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All drawings appearing in this Recommendation have been done in Autocad.
Recommendation Q.551
TRANSMISSION CHARACTERISTICS OF DIGITAL EXCHANGES
1 Introduction
1.1 General
The field of application of this Recommendation is found in Recommendation
Q.500.
Note - A high percentage of international calls will have a digital PABX
included in the connection. Therefore, Recommendations Q.551-Q.554 are applicable
also for digital PABXs with regard to those specific transmission parameters
affecting the transmission quality of an international call, for instance
Loudness Ratings, noise, talker's and listener's sidetone, echo and stability.
These Recommendations primarily concern digital PABXs connected digitally to the
international network. However, Administrations may find some of the
specification details useful for digital PABXs connected by analogue means to the
international network.
The signals taken into consideration are passed through the following
interfaces as described in Recommendations Q.511 and Q.512 and Figures 1/Q.551
and 2/Q.551.
- Interface A is for primary digital signals at 2048 kbit/s or 1544
kbit/s.
- Interface B is for secondary digital signals at 8448 kbit/s or 6312
kbit/s.
- Interface C comprises both 4-wire and 2-wire analogue trunk interfaces.
Interfaces C1 4-wire and C2 2-wire represent possible applications of
interface C in Figure 1/Q.511.
Interface C1 repres a 4-wire and interface C2 a
2-wire analogue trunk interface. For practical reasons, C1 and C2
are further sub-divided into C11, C12, C13, C21 and C22.
C11 interfaces channel translating equipment. C12 and C13
interface 4-wire analogue exchanges; C12 via the relay sets, C13
directly to the switching stages.
C21 is an interface applicable when a 2-wire circuit connects
a digital transit exchange with a local exchange, analogue or
digital. C22 is an interface applicable when a 2-wire circuit
connects local exchanges, analogue and/or digital.
See Figures 1/Q.551 and 2/Q.551 for illustration of the
principles.
- Interface V is for digital subscriber line
access.
- Interface type Z is for analogue subscriber
line access.
Note 1 - Remote analogue exchange concentrators and analogue PABXs may use
interface Z for access to a digital exchange.
Note 2 - In the future, differences in circuit configurations with respect
to transmission parameters may cause a sub-division of the Z interface.
Figure 1/Q.551 - T1107790-87
Figure 2/Q.551 - T1107800-87
Also for ports other than those designated Z, there can exist types whose
transmission characteristics have not been defined, even though they may be
recognized as being in practical operation. This may be due to CCITT not having
considered their international standardization justified, e.g., because of their
limited use, or because their function is to coordinate with pre-existing
national standards. However, later additions to recommended interfaces are not
ruled out (e.g., 4-wire subscriber interface).
Interface types V and Z may appear remote from the exchange through the
use of digital transmission facilities. When this occurs, there should be no
impact on transmission parameters other than delay. Transmission parameters
associated with interface Z include the effects of the equipment provided for
interfacing the analogue subscriber line to the digital switching network of the
exchange.
Multislot channels are not considered in this Recommendation. This
requires further study.
Fascicle VI.5 - Rec. Q.551 PAGE1
It is necessary to ensure that representative feeding currents are flowing
during the measurements of all of these transmission parameters. These feeding
currents can contribute to noise, distortion, crosstalk, variation of gain with
input level, etc. Therefore, appropriate allowances for this must be made. In
some cases, where indicated, the permissible limits quoted include these
allowances.
PAGE2 Fascicle VI.5 - Rec. Q.551
Detailed transmission characteristics for these interfaces are provided in
the following Recommendations:
- Recommendation Q.552 for 2-wire analogue interfaces
- Recommendation Q.553 for 4-wire analogue interfaces
- Recommendation Q.554 for digital interfaces.
In the present Recommendations, values given for transmission
characteristics relate to the path from an exchange test point to an exchange
interface and vice-versa; the overall characteristics for connections involving
two interfaces can in most cases be obtained by suitably combining these values
(see Recommendation Q.551, S 3).
In the future, other interfaces may be defined.
At this time these Recommendations consider analogue signals which are
encoded in accordance with Recommendation G.711. Other coding laws may be defined
in the future and these Recommendations will need to take them into account.
The transmission characteristics of voice-frequency (VF) connections
through a digital transit exchange should in principle provide performance in
accordance with Recommendations G.712, G.713 and, where applicable, Q.45 bis (see
also Recommendation G.142).
