SIST-V ETSI/EG 202 057-2 V1.3.2:2011

ETSI Guide
Speech and multimedia Transmission Quality (STQ);
User related QoS parameter definitions and measurements;
Part 2: Voice telephony, Group 3 fax,
modem data services and SMS
2 ETSI EG 202 057-2 V1.3.2 (2011-04)

Reference
REG/STQ-00149-2
Keywords
data, fax, modem, QoS, quality, user, voice
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3 ETSI EG 202 057-2 V1.3.2 (2011-04)
Contents
Intellectual Property Rights . 5
Foreword . 5
1 Scope . 7
2 References . 7
2.1 Normative references . 7
2.2 Informative references . 8
3 Definitions and abbreviations . 9
3.1 Definitions . 9
3.2 Abbreviations . 11
4 General considerations . 12
4.1 Services covered . 12
4.2 Use of the parameters . 12
4.3 Reporting for different classes of customers . 12
4.4 Non standard levels of QoS . 13
4.5 Reporting for directly - and indirectly-serviced customers . 13
4.6 Data processing issues . 13
4.7 Data collection period . 13
4.8 Sampling and test calls . 14
4.9 Comparability of measurements . 14
4.10 Publication of QoS parameters . 14
5 QoS parameters for voice, data and fax services accessed via the PTN and SMS . 14
5.1 Unsuccessful call ratio . 16
5.1.1 Definition . 16
5.1.2 Application . 16
5.1.3 Measurement and statistics . 16
5.1.4 Further considerations. 17
5.2 Call set up time . 18
5.2.1 Definition . 18
5.2.2 Application . 18
5.2.3 Measurement and statistics . 18
5.2.4 Further considerations. 19
5.3 Speech connection quality . 20
5.3.1 General considerations . 20
5.3.2 Evaluation methods. 21
5.3.2.1 Methods based on intrusive measurements . 21
5.3.2.2 Methods based on non-intrusive measurements . 22
5.3.2.3 Methods based on the usage of the E model . 22
5.3.3 Practical Implementation of parametrical model . 23
5.3.3.1 Results . 24
5.3.3.2 Further considerations . 24
5.3.4 Application . 24
5.4 Fax connection quality . 24
5.4.1 Definition . 24
5.4.2 Application . 25
5.4.3 Measurement and statistics . 25
5.5 Data rate of dial-up access to the Internet . 25
5.5.1 Definition . 25
5.5.2 Application . 26
5.5.3 Measurement and statistics . 26
5.5.4 Further considerations. 26
5.6 Short Message Service (SMS) QoS parameters . 27
5.6.1 Successful SMS Ratio . 27
5.6.1.1 Definition . 27
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4 ETSI EG 202 057-2 V1.3.2 (2011-04)
5.6.1.2 Application . 27
5.6.1.3 Measurement and statistics . 27
5.6.1.4 Further considerations . 28
5.6.2 Completion Rate for SMS . 28
5.6.2.1 Definition . 28
5.6.2.2 Application . 28
5.6.2.3 Measurement and statistics . 28
5.6.3 End-to-End delivery time for SMS . 28
5.6.3.1 Definition . 28
5.6.3.2 Application . 29
5.6.3.3 Measurement and statistics . 29
Annex A: Combination of weekly or monthly results . 30
Annex B: Further explanation of "X % of" . 31
Annex C: Decision about the success of a call attempt . 32
Annex D: Relationship between the accuracy of the estimator of the unsuccessful call ratio
and the number of calls to be observed . 34
D.1 Theory . 34
D.2 Guidance . 36
Annex E: Method of calculating the number of observations required for measures of time . 38
Annex F: Standardized test chart for fax connection quality . 39
Annex G: Guidance on the selection of representative samples and test calls . 42
G.1 Specific considerations for determining measurement locations and their distribution for voice
quality measurements . 42
Annex H: Application examples of measurement location determination . 45
H.1 Example for France . 45
H.2 Example for Switzerland . 47
Annex I: Calculation of the Design figure of merit for speech quality . 49
I.1 The reference terminals . 49
I.2 The network performance characteristics . 49
I.2.1 Questions for fixed operators . 50
I.2.2 Questions for mobile operators . 52
I.2.3 Treatment of echo . 56
I.3 Calculating the end-to-end performance . 56
I.4 Terminals . 58
Annex J: Bibliography . 59
History . 60

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5 ETSI EG 202 057-2 V1.3.2 (2011-04)
Intellectual Property Rights
IPRs essential or potentially essential to the present document may have been declared to ETSI. The information
pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be found
in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in
respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the ETSI Web
server (http://webapp.etsi.org/IPR/home.asp).
