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C:\WINWORD\CCITTREC.DOT_______________
Recommendation G.763
ANNEX B
(to Recommendation G.763)
B.1 An example of a DLC double averaging technique
The average number of encoding bits per sample is obtained using a double
average process.
a) The first stage averaging is computed at discrete time instances once every n
DCME frames, where n is operator selectable (n=4, 16, 32, 64 or 128). The
result of the computation is the ensemble average <Se> taken over the
ensemble of BCs which are carrying voice traffic and will result in one of
the following possible outcomes:
û <Se>=4 for N ú M
û <Se>=4M/N for 3N/4 ú M < N
with M= total number of 4 bit bearer time slots in the pool which are not
used for voice band data, bit banks and 64kbit/s on-demand traffic
counted in the measurement frame.
N= total number of connected active voice ITs, counted in the measurement
frame.
Two ensemble averages should be determined:
û <Sea>ûwhich is the actual measured ensemble average of encoding bits/
sample <Se> based on <Sea>ûactual counts of M and N.
û <Sep>ûwhich is the predicted ensemble average of encoding bits/sample
<Se> based on the <Sep>ûactual count of N and a reduced count of Mû
2.
b) The second stage averaging should be a moving discrete time averaging of
<Sea> and <Sep>:
û Staûwhich is the moving discrete time average of 100 consecutive values
of <Sea>.
û Stpûwhich is the moving discrete time average of 100 consecutive values
of <Sep>.
The value of Sta may be used as a measure of the average number of encod-
ing bits/sample when determining the dynamic load control condition for
voice and voice band data channels.
The value of Stp may be used as a measure of the average number of encod-
ing bits/sample when determining the dynamic load control condition for
on-demand 64kbit/s channels.
B.2 Transmit activity detector threshold and operate time characteristic
A typical response to a sinusoidal stimulus signal in the band 300 to 3400 Hz
will be as given below:
Average signal power (see Note) Operate time
< -40 dBm0 OFF
│ -40 dBm0, ú -30 dBm0 Figure B-1/G.763
> -30 dBm0 2 ms < t < 4 ms
The operate time requirements will be satisfied while permitting tolerances on
the average signal power of any stimulus signal in the frequency band at boundary con-
ditions as follows:
-40 dBm0 + 1.5 dB
-30 dBm0 + 1.0 dB
A typical rate of change of the transmit activity detector adaptive threshold will
be between 2.5 dB/s and 20.0dB/s.
Note û The activity detector should not indicate activity for idle channel noise
less than -40 dBm0.
FIG. B-1/G.763
B.3 Data/speech discriminator
Functionally, the data/speech (D/S) discriminator determines whether the activ-
ity on each transmit IT is speech or data and provides a speech/data indication to the
hangover control and signal classification process.
The implementation of the D/S discriminator may be performed by a combina-
tion of spectral analysis and 2100Hz tone detection.
The following requirements should be met with the modem types and bit rates
given in Table 7/G.763.
B.3.1 Output conditions
The D/S discriminator analyzes the activity on each transmit IT and provides
the following output conditions:
Activity Output condition
Speech ôVoiceö
Tones except 2100 Hz ôVoiceö
Data signal (see Note) ôDataö
2100 Hz ôDataö
Note û V.23 modem signals may be classified as either voice or data dependent
upon the design of the data/speech discriminator.
The D/S discriminator provides a continuous output condition indicating the
presence of either speech or data on the ITs. The current output condition should be
maintained upon termination of activity on the IT or until the output condition of a sub-
sequent activity is determined. Upon system start-up or map change, the D/S discrimi-
nator should be reset to ôVoiceö.
B.3.2 Accuracy
The missed detection probability of data as speech or speech as data should be
less than 0.5%.
B.3.3 Response time
The D/S discriminator should update its output condition within 200ms after
any of the following changes in the signal characteristics on the IT:
û Inactive-to-speech
û Inactive-to-data
û Speech-to-data
û Data-to-speech
B.3.4 2100 Hz tone detection
The 2100 Hz tone detector should meet the following requirements:
û Frequency range of tone: 2100 + 21 Hz
û Minimum amplitude of tone: -25 dBm0
û Response time: < 100 ms (for further study)
B.4 2400 Hz tone detector
The 2400 Hz tone detector should meet the following requirements:
û Frequency of tone: 2400 Hz + 15 Hz
û Minimum amplitude of tone: -25 dBm0
û Response time: < 50 ms
û Missed detection probability: < 0.5%.