The principles of Recommendation G.142 and the limits of Recommendations
G.714 and G.715 have been used as a basis for establishing the transmission
characteristics for analogue voice frequency connections specified in SS 2 and 3
of Recommendations Q.552 and Q.553, respectively. The limit values are not
necessarily identical with those specified in the G-Series, since in the case of
a connection through the exchange, additional allowances generally have been made
for cabling (see S 2). The principles of Recommendations G.714 and G.715 have
been used for the analogue/digital test connections referred to in SS 2 and 3 of
Recommendations Q.552 and Q.553, respectively.
The values given are to be considered as either "design" or "performance
objectives" according to the explanations of the terms given in Recommendation
G.102 (Transmission performance and objectives and recommendations) and the
particular context.
The specification clauses in this Recommendation exclude the effects of
auxiliary functions such as echo suppression, echo cancellation or transmission
to the subscriber of metering impulses, or of non-telephony functions such as
telemetering over the subscriber pair.
1.2 Definitions
1.2.1 Exchange test points, exchange input and output and half-connections
1.2.1.1 exchange test points
The exchange test points shown in Figure 1/Q.551 are defined for
specification purposes. They may not physically exist in an exchange but may be
accessed via the digital switching network. In this case, some or all of the
switching network will be included in the path from the exchange interface to the
access points.
The transmission parameters affected by this means of access are the
absolute group delay and possibly jitter and wander and bit error ratio. For most
other parameters, either the exchange test points or the access points are
located such that end-to-end performance can be determined by suitably combining
performances between each interface and either the exchange test points or the
access points.
1.2.1.2 exchange input and output
The exchange input and output for a connection through a digital exchange
are located at the interfaces identified in S 1.1 and shown in Figures 1/Q.551
and 2/Q.551.
The exact position of each of these points depends on national practice,
and it is not necessary for the CCITT to define it.
However, the applicability of recommended values to points arbitrarily
located is subject to certain restrictions:
- for analogue interfaces, as referred to in S 2 of this Recommendation
(maximum length of exchange cabling between exchange equipment ports
and the interface);
- for digital interfaces, as also mentioned in S 2 (maximum loss between
exchange interfaces and connected equipment, e.g., digital line or
higher order multiplex equipment).
1.2.1.3 Half-connections
input connection - A unidirectional path from an interface of a digital
Fascicle VI.5 - Rec. Q.551 PAGE1
exchange to an exchange test point.
output connection - A unidirectional path from an exchange test point to
an interface of a digital exchange.
half connection - A bidirectional path comprised of an input connection
and an output connection, both having the same exchange interface.
Note 1 - These terms may be qualified by the words analogue or digital,
the qualification signifying the property of the exchange interface.
Note 2 - An analogue input (output) (half) connection may be further
qualified by the words 2-wire or 4-wire.
Note 3 - Refer to Recommendation Q.9 for additional information.
1.2.2 Relative levels
1.2.2.1 Exchange test points
The nominal relative level at the input and output exchange test points is
assigned the value 0 dBr.
1.2.2.2 Analogue interfaces
The nominal relative level at the exchange input point is designated Li.
The nominal relative level at the exchange output point is designated Lo.
1.2.2.3 Digital interfaces
The relative level to be associated with a point in a digital path
carrying a digital bit stream generated by a coder lined-up in accordance with
the principles of Recommendation G.101 is determined by the value of the digital
loss or gain between the output of the coder and the point considered.
If there is no such loss or gain the relative levels at the exchange input
and output points (i.e., digital interfaces V, A and B) are by convention said to
be 0 dBr. For further information, see Recommendation G.101, S 5.3.2.4.
Note - The digital level may be established using measuring equipment in
accordance with Recommendation O.133.
Relative level has no meaning for digital bit streams that are not derived
from real or simulated analogue sources.
1.2.3 Measurement conditions
1.2.3.1 Common measurement conditions
All digital signal processing devices which affect bit integrity of the 64
kbit/s path (e.g., digital loss pads, code converters, digital echo control
devices, digital speech interpolation apparatus or all-zero suppressors) must be
rendered inoperative when measuring the transmission parameters of this
Recommendation. However, if the nominal transmission loss, NL, for speech
connections is implemented by a digital loss pad, the loss pad must not be
inoperative for the output connection when measuring parameters dependent on NL.
Where measuring transmission parameters between 2-wire ports is considered
necessary, the opposite direction of transmission must be interrupted in order to
avoid disturbing effects due to reflections at hybrids.