Pursuant to the ETSI IPR Policy, no investigation, including IPR searches, has been carried out by ETSI. No guarantee
can be given as to the existence of other IPRs not referenced in ETSI SR 000 314 (or the updates on the ETSI Web
server) which are, or may be, or may become, essential to the present document.
Foreword
This ETSI Guide (EG) has been produced by ETSI Technical Committee Speech and multimedia Transmission Quality
(STQ).
The present document is part 2 of a multi-part deliverable covering Speech and multimedia Transmission Quality
(STQ); User related QoS parameter definitions and measurements, as identified below:
Part 1: "General";
Part 2: "Voice telephony, Group 3 fax, modem data services and SMS";
Part 3: "QoS parameters specific to Public Land Mobile Networks (PLMN)";
Part 4: "Internet access".
EG 202 057-1 [i.5] contains general user related QoS parameter definitions and measurement methods that can be
applied to any service.
EG 202 057-2 (the present document) contains user related QoS parameter definitions and measurement methods for
voice, Group 3 fax, modem data services and SMS accessed via the public telecommunication network. The data
parameters are specified for the case where an ITU-T Recommendations V.90 [i.24] and V.92 [i.25], compliant modem
is used since this kind of modem is in common use.
EG 202 057-3 [i.6] contains user related QoS parameter definitions and measurement methods specific to public land
mobile networks (PLMN).
EG 202 057-4 [i.7] contains user related QoS parameter definitions and measurement methods specific to Internet
access.
The present document takes into account as far as practicable the following eight principles:
1) QoS parameters should be easily understood by the public, and be useful and important to them.
2) All parameters are applicable at the network termination point (where appropriate).
3) Where measurements are possible they should be made on the customer's premises, using in-service lines.
NOTE: Literally principles 2 and 3 imply that all measurements should be carried out at the NTP, which would
require co-operation by users and be excessively intrusive, as it would require many visits to the premises
of users. Measurements at the subscriber side of the local exchange (e.g. at the MDF or other possible
connection point/distribution frame in the access network) generally give an adequate representation of
the quality that would be perceived at the NTP for the parameters defined in the present document, and so
this approach is used because it is more practicable and meets the underlying objectives of these
principles.
4) To be as realistic as possible, real traffic rather than test calls should be used as a basis of the measurements,
wherever possible.
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5) Parameters should be capable of verification by independent organizations. This verification might be made by
direct measurements or by audit of service provider's measurements.
6) The accuracy of QoS values should be set to a level consistent with measurement methods being as simple as
possible with costs as low as possible.
7) The parameters are designed for both statistical and individual application. The statistical values should be
derived by the application of a simple statistical function to the individual values. The statistical function
should be specified in this multi-part deliverable. This multi-part deliverable should also contain guidelines on
how statistically significant samples should be selected.
8) The statistical functions should be designed so QoS figures from different service providers can be compared
easily by users and in particular consumers.
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1 Scope
The present document contains definitions and measurement methods for a range of user perceivable Quality of Service
(QoS) parameters. The purpose of these parameters is to define objective and comparable measures of the QoS
delivered to users/customers for use by users/customers. The present document applies to any telecommunication
service, however, some parameters may have a limited application.
The present document is intended to provide a menu from which individual items can be selected. There is no
obligation to use any or all of the parameters.
The QoS parameters are related primarily to services and service features and not to the technology used to provide the
services. Therefore the parameters should be capable of use when the services are provided on new technologies such as
IP and ATM or other packet switched technologies as well as on circuit switched technologies.
The establishment of target values for QoS is beyond the scope of the present document. The QoS parameters listed in
the present document are also not intended to assess the complete QoS of a telecommunication service. The present
document provides a set of QoS parameters that covers specific user related QoS aspects rather than a complete list of
QoS parameters. This set has been chosen to address areas where monitoring of QoS is likely to be most worthwhile,
i.e. the areas that are most likely to be affected by any QoS problems.
If stakeholders wish to examine other QoS aspects they are recommended to follow the general approach of the present
document - as far as practicable - as a basis for the development of definitions and measurement methods for new
specific QoS parameters.
