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All drawings appearing in this Recommendation have been done in Autocad.
Recommendation Q.543
DIGITAL EXCHANGE PERFORMANCE DESIGN OBJECTIVES
1 General
This Recommendation applies to digital local, combined, transit and
international exchanges for telephony in Integrated Digital Networks (IDN) and
mixed (analogue/digital) networks, and also to local, combined, transit and
international exchanges in an Integrated Services Digital Network (ISDN).
The field of application of this Recommendation is more fully defined in
Recommendation Q.500. As to the application in an ISDN, transit connections and
exchange connections types I, II, III and IV as defined in Recommendation Q.522
are covered (Notes 1 and 2). Other types of connection and variants of these
connections may be feasible in ISDN and will be the subject of further study.
These performance design objectives are applicable to all exchange
implementations at all points in the growth cycle up to the maximum size. These
reference loads and performance objectives may be used by manufacturers in
designing digital switching systems and by Administrations in evaluating a
specific exchange design or for comparing different exchange designs for
potential use in the Administration's intended implementation.
These recommended performance design objectives relate to the technical
capabilities of exchange design. They are intended to assure that exchanges
operating in their intended implementation will be capable of supporting the
network grades of service recommended in the E.500-series of Recommendations and
will offer a level of performance consistent with the overall network performance
objectives given in the I-series of Recommendations. The recommended parameters
are design objectives which should not be construed to be grade of service or
operating requirements. In actual operation, exchanges will be engineered to
provide adequate grades of service as economically as possible and the
performance requirements (delays, blocking, etc.) of the exchange in operation
will differ from the recommended values for these performance design objectives.
2 Performance design objectives
2.1 Reference loads
The given reference loads are traffic load conditions under which the
performance design objectives stated in SS 2.2 to 2.7 are to be met. In order to
have a comprehensive characterization of exchange reference loads, supplementary
services and other types of services must be taken into account. Administrations
may specify hypothetical models for use in computing exchange loading. These
models should characterize the sets of traffic parameters and services that are
considered to be typical in the intended application of the exchange, and should
include the traffic mix (originating-internal, originating-outgoing,
incoming-terminating, transit, abandoned, busy non-answer, etc.), the mix of
service classes (residential, business, PABX, coin, etc.), the types and volume
of supplementary services (call waiting, call forwarding, etc.) and any other
pertinent characteristics. Using the above information, it should be possible to
"engineer" the exchange to produce the model. It should also be possible to
determine the maximum size of the exchange by the computations discussed in S
2.1.4.
Reference load A is intended to represent the normal upper mean level of
activity which Administrations would wish to provide for on customer lines and
inter-exchange activities. Reference load B is intended to represent an increased
level beyond normal planned activity levels. (Recommendations E.500 and E.520
recommended that the normal provisioning of international circuits in automatic
and semi-automatic operation be based on a particular loss probability during the
mean busy hour and the average traffic estimated for the "five busiest days" as
set down in Recommendation E.500.)
Note 1 - For the time being, the following definitions and corresponding
values are only applicable to 64 kbit/s circuit switched connections, i.e.,
including transit connections and connection types I, II and III option a). Other
rates and transfer modes require further study.
Note 2 - The applicability of this document to connections originating or
terminating on PABXs is for further study.
2.1.1 Reference load on incoming interexchange circuits
a) Reference load A
- 0.7 erlangs average occupancy on all incoming circuits
Call attempts/h =eq \f(0.7 x number of incoming circuits,Average holding time in
Fascicle VI.5 - Rec. Q.543 PAGE1
hours)
Note - Ineffective call attempts must be included in reference call
attempts.
b) Reference load B
- 0.8 erlangs average occupancy on all incoming circuits
with 1.2 times the call attempts/h for reference load A.
2.1.2 Reference load on subscriber lines (originating traffic)
Characteristics of traffic offered to local exchanges vary widely
depending upon factors such as the proportions of residence and business lines
that are served. The following Table 1/Q.543 provides reference load
characteristics for lines typical of four possible local exchange applications.
Also provided are representative ISDN cases which are discussed below.
Administrations may elect to use other models and/or loads that are more suitable
for their intended application.
In the following text, ISDN lines will be referred to as digital lines and
non-ISDN lines as analogue lines.
2.1.2.1 Reference load A
TABLE 1/Q.543
Subscriber line traffic model
a) Non-ISDN subscriber lines with or without supplementary services
Exchange type Average traffic Average BHCA
intensity
W 0.03 E 1.2
X 0.06 E 2.4
Y 0.10 E 4.8
Z 0.17 E 6.8
PAGE22 Fascicle VI.5 - Rec. Q.543
b) ISDN digital subscriber access 2B + D
The following ISDN models and traffic parameters are provisional and may
be revised in subsequent study periods.
Line type Average traffic Average BHCA per Average packets per second per
intensity per B B channel D channel
channel
Y` 0.05 E 2 0.05 (signalling) + Data
packets a)
Y`` 0.10 E 4 0.1 (signalling) + Data
packets a)
Y``` 0.55 E 2 0.05 (signalling) + Data
packets a)
BHCA Busy hour call attempts.
a) Data packet rates are for further study. These include teleaction and packet
services data.
Even though only limited ISDN traffic data is available, the specification
of the corresponding reference loads remains an important factor in exchange
evaluation. For the case of digital subscriber lines in Table 1/Q.543 b), access
is assumed to utilize the Basic Access with 2B + D channels. The B channels are
available for circuit-switched calls, while the D channel is used to carry
signalling information or may be used to carry teleaction data and packet
switched data. It is assumed that digital lines typically carry traffic
comparable with the heavy-traffic analogue lines designated as case Y in Table
1/Q.543 a). Three cases representing likely ISDN applications are included in the
table.
Case Y` traffic per pair of B channels comparable to 1 Case Y line.