In addition, a quiet code, i.e., a PCM signal corresponding to decoder
output value 0 (m-law) or output value 1 (A-law), with the sign bit in a fixed
state should be applied to the exchange test point.
Note - These patterns are slightly different from the idle code produced
by an exchange (see e.g., Recommendation Q.522, S 2.12).
PAGE2 Fascicle VI.5 - Rec. Q.551
1.2.3.2 Reference frequency
For the reference frequency, Recommendation O.6 applies:
- A reference test frequency of 1020 Hz is recommended for test frequency
generating circuits or instruments that provide reference test
frequencies. The specified frequency tolerance should be +2 to -7 Hz.
1.2.3.3 Impedance
Unless otherwise specified, measurements at analogue interfaces shall be
made under nominally matched conditions.
Note - The preferred interpretation of this statement should be that the
nominal exchange impedance should be used as the internal impedance of the
analogue test generator and the analogue level meter. However, under some
circumstances it may be preferable to use a low impedance generator and a high
impedance meter which corresponds to an exact matching to the actual exchange
impedance. (Losses measured according to the two methods will only differ by a
small amount, in the same order of magnitude as the loss of a very short
subscriber cable.)
1.2.3.4 Test levels at analogue interfaces
At the reference frequency, test levels are defined in terms of the
apparent power relative to 1 mW.
Where no value is given, the test level should be -10 dBm0.
At frequencies different from the reference frequency, test levels are
defined as having the same voltage as the test level at the reference frequency.
Measurements are based on the use of a test generator with a
frequency-independent EMF.
The above considerations are primarily concerned with measurements at
discrete frequencies. Their impact on the measurement at interfaces with complex
impedances of broadband signals (e.g., random or quasi-random noise with defined
spectral intensity) and vice versa needs further study.
1.2.4 Transmission loss
1.2.4.1 Nominal transmission loss
A connection through the exchange (see Figure 1/Q.551) is established by
connecting in both directions an input located at one interface to an output
located at another interface.
The nominal transmission loss for a connection through an exchange is
equal to the difference of the relative levels at the input and the output.
NL = (Li - Lo) dB
The nominal transmission loss between the input at an analogue interface
and the exchange test point is defined as:
NLi = Li
The nominal transmission loss between the exchange test point and the
output of an analogue interface is defined as:
NLo = -Lo
This is equal to the nominal "composite loss" (see definition in Blue
Book, Fascicle I.3) at the reference frequency. See also Recommendation G.101, S
5.3 and Supplement No. 1 in Fascicle VI.5 of the CCITT Blue Book.
Note 1 - The nominal transmission loss, NL, may be implemented by an
analogue loss pad. It may also be implemented by a digital loss pad. In the
latter case, the digital loss pad may be on the incoming side of the digital
switching network, or on the outgoing side of the digital switching network or
both.
As a general principle, the use of digital loss pads should be avoided
because bit integrity is lost for digital services and additional transmission
impairments are introduced for analogue services.
However, it is recognized that during the transition stage to a completely
digital network, existing national transmission plans may require digital pads to
be inserted for speech.
In addition, connections in a future ISDN used for voice can be expected
to contain other devices which destroy bit integrity of the 64 kbit/s path (e.g.,
code converters, digital echo control devices, digital speech interpolation
apparatus, or all-zero-suppressors). Provision must be made to render all such
devices inoperative when necessary. See Recommendation Q.521, S 4.3.7.
Note 2 - The nominal transmission loss of the exchange may be different in
the two directions.
1.2.5 Attenuation frequency distortion
The attenuation frequency distortion (loss distortion) is the logarithmic
Fascicle VI.5 - Rec. Q.551 PAGE1
ratio of output voltage at the reference frequency (nominally 1020 Hz), U(1020
Hz), divided by its value at frequency f, U(f):
LD = 20 log eq \f( U(1020 Hz), U(f))
See Recommendation G.101, S 5.3 and Supplement No. 1 in Fascicle VI.5 of
the CCITT Blue Book.
1.2.6 Digital parameters
1.2.6.1 bit integrity
The property of a digital half connection of a digital exchange in which
the binary values and the sequence of the bits in an octet at the input of the
half connection are reproduced exactly at the output.
Note - Digital processing devices such as A/m law converters, echo
suppressors and digital pads must be disabled to provide bit integrity.