B.5 Speech detector/echo control device interactions
Consideration must be given to minimizing excessive channel loading which
may exist as the result of network interactions between the DCME speech detector and
an echo control device (see FigureB-2/G.763).
If the DCME utilizes an adaptive threshold speech detector, interaction between
the speech detector threshold adjustment and the echo control operation may generate
excessive activity in the channel. The echo control device modulates the terrestrial cir-
cuit noise accumulated between the telephone and the send-input port of the echo con-
trol device. The adaptive threshold speech detector may falsely classify this change in
terrestrial circuit noise as speech and increase the load on the DCME. This will
increase the occurrence of overload channels and/or freeze-out, thereby degrading the
performance in the baseband channel. This interaction occurs as follows:
a) Receive speech arrives at the receive input (Rin) port of the echo control unit.
b) The echo suppression switch or canceller centre clipper activates, stopping
the echo or residual echo and attenuating the near-end-generated analogue
terrestrial noise (N1) present at the send input (Sin) port.
c) If very little noise is generated between the echo control send output (Sout)
port and the DCME speech detector input, the speech detector threshold will
adapt to its minimum level (typically -50dBm0).
d) When the receive speech stops, after a suitable echo control unit handover
time the echo suppression switch or canceller centre clipper will close and
the near-end-generated terrestrial noise, as seen by the DCME speech detec-
tor will reappear as a step change in noise level.
e) This step change in noise level may exceed the speech detector threshold,
causing the DCME to transmit a noise spurt as if it were speech. The noise
spurt duration will be a function of the adaptation speed of the speech detec-
tor and the near-end-generated terrestrial noise level.
This sequence will be repeated for every speech spurt and will produce a very annoy-
ing speech-correlated noise spurt heard by the far-end talkers every time they stop
speaking.
This interaction is not limited to single echo control device network configurations. A
typical network configuration with multiple echo control devices interacting with a
DCME speech detector is shown in FigureB-3/G.763. In this configuration, the
DCME speech detector may respond to unit step increases in noise power which result
from echo suppressor switch or echo canceller centre clipper activations in the send
paths of echo control devices 1 and 3. The DCME speech detector will first experience
a unit step increase in noise power from echo control device 3 switch activation, fol-
lowed by a second step increase from echo control device 1 switch activation. The
extent to which the DCME speech detector incorrectly responds to these step increases
in noise power will be a function of the noise power levels N1, N2, N3, and N4, and the
specific DCME speech detector threshold adaptation algorithm. For example, the dual
step increases in noise presented to the DCME speech detector which result from
switch or centre clipper activation at locations 1 and 3 will be masked if the power
level of N4 is excessively high. Likewise, high noise power levels at N2 or N3 may
mask step increases in noise power caused by echo control unit1.
There are several methods for dealing with the interactions between the echo control
devices and the DCME speech detector. In one approach, the echo control device could
be modified to monitor the terrestrial-generated noise at the send-input port. When the
send transmission path is broken, noise at the proper level is injected into the
send-output toward the DCME, keeping the noise seen by the speech detector at a con-
stant level and avoiding speech detector activation. This approach is unacceptable due
to the number of different echo control devices in use and the uniqueness of the appli-
cation. In a second approach, the speech detector adaptive threshold would be frozen in
the presence of speech on the corresponding receive channel. A third approach would
be to specify an adaptive speech detector with a fast adaptation feature which would
track step changes in noise level and minimize the noise spurts.
The transmit activity detector threshold should not adapt to Gaussian noise level varia-
tions which are due to the action of echo suppressors or echo cancellers. This may be
accomplished by any means which is functionally equivalent to providing the transmit
activity detector with a threshold inhibit signal from a receive activity detector when
activity is present on the receive channel (see º12.4).
Fig. B-2/G.763 = 7,5 cm
Fig. B-3/G.763 = 6 cm
B.6 Timing synchronization
The following figures provide a number of examples of Doppler and plesio-
chronous slip buffer placements for a variety of network synchronization schemes. In
the figures it is assumed that all buffers will derive their write clocks from the input bit
stream.
B.6.1 Point-to-point operation
B.6.1.1 Terrestrial operation within a national network
Figures B-4/G.763 and B-5/G.763 show methods of DCME terminal synchroni-
zation for operation within a national network.
Fig. B-4 y B-5/G.763 = 12 cm
B.6.1.2 Terrestrial operation between national networks
Figures B-6/G.763, B-7/G.763 and B-8/G.763 show methods of terminal syn-
chronization for operation between national networks via terrestrial networks. Plesio-
chronous buffers are required for networks as shown in FiguresB-6/G.763 and B-7/
G.763. Figure B-8/G.763 utilizes loop timing and therefore does not require plesio-
chronous buffering.