Case Y`` traffic per pair of B channels comparable to 2 Case Y lines.
Case Y``` traffic per pair of B channels comparable 1 Case Y line plus
some very high traffic (e.g., circuit switched data traffic at 1
erlang).
Each of these digital lines also carries the associated ISDN signalling
and data services on the D channel. For the circuit switched calling rates
specified in Table 1/Q.543 b), ISDN signalling is expected to contribute less
than 0.05 packet per second per digital subscriber line. The packet rates for D
channel ISDN data services can be much larger than this; however, these are left
for further study.
2.1.2.2 Reference load B
Reference load B is defined as a traffic increase over reference load A
of:
+25% in erlangs, with +35% in BHCA.
Reference load B levels for D channel activity are for further study.
2.1.3 Impact of supplementary services
If the reference model exchange assumes that significant use is made of
supplementary services, the performance of the exchange can be strongly affected,
especially in exchange designs where processor capacity can become a limiting
item. The performance delays recommended in SS 2.3 and 2.4 can be significantly
lengthened at a given call load under such circumstances. The Administration or
Operating Agency defining the reference model should estimate the fractions of
calls which use various supplementary services so that an average processor
impact relative to a basic telephone call can be calculated (e.g., possibly by a
methodology similar to that of Annex A to this Recommendation).
2.1.4 Exchange capacity
In order to evaluate and compare exchange designs, an Administration will
usually want to know the maximum possible size of the exchange for the intended
implementation. While several factors may limit exchange capacity, processing
capacity will frequently be the limiting factor. The maximum possible number of
lines and circuits served by an exchange, while meeting performance objectives,
will depend on the mix, volumes and types of traffic and the services expected in
the particular implementation.
Two methods of determining exchange processing capacity are provided in
the annexes to this Recommendation:
- Annex A provides an example of methodology for computing processing
capacity of an exchange using information provided by the manufacturer
and estimates of traffic mix and load provided by the Administration.
Fascicle VI.5 - Rec. Q.543 PAGE1
- Annex B provides an example of methodology for estimating the capacity
of an exchange by making projections from measurements made on a
functioning exchange in the laboratory or in the field. The test
exchange must be representative of mix and load of traffic and services
expected at maximum size.
2.1.5 Reference loads on other accesses and interfaces
At this time, other applications, such as n x 64 kbit/s on the Primary
Rate Interface, are left for further study.
2.2 inadequately handled call attempts
2.2.1 Definition
Inadequately handled call attempts are attempts which are blocked (as
defined in the E.600-series of Recommendations) or are excessively delayed within
the exchange. "Excessive delays" are those that are greater than three times the
"0.95 probability of not exceeding" values recommended in the tables in SS 2.3
and 2.4. (See Note.)
For originating and transit calls, this inadequately handled call attempt
parameter applies only when there is at least one appropriate outlet available.
Note - Provisionally, call request delay is not included in this
parameter. Further study is required.
2.2.2 Probability of inadequately handled call attempts occurring
The values in Table 2/Q.543 are recommended.
TABLE 2/Q.543
Type of connection Reference load A Reference load B
Internal 10-2 4 x 10-2
Originating 5 x 10-3 3 x 10-2
Terminating 5 x 10-3 3 x 10-2
Transit 10-3 10-2
2.3 Delay probability - non-ISDN or mixed (ISDN - non-ISDN
environmentt
The non-ISDN environment is composed of analogue subscriber lines and/or
circuits that use either channel associated or common channel signalling.
The ISDN environment is composed of digital (ISDN) subscriber lines and/or
circuits that use common channel signalling.
This section defines delay parameters related to non-ISDN environment and
mixed (ISDN - non-ISDN) environment.
When a delay parameter in this section is also applicable to the pure ISDN
environment, a reference to the appropriate part of S 2.4 (delay probability -
ISDN environment) is provided.
In the following delay parameters, it is understood that delay timing
begins when the signal is "recognizable", that is, after the completion of signal
verification, where applicable. It does not include line-dependent delays for the
recognition of induced voltage conditions or line transients.
The term "mean value" is understood to be the expected value in the
probabilistic sense.
Where several messages are received at the exchange from a digital
subscriber line signalling system (e.g., several alert messages are received from
a multi-user configuration), the message that is accepted for call handling is
the one considered in determining the start of a given delay interval.
Where common channel signalling (including inter-exchange and subscriber
line signalling) is involved, the terms "received from" and "passed to" the
signalling system are used. For CCITT Signalling System No. 7, this is designated
as the instant the information is exchanged between the signalling data link
(layer 1) and the signalling link functions (layer 2). For digital subscriber
line signalling, this is designated as the instant the information is exchanged
by means of primitives between the data link layer (layer 2) and the network
layer (layer 3). Thus, the time intervals exclude the above layer 1 (CCITT
Signalling System No. 7), and layer 2 (D channel) times. They do, however,
include queuing delays that occur in the absence of disturbances but not any
queuing delays that occur in the absence of disturbances but not any queuing
delays caused by re-transmission.
2.3.1 incoming response delay - transit a d terminating incoming
traffic connections
Incoming response delay is a characteristic that is applicable where
channel associated signalling is used. It is defined as the interval from the
instant an incoming circuit seizure signal is recognizable until a
PAGE22 Fascicle VI.5 - Rec. Q.543
proceed-to-send signal is sent backwards by the exchange.
The values in Table 3/Q.543 are recommended.
TABLE 3/Q.543
Reference load A Reference load B
Mean value ú 300 ms ú 400 ms
0.95 probability of not 400 ms 600 ms
exceeding
2.3.2 local exchange call request delay - originating outgoing and
internal traffic connections
2.3.2.1 For ANALOGUE SUBSCRIBER LINES, call request delay is defined as the
interval from the instant when the off-hook condition is recognizable at the
subscriber line interface of the exchange until the exchange begins to apply dial
tone to the line. The call request delay interval is assumed to correspond to the
period at the beginning of a call attempt during which the exchange is unable to
receive any call address information from the subscriber.
