QUALITY OF SERVICES (QOS) QUANTIFICATION
FOR SELECTED TELEMATIC SYSTEMS
OF HIGHWAY TRANSPORTATION
Wawrzyński W., Siergiejczyk M.
Technical University of Warsaw Faculty of Transportation
Abstract: The paper presents problems of quantification for quality of services with respect to
selected telematic systems that are applicable to roadway transportation. As about the SOS telecommunication systems, the numerical parameters related to network reliability and its transmission performance were considered as the most representative ones. However, for the roadway navigation and location systems other parameters were stated as crucial ones, namely precision of determining the client position, time delay intervals and operational performance of the navigation software. Finally, it was concluded that the suggested quantificational parameters are well-suited for assessment of the operational properties of telematic systems in terms of their applicability to practical purposes.
1. Introduction
Telematic systems for roadway transportation represent a set of telecommunication equipment and computer systems (hardware and software) along with general-purpose electronic appliances. All the components are interconnected by means of facilities related to information transmission and processing and are deployed along the highway and within its closest vicinity. Moreover, the entire system operates for a specific purpose. The aim of such a system is to assure the appropriate level of traffic safety as well as desired efficiency and throughput of the transportation process that uses a highway as an inland transportation link. The system incorporates a number of components [5] that execute appropriate function, but the overall goals of the system include the sufficient Quality of Services (QoS) that has to be achieved. The Quality of Services is defined as a degree of customers’ satisfaction that results from providing of the comprehensive services. Thus, it is a very wide term that in case of e.g. telecommunication networks refers to the entire chain of services from one end to the other, i.e. from a service provider to a client.
2. Quality of Services provided by highway SOS telecommunication
systems
2.1. Architecture and operation principle of a highway SOS telecommunication system
Any highway SOS telecommunication system is designed for transmission of calls from clients that travel down the specific section of the highway. The calls are put through to the
supervising centre of the highway section and may concern failures, breakdowns, crashes, collisions etc.
There are three major components that make up the SOS telecommunication system (Fig. 1).
• SOS posts, deployed along the highway from where calls can be made. The
posts are located at every 2 km on the both sides of the highway or every 2 km alternatively on one side or the other.
• the Supervising Centre of the SOS telecom system that assures right operation of the network and handles the received information, i.e. answers the calls and forwards the information to relevant emergency services, etc.,
• the transmission carrier – copper cables and fibre-optic cables,
The SOS posts are furnished with equipment that enables two-way connection (making a call and voice communication) or one-way connection (merely making a call by means of the emergency pushbutton). The system user presses the emergency pushbutton installed on the SOS post and sends the message to the Supervising Centre. The transmitted message contains information that makes it possible to identify the post where the message has been sent from. That is why the location of the event is immediately known to the operator, who recognizes the number and location of the specific posts where the call has been made from and is able to relay the message to appropriate services as well as to activate remotely the blinking lamp that is installed on the post. In case of the two-way connection the operator can callback to the specific post and contacts the user.
Fig.1. Architecture of the SOS telecommunication network
To the Supervising Centre fibre-optic Cable Cable SOS posts Electronic interface fibre-optic Switching hub
According to [4], the quality of services is determined by the set of the service features that define the degree to which the client’s demands of specific services are satisfied. Thus, in case of the highway SOS communication system, there are two components that affect the service quality:
reliability of the network operation, quality of the SOS signal transmission.
2.2. Quality-related parameters that affect the reliability of the SOS communication system
For any telecommunication system (including the SOS one) the reliability of the information transmission is defined as such feature of the network that makes it possible to establish and maintain communication links between the active subscribers during the specific period of time and under the certain operating conditions. However, the overall reliability of the network depends on reliability of its components and the reliability-related structure of the network.
