DOT Network

Maintainer

Figure 3 Top-level use-case diagram.

1 Vehicle Presence Detector – User: Motor Vehicle

2 Pedestrian Presence Detector – User: Pedestrian

3 Emergency Vehicle Override – User: Emergency Vehicle

4 Manual Override – User: Traffic Control Officer

5 Remote Override – User: DOT Officer

6 Maintenance Interface – User: Maintainer

5 Hardware watchdog

6 Uninterruptible power supply

3.1.3 Software Interfaces

1 RTOS API calls.

2 Hardware resource manager interfaces.

pedestrianTrafficStandard

trafficHistory incidentLog approach

pedestrianRequestPushbutton remoteOverride

intersectionController networkInterface

transponderInterface

1 1

1

1

2 2

4

Figure 4 Preliminary intersection controller class diagram.

202 4 SOFTWARE REQUIREMENTS ENGINEERING

3.2.1 Intersection Controller

The Intersection Controller is responsible for managing the following functions:

1 Initialization.

2 Instantiation of contained objects.

3 Control of the intersection vehicle traffic standards.

4 Control of the intersection pedestrian traffic standards.

5 Collection and processing of traffic history from all approaches.

6 Adaptive control of intersection timings in response to traffic flow.

7 Actuated control of intersection in response to vehicle presence.

8 Timed control of intersection in response to a fixed scheme.

9 Handling of pedestrian crossing requests.

10 Handling of emergency vehicle pre-emption.

11 Intersection control in response to manual override commands.

12 Intersection control in response to remote override commands.

13 Management of traffic history and incident log databases.

14 Handling of maintenance access requests from the maintenance port.

15 Handling of maintenance access requests from the DOT WAN (wide area network).

Table 2 below illustrates the attributes, methods, and events of the Interface Controller class and Figure 5 illustrates the controller functional sequencing.

Table 2 Intersection controller class

Intersection Controller

Manual Override Represents the Manual Override console.

Remote Override Represents the Remote Software console.

Traffic History Contains the traffic history for up to at least seven

(7) days.

Incident Log Contains the incident log for up to at least seven

(7) days.

Network Interface Object that provides an interface from the Network

resource manager (driver) to the Intersection Controller object.

Emergency Vehicle

Interface

Object that provides an interface between the Emergency Vehicle transponder and the Intersection Controller object.

Controller.

Table 2 (continued )

Intersection Controller

Priority Relative priority of the approaches.

Cycle Time Time to complete a full traversal of all intersection

phases.

Splits Array of numbers defining the fraction of the cycle

time allocated to each phase.

Current Phase Current intersection phase.

Phase Time Remaining Time remaining until the intersection moves to the

next phase in the sequence.

Detected Green Signal

Safety Relay State

This holds the actual state of the Green Signal Safety Relay.

Override Canceled Fires when Overrides are deactivated.

Watchdog Timeout Fires on a watchdog trip.

Error Fires when an error occurs Takes the Error code is a

parameter.

The corresponding traffic standard aspects are shown in Figure 6 below.

Flashing DONT′T WALK

Fashing Yellow

Flashing Red Phase

DONT WALK

DONT WALK

DONT WALK

DONT WALK

DONT WALK

DONT WALK

DONT WALK

DONT WALK

DONT WALK

WALK

DONT WALK

DONT WALK

DONT WALK

DONT WALK

DONT

DONT WALK

DONT WALK

DONT WALK

DONT WALK

DONT WALK

DONT WALK WALK

Figure 6 Traffic standard aspects for each phase.

206 4 SOFTWARE REQUIREMENTS ENGINEERING

3.2.2 Approach

This is the programmatic representation of an intersection approach.

The Approach object is responsible for managing the following functions:

1 Instantiation of contained objects.

2 Control of the traffic standards associated with the approach.

3 Handling of pedestrian crossing events.

4 Handling of loop detector entry and exit events.

5 Tracking the vehicle count.

Table 3 below illustrates the attributes, methods, and events of the Approach class.