The set of QoS parameters is designed to be understood by the users of various telecommunications services. Sub-sets
of these parameters can be selected for use in different circumstances. For example a specific parameter might be
relevant for many users in some countries or markets but the same parameter might not be of relevance in others.
Therefore stakeholders - users, customers, regulators, service providers, network operators and other parties interested
in the use of QoS parameters - should decide in co-operation, which parameters and which measures should be used in
their particular situation. This decision should take account of:
• The precise purpose for which they will be used.
• The general level of quality achieved by most operators/providers.
• The degree to which the parameters will provide a reliable comparison of performance.
• The cost of measuring and reporting each parameter.
2 References
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
reference document (including any amendments) applies.
Referenced documents which are not found to be publicly available in the expected location might be found at
http://docbox.etsi.org/Reference.
NOTE: While any hyperlinks included in this clause were valid at the time of publication ETSI cannot guarantee
their long term validity.
2.1 Normative references
The following referenced documents are necessary for the application of the present document.
Not applicable.
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2.2 Informative references
The following referenced documents are not necessary for the application of the present document but they assist the
user with regard to a particular subject area.
[i.1] ETSI EG 201 769: "Speech Processing, Transmission and Quality Aspects (STQ); QoS parameter
definitions and measurements; Parameters for voice telephony service required under the ONP
Voice Telephony Directive 98/10/EC".
[i.2] ETSI ETS 300 905: "Digital cellular telecommunications system (Phase 2+) (GSM); Teleservices
supported by a GSM Public Land Mobile Network (PLMN) (GSM 02.03)".
[i.3] ETSI EN 300 659 (all parts) : "Access and Terminals (AT); Analogue access to the Public
Switched Telephone Network (PSTN); Subscriber line protocol over the local loop for display
(and related) services".
[i.4] ETSI EG 201 377-1: "Speech processing, Transmission and Quality aspects (STQ); Specification
and measurement of speech transmission quality; Part 1: Introduction to objective comparison
measurement methods for one-way speech quality across networks".
[i.5] ETSI EG 202 057-1: "Speech Processing, Transmission and Quality Aspects (STQ); User related
QoS parameter definitions and measurements; Part 1: General".
[i.6] ETSI EG 202 057-3: "Speech Processing, Transmission and Quality Aspects (STQ); User related
QoS parameter definitions and measurements; Part 3: QoS parameters specific to Public Land
Mobile Networks (PLMN)".
[i.7] ETSI EG 202 057-4: "Speech Processing, Transmission and Quality Aspects (STQ); User related
QoS parameter definitions and measurements; Part 4: Internet access".
[i.8] ETSI TR 101 949: "Speech Processing, Transmission and Quality Aspects (STQ); QoS parameter
definitions and measurements for use in network-to-network narrowband interconnection".
[i.9] ETSI TS 102 250-6: "Speech Processing, Transmission and Quality Aspects (STQ); QoS aspects
for popular services in GSM and 3G networks; Part 6: Post processing and statistical methods".
[i.10] Directive 98/10/EC of the European Parliament and of the Council of 26 February 1998 on the
application of open network provision (ONP) to voice telephony and on universal service for
telecommunications in a competitive environment.
[i.11] ITU-T Recommendation E.180: "Technical characteristics of tones for the telephone service".
[i.12] ITU-T Recommendation E.425: "Internal automatic observations".
[i.13] ITU-T Recommendation E.451: "Facsimile call cut-off performance".
[i.14] ITU-T Recommendation E.452: "Facsimile modem speed reductions and transaction time".
[i.15] ITU-T Recommendation E.453: "Facsimile image quality as corrupted by transmission-induced
scan line errors".
[i.16] ITU-T Recommendation E.800: "Terms and definitions related to quality of service and network
performance including dependability".
[i.17] ITU-T Recommendation G.107: "The E-model, a computational model for use in transmission
planning".
[i.18] ITU-T Recommendation P.862.1: "Mapping function for transforming P.862 raw result scores to
MOS-LQO".
[i.19] ITU-T Recommendation P.862.2: "Wideband extension to Recommendation P.862 for the
assessment of wideband telephone networks and speech codecs".
[i.20] ITU-T Recommendation G.109: "Definition of categories of speech transmission quality".