With respect to the reliability evaluation of the SOS communication network the operability criterion is adopted as the capability to establish the “peer-to-peer” connection, i.e. to establish the <a,b> link, for any a,b
W, where W represents the set of network nodes. In such a case the value of the reliability factor Pa,b{
(x)=l} is limited from the upper side by reliability of the end-point nodes [3]. Therefore:Pa,b{
(x) = l} = P{
(a) = l}
P{
(b) = l}where:
(x) - the structural function of the network reliability, whereas the network reliability status is defined by the vector x,
(b) - the structural function for the reliability of the end-point nodeHowever, the accurate evaluation of the communication system takes also account for the influence of the reliability-related structure of the bipolar network between the nodes a and b. Thus:
Pa,b{
(x) = l} = P{
(a) = l}
P{
(b) = l}
Pa,b{
(x*) = l}where: x* - the status vector of the bipolar network, with exclusion of the a and b nodes, i.e. the nodes that are indicated by the operability criterion of the network. Taking account for the familiarity with the reliability-related structure of the network nodes such as the SOS post and the operator’s terminal in the Supervising Centre as well as with the reliability-related structure of the bipolar network between these two nodes, the overall reliability of the telecommunication SOS network for transmission of a message from a single SOS post can be expressed by the following formula:
RKC(t) = RKA(t)
RKT(t)
RTCN(t)where: RKC(t) - reliability of the telecommunication SOS network between a single SOS post and the operator’s terminal in the Supervising Centre,
RKA(t) - reliability of the SOS post,
RKT(t) - reliability of the transmission channel between the SOS post and the operator’s terminal in the Supervising Centre (copper cables and light pipe),
RTCN(t) - reliability of the operator’s terminal in the Supervising Centre.
The important issue related to calculation of reliability for the telecommunication SOS network is the estimation of reliability parameters for both the SOS post and the operator’s terminal. The SOS post is equipped with electronic module assembled on a common PCB. In case of any defect the board is replaced with a new one, so the dependability model of the electronic equipment can be adopted for further calculations. The operator’s terminal in the Supervising Centre for emergency communication is composed of a PC-type computer that is connected the set of emergency phone lines. Every set comprises a central unit (a hub) and branch units for serial transmission. In this case the evaluation methods that are used for calculation of microprocessor systems reliability should be applied provided that these methods take account for both hardware and software reliability.
The interrelationships that are presented above allow making the conclusion that the crucial requirements that determine reliability of such system include:
- making sure that the individual components of the network nodes (SOS
posts, operator’s terminals) are of high reliability,
- development of such reliability-related structure of the network that it would only slightly distort transmission of information.
The first requirement is fulfilled by reducing the distance between the network nodes to the minimum possible value. The second one imposes the need to solve the allocation problem for reliability issues with respect to individual interconnections between the network nodes. For that purpose the function Pa,b{
(x*) = l} = f(
/km) can be used, where
/km is the operational failure rate per one kilometer of a transmission channel between the network nodes.2.3. Quality-related parameters that concern transmission of messages via the SOS network
The second important feature of the highway SOS telecommunication network is the error-free transmission of messages that are sent from the SOS post the call was made from. It results from the fact that the message contains identification number that specifies the post within the network structure and determines its geographic location on the highway section.
In this case, establishing the Quality of Service determinants take account for specific properties of individual types of services. For data transmission, as it is the transmission type that is initiated in case of an emergency call from any SOS post, the error rate per character Pbz can be used as a quantification parameter for transmission quality. Hence,
z
z
P
bbz
where zb - number of characters that have been correctly received and interpreted by the
operator’s terminal in the Supervising Centre;
z - number of characters that have been sent by the transmission terminal of
an SOS post,
For voice transmission, as is in the case when the voice connection is established between the operator of the Supervising Centre and the voice terminal of the SOS post, the transmission quality can be measured as understandability of individual syllables. The understandability of syllables Qs for single service of voice connection can be expressed
in the following way:
n n
Q c
s
where nc - number of syllables correctly received by the operator’s terminal; n - total number of syllables sent by the terminal,
For digital channels the commonly used parameter, as is the bit (elementary) error rate (Pe) can be used. However, in case of an analog channel, the quality criterion should
generally include threshold values for a defined set of channel parameters (e.g. bandwidth, signal suppression, noise level, pulse interference in case of commutated channels). The
Pe value can be calculated as a ratio:
n o e e e P
where eo - number of correctly received bits; en - total number of transmitted bits.