2 Characteristics of interfaces
The interfaces taken into account are those of Figures 1/Q.511 and
1/Q.551. For voice-frequency interfaces (C and Z), the electrical parameters
refer to the appropriate distribution frame (DF), on the assumption that the
length of the cabling between the DF and the actual exchange does not exceed 100
m (exchange cables). In this respect, Recommendation Q.45 bis S 3 applies. For
corresponding limitations on the location of digital interfaces, see
Recommendation G.703.
2.1 Two-wire analogue interfaces
Detailed transmission characteristics of 2-wire analogue interfaces are
provided in Recommendation Q.552.
2.1.1 Interface Z
The interface Z provides for the connection of analogue subscriber lines
and will carry signals such as speech, voice-band analogue data and
multi-frequency push button signals, etc. In addition, the interface Z must
provide for DC feeding the subscriber set and ordinary functions such as DC
signalling, ringing, metering, etc., where appropriate.
Other extraordinary (supplementary) functions, as mentioned in S 1.1
above, are not considered as forming part of the exchange but rather of the line,
i.e. included on the exchange side. Since the interface Z ordinarily terminates
the subscriber line, it is necessary to control the impedance and unbalance about
earth. (While this will also be true of equipment providing supplementary
functions, its specification is not dealt with here.)
When the Z interface is used as an extension line interface of a digitally
connected digital PABX, additional functions may be required to provide special
features of the PABX. If the extension line is entirely contained within a
building some attributes of the PABX such as longitudinal conversion loss, may no
longer need to be specified and others may take special values.
PAGE2 Fascicle VI.5 - Rec. Q.551
2.1.2 Interface C2
The interface C2 provides for the connection of 2-wire analogue circuits
to other exchanges.
The interface C21 provides the termination of outgoing and incoming
international long-distance connections and possibly national connections also
with the exchange acting as a transit switch (see Figure 2/Q.551).
The interface C22 provides for the connection of a 2-wire trunk line.
Typical is the interconnection of a Z interface with a C22 interface in a local
exchange for routings through the existing 2-wire analogue trunk network. A C22
interface cannot be part of the international 4-wire chain.
2.2 Four-wire analogue interfaces
Detailed transmission characteristics of 4-wire analogue interfaces are
provided in Recommendation Q.553.
2.2.1 Interface C1
The interface C1 provides for the connection of 4-wire analogue circuits
to other exchanges.
According to Figure 1/Q.551, the interface C11 of a digital exchange is
intended for connection to the channel translating equipment of an FDM system.
According to Figure 1/Q.551, the interface C12 of a digital exchange is
intended for connection to the incoming and outgoing relay set of an analogue
4-wire exchange (see Figure 1/Q.45 bis).
According to Figure 1/Q.551, the interface C13 of a digital exchange is
intended for connection to a 4-wire analogue switching stage (see Figure 1/G.142,
case 5.)
2.3 Digital interfaces
Detailed transmission characteristics of digital interfaces are provided
in Recommendation Q.554.
2.3.1 Interface A
The interface A operating at a rate of 1544 kbit/s or 2048 kbit/s provides
for the digital connection of circuits to other exchanges.
2.3.2 Interface B
The interface B operating at a rate of 6312 kbit/s or 8448 kbit/s provides
for the digital connection of circuits to other exchanges.
2.3.3 V-type interfaces
V-type interfaces provide for digital subscriber line access.
V-type interfaces allow the connection to the exchange of a digital
subscriber line capable of providing digital subscriber side access for ISDN. The
different variants, V2, V3 etc., are described in Recommendation Q.512 S 3. It
will be seen that the differences lie essentially in multiplexing and in the
associated signalling facilities, the transmission requirements being
substantially identical, i.e., providing 64 kbit/s B channels, with bit integrity
assumed unless the transmission plan specifically requires otherwise. See also
Recommendation Q.554, S 2.5.
Note - The designation "V1" is applied to a reference point which is
connected to a basic access digital section.
3 Voice frequency parameters of a connection between two interfaces of the
same exchange
3.1 General
This section of Recommendation Q.551 provides guidance on obtaining the
overall characteristics for connections between two interfaces of the same
exchange. For overall connections involving one or more digital interfaces, the
results may be interpreted by assuming that ideal send and receive sides (see
Recommendations G.714 and Q.715) are connected to the digital inputs and outputs,
respectively.
In this section, transmission parameters relating to the path from an
exchange interface to an exchange test point will be referred to as input
parameters. Transmission parameters relating to the path from an exchange test
point to an exchange interface will be referred to as output parameters.
3.2 Transmission loss through the exchange
The transmission loss through the exchange is equal to the algebraic sum
of the input transmission loss and the output transmission loss.
