home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Internet Standards
/
CD2.mdf
/
ccitt
/
1992
/
e
/
e855.asc
< prev
next >
Wrap
Text File
|
1991-12-30
|
10KB
|
214 lines
All drawings appearing in this Recommendation have been done in Autocad.
Recommendation E.855
CONNECTION INTEGRITY OBJECTIVE FOR INTERNATIONAL
TELEPHONE SERVICE
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 telecommunication services,
specially telephone services.
The CCITT,
considering
(a) that users of the telephone service can perceive the speech loss due to
transmission interruptions with durations shorter than 10 seconds;
Note - Transmission interruptions with durations longer than or equal to
10 seconds in a conversation phase are not tolerable by telephone users (Annex
A). Such transmission interruptions are considered as a premature release of the
connection as defined in Recommendation E.850.
(b) that speech loss causing transmission interruptions are caused by a
change beyond given limits for a given period of time in one or more parameters,
e.g. power level, noise level, signal-to-noise ratio, bit error ratio, etc.;
(c) that the objective should take into account the expectations of the
users for quality of voice communications as well as the capabilities of current
technologies;
(d) that the objective should take into account the concerns of network
planners and system planners, provide useful guidance to each and that it can be
used by Administrations in a consistent way to measure transmission
interruptions;
(e) that the objective should be in conformity with other Recommendations;
(f) the definition of interruption as given in Recommendation E.800,
recommends
1 Definitions
1.1 connection integrity for telephone service
The degree to which an established telephone connection is offered without
excessive transmission interruptions.
1.2 mean time between interruptions (MTBI)
The expectation of the time between interruptions. The time between
interruptions is the time duration between the end of one interruption and the
beginning of the next.
1.3 mean interruption duration (MID)
The expectation of interruption duration.
1.4 transmission interruption
Temporary inability of the user-to-user transmission path to be provided
persisting for less than 10 seconds (maximum duration) and more than another
given time duration (or minimum duration) characterized by a reduction below a
certain threshold in received signal power level. The minimum duration of
transmission interruption and the
Fascicle II.3 - Rec. E.855 PAGE1
minimum power threshold are for further study. Transmission interruptions caused
by changes beyond certain thresholds of other parameters essential to connection
integrity e.g. noise level, signal-to-distortion ratio, are for further study.
2 A measure to quantify telephone connection integrity performance
The measure to be used shall be the complement of connection integrity,
namely the probability of speech loss, Pi, which is tolerable to telephone users
due to transmission interruptions with durations shorter than 10 s. The estimator
of the speech loss probability, Pie, is the ratio of accumulated transmission
interruption duration to the total observation period of time.
eq Pie = \i\su(i=1,N, ) TDi/T
where T is the observation time and TDi is the time duration of the i th
transmission interruption of N transmission interruptions measured during T (see
Annex B).
Note - There are two major parameters: time between interruptions (or
frequency) and duration to specify characteristics of transmission interruptions.
Those parameters should be easy to observe from the practical point of view.
Actually it seems very difficult to measure very short duration of transmission
interruptions in analogue networks and to separate interruptions from burst
errors in digital networks.
3 Overall objective for speech loss probability
The provisional objective for Pi shall be such that the performance is
better than the value given below:
Pi = x (to be defined with further study)
Note - A percentage of speech loss of less than 0.5% due to transmission
interruptions with durations shorter than 10 s (Annex C) in a conversation phase
is assumed to be tolerable to telephone users.
4 Allocation of the overall objective
From a practical point of view, instead of Pi, the value eq \f( Pi,1 - Pi)
should be allocated to various network components.
The method to allocate this value is for further study.
ANNEX A
(to Recommendation E.855)
Tolerability of telephone user to transmission interruptions
lasting several seconds or more
A.1 Measure
The time interval between the start of transmission interruption occurring
in the middle of a conversation and the abandoning of the disturbed call either
by a calling or a called party is used as a measure to assess or evaluate the
tolerability of telephone users.
A.2 Measuring method
Fifty intra-office calls were selected at random and deliberately
interrupted by callers soon after the calls had been established, and time
intervals between the start of transmission interruption and the release of calls
by the called party were measured.
A.3 Results of tests
The distribution of the durations of interruptions which forced the
telephone users to give up their established calls is depicted in Figure
A-1/E.855. The distribution curve is well approximated by an exponential
distribution function with the mean value of 17.26 sec.
The figure shows that 50% of users released the established calls when the
interruption lasted longer than 11.96 sec.
Figure A-1/E.855 - CCITT 58280
ANNEX B
(to Recommendation E.855)
Relationship between speech loss probability and its estimation
The following relationship exists between the speech loss probability (Pi)
and its estimator (Pie):
eq \o\ac(lim,\s\do10(T -> Ñ)) Pie = \o\ac(lim,\s\do10(T -> Ñ))
\i\su(k=1,K, ) \f(TDk,T) = Pi
if such limit exists and where T is the observation period of time and TDk is the
duration of the k th transmission interruption of K transmission interruptions
over T.
It should be noted that there is also the following relationship:
Pi = eq \f( r, 1 + r) , r = L i=1 eq \f( MIDi, MTBIi)
PAGE4 Fascicle II.3 - Rec. E.855
where, MIDi is the mean time duration of transmission interruption caused by the
i th component of a telephone connection and MTBIi is the mean time between
interruptions caused by the i th component of the connection, under the
assumption that the transmission interruption duration and the time between
transmission interruptions are exponentially distributed (see also Figure
B-1/E.855).
Figure B-1/E.855 - T0201430-88
ANNEX C
(to Recommendation E.855)
Quality of speech impaired by short interruptions
C.1 Measure
The subjective opinion is used as a measure to assess or evaluate speech
quality impaired by short interruptions with durations shorter than 1 s.
C.2 Measuring method
Recommendation P.77 was applied for this subjective evaluation with five
grade opinion scores (Excellent = 4, Good = 3, Satisfactory = 2, Poor = 1 and
Unacceptable = 0). The test procedure was comprised of a 40-second text tape
recorded in Japanese spoken by a female, which was listened to by 20 test
subjects through indoor test circuits with a transmission interruption generator.
C.3 Test results
The relationship between frequency and duration of transmission
interruptions for a given Mean Opinion Score (MOS) is depicted in Figure
C-1/E.855.
The dotted line in this figure shows the locus of "frequency x duration =
0.5%" which is considered as a permissible limit of the freeze out rate or the
percentage of speech loss for designing Digital Speech Interpolation (DSI) and
Time Assignment Speech Interpolation (TASI) equipment.
Note - The product of frequency and duration of the short interruption is
identical to P [= MID/(MTBI + MID)] in Annex B.
Figure C-1/E.855
Fascicle II.3 - Rec. E.855 PAGE1