Fig. B-6/G.763 = 6,5 cm
Fig. B-7 y B-8/G.763 = 13 cm
B.6.1.3 Satellite operation between national networks based upon continuous digital
carrier type services
Figures B-9/G.763, B-10/G.763, B-11/G.763 and B-12/G.763 show methods of
terminal synchronization for operation between national networks over a satellite link
based upon asynchronous continuous digital carrier type services. FigureB-9/G.763
introduces controlled slips between the DCMEs which are limited to 1 in 70days if
G.811 clocks are available in both networks. The configuration shown in FiguresB-10/
G.763, B-11/G.763 and B-12/G.763 permit slip free operation between the DCMEs.
Fig. B-9, B-10 y Fig. B-11/G.763 = 6,5 cm
B-12 /G.763 = 6,5 cm
B.6.1.4 Satellite operation between national networks based upon TDMA type services
Figures B-13/G.763 and B-14/G.763 show a method of DCME terminal syn-
chronization for operation between national networks over a satellite link based on
TDMA-type services. An appropriate interface is provided in the TDMA terminal to
permit interfacing the DCME with and without multi-clique over a primary multiplex
port. The configuration shown in FigureB-13/G.763 permits slip free operation
between the DCMEs.
B.6.2 Multi-clique operation
B.6.2.1 Terrestrial operation within a national network
Figure B-15/G.763 shows a method of DCME terminal synchronization for
operation within a national network. The cross connect function provides a means of
assembling the received multi-clique pools on to a single primary multiplex.
B.6.2.2 Terrestrial operation between national networks
Figure B-16/G.763 shows a method of DCME terminal synchronization for
operation between national networks via terrestrial facilities. Plesiochronous buffers
are required to resolve timing differences between the various plesiochronous net-
works. Due to the multiple source nature of the multi-clique configuration, the plesio-
chronous buffers must be placed before the cross connect function.
B.6.2.3 Satellite operation between national networks based upon continuous carrier
type services
Figure B-17/G.763 shows a method of DCME terminal synchronization for
operation between national networks based on continuous digital satellite carriers. Ple-
siochronous/doppler buffers are required to resolve timing differences between the var-
ious plesiochronous networks and to remove satellite induced doppler shifts on the
received data streams. Due to the multiple source nature of the multi-clique configura-
tion, the plesiochronous/doppler buffers must be placed before the cross connect func-
tion.
Fig. B-13/G.763 = 22 cm
Fig. B-14G.763 = 22 cm
Fig. B-15/G.763 = 22 cm
Fig. B-16/G.763 = 22 cm
Fig. B-17/G.763 = 22 cm
B.6.3 Multi-destination operation
B.6.3.1 Terrestrial operation within a national network
Figure B-18/G.763 shows a method of DCME terminal synchronization for
operation within a national network. The received data streams are assumed to origi-
nate from mutually synchronized sources.
B.6.3.2 Terrestrial operation between national networks
Figure B-19/G.763 shows a method of DCME terminal synchronization for
operation between national networks via terrestrial facilities. Plesiochronous buffers
are required to resolve timing differences between the various plesiochronous net-
works. Due to the multiple source nature of the multi- destination configuration, the
plesiochronous buffers must be placed before the DCME receive function.
B.6.3.3 Satellite operation between national networks based upon continuous carrier
type services
Figure B-20/G.763 shows a method of DCME terminal synchronization for
operation between national networks based on continuous digital satellite carriers. Ple-
siochronous/doppler buffers are required to resolve timing differences between the ple-
siochronous networks and to remove satellite induced doppler shifts on the received
data streams. Due to the multiple source nature of the receive signals in the multi-desti-
nation configuration, the plesiochronous/doppler buffers must be placed before the
DCME receiver.
B.6.3.4 Satellite operation between national networks based upon TDMA-type services
Figures B-21/G.763 and B-22/G.763 show a method of DCME terminal syn-
chronization for operation between national networks over a satellite link based on
TDMA-type services. An appropriate interface is provided in the TDMA terminal to
permit interfacing the DCME over a primary multiplex port. The configuration shown
in FigureB-21/G.763 permits slip free operation between the DCMEs.