The overall characteristic for the following parameters can be obtained
the same way.
- short-term variation of loss with time;
- attenuation/frequency distortion;
Fascicle VI.5 - Rec. Q.551 PAGE1
- variation of gain with input level.
3.3 Group delay
3.3.1 Absolute group delay
"Absolute group delay" refers to the minimum group delay measured in the
frequency band 500-2800 Hz.
The absolute group delay through an exchange will very much depend on the
exchange architecture and the types of connections involved. Table 1/Q.551 gives
estimated mean and 0.95 probability of not exceeding values of round trip delay
between interfaces exemplified in Figure 3/Q.551. These values may not be
applicable to digital PABXs.
The absolute group delay includes delay due to electronic devices such as
frame aligners and time stages of the switching matrix but does not include
delays due to ancillary functions, such as echo suppression or echo cancellation.
TABLE 1/Q.551
Round trip delay between interfaces as depicted in Figure 3/Q.551
Figure Mean ms 0.95 probability of
not exceeding ms
a) 900 1500
b) 1950 2700
c) 1650 2500
d) 3000 3900
e) 2700 3700
f) 2400 3500
Note 1 - These values for the absolute group delays are
applicable under reference load A conditions as defined
in Recommendation Q.543.
Note 2 - These values do not include the propagation
delay associated with transmission across the link
between the main part and any remotely located parts of
a digital local exchange.
Figure 3/Q.551 - T1105720-87
3.3.2 Group delay distortion
The total group delay distortion is equal to the sum of the input and the
output group delay distortions.
3.4 Noise and total distortion
When evaluating the exchange noise characteristics, it is necessary to
consider two components of noise. One of these arises from the PCM translating
process, the other from analogue sources e.g., signalling circuits, exchange
power supply, line power feeding on both sides of a connection between two
interfaces through the same exchange.
The noise arising from the PCM translating process is limited by
Recommendation G.712, the noise from analogue sources by Recommendation G.123.
This applies to both weighted noise and total distortion. The requirements for
weighted noise and total distortion for connections between the same interfaces
and through the same exchange are of value for test purposes.
In real connections through the network, usually several connections
between different exchanges with different levels and different interfaces apply.
This would result in very complex calculations for the overall noise contribution
and cannot be handled in a simple way. Consideration of the contribution of noise
and total distortion for each individual half connection as specified in
Recommendations Q.552 and Q.553 for the case in question should be preferred.
3.4.1 Weighted noise
The total psophometric noise power allowed at a Z interface contributed by
a whole connection through the exchange Z-Z is approximated by the formula:
PTN = PANeq \b\bc\( ( 1+10\f( Lo - Li, 10)) + 10eq \f( 90 + LIN + Lo, 10) pWp
respective the total noise level
PAGE2 Fascicle VI.5 - Rec. Q.551
LTN = 10 logeq \b\bc\( (\f( PTN, 1 pW)) - 90 dBmp
where
PTN : Total weighted noise power of a whole connection through the local
digital exchange Z-Z.
PAN : Weighted noise power caused by analogue functions according to
Recommendation G.123, Annex A, i.e., 200 pWp.
Lo : Output relative level at the Z interface.
Li : Input relative level at the Z interface of the same exchange.
LIN : Weighted noise (idle channel noise) for PCM translating equipment
according to Recommendation G.712, i.e., -65 dBm0p
LTN : Total weighted noise level of a whole connection through the local
digital exchange Z-Z.
Alternatively the same PTN and LTN can be obtained by adding the relevant
values for input and output connections at Z interfaces according to
Recommendation Q.552, S 3.3.2.1, observing that the values for LINi and LINo are
different from LIN.
However, a small difference in the numerical results occurs due to
approximation errors between LIN on the one hand compared with LINi and LINo on
the other.
For the C2 interfaces, similar considerations can be made to obtain the
allowed psophometric noise power.
Either calculating the idle channel noise according to Recommendation
G.712 (to be maximum -65 dBm0p) together with the analogue noise according to
Recommendation G.123 (to be maximum -67 dBm0p) which results in approximately -63
dBm0p. Alternatively, the allowed values for the input and output connections
according to Recommendation Q.553, S 3.2.2.1 for equipment with signalling on the
speech wires can be combined, giving again approximately -63 dBm0p.