However, evaluation of data transmission correctness via the tract that links SOS posts with the Supervising Centre presents a separate problem that refers to quality of services for SOS networks. It is because the same transmission tract is used for transfer of information from other equipment and telematic systems that are deployed down the highway. Amount of information that has to be transmitted via that tract is pretty large as the tract enables establishing many concurrent links (transmission channels) and technologies of package networks are used for data transmission.
Calculation of the so-called load availability to data transmission can serve as a method that is applicable to evaluation of transmission quality via the transmission tract down a highway, as is proposed in [6]. Calculation of that parameter is justified in case when any part of data from a certain post is transmitted to the Supervising Centre via different tracts, with various values of load availability. The package networks that use the IP protocol can serve as an example of such solutions. Load availability via a teletransmission tract that is defined in such a manner can be defined as the weighted average of load availability values for individual links and is expressed by the formula [6]:
D A =
i pi i pi pi D D Awhere AD - load availability for data transmission between two nodes of the networks be
means of i links,
Ap - load availability of the ith link,
i pi
D - transmission rate between two nodes.
It is worth noting that the relation min {Ai} <AD< max {Ai} is always true.
Although calculation of that parameter is considered as unjustified from the viewpoint of the reliability theory, this approach seems to be promising for practical application, as the results presented in [6] show, and can be used for analysis of convergent networks where the IP protocol is applied.
3. Quality of services for a roadway navigation systems
The parameters that affect quality of services for the process of roadway navigation QoSRN depend on the precision for determining the position where a certain vehicle is at the specific moment of time (ΔP), delay intervals (ΔT) attributable to the location system as well as operational performance of the navigation software (ΔS). Thus:
QoSRN=<P,T,S>
3.1. Quality-related parameters attributable to the precision of location systems
Making assessment how accurate the navigation receivers (locators) are able to determine locations of specific objects depends on the adopted operation modes, i.e. 3D (location in space) or 2D (location on a two-dimensional surface).
As about the 3D mode, the location accuracy is defined via the spatial parameter: Position
Dilution of Precision (PDOP), which is determined by means of the equation below [1]:
2 2 2 1 h PDOP
where: σp - standard deviation for measurement results of distances between
individual satellites and the receiver, σ2φ- variance for geographic latitude , σ2λ- variance for geographic longitude λ,
σ2
h variance for ecliptic elevation h. 1
1W oryginale jest wysokość elipsoidalna, ale to chyba wynik błędnej retranslacji, ecliptic – elliptic. Ekliptyczny
While operating in the two-dimensional (2D) mode, the accuracy is defined by the horizontal coefficient HDOP - Horizontal Dilution of Precision, which can be calculated by means of the equation [1]:
2 2 1 HDOP
where all the variables are the same as in the preceding formula.
Specific values of that coefficient can be associated with information about precision level and deviations for location determining (e.g. for a selected receiver the value HDOP = 3 corresponds to the deviation of 15 m). Obviously, the lower the HDOP parameters, the more accurate location determining is possible.
There exist some solutions for location systems that make it possible to specify the maximum permissible limit thresholds for a certain accuracy parameter, which enables specification of the acceptable deviation for location determining. If the allowed limit is exceeded, the fact is signaled by triggering an appropriate alarm (sending a message) related to the situation occurred.
Accuracy of determining the client location is also affected by the receiver capability to operate in the differential mode. Such a receiver feature, if available, makes it possible to determine the client location with the deviation less than several meters. However, connecting an additional antenna to the locator is then the indispensable provision as the antenna is able to receive correcting factors from reference stations.
3.2. Quality-related parameters attributable to time delays of location determining.
The time that expires until the first readout of the location is possible (acquisition time) is considered in two aspects. Depending on the type of information stored in the receiver memory the cold start time and the hot start time can be distinguished.
