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All drawings appearing in this Recommendation have been done in Autocad.
Recommendation E.8451)
CONNECTION ACCESSIBILITY OBJECTIVE FOR THE
INTERNATIONAL TELEPHONE SERVICE2)
Introduction
This Recommendation is one of a set of closely related Recommendations,
comprising Recommendations E.810, E.830, E.845, E.850 and E.855 concerned with
the accessibility, retainability and integrity of telephone services.
Preamble
This Recommendation provides an overall end-to-end connection
accessibility (availability) objective for international switched telephone
service.
Connection accessi is a component of
service accessibility as defined in
Recommendation E.800.
This Recommendation contains a measure of connection
accessibility, an objective, and an allocation of the objective to
the national systems and international chain of international
connections. The Recommendation also relates the overall end-to-end
performance to the reliability and availability of circuits and
exchanges in a way useful for network design purposes.
The objective includes the effects of equipment faults and
traffic congestion.
The CCITT,
considering
(a) that connection accessibility is defined in Recommendation E.800;
(b) that customers rank connection inaccessibility as one of the most
annoying of call set-up impairments;
(c) that an objective for connection accessibility which takes into account
customer opinion about the call set-up phase is consistent with other
Recommendations which have recommended an objective for service retainability based, in part, on customer opinion;
(d) that connection accessibility will not be constant over
time, even for a particular calling and called line pair. One
suitable measure is a long-term average network connection failure
probability. (Other suitable measures may also be required.);
(e) that the overall objective for connection accessibility
should be allocable to the national systems and the international
chain of the international connection;
(f) that the objective should take into account the concerns
of network planners and system designers, provide useful guidance
to both and may be used by Administrations in providing a method
for verifying whether or not network performance is acceptable;
(g) that the overall connection accessibility should be
controlled by the accessibility performances of individual
exchanges and circuits, and that to obtain this control, the
overall connection accessibility must be mathematically linked to
the equipment availability and reliability,
1) Formerly G.180, in Red Book, Fascicle III.1.
2) Some of the terms in this Recommendation, for example, the noun "measure", are used in
the sense of their definition given in Recommendation E.800.
Fascicle II.3 - Rec. E.845 PAGE1
recommends
1 Measure of connection accessibility
Connection accessibility shall be measured using the long-term average
network con failure probability, which is
the complement of the connection access probability as defined in Recommendation E.800.
The network connection failure probability PNCF can be
estimated by using the following formula:
PNCF = eq \f( QN,N)
where QN is the number of unsuccessful connection access attempts and N is the
total number of connection access attempts in some time period (to be
determined).
A method for estimating the required call sample size is contained in
Annex A.
For purposes of network design, the network connection failure
probability, PNCF, can also be calculated using the method outlined in Annex B.
Annex C describes how the busy and non-busy hours affect the network connection
failure (NCF).
Note 1 - Those unsuccessful connection access attempts reflecting failure
of the network to work properly, from the user's perspective, are called network
connect failure. They are call failures an
astute caller can determine and are caused by network faults and
congestion. A network connection failure is any valid bid for
service which receives one of the following network responses:
1) dial tone returned after dialling is completed;
2) no ring and no answer;
3) all circuits busy signal or announcement;
4) connection to the wrong number (misrouting);
5) double connection.
This list may not be exhaustive.
Note 2 - This definition of network connection failure is based on the response the caller can hear.
Note 3 - There are two generic causes of network connection
failures: equipment faults and traffic congestion.
Note 4 - The averaging interval (to be determined) used for
estimating the connection failure probability shall include normal
and peak hour traffic periods. In the event of exceptionally high
traffic demand (public holiday, natural disaster, etc.) failure
rates higher than the objective may be tolerated.
Note 5 - The network connection failure probability should be
estimated by Administrations in a manner consistent with obtaining,
from the Administration's point of view, reasonably accurate
estimates.
2 Objective for connection accessibility
Connection accessibility is acceptable if the long-term average connection
failure probability, expressed as a percentage, does not exceed a value (overall
average for all international calls) of A% to B% (values to be determined).
Additionally, the long-term average failure probability at any single
international homing exchange should never exceed C% (value to be determined).