The values in Table 4/Q.543 are recommended.
TABLE 4/Q.543
Reference load A Reference load B
Mean value ú 400 ms ú 800 ms
0.95 probability of not 600 ms 1000 ms
exceeding
Note - The above values are understood to apply when a continuous tone, i.e.,
without a cadence, is used and do not include delays caused by functions such as
line tests, which may be used in national networks.
2.3.2.2 For DIGITAL SUBSCRIBER LINES using overlap sending, call request delay
is defined as the interval from the instant at which the SETUP message has been
received from the subscriber signalling system until the SETUP ACKNOWLEDGE
message is pased back to the subscriber signalling system.
Note - In this case this parameter is equivalent to the user signalling
acknowledgement delay (see S 2.4.1).
The values in Table 5/Q.543 are recommended.
TABLE 5/Q.543
Reference load A Reference load B
Mean value ú 400 ms ú 800 ms
0.95 probability of not
Fascicle VI.5 - Rec. Q.543 PAGE1
exceeding 600 ms 1000 ms
2.3.2.3 For DIGITAL SUBSCRIBER LINES using en-bloc sending, call request delay
is defined as the interval from the instant at which the SETUP message is
received from the subscriber signalling system until the call proceeding message
is passed back to the subscriber signalling system.
The values in Table 6/Q.543 are recommended.
TABLE 6/Q.543
Reference load A Reference load B
Mean value ú 600 ms ú 900 ms
0.95 probability of not 800 ms 1200 ms
exceeding
2.3.3 exchange call set-up delay - transit and originating outgoing
traffic connections
Exchange call set-up delay is defined as the interval from the instant
that the information is required for outgoing circuit selection is available for
processing in the exchange, or the signalling information required for call
set-up is received from the signalling system, until the instant when the seizing
signal has been sent to the subsequent exchange or the corresponding signalling
information is passed to the signalling system.
2.3.3.1 Exchange call set-up delay for transit connections
2.3.3.1.1 For transit traffic connections that involve circuits that use channel
associated signalling or a mix of channel associated and common channel
signalling, the values in Table 7/Q.543 are recommended.
TABLE 7/Q.543
Reference load A Reference load B
Mean value ú 250 ms ú 400 ms
0.95 probability of not 300 ms 600 ms
exceeding
2.3.3.1.2 For transit traffic connections between circuits that use CCITT
Signalling System No. 7 signalling exclusively, the requirements of the
appropriate signalling system Recommendation should apply, e.g. CCITT
Recommendations Q.725 and Q.766 for Tcu value (case of a processing intensive
message).
2.3.3.2 Exchange call set-up delay for originating outgoing traffic connections
2.3.3.2.1 For outgoing traffic connections originating from ANALOGUE SUBSCRIBER
LINES, the values in Table 8/Q.543 are recommended.
TABLE 8/Q.543
Reference load A Reference load B
PAGE22 Fascicle VI.5 - Rec. Q.543
Mean value ú 300 ms ú 500 ms
0.95 probability of not 400 ms 800 ms
exceeding
2.3.3.2.2 For outgoing traffic connections originating from DIGITAL SUBSCRIBER
LINES using overlap sending, the time interval starts when the INFORMATION
message received contains a "sending complete indication" or when the address
information necessary for call set-up is complete.
The values in Table 9/Q.543 are recommended.
TABLE 9/Q.543
Reference load A Reference load B
Mean value ú 400 ms ú 600 ms
0.95 probability of not 600 ms 1000 ms
exceeding
2.3.3.2.3 For outgoing traffic connections originating from DIGITAL SUBSCRIBER
LINES using en-bloc sending, the time interval starts when the SETUP message has
been received from the digital subscriber signalling system.
The values in Table 10/Q.543 are recommended.
TABLE 10/Q.543
Reference load A Reference load B
Mean value ú 600 ms ú 800 ms
0.95 probability of not 800 ms 1200 ms
exceeding
2.3.4 through-connection delay
Through-connection delay is defined as the interval from the instant at
which the information required for setting up a through-connection is available
for processing in an exchange, or the signalling information required for setting
up a through-connection is received from the signalling system, to the instant at
which the appropriate transmission path is available for carrying traffic between
the incoming and outgoing exchange terminations.
The exchange through-connection delay does not include an inter-office
continuity check, if provided, but does include a cross-office check if one
occurs during the defined interval.
When the through-connection is established during call set-up, the
recommended values for exchange call set-up delay apply. When the
through-connection in an exchange is not established during the exchange call
set-up interval, the through-connection delay may then contribute to the network
call set-up delay.
2.3.4.1 For transit and originating outgoing traffic connections
The values in Table 11/Q.543 are recommended.
TABLE 11/Q.543
Fascicle VI.5 - Rec. Q.543 PAGE1
Reference load A Reference load B
Without With Without With
ancillary ancillary ancillary ancillary
equipment equipment equipment equipment
Mean value ú 250 ms ú 350 ms ú 400 ms ú 500 ms
0.95% probability of not 300 ms 500 ms 600 ms 800 ms
exceeding
The requirements for multi-slot connections require further study.
2.3.4.2 For internal and terminating traffic connections
For connections terminating on ANALOGUE SUBSCRIBER LINES, the
through-connection delay is the interval from the instant at which the called
subscriber off-hook condition (answer) is recognizable at the subscriber line
interface of the exchange until the through-connection is established and
available for the carrying traffic or a consequent signal is sent backwards by
the exchange.
The maximum values applying to this parameter are included with those for
incoming call indication sending delay in S 2.3.5.