Table 3 Approach class

Approach

Attributes Pedestrian Traffic Standard Object representing the two pedestrian traffic

standards associated with the approach Vehicle Traffic Standards Object representing the two vehicle traffic

standards associated with the approach Pedestrian Service Button Object representing the two pedestrian service

pushbuttons associated with the approach Vehicle Presence Detector Object representing the proximity detection

loop, located at the stop line, associated with the approach.

Vehicle Count Count of vehicles passing through the

approach.

Indication Array used to store the indications actually

being displayed on all associated traffic standards.

Current Aspect Current commanded aspect corresponding to

the Intersection Controller phase.

Speed Limit Value (in km/h) of the speed limit associated

with the approach.

Aspect.

signals from the current sensor hardware resource manager.

Increment Count Increase the vehicle count by 1.

3.2.3 Pedestrian Traffic Standard

This is the programmatic representation of a pedestrian crossing signal.

The Pedestrian Traffic Standard object is responsible for managing the following functions:

1 Displaying the commanded indication aspect from the Approach.

2 Determining the indication actually displayed.

Table 4 below illustrates the attributes, methods, and events of the Pedestrian Service Button class.

Table 4 Pedestrian traffic standard class

Pedestrian Traffic Standard

Get Indication Get the actual displayed indication based

on signals from the current sensor hardware resource manager.

3.2.4 Vehicle Traffic Standard

This is the programmatic representation of a vehicle traffic signal.

The Vehicle Traffic Standard object is responsible for managing the ing functions:

follow-1 Displaying the commanded aspect from the Intersection Controller.

2 Determining the aspect actually displayed.

208 4 SOFTWARE REQUIREMENTS ENGINEERING

Table 5 below illustrates the attributes, methods, and events of the Vehicle Traffic Standard class.

Table 5 Vehicle traffic standard class

Vehicle Traffic Standard

Get Indication Get the actual displayed indication based on

signals from the current sensor hardware resource manager.

3.2.5 Pedestrian Service Button

This is an object representing the set of pushbutton consoles located on opposite sides of the crosswalk associated with an approach.

The Pedestrian Service Button object is responsible for managing the following functions:

1 Filtering of pushbutton service requests.

2 Generation of Pedestrian Service Request event.

Table 6 below illustrates the attributes, methods, and events of the Pedestrian Service Button class.

Table 6 Pedestrian service button class

Pedestrian Service Button

Attributes Request Masked Indicates whether pedestrian service

pushbutton signals should be ignored or processed.

Request State Indicates whether or not a pedestrian service

request is active.

Methods Set Request State In response to a signal from the pushbutton

hardware resource manager, determine whether or not to modify the Request State and raise an event.

Reset Request State Clear the Request State.

3.2.6 Vehicle Presence Detector

This is an object representing the proximity detection loop located near the stop line associated with an approach The object class is based on the Pedestrian Service Button class.

The Vehicle Presence Detector object is responsible for managing the following functions:

1 Filtering of vehicle service requests (ACTUATED mode).

2 Generation of Vehicle Service Request event (ACTUATED mode).

3 Maintenance of the vehicle count statistic (FIXED, ACTUATED, and TIVE mode).

ADAP-Table 7 below illustrates the attributes, methods, and events of the Pedestrian Service Button class.

Table 7 Vehicle presence detector class

Vehicle Presence Detector

Attributes Request State Indicates whether or not a vehicle service request is

active (ACTUATED mode).

Methods Set Request State Set the Request State.

Reset Request State Clear the Request State.

Events Vehicle Entry Indicates that the detector loop is occupied.

Vehicle Exit Indicates that the detector loop is no longer occupied.

3.2.7 Manual Override

This is an object representing the set of pushbuttons on the manual override console.