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[i.21] ITU-T Recommendation T.4: "Standardization of Group 3 facsimile terminals for document
transmission".
[i.22] ITU-T Recommendation I.210: "Principles of telecommunication services supported by an ISDN
and the means to describe them".
[i.23] ITU-T Recommendation T.22: "Standardized test charts for document facsimile transmissions".
[i.24] ITU-T Recommendation V.90: "A digital modem and analogue modem pair for use on the Public
Switched Telephone Network (PSTN) at data signalling rates of up to 56 000 bit/s downstream
and up to 33 600 bit/s upstream".
[i.25] ITU-T Recommendation V.92: "Enhancements to Recommendation V.90".
[i.26] ITU-T Recommendation Q.850: "Usage of cause and location in the Digital Subscriber Signalling
System No. 1 and the Signalling System No. 7 ISDN User Part".
[i.27] ITU-T Recommendation P.862: "Perceptual evaluation of speech quality (PESQ): An objective
method for end-to-end speech quality assessment of narrow-band telephone networks and speech
codecs".
[i.28] ITU-T Recommendation P.561: "In-service non-intrusive measurement device - Voice service
measurements".
[i.29] ITU-T Recommendation P.562: "Analysis and interpretation of INMD voice-service
measurements".
[i.30] ITU-T Recommendation P.563: "Single-ended method for objective speech quality assessment in
narrow-band telephony applications".
[i.31] ITU-T Recommendation G.114: "One-way transmission time".
[i.32] ITU-T Recommendation G.113: "Transmission impairments due to speech processing".
3 Definitions and abbreviations
3.1 Definitions
For the purposes of the present document, the following terms and definitions apply:
access line: connection from the Network Termination Point (NTP) to the entry point to the local switch or remote
concentrator, whichever is the nearer
NOTE: In many cases this is the main distribution frame.
access network operator: organization that provides the access line
NOTE: In many cases the access network operator will be the direct service provider, but if the line is unbundled,
the direct service provider would be a separate organization.
call by call carrier selection: form of carrier selection where the user dials a carrier access code to indicate which
carrier is to route the call
carrier access code: code that the user may or needs to dial before the national (significant) number when dialling an
access line in another telecommunications network, so that the call is routed by the carrier of his choice
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customer: party that pays for the telecommunication service(s) provided
NOTE: Customers can generally be categorized as business or residential; the definition of business and
residential customers is left to individual service providers. Service providers who receive interconnect
services from other service providers are not considered to be customers for the purpose of the present
document. The term "customer" is equivalent to "subscriber", which is used in Directive 98/10/EC [i.10].
data service: telecommunications service involving the transport of data via the PTN such that any user can use
equipment connected to a network termination point to exchange data with another user of equipment connected to
another termination point
direct service: service where the service provider that provides the telecommunication service(s) also provides the
access network or rents an unswitched local loop (unbundled local loop) to use for the provision of the service to the
customer
fax service: telecommunications service of transport of facsimile via the PTN such that any user can use equipment
connected to a network termination point to exchange facsimiles with another user of equipment connected to another
termination point
indirect service: service where the service provider that provides the telecommunication service(s) does not provide the
access network but is selected by the customer or user using a form of call by call carrier selection or carrier
preselection
network operator: organization that provides a network for the provision of a public telecommunication service
NOTE: If the same organization also offers services it also becomes a service provider.
Network Termination Point (NTP): physical point at which a user is provided with access to a public
telecommunications network
ported number: subscriber number (directory number) where the location of the NTP and/or the identity of the service
provider has changed after the number was originally allocated
preselection: form of carrier selection where the customer informs his access network operator which carrier is to route
all or a particular subset of his calls, unless call by call carrier selection is used
Public Telecommunications Network (PTN): telecommunications network used wholly or partly for the provision of
publicly available telecommunications services
Quality of Service (QoS): collective effect of service performance which determines the degree of satisfaction of a user
of the service
NOTE: See ITU-T Recommendation E.800 [i.16].
service provider: organization that offers a telecommunication service to the customer and/or user
NOTE: A service provider need not be a network operator.