For connections terminating on DIGITAL SUBSCRIBER LINES, the
through-connection delay is the interval from the instant at which the CONNECT
message is received from the signalling system until the through-connection is
established and available for carrying traffic as those indicated by passing to
the respective signalling systems of the ANSWER and CONNECT ACKNOWLEDGE messages.
The values in Table 12/Q.543 are recommended.
TABLE 12/Q.543
Reference load A Reference load B
Mean value ú 250 ms ú 400 ms
0.95 probability of not 300 ms 600 ms
exceeding
2.3.5 incoming call indication sending delay - (for terminating and
internal traffic connections)
2.3.5.1 For calls terminating on ANALOGUE SUBSCRIBER LINES, the incoming call
indication sending delay is defined as the interval from the instant when the
last digit of the called number is available for processing in the exchange until
the instant that ringing signal is applied by the exchange to the called
subscriber line.
It is recommended that the sum of the values for ringing signal sending
delay and through-connection delay for internal and teminating traffic connection
should not exceed the values in Table 13/Q.543. In addition, it is recommended
that the value of the incoming call indication sending delay should not exceed
90% of these values nor the though-connection delay exceed 35% of these values.
TABLE 13/Q.543
PAGE22 Fascicle VI.5 - Rec. Q.543
Reference load A Reference load B
Mean value ú 650 ms ú 1000 ms
0.95 probability of not 900 ms 1600 ms
exceeding
Note - The above values assume that "immediate" ringing is applied and do not
include delays caused by functions such as line tests, which may be used in
national networks.
2.3.5.2 For calls terminating on DIGITAL SUBSCRIBER LINES, the incoming call
indication sending delay is defined as the interval from the instant at which the
necessary signalling information is received from the signalling system to the
instant at which the SETUP message is passed to the signalling system of the
called digital subscriber line.
In the case of overlap sending in the incoming signalling system, the
values in Table 14/Q.543 are recommended.
TABLE 14/Q.543
Reference load A Reference load B
Mean value ú 400 ms ú 600 ms
0.95 probability of not 600 ms 1000 ms
exceeding
In the case of en-bloc sending in the incoming signalling system, the
values in Table 15/Q.543 are recommended.
TABLE 15/Q.543
Reference load A Reference load B
Mean value ú 600 ms ú 800 ms
0.95 probability of not 800 ms 1200 ms
exceeding
2.3.6 Alerting sending delay - terminating and internal traffic
connections
2.3.6.1 alerting sending delay for terminating traffic
Fascicle VI.5 - Rec. Q.543 PAGE1
2.3.6.1.1 For calls terminating on ANALOGUE SUBSCRIBER LINES, alerting sending
delay is defined as the interval from the instant when the last digit is
available for processing in the exchange until the ringing tone is sent backwards
toward the calling user.
The values in Table 13/Q.543 are recommended.
2.3.6.1.2 For calls termining on DIGITAL SUBSCRIBER LINES, the alerting sending
delay is defined as the interval from the instant that an ALERTING message is
received from the digital subscriber line signalling system to the instant at
which an ADDRESS COMPLETE message is passed to the interexchange signalling
system or ringing tone is sent backward toward the calling user.
The values in Table 16/Q.543 are recommended.
TABLE 16/Q.543
Reference load A Reference load B
Mean value ú 200 ms ú 350 ms
0.95 probability of not 400 ms 700 ms
exceeding
2.3.6.2 alerting sending delay for internal traffic
2.3.6.2.1 For calls terminating on ANALOGUE SUBSCRIBER LINES, alerting sending
delay is defined as the interval from the instant that the signalling information
is available for processing in the exchange until ringing tone is applied to an
ANALOGUE calling subscriber line or an ALERTING message is sent to a DIGITAL
calling subscriber line signalling system.
For calls from ANALOGUE SUBSCRIBER LINES to ANALOGUE SUBSCRIBER LINES, the
values in Table 13/Q.543 are recommended.
For calls from DIGITAL SUBSCRIBER LINES to ANALOGUE SUBSCRIBER LINES, the
values in Table 17/Q.543 are recommended.
TABLE 17/Q.543
Reference load A Reference load B
Mean value ú 300 ms ú 500 ms
0.95 probability of not 500 ms 800 ms
exceeding
2.3.6.2.2 For internal calls terminating on DIGITAL SUBSCRIBER LINES originating
from ANALOGUE SUBSCRIBER LINES, alerting sending delay is defined as the interval
from the instant that an alerting message is received from the signalling system
of the called subscriber's line until ringing tone is applied to the calling
subscriber line.
The values in Table 13/Q.543 are recommended.
Alerting sending delay on internal calls between DIGITAL SUBSCRIBER LINES
are covered by Table 28/Q.543.
2.3.7 ringing tripping delay - intern l and terminating traffic
connections
Ringing tripping delay is a characteristic that is applicable for calls
terminating on ANALOGUE SUBSCRIBER LINES only. It is defined as the interval from
the instant that the called subscriber off-hook condition is reconizable at the
subscriber line interface until the ringing signal at the same interface is
suppressed.
The values in Table 18/Q.543 are recommended.
TABLE 18/Q.543
Reference load A Reference load B
Mean value ú 100 ms ú 150 ms
0.95 probability of not
PAGE22 Fascicle VI.5 - Rec. Q.543
exceeding 150 ms 200 ms
2.3.8 exchange call release delay
Exchange call release delay is the interval from the instant at which the
last information required for releasing a connection is available for processing
in the exchange to the instant that the switching network through-connection in
the exchange is no longer available for carrying traffic and the disconnection
signal is sent to the subsequent exchange, if applicable. This interval does not
include the time taken to detect the release signal, which might become
significant during certain failure conditions, e.g., transmission system
failures.
2.3.8.1 For transit traffic connections involving circuits using channel
associated signalling or a mix of channel associated and common channel
signalling, the values in Table 19/Q.543 are recommended.