3.4.2 Total distortion including quantizing distortion
The method shown below uses the sinusoidal test signal with the reference
frequency of 1020 Hz as specified in Recommendation O.132. The ratio of
signal-to-total-distortion power for a whole connection through the exchange is
given by the formula:
eq \f(S, NT) = LS + Lo - 10 logeq \b\bc\( ( 10\f( LS + Lo - S/N, 10) + 10\f(
LN, 10))
Fascicle VI.5 - Rec. Q.551 PAGE1
where
S/NT : resulting signal-to-total distortion ratio for a whole
connection through a digital exchange.
LS : signal level of the measuring signal in dBm0.
Lo : output relative level of the local exchange in dBr.
S/N : signal-to-total distortion ratio for PCM translating equipment
in Recommendation G.712 (whole connection).
LN : Weighted noise caused by analogue functions according to
Recommendation G.123, Annex A, i.e., -67 dBmp.
Note - No band limiting effect on the noise by the encoding process was
taken into account to compensate for overall effects. Thus the calculation above
is assumed to give the worst case requirements.
This calculation of S/NT applies to both Z and C2 interfaces.
Total distortion including quantizing distortion using the noise method as
specified in Recommendation O.131 will be the subject of further study.
3.5 Crosstalk
Where measurement of the signal to crosstalk ratio between any two
complete connections (analogue to analogue) through the exchange is considered
necessary, a sine wave test signal at the reference frequency of 1020 Hz and at a
level of 0 dBm0 is applied to the analogue 2-wire or 4-wire interface of one
connection. An auxiliary low level activation signal, for example a band limited
noise signal (see Recommendation O.131) at a level in the range
-50 to -60 dBm0 is injected into the input of the connection to be measured. The
level produced in any other connection should not exceed -65 dBm0 (value to be
further studied).
Care must be taken on the choice of frequency and the filtering
characteristics of the selective measuring equipment, in order to avoid that the
activating signal and noise affects the accuracy of the crosstalk measurement.
This measurement arrangement is shown in Figure 4/Q.551.
Note 1 - The go to return crosstalk of 4-wire connections is covered by
Recommendation Q.553 SS 3.1.4.1.2 and 3.1.4.2.2.
Note 2 - Measurement of NEXT is not required, as it is the same as in a
half-connection.
Note 3 - If it is not possible without considerable difficulty to break
the return path of the 4-wire loop, reflection should be minimized by making the
terminating impedance and the balance impedance equal.
Note 4 - Further study is required to determine whether MORE STRINGENT
LIMITS or measurements at additional frequencies should be specified.
3.6 Discrimination against out-of-band signals applied to the input interface
The values for these parameters for a complete connection through an
exchange are identical to the corresponding values for a half connection. See
Recommendation Q.552, S 3.1.6 and Recommendation Q.553, S 3.1.6.
Figure 4/Q.551 - T1108990-87
3.7 Spurious out-of-band signals received at the output interface
The values for these parameters for a complete connection through an
exchange are identical to the corresponding values for a half connection. See
Recommendation Q.552, S 3.1.7 and Recommendation Q.553, S 3.1.7.
3.8 Echo and stability
When a complete connection, comprised of a 2-wire analogue half connection
and a 4-wire half connection, terminates the international chain, the total
stability loss of the national extension is provided by the 2-wire analogue half
connection. See Recommendation Q.552, S 3.1.8.
If in a digital exchange (including PABXs), 2-wire half connections (Z or
C2 interfaces) cooperate in such a way that an additional 2-wire-4-wire-2-wire is
included as part of an international connection, then Recommendation G.122
concerning echo, stability and especially effects of listener echo has to be
fulfilled.
The effects of listener echo depend on the maximum total number of loops
in a complete connection. Listener echo signals:
- can lead to objectionable "hollowness" in voice communications, and
- can impair the bit error ratio of received voice-band data signals.
4 Exchange transfer function - jitter and wander
The exchange transfer function relates wander at the output of the
exchange to wander at the inputs used for synchronization purposes. It is
PAGE2 Fascicle VI.5 - Rec. Q.551
recognized that the approach of using the exchange transfer function to specify
the performance of an exchange is not applicable to all implementations (e.g.,
when mutual synchronization methods are used). The exchange transfer mask is
similar to that of a low pass filter with a maximum gain of 0.2 dB, a break point
at 0.1 Hz and slope of 6 dB/octave as shown in Figure 5/Q.551.
The higher frequency (jitter) portion of the mask is undefined, but must
provide significant attenuation above 100 Hz.
Figure 5/Q.551 - CCITT 35941
Fascicle VI.5 - Rec. Q.551 PAGE1