Short Message Service (SMS): telecommunications service involving the transport of a short alphanumeric message
(160 alphanumeric characters) via the PTN such that any user can use equipment connected to a network termination
point to exchange these messages with another user of equipment connected to another termination point
NOTE: See also ETS 300 905 [i.2] (GSM networks) and EN 300 659 [i.3] (fixed networks).
stakeholder: party having an interest in the level of quality of a service
supplementary service: additional service that modifies or supplements a basic telecommunication service
NOTE: Consequently, it cannot be offered to a customer as a stand-alone service; it has to be offered in
association with a basic telecommunication service. The same supplementary service may be common to
a number of basic telecommunication services. (See ITU-T Recommendation I.210 [i.22]).
telecommunications: technical process of sending, transmitting and receiving any kind of message in the form of signs,
voice, images or sounds by means of telecommunications systems
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telecommunication services: provision of telecommunications and the provision of other additional services that are
closely related to the provision of telecommunications like
EXAMPLE: Billing, directory services.
telecommunications systems: technical equipment or systems capable of sending, transmitting, switching, receiving,
steering or controlling as messages identifiable electromagnetic signals
user: individuals, including consumers, or organizations using or requesting publicly available telecommunications
services
NOTE: See Directive 98/10/EC [i.10].
voice service: telecommunications service of direct transport of real-time speech via the PTN such that any user can use
equipment connected to a network termination point to communicate with another user of equipment connected to
another termination point
3.2 Abbreviations
For the purposes of the present document, the following abbreviations apply:
AMR Adaptive Multi-Rate
ATM Asynchronous Transfer Mode
BAS Broadband Access Server
CPU Central Processing Unit
DECT Digital European Cordless Telephone
DSLAM Digital Subscriber Line Access Multiplexor
DSP Digital Speech Processing
EFR Enhanced Full Rate
FR Full Rate
GMSC Gateway Mobile Switching Centre
GSM Global System for Mobile communications
HLR Home Location Register
INMD In-service Non-intrusive Measurement Device
IP Internet Protocol
ISDN Integrated Services Digital Network
ISP Internet Service Provider
LQO Listening Quality Objective
MDF Main Distribution Frame
MGCP Media Gateway Control Protocol
MOS Mean Opinion Score
MSC Mobile Switching Centre
MVNO Mobile Virtual Network Operator
NER Network Effectiveness Ratio
NRA National Regulatory Authority
NTP Network Termination Point
PCM Pulse Code Modulated/Pulse Code Modulation
PLC Packet Loss Concealment
PLMN Public Land Mobile Network
PSTN Public Switched Telephone Network
PTN Public Telecommunications Network
QDU Quantization Distortion Unit
QoS Quality of Service
RLR Receive Loudness Rating
RTP Real Time Protocol
SAP Service Access Point
SIP Session Initiation Protocol
SLR Send Loudness Rating
SMS Short Message Service
STMR SideTone Masking Rating
STQ Speech Transmission and Quality (Technical Committee)
TELR Talker Echo Loudness Rating
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TFO Tandem Free Operation
TrFO Transcoder Free Operation
TrFO Transcoder Free Operation
VAD Voice Activity Detection
VoIP Voice over Internet Protocol
4 General considerations
4.1 Services covered
The QoS parameters of the present document cover aspects of telecommunications services which are typically
provided via the public telecommunications network such as voice, fax or data services. These services may be accessed
via terminals connected to fixed network termination points or via mobile accesses e.g. GSM.
The definitions and measurement methods of the QoS parameters were elaborated primarily in order to assess QoS
aspects of "standard" telecommunication services. Therefore mainly common aspects and applications of
telecommunication services were considered and are reflected in the present parameters. In principle the QoS
parameters may also be used for the investigation of special or non-standard telecommunication services but further
enhancements/additions to the definitions and measurements methods may be necessary.
Most parameters are in principle applicable to any service provided via the public telecommunication network. Some
parameters are however only applicable to specific services depending on technical aspects of the provision of those
services, e.g. mobile, data, fixed NTP. Depending on the set of QoS parameters used by the stakeholders the scope of
the services covered may vary.
The parameters are end-user/customer and end-to-end orientated and are not intended to address the quality of
interconnect services explicitly. Any dependence on interconnect services is included implicitly in the measures of QoS
provided to the end user. Separate Guides in this series deal with the QoS of interconnect arrangements.
In many cases the provider of telecommunications services to the customer may depend on other providers for part of
the service. An example is an international call where several service providers are normally involved. In such cases the
provider of the service to the customer is responsible for all elements for which it receives payment from the customer.