TABLE 19/Q.543
Reference load A Reference load B
Mean value ú 250 ms ú 400 ms
0.95 probability of not 300 ms 700 ms
exceeding
For transit traffic connections involving circuits using CCITT Signalling
System No. 7 signalling exclusively, the values in Table 35/Q.543 are
recommended.
2.3.8.7 For originating, terminating and internal traffic connections, the
values in Table 20/Q.543 are recommended.
TABLE 20/Q.543
Reference load A Reference load B
Mean value ú 250 ms ú 400 ms
0.95 probability of not 300 ms 700 ms
exceeding
2.3.9 exchange signalling transfer delay - other than answer signal
Exchange signalling transfer delay is the time taken by the exchange to
transfer a signal, no other exchange action being required. It is defined as the
interval from the instant that the incoming signal is recognizable, or the
signalling information is received from the signalling system, until the instant
when the corresponding outgoing signal has been transmitted, or the appropriate
signalling information is passed to the signalling system.
2.3.9.1 For transit traffic connections involving circuits using channel
associated signalling or a mix of channel associated and common channel
signalling, the values in Table 21/Q.543 are recommended.
TABLE 21/Q.543
Reference load A Reference load B
Mean value
Fascicle VI.5 - Rec. Q.543 PAGE1
ú 100 ms ú 150 ms
0.95 probability of not 150 ms 300 ms
exceeding
For transit traffic connections between circuits that use CCITT Signalling
System No. 7 signalling exclusively, the requirements of the appropriate
signalling system Recommendations should apply, e.g., CCITT Recommendations
Q.725/Q.726 for Tcu value (case of a simple message).
2.3.9.2 Exchange signalling transfer delay for originating, terminating and
internal traffic involving a mix of ANALOGUE and DIGITAL SUBSCRIBER LINES is left
for further study. Exchange signal transfer delay between DIGITAL SUBSCRIBER
signalling systems or between DIGITAL SUBSCRIBER LINE signalling systems and
CCITT Signalling System No. 7 is covered in S 2.4.2.
2.3.10 answer sending delay
Answer sending delay is defined as the interval from the instant that the
answer indication is received at the exchange to the instant that the answer
indication is passed on by the exchange toward the calling user. The objective of
this parameter is to minimize the possible interruption of the transmission path
for any significant interval during the initial response by the called user.
2.3.10.1 For transit traffic involving circuits that use channel associated
signalling or a mix of channel associated and common channel signalling, the
values in Table 22/Q.543 are recommended.
TABLE 22/Q.543
Reference load A Reference load B
Mean value ú 100 ms ú 150 ms
0.95 probability of not 150 ms 300 ms
exceeding
More stringent values are recommended where in-band line signalling may be
encountered in the national part of a built-up connection. The recommended values
are given in Table 23/Q.543.
TABLE 23/Q.543
Reference load A Reference load B
Mean value ú 50 ms ú 90 ms
0.95 probability of not 100 ms 180 ms
exceeding
For transit traffic connections involving circuits that use CCITT
Signalling System No. 7 exclusively, the requirements of the appropriate
signalling system Recommendations should apply, e.g., CCITT Recommendations Q.725
and Q.766 for Tcu value (case of a simple message).
2.3.10.2 For connections in a terminating exchange, exchange answer sending
delay is defined as the interval from the instant that the off-hook condition is
recognizable at the ANALOGUE SUBSCRIBER LINE interface on an incoming call or a
CONNECT message is received from a DIGITAL SUBSCRIBER LINE signalling system
until the instant that an answer indication is sent back toward the calling user.
The values in Table 24/Q.543 are recommended.
TABLE 24/Q.543
Reference load A Reference load B
PAGE22 Fascicle VI.5 - Rec. Q.543
Mean value ú 250 ms ú 350 ms
0.95 probability of not 300 ms 700 ms
exceeding
2.3.10.3 For connections in an originating exchange, exchange answer sending
delay is defined as the interval from the instant that the answer indication is
received from the outgoing circuit signalling system or in the case of an
internal call, from the called subscriber's line, until the instant that the
answer indication is sent to the calling user. In the case of a call originated
from a DIGITAL SUBSCRIBER LINE, the answer indication is a CONNECT message that
is sent to the DIGITAL SUBSCRIBER LINE signalling system. If an ANALOGUE
SUBSCRIBER LINE originated the call, the answer indication may not be sent.
The values in Table 25/Q.543 are recommended.
TABLE 25/Q.543
Reference load A Reference load B
Mean value ú 250 ms ú 400 ms
0.95 probability of not 300 ms 700 ms
exceeding
For ISDN operation involving DIGITAL SUBSCRIBER LINES and CCITT Signalling
System No. 7 exclusively, the values in Table 28/Q.543 are recommended.
2.3.11 timing for start of charging (circuit switched calls)
When required, timing for charging at the exchange where this function is
performed, shall begin after receipt of an ANSWER indication from a connecting
exchange or the called user. The start of timing for charging should occur within
the intervals recommended in Table 26/Q.543.
TABLE 26/Q.543
Reference load A Reference load B
Mean value ú 100 ms ú 175 ms
0.95 probability of not 200 ms 350 ms
exceeding
2.4 Delay probability - ISDN environment
The following notes apply to the delay parameters included in this
section:
1) The term "mean value" is understood as the expected value in the
probabilistic sense.
2) Where several messages are received at the exchange from a digital
Fascicle VI.5 - Rec. Q.543 PAGE1
subscriber line signalling system (e.g. several alert messages are
received from a multi-user configuration), the message that is accepted
for call handling is the one considered in determining the start of a
given delay interval.