210 4 SOFTWARE REQUIREMENTS ENGINEERING

The Manual Override object is responsible for managing the following functions:

1 Triggering the appropriate mode change.

2 Generation and handling of events required to control intersection phase Table 8 below illustrates the attributes, methods, and events of the Manual Over- ride class.

Table 8 Manual override class

Manual Override

Events Override Activated Fires when the override is activated.

Override Canceled Fires when the override is deactivated.

Advance Phase Fires in response to the ADVANCE button on the

override console being pressed.

3.2.8 Remote Override

This is an object representing the commands available on the Remote Software console Additionally, the object provides an interface for remote access to and update of intersection traffic data and cycle parameters for coordinated intersection control (option).

The Remote Override object is responsible for managing the following functions:

1 Triggering the appropriate mode change.

2 Generation and handling of events required to control intersection phase Table 9 below illustrates the attributes, methods, and events of the Remote Over- ride class.

Table 9 Remote override class

Remote Override

Methods Process Command Processes the events generated by the object,

modifying the appropriate attribute or calling the appropriate method of the Intersection Controller object.

Table 9 (continued )

Remote Override

Get Status Retrieves the all parameter and other status data used

as inputs to the Calculate Cycle Parameters adaptive control algorithm.

Set Parameters Sets the cycle timing parameters as calculated by the

remote host.

Events Override Activated Fires when the override is activated.

Override Canceled Fires when the override is deactivated.

Advance Phase Fires in response to the ADVANCE command from

the Remote Software console.

3.2.9 Emergency Vehicle Interface

This is an object that manages the wireless transponder interface to authorized emergency vehicles and accesses the Intersection Control object in order to display the correct traffic signals, allowing the emergency vehicle priority access to the intersection.

The Emergency Vehicle Interface object is responsible for managing the following functions:

1 Triggering the appropriate mode change.

2 Reception of emergency vehicle preemption requests.

3 Decryption and validation of emergency vehicle preemption requests.

4 Generation and handling of events required to control intersection phase.

Table 10 below illustrates the attributes, methods, and events of the Emergency Vehicle Interface class.

Table 10 Emergency vehicle interface class

Emergency Vehicle Interface

Attributes None None

(continued)

212 4 SOFTWARE REQUIREMENTS ENGINEERING

Table 10 (continued )

Emergency Vehicle Interface

Events Preempt Activated Fires when preemption is activated.

Preempt Canceled Fires when preemption is deactivated.

Preempt Timeout Fires when the preempt cancellation timeout interval

expires.

3.2.10 Network Interface

This is an object that manages communication via the Ethernet port.

The Network Interface object is responsible for managing the following functions:

1 Routing control messages to the appropriate objects.

2 Transferring traffic history and incident log data.

3 Management of maintenance operations.

Table 11 below illustrates the attributes, methods, and events of the Network Interface class.

Table 11 Network interface class

Emergency Vehicle Interface

Methods Process Message Analyzes and routes network messages.

Receive Message Receives network messages.

Send Message Sends network messages.

3.2.11 Traffic History

This is an object that manages the stored traffic history.

The Traffic History object is responsible for managing the following functions:

1 Storage and retrieval of traffic history database records.

2 Clearing of traffic history in response to a command from a remote host.

Table 12 below illustrates the attributes, methods, and events of the Traffic tory class.

His-Table 12 Traffic history class

Traffic History

Attributes Record An array of structures, each of which holds a

single traffic history record.

First Record Index of the first active record.

Last Record Index of the record most recently added.

Record Pointer Index used to sequence through the Traffic

Move Record Pointer Moves record pointer as specified.

Clear Database Returns the database to an empty state.

Database Full Fires when all allocated space for the database is

used Since the database is a FIFO structure, records will begin to be overwritten.

3.2.12 Incident Log

This is an object that manages the stored incident log.

The Incident Log object is responsible for managing the following functions:

1 Storage and retrieval of incident log database records.

2 Clearing of incident in response to a command from a remote host.

Incidents are generated by the following events:

1 Error conditions.

2 Traffic History database full.

3 System resets.