Note - Possible values for A, B and C are in the range of 10% to 20%.
3 Allocation of the overall objective to the national systems
and international chain
The network connection failure probability objective shall be apportioned
as follows:
X% to the originating national system,
Y% to the international chain,
Z% to the terminating national system,
where X + Y + Z = P, and P is the overall objective stated in S 2.
Note 1 - The connection access attempt may fail in the national systems or
the international chain of the connection.
Note 2 - The objective takes into account all means of "defense" of the
network against failure to complete the connection, including alternate routing,
if used.
Note 3 - The network connection failure probability of the national
systems or international chain is defined as the probability that the call access
attempt will fail because of some problem (equipment fault or congestion) in the
systems or chain.
PAGE6 Fascicle II.3 - Rec. E.845
Note 4 - Values for X, Y and Z are in the range of 3% to 7%.
ANNEX A
(to Recommendation E.845)
Method for selecting the required call sample size, N
The network connection failure probability shall be estimated by
Administrations in a manner consistent with obtaining reasonably accurate
estimates.
The number of call access attempts sampled shall be sufficiently large to
obtain a good estimate of the probability.
A method of picking a sample size N could be used which could produce a
maximum error of measurement, e, (to be determined) with confidence level, a (to
be determined).
Recommendation E.850 contains a method for estimating the sample size
required to estimate cutoff call probability. This method should be studied for
application here.
ANNEX B
(to Recommendation E.845)
Method for relating overall network connection failure
probability to the reliability and availability
performance of exchanges and circuits
The following equation gives the relationship between the overall network
connection failure probability, PNCF, and the probabilities of connection failure
in the national systems and international chain of the connection:
PNCF = 1 - (1 -POE)(1 - PI)(1 - PTE)
where POE is the probability that the access attempt fails in the originating
national system, PI is the probability of failure in the international chain and
PTE is the probability of failure in the terminating national system.
Hypothetical reference connections for the three parts of an international
connection are shown in Figure B-1/E.845. The proportion of calls (Fn) which are
routed over the parts are also given in the figure. The values are taken from
Table 1/G.101.
The probability that a connection access attempt fails in either of the
parts is given by the following equations:
eq POE = 1 - \i\su(n=1,5, )Fn (1 - Pc)n(1 - Ps)n
eq PI = 1 - \i\su(n=1,2, )Fn (1 - P\s(`,c))n(1 - P\s(`,n))n+1
eq PTE = 1 - \i\su(n=1,5, )Fn (1 - P\s(",c))n(1 - P\s(",s)n
where n is the number of circuits in a selected part. Fn is the call frequency
for an n-circuit system or chain (from Figure B-1/E.845).
Figure B-1/E.845 - CCITT 85790
Pc, P`c and P"c are the probabilities that the connection access fails in
the originating system, international chain or terminating system circuits,
respectively. (It is assumed here for simplicity that all circuits in a system or
chain have the same probability of failure. However, this is not a requirement.)
Ps, P`s and P"s are the probabilities that the connection access attempt
fails in the originating system, international chain (note that ISC is assumed
part of the international chain) or terminating system exchanges, respectively.
(For simplicity, all exchanges are assumed to have the same failure probability,
but this is not a requirement.)
A circuit or exchange can cause a network connection failure for one of
three reasons:
1) The call is blocked because of congestion. The probability of blockage
is PCB and PSB for circuits and exchanges, respectively.
2) The circuit or exchange fails during the call set-up time. The
probability of such a failure is PCF and PSF for circuits and
exchanges, respectively.
3) The circuit or exchange is unavailable to arriving calls, so all calls
arriving during the downtime fail to be completed. These probabilities
are PCD and PSD for circuits and exchanges, respectively.
The probability that a circuit or exchange causes a network connection
failure is given by the following equations, respectively:
PC = 1 - (1 - PCB)(1 - PCF)(1 - PCD)
PS = 1 - (1 - PSB)(1 - PSF)(1 - PSD)
The failure probabilities PCF and PSF can be expressed in terms of the
long-term mean failure intensities Zc and Zs of circuits and exchanges,
Fascicle II.3 - Rec. E.845 PAGE1
respectively, by the following equations:
PCF = Zc Ts
PSF = Zs Ts,
where Ts is the long-term average call set-up time.