3) The terms "received from" and "passed to" the signalling system are
used. For CCITT Signalling System No. 7 this is designated as the
instant the information is exchanged between the signalling data link
(layer 1) and the signalling link functions (layer 2). For digital
subscriber line signalling, this is designated as the instant the
information is exchanged by means of primitives between the data link
layer (layer 2) and the network layer (layer 3). Thus, the time
intervals exclude the above layer 1 (CCITT Signalling System No. 7) and
layer 2 (D channel) times. They do, however, include queuing delays
that occur in the absence of disturbances but not any queuing delays
caused be re-transmission.
2.4.1 user signalling acknowledgement delay
User signalling acknowledgement delay is the interval from the instant a
user signalling message has been received from the subscriber line signalling
system until a message acknowledging the receipt of that message is passed back
from the exchange to the user line signalling system. Examples of such messages
are SETUP ACKNOWLEDGEMENT TO SETUP, CONNECT ACKNOWLEDGEMENT to CONNECT and
RELEASE ACKNOWLEDGEMENT to RELEASE.
The values in Table 27/Q.543 are recommended.
TABLE 27/Q.543
Reference load A Reference load B
Mean value ú 400 ms ú 800 ms
0.95 probability of not 600 ms 1000 ms
exceeding
2.4.2 signalling transfer delay
The exchange signalling transfer delay is the time taken for the exchange
to transfer a message from one signalling system to another with minimal or no
other exchange actions required. The interval is measured from the instant that a
message is received from a signalling system until the moment the corresponding
message is passed to another signalling system. Examples of messages are ALERT to
ADDRESS COMPLETE, ADDRESS COMPLETE to ADDRESS COMPLETE, CONNECT to ANSWER,
RELEASE to DISCONNECT, etc.
The values in Table 28/Q.543 are recommended for originating and
terminating connections.
TABLE 28/Q.543
Reference load A Reference load B
Mean value ú 200 ms ú 350 ms
0.95 probability of not 400 ms 700 ms
exceeding
For transit connections, the requirements of the appropriate signalling
system Recommendation should apply, e.g. CCITT Recommendations Q.725 and Q.766
for Tcu value (case of a simple message).
Note - User-to-user signalling may imply additional functions in the
exchanges, e.g. charging, flow control, etc. The requirements for user-to-user
signalling transfer delay and the impact of user-to-user signalling on exchange
performance is for further study.
2.4.3 call set up delay
Call set up delay is defined as the interval from the instant when the
signalling information required for outgoing circuit selection is received from
the incoming signalling system until the instant when the corresponding
signalling information is passed to the outgoing signalling system.
2.4.3.1 For originating 64 kbit/s circuit switched connections (types I, II and
III option a).
i) If overlap sending is used, the interval starts when the information
message received contains a "sending complete" indication or the
address information for call set up is complete.
ii) If en-bloc sending is used, the time interval starts when the SETUP
message has been received from the user signalling system.
For call attempts using overlap sending, the values in Table 29/Q.543 are
recommended.
TABLE 29/Q.543
PAGE22 Fascicle VI.5 - Rec. Q.543
Reference load a Reference load B
Mean value ú 400 ms ú 600 ms
0.95 probability of not 600 ms 1000 ms
exceeding
For call attempts using en-bloc sending, the values in Table 30/Q.543 are
recommended.
TABLE 30/Q.543
Reference load A Reference load B
Mean value ú 600 ms ú 800 ms
0.95 probability of not 800 ms 1200 ms
exceeding
2.4.3.2 For originating supplementary service call attempts:
for further study.
2.4.3.3 For transit 64 kbit/s circuit switched connections between circuits
that use CCITT Signalling System No. 7, the requirements of CCITT Recommendations
Q.725 and Q.766 should apply for Tcu value (case of a processing intensive
message).
2.4.4 through connection delay
2.4.4.1 For originating outgoing and transit traffic 64 kbit/s switched circuit
connections, through connection delay is defined as the interval from the instant
that the signalling information required for setting up a connection through the
exchange is received from the incoming signalling system to the instant that the
transmission path is available for carrying traffic between the incoming and
outgoing terminations on the exchange.
Usually, both directions of transmission will be switched through at the
same time. However, at an originating exchange, on certain calls, there may be a
requirement to effect switch through in two stages, one direction at a time. In
this case, different signalling messages will initiate the two stages of switch
through and the recommended delay applies to each stage of switch through.
The values in Table 31/Q.543 are recommended.
TABLE 31/Q.543
Reference load A Reference load B
Without With Without With
ancillary ancillary ancillary ancillary
function function function function
Mean value ú 250 ms ú 350 ms
Fascicle VI.5 - Rec. Q.543 PAGE1
ú 400 ms ú 500 ms
0.95 probability of not 300 ms 500 ms 600 ms 800 ms
exceeding
2.4.4.2 For internal and terminating traffic 64 kbit/s switched circuit
connections the through connection delay is defined as the interval from the
instant that the CONNECT message is received from the called line signalling
system until the through connection is established and available for carrying
traffic and the ANSWER and CONNECT ACKNOWLEDGEMENT messages have been passed to
the appropriate signalling systems.
The values in Table 32/Q.543 are recommended.
TABLE 32/Q.543
Reference load A Reference load B
Mean value ú 250 ms ú 400 ms
0.95 probability of not 300 ms 600 ms
exceeding
traf
traffic connections)
The incoming call indication sending delay is defined as the interval from
the instant at which the necessary signalling information is received from the
signalling system to the instant at which the SETUP message is passed to the
signalling system of the called subscriber line.
In the case of overlap sending in the incoming signalling system, the
values in Table 33/Q.543 are recommended.
TABLE 33/Q.543
Reference load A Reference load B
Mean value ú 400 ms ú 600 ms
0.95 probability of not 600 ms 1000 ms
exceeding
In the case of en-bloc sending in the incoming signalling system, the
values in Table 34/Q.543 are recommended.