The cold start time is the essential parameter and plays the important role when no information on the current time, current location of a client, satellite ephemerides and the system log have been previously stored into the receiver memory. It is the case when the receiver is activated for the first time or its memory content has been wiped out by any reason. In such a case the cold start time can reach even up to a dozen of minutes. In case of the hot start procedure the receiver memory already contains information related to the time, location and the system log, but current satellite ephemerides are absent. Therefore the hot start time depends on whether the current position is not different from the previous one that has been determined by the GPS receiver as well as
on the time that has expired from the moment when the receiver was switched off. It is why the hot start time may vary from dozen odd to several dozen of seconds.
The reacquisition time is defined as the time interval required by the receiver to redefine the location in case of unexpected and unpredictable decay of the signal that is received from at least one satellite from among all of those used for calculation of the geographical location. On average, that time interval equals few seconds.
The frequency of location updating is defined as the time interval where the receiver commences to determine its location. It is the standardized parameter and in most cases equals 1 second.
Another significant parameter is the receiver continuity to trace its location. Tracing interrupts in the location system can be affected by movement dynamics of the vehicle where the locator is installed.
Determination of the vector of vehicle velocity (in other words the travel direction and speed) is carried out on the basis of Doppler’s effect or on the basis of subsequent readouts of the locator position. Both methods can also be combined. The GPS system has no constraints that might be imposed by its operation principle with respect to the range of speed readouts for traveling vehicles. However, the requirements of the system operator restrict operation of the locators to the speed of 1665 km/h and the height of 18 km [1] (due to the need to disable uncontrolled application of the system for launching of missiles). Obviously, the range entirely covers the needs of individual and collective transportation means, both on the land and by air. Hence, the only adjustment that has to be made by a client it to setup the maximum vehicle performances in terms of its speed and maximum available accelerations. Tracing interrupts may happen due to the tracing breakdown by the tracing loop of the carrier frequency and the receiver code that can be the result of incorrect adjustments.
3.3. Quality-related problems connected with operational friendliness of the navigation software
To make assessment of that group of quality properties it is essential to study the relationships between the coordinate reference system of a receiver and the reference system of the electronic map. In case of receivers, the WGS-84 (Word Geodetic System
1984) is used as the coordinate reference system and the receivers show their location on
the background of that system. In order to correctly locate a vehicle on an electronic map it is necessary to make the comparison between the reference system, which is used by the locator to determine its geographic coordinates, and the coordinate system that had been used for the development of maps. The location that is read from the locator is indicated on the map and then such information is provided to a client. It means that the uncertainty of locations that are shown on geographic maps can be actually much higher than the
deviations for geographic coordinates determined in the mentioned reference system by the GPS receiver itself.
Another quality-related parameter that is classified in this group is quality of navigation software in terms of its ability to display electronic maps that serve as background for road navigation. The electronic maps can be stored in the locator memory by uploading them from CD-ROMs whilst the navigation software makes it possible to record a specific number of coordinates for so called waypoints. The waypoints are understood as characteristic points that are used for voyage routing. The locators can keep a dozen of pre-programmed routes whereas each route may incorporate dozen odd or even several dozen of waypoints. In addition, the GPS locators with sophisticated software have various facilities beneficial for clients, such as routing optimization with consideration to current parameters of roads, optimization of fuel consumption, etc.,
4. Conclusion
Rapid development of telecom and telematic technologies results in the main objective to be achieved by the providers of telecom services is clients’ satisfaction with the offered services. Great many new telematic services require reaching and maintaining a permanently high level of service quality. Therefore, the establishing of the quantitative grades for Quality of Services (QoS) is becoming the problem of extreme importance. The QoS characteristics are used as a tool for grading of certain services, just as they are perceived from the client viewpoint.
In case of telematic services the term of their quality is not ambiguous and can be defined in many ways. Nowadays, determination of telematic service quality tends to pay more and more attention to match the client expectations, successful execution of tasks related to traffic organization and management as well as providing safe and reliable communication is case of emergency, when users of transportation systems are under a direct threat to their life or health. Thus, the statement that numerous and versatile parameters should be used for making assessments on quality of services in telematic transportation systems seems to be justified as different parameters are used to specify the most important tasks that are exercised by those systems.
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[2] ITU-T Recommendations E.800: Terms and definitions related to quality of service and network performance including dependability. Approved in 1994.08. Status: In Force
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