Similarly, the failure probabilities PCD and PSD can be expressed in terms
of the long-term mean accumulated downtime (MADT)c and (MADT)s of circuits and
exchanges, respectively, by the following equations:
PCD = eq \f( (MADT)c x ac,K x N)
PSD = eq \f( (MADT)s x as,K x N)
ac and as are the long-term average call arrival rates for circuits and
exchanges, respectively, and N is the long-term average number of call attempts
(in some interval, such as one year).
K is a constant equal to the number of units of time (minutes or seconds)
used to express the downtime in the long-term averaging interval selected (such
as a year).
For example, if the downtime is expressed in minutes and the averaging
interval is one year, then K = 525 600 min./year.
PAGE6 Fascicle II.3 - Rec. E.845
ANNEX C
(to Recommendation E.845)
Effects of busy hours and non-busy hours on the network connection
failure
The two major components of network connection failure (NCF) are the
blocking rate due to congestion and connection access attempt failures due to
equipment faults. Equipment faults are further divided into major and minor
faults. These components affect NCF differently.
C.1 Influences of faults
Faults of subsystems in a telephone network may be divided into two
categories, according to their influence on network performance. Table C-1/E.845
shows two fault categories: major and minor.
TABLE C-1/E.845
Failure category Definition Network components
Major (considerable) Fault wherein a connection access attempt Subscriber line,
influence fault encounters a situation such that service subscriber terminal
degradation of network component(s) lasts for b), exchange,
some period of time, owing to large scale
failure of equipment, and a subscriber cannot
be assured of normal service.
Minor (less Small scale fault wherein a connection access transmission line,
important) influence attempt is handled incorrectly and encounters service center
fault a) no signal (e.g. dial tone, ring-back tone),
no connection, low level speech signal, etc.,
i.e. less important service degradation is
experienced
a) Intermittent fault is excluded and its treatment is an unresolved problem.
b) In some Administrations the subscriber terminal is not considered a network
component.
C.2 Relationship between NCF, congestion and fault
Congestion-related NCF depends on the traffic offered to a system being
considered (a switching system, a network, etc.).
The effects of a minor fault will be considered as so-called white noise
where the absolute value is small and fluctuates at random.
The effects of a major (complete) fault depend on the offered traffic
volume at the time of fault. If a major fault occurred during busy hours, there
would be an extremely high value for NCF. Conversely, a major fault during
non-busy hours will merely yield a small NCF, no matter how large the affected
system is. This is because the traffic load itself is small. Since it is usually
expected that major faults will be very rare, NCF characteristics under major
fault conditions are different from those under minor fault conditions which may
be daily occurrences.
C.3 Long-term NCF (averaged throughout a year)
The long-term NCF concerned with traffic congestion during non-busy hours
will be much smaller than that during busy hours. Since both cumulative call
failures Nf and total calls offered No during non-busy hours are much smaller
than those during busy hours, the averaged 24-hour NCF including non-busy and
busy hours effects will not be much different from the busy hour NCF.
A major fault can be identified but a minor fault cannot be specified
correctly when network operators maintain network equipment. By measuring
long-term NCF during non-busy hours, the effect of minor faults can be estimated
because NCF during non-busy hours is attributed not to traffic congestion but to
minor faults.
C.4 NCF and busy hour pattern
In a country (international region) with several standard time zones,
there will be several busy hours. In such cases, a connection in the network may
include busy and non-busy network components. Thus, an averaged 24-hour NCF would
be helpful to administer a network with different time zones.
However, the averaged 24-hour NCF does not seem to be appropriate to
administer a network having only one standard time zone because its fault-related
term is too small to affect the total NCF, and it might be too late by the time
an extraordinary NCF value has been detected. The NCF averaged during non-busy
hours would be one measure for monitoring the effect of equipment faults (minor
faults) on subscribers, since this will become a major factor during non-busy
hours.
Fascicle II.3 - Rec. E.845 PAGE1
PAGE6 Fascicle II.3 - Rec. E.845