TABLE 34/Q.543
PAGE22 Fascicle VI.5 - Rec. Q.543
Reference load A Reference load B
Mean value ú 600 ms ú 800 ms
0.95 probability of not 800 ms 1200 ms
exceeding
2.4.6 connection release delay
Connection release delay is defined as the interval from the instant when
DISCONNECT or RELEASE message is received from a signalling system until the
instant when the connection is no longer available for use on the call (and is
available for use on another call) and a corresponding RELEASE or DISCONNECT
message is passed to the other signalling system involved in the connection.
The values in Table 35/Q.543 are recommended.
TABLE 35/Q.543
Reference load A Reference load B
Mean value ú 250 ms ú 400 ms
0.95 probability of not 300 ms 700 ms
exceeding
2.4.7 Call clearing delay
Disconnect and call clearing will usually be performed at the same time.
However, on certain calls it may be necessary for an exchange to retain call
references after disconnect has occurred, until a clearing message is received.
The exchange may then discard the call reference information. The corresponding
RELEASE message must be passed on to other involved signalling systems in the
interval allowed for signalling transfer delay (see S 2.4.2).
2.4.8 Timing for start of charging (circuit switched calls)
When required, timing for charging at the exchange where this function is
performed, shall begin after receipt of an ANSWER indication from a connecting
exchange or the called user. The start of timing for charging should occur within
the intervals recommended in Table 36/Q.543.
TABLE 36/Q.543
Reference load A Reference load B
Mean value ú 100 ms ú 175 ms
0.95 probability of not 200 ms 350 ms
exceeding
Fascicle VI.5 - Rec. Q.543 PAGE1
2.5 Call processing performance objectives
2.5.1 64 kbit/s switched connections
2.5.1.1 Premature release
The probability that an exchange malfunction will result in the premature
release of an established connection in any one minute interval should be:
P ú 2 x 10-5
2.5.1.2 Release failure
The probability that an exchange malfunction will prevent the required
release of a connection should be:
P ú 2 x 10-5
2.5.1.3 Incorrect charging or accounting
The probability of a call attempt receiving incorrect charging or
accounting treatment due to an exchange malfunction should be:
P ú 10-4
PAGE22 Fascicle VI.5 - Rec. Q.543
2.5.1.4 Misrouting
The probability of a call attempt misrouted following receipt by the
exchange of a valid address should be:
P ú 10-4
2.5.1.5 No tone
The probability of a call attempt encountering no tone following receipt
of a valid address by the exchange should be:
P ú 10-4
2.5.1.6 Other failures
The probability of the exchange causing a call failure for any other
reason not identified specifically above should be:
P ú 10-4
2.5.2 64 kbit/s semi-permanent connections
This requires further study taking into consideration:
- need to recognize an interruption;
- probability of an interruption;
- requirements for re-establishment of interrupted connection;
- any other unique requirements.
2.5.3 n x 64 kbit/s switched connections
To be recommended if/when specific services are defined.
2.5.4 n x 64 kbit/s semi-permanent connections
To be recommended if/when specific services are defined.
2.6 Transmission performance
2.6.1 64 kbit/s switched connections
The probability of a connection being established with an unacceptable
transmission quality across the exchange should be:
P(Unacceptable transmission) ú 10-5
The transmission quality across the exchange is said to be unacceptable
when the bit error ratio is above the alarm condition.
Note - The alarm condition has yet to be defined.
2.6.2 64 kbit/s semi-permanent connections
To be recommended.
2.6.3 n x 64 kbit/s switched connections
To be recommended, if/when specific services are defined.
2.6.4 n x 64 kbit/s semi-permanent connections
To be recommended if/when specific services are defined.
2.7 Slip rate
2.7.1 Normal conditions
The slip rate under normal conditions is covered in Recommendation Q.541.
2.7.2 Temporary loss of timing control
The case of temporary loss of timing control corresponds to the "holdover
operation" defined and recommended in Recommendation G.812. The allowable slip
rate will correspond to the maximum relative TIE also recommended therein.
2.7.3 Abnormal conditions at the exchange input
The slip rate in case of abnormal conditions (wide phase diviations, etc.)
at the exchange input is the subject of further study taking into account the
requirements of Recommendation G.823.
3 Exchange performance during overload conditions
This section applies to digital exchanges operating during periods when
the number of call attempts presented to the exchange exceeds its call processing
capacity for a significant period of time, excluding momentary peaks. Under these
conditions the exchange is said to be operating in an overload condition.
This Recommendation identifies requirements for exchange performance
during overload and for overload mechanisms in the exchange. Network management
functions to be supported by an exchange are defined in Recommendation Q.542, S
5.
3.1 Explanation of terms used in definition of overload parameters
- load: the total number of call attempts presented to an exchange during
a given interval of time (i.e. offered load)
- overload: that part of the total load offered to an exchange, in excess
of the engineered traffic processing capacity of the exchange. Overload
is usually expressed as a percentage of engineered capacity.
- throughput: the number of call attempts processed successfully by an
exchange per unit time.
- engineered capacity: the mean offered load at which the exchange just
Fascicle VI.5 - Rec. Q.543 PAGE1
meets all grade of service requirements used by the Administration to
engineer the exchange.
3.2 Call processing performance during overload
An exchange must continue to process a specified load even when the
offered call attempts exceed its available call processing capacity. The number
of call attempts handled during an overload condition should not be significantly
lower than the engineered capacity of the exchange for a specified Grade Of
Service (GOS), as noted in S 3.7.
Two basic requirements for exchange performance during overload are:
- to maintain adequate exchange throughput in sustained overload
- to react sufficiently quickly to load peaks and the sudden onset of
overload.