In order to provide satisfactory QoS, this service provider will need to ensure that adequate QoS is provided by the
other interconnected service providers. QoS figures for the responsible service provider will reflect both its own
capability and that of the interconnected service providers.
4.2 Use of the parameters
The parameters may be used for various purposes including:
• Specifying the level of quality of service in customer telecommunication service contracts or in the description
or terms and conditions of the service.
• Comparing the quality of service of different service providers.
• Comparing the quality of service aspects of different service offers.
• Preparing long term studies on the quality of service aspects of a specific service.
4.3 Reporting for different classes of customers
For each parameter, statistics may be produced or requested that are aggregated over all classes of customer or, where a
distinction between different classes is desired, e.g. residential and business, separate statistics may be used, or both.
This recognizes the voluntary nature of these measures and the fact that some stakeholders may only wish to target
specific sections or to provide a rough overview of the market.
NOTE: Due to the fact that a variety of different service offers is available at the market, it is not always possible
to clearly distinguish between classes of customers like residential or business. Furthermore it may not be
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fair to compare different service offers on the basis of different classes of customers because the results
may be misleading. Also statistics may be falsified when aggregating over all classes of customers. See
also clause 4.9.
4.4 Non standard levels of QoS
Statistics produced should normally be based on the standard level of QoS for each telecommunication service. The
standard level is defined in the terms and conditions of the services as published by the service providers. Stakeholders
may choose to produce or request specific statistics for cases where customers are able to pay more for enhanced or less
for lower QoS. It is recommended to provide additional information on the kind and scope of services the QoS statistics
are referring to when covering non-standard levels of QoS.
4.5 Reporting for directly - and indirectly-serviced customers
The principle used is that the service provider who charges the customer should be responsible for the quality of the
service and for providing QoS statistics relevant to the service provided. Thus, in the case of carrier selection, the
indirect service provider has the responsibility for QoS and provision of QoS statistics when it is selected to carry a call.
For each parameter in clause 5 a statement is made on whether it is applicable to indirect services.
Some service providers provide both direct and indirect services. Where there are likely to be significantly different
levels of performance for these two service types or where the services are understood as being two different not
comparable service offers (even though the same telecommunication service is offered), the production of separate
statistics for each service type is recommended.
The treatment of direct and indirect services is summarized in the last column of table 1.
NOTE: Where only a combined statistic for both types of service is specified, separate statistics for each service
type may be provided in addition if the stakeholders to do so.
4.6 Data processing issues
Where the measures are based on all actual occurrences rather than samples, the measuring party may prefer to process
data on a weekly or monthly basis, discard the detailed data and use a statistical method such as that specified in
annex A for combining the weekly or monthly results.
For one parameter the statistic required is "X % of .". This statistic is explained in annex B.
In some cases disasters, freak weather, etc. may distort measured QoS figures. Such occurrences may not necessarily
damage a network, but could degrade QoS by inducing exceptional traffic levels etc. In these cases, service providers
should provide the measured QoS and may additionally provide a second figure which excludes the effects of the
exceptional circumstances. A note clearly explaining the difference should also be provided. Service providers covering
large geographical areas are likely to be more prone to these effects than service providers serving smaller areas. The
effect on the reported QoS of a service provider covering a small area is likely to be more severe, however, should such
an event occur.
4.7 Data collection period
Where the measurements are to be used for long term comparisons, it is recommended that QoS data should be
collected and calculated on a quarterly basis starting on 1 January, 1 April, 1 July and 1 October.
Stakeholders may also decide to use longer or shorter data collection periods. For most QoS parameters a data
collection period on a quarterly basis is suitable, and will provide adequately up-to-date information. But there may also
be cases were a longer period is more practicable, e.g. extensive customer surveys. Shorter periods are advisable for
QoS aspects where frequent and fast changes in quality are likely to occur.
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4.8 Sampling and test calls
Where sampling and test calls are used the approach should ensure that the results adequately reflect the QoS perceived
by customers for the period under review.
Guidance on the choice of adequate test calls with respect to choice of origin, destination, traffic variations etc. may be
found in annex G.
4.9 Comparability of measurements
The following issues may affect the comparability of the measurement results:
• Measurement methods may be implemented differently (e.g. use of real traffic vs. test calls, choice of
representative connections).
• Measurements based on signalling information or tones may be unreliable because the signalling systems and
tones are not implemented in a fully standardized manner (e.g. diff
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