As the offered load increases beyond the engineered attempt capacity of
the exchange, the throughput or the carried attempt load may exhibit a behaviour
shown by curve A in Figure 1/Q.543, i.e. processor throughput may be reduced
drastically if the offered load increases well beyond the engineered load. Curve
B in Figure 1/Q.543 represents the maximum throughput, where the throughput
remains at the nominal design level under overload. Appropriate overload
protection mechanisms should be included in the overall exchange design so that
the throughput performance of the processor under overload resembles the curve C
in Figure 1/Q.543.
Figure 1/Q.543 - T1106950-87
3.3 Engineered exchange capacity
Exchange engineered capacity is the maximum load that the exchange can
handle while operating in a "normal" mode (i.e. performing all required operating
and administrative functions) while meeting performance requirements specified in
S 2 or those specified by the Administration. It is not necessarily the point of
maximum throughput (see Figure 1/Q.543).
Overload controls, when applied, may have a significant effect on exchange
capacity. Overload throughput performance should be specified relative to the
engineered capacity of the exchange when overload controls are operating.
3.4 Overload control strategy
An effective overload control strategy will prevent the rapid decrease in
processed call attempts with increasing overload (see Curve A in Figure 1/Q.543);
the relatively gradual decrease with overload controls enabled (Curve C in Figure
1/Q.543) is due to the increasing processing overhead in exercising the overload
controls.
Overload is defined as the level of call attempts offered to the exchange
in excess of the exchange engineered capacity. For example, when the exchange is
offered call attempts at a rate of 10% greater than the engineered capacity, the
exchange is said to have 10% overload.
The exchange throughput at an overload of Y% above the engineered capacity
load should be at least X% of the throughput at engineered capacity. This concept
is shown in Figure 2/Q.543 which shows the region of unacceptable throughput
performance. Any throughput curve which remains above the X% level until reaching
the point of Y% overload is acceptable. The recommended values are Y = 50% and X
= 90%. Beyond Y% overload the exchange should continue to process calls in an
acceptable manner.
As long as the level of overload does not exceed Y% above the exchange
engineered capacity, then the exchange throughput should be no less than X% of
engineered capacity, as depicted in Figure 2/X.543.
Measurements that can provide data as the basis for calculation of X and
Y, are identified in S 3.8.
Figure 2/Q.543 - T1106960-87
3.5 Detection of overload
The exchange should incorporate suitable means for detecting overload
conditions.
The onset of an overload state should be recognized by the exchange
processing logic which in turn will invoke strategies to avoid a severe
degradation in throughput load. During overload, both severe delays and
processing delays will increase and will normally exceed the performance
objectives given for Reference load B.
Overload indications may, for example, be provided by: a continuous
PAGE22 Fascicle VI.5 - Rec. Q.543
measurement of the occupancy of the resources used for call handling over short
periods (e.g. a few seconds); monitoring the queue lengths for the various call
handling processes, etc. Overload control activation indications should be given
to the administration staff.
3.6 Overload protection
The internal overload control methods used in an exchange are dependant on
the particular technical arrangement of the switching system, and are not subject
to CCITT Recommendations. Overload controls used in conjunction with adjacent
exchanges are discussed under "Network management design objectives" in
Recommendation Q.542, S 5.
In order to reduce the load on the exchange caused by calls that cannot be
processed during overload, it may be necessary to discourage further attempts by
customers during this situation. Methods used to achieve this reduction should
not significantly increase the load on exchange processors, as for example,
routing calls to recorded announcements.
Overload controls, once applied, should be removed as quickly as possible
when the degree of overload reduces, consistent with the need to avoid
oscillatory behaviour which might prolong the period of degraded service.
As a guideline to providing service during overload conditions, the
following general principles are applicable:
- give preference to the processing of terminating calls,
- give preference to priority class lines, calls to priority destinations
based on digit analysis and incoming calls with priority indications
in, for example, the Initial Address Message of a call using CCITT
Signalling System No. 7, if an essential service protection capability
has been invoked,
Fascicle VI.5 - Rec. Q.543 PAGE1
- defer some or all activities non-essential to handling offered traffic;
examples are some administration and maintenance processes in the
exchange. (Nevertheless the man-machine communications essential for
priority operational tasks should always be preserved. In particular,
network management terminals and functions associated with interfaces
to network management support systems should be afforded high priority,
since network management actions can play an important role in reducing
exchange overloads),
- maintain normal charging and supervisory functions, and established
connections until the receipt of the appropriate release signal,
- assign priorities to specific exchange measurements, such that low
priority measurements cease at a predetermined level of congestion.
Higher priority measurements may be ceased at a higher level of
congestion, or may be run continuously, depending on their importance
to the call handling functions,
- give preference to calls already being processed, before accepting new
calls.
3.7 Grade of service during overload
In general the overall grade of service seen by the subscribers will
deteriorate when the exchange experiences severe overload conditions and the
overload protection mechanisms have been invoked. This may be due to the fact
that the overload protection procedures may require that the exchange not accept
all the call attempts offered.
Accepted calls may or may not receive a grade of service equal to that
received by calls at Reference load B of S 2. In terms of the exchange overload
performance, it is sufficient that calls be accepted in such a way that
throughput is maximized.
3.8 Performance monitoring during overload control activation
The operational measurements in the exchange should be sufficient to
determine the number of call attempts accepted by the exchange, and the number
that are successfully being completed, from the exchange point-of-view. Separate
measurements should be available to count the number of attempts rejected by the
exchange during overload, so that the total load can be estimated.
An accepted call attempt is defined to be a call attempt which is accepted
for processing by the exchange. This does not necessarily mean that an accepted
call attempt will complete or receive an acceptable grade of service.
The call completion rate can vary statistically with time, according to
the specific call attempt acceptance process invoked by the overload controls.
Therefore the call completion rate estimated from the operational measurements
needs to be taken over a sufficiently long period of time to verify conformance
to the X% throughput requirement.
PAGE22 Fascicle VI.5 - Rec. Q.543