B.7 Performance
B.7.1 Speech performance (provisional)
Recommendation P.84 describes a subjective test method for comparing the per-
formance of DCME systems against suitable reference conditions for carefully defined
input signals. RecommendationP.84 consists of listening tests and is the recommended
source of information about subjective testing of DCME. These tests are a first step and
do not preclude the need for conversational tests.
It is recommended that a fixed delay be inserted in the transmit speech path to
reduce the probability of front end clipping. This delay compensates for activity detec-
tion time and DCME assignment message connection delay. The delay should be such
as to assure that the main speech spurt clipping is less than 5ms.
B.7.2 Voice band data performance
Extensive testing has demonstrated satisfactory voice band data performance
for the 40kbit/s algorithm specified in RecommendationG.726 for voice band data
rates up to 9600bit/s.
Voice band data at rates up to 12 000 bit/s can be accommodated by 40kbit/s
ADPCM. The performance of V.33 modems operating at 14400bit/s over 40kbit/s
ADPCM is for further study. Selection of a 64 kbit/s unrestricted channel through a
DCME is also possible and may be used for V.33 modems operating at 14400bits.
Fig. B-18/G.763 = 22 cm
Fig. B-19/G.763 = 22 cm
Fig. B-20/G.763 = 22 cm
Fig. B-21/G.763 = 22 cm
Fig. B-22/G.763 = 22 cm
Supplement No. 1
DCME TUTORIAL
(to Recommendation G.763)
1 Use of digital circuit multiplication system (DCMS)
DCMS provide the means to reduce the cost of transmission (e.g. long distance
transmission) by making use of the combination of digital speech interpolation (DSI)
and low rate encoding (LRE) techniques.
DSI is used to concentrate a number of input channels (generally referred to as
trunk channels) onto a smaller number of output channels (generally referred to as
bearer channels). It does this by connecting a trunk channel to a bearer channel only for
the period that the trunk channel is active, i.e.is carrying a burst of speech or voice-
band data. Since in average conversations one direction of transmission is active only
for 30% to 40% of the time, if the number of trunks is large the statistics of the speech
and silence distributions will permit a significantly smaller number of bearer channels
(bearer channel pool) to be used. Control information must also be passed between the
terminals to make sure that bearer and trunk channel assignments at each end remain
synchronized.
LRE uses digital filtering techniques to construct an estimate of the waveform
at both the encoder and the decoder. Since the actual information rate of speech is
much lower than the channel Nyquist rate the link used between the LRE encoder and
the decoder can operate at a rate which is dependent mainly on the quality of the mod-
els and the permissible amount of transmission degradation. The CCITT has standard-
ized in RecommendationsG.726 andG.727 a type of LRE known as ADPCM, the
performance of which has been extensively characterized. DCME uses the ADPCM
defined in RecommendationG.726.
Facsimile compression uses recognition and decoding of some or all of the
voice-band signals sent by the modem to enable the sub-multiplexing of the digital
information from a number of trunk channels onto a reduced number of bearer chan-
nels with the object of enhancing both the quality and the efficiency of transmission as
compared to rate reduction of the signals using ADPCM. This is under study.
The simplest way to use DCMS is in the single destination mode as shown in
Figure 1/G.763. This mode of operation is most economic for the largest routes. For
smaller routes there are two options:
û operation in multi-clique mode,
û operation in multi-destination mode.
Operation in multi-clique mode, see Figure 2/G.763, divides the bearer channels into a
number of blocks or cliques, each associated with a different route. There is normally a
fixed boundary between cliques, and trunk/bearer channel assignments are generally
carried in a control channel within the clique to which they refer. This limits the
dynamic processing of received channels to those which are contained in the wanted
clique; selection of the wanted clique channels can be done using a simple static digital
switch without reference to the assignment information. With a 2048kbit/s bearer sys-
tem in multi-clique DCMS the statistics of the DSI are unpromising with more than
three routes. RecommendationG.763 provides for two cliques.
Operation in multi-destination mode, see Figure 3/G.763, permits any bearer channel
to be associated with any trunk channel of any of a number of different routes. There is
no segregation of routes on the bearer, and therefore at the receive terminal it is impos-
sible to select the wanted channels without reference to the assignment information.
Multi-destination mode is economic for very small routes via satellite, but practical dif-
ficulties limit the number of routes which it is desirable to have on a single DCMS.
2 Location
Location of DCME depends on its use. Equipment used in single destination
mode or in multi-clique mode can in general be located at:
û ISC,
û earth station,
û cable head,
without significant restrictions. Equipment used in the multi-clique mode will typically
be located at the ISC so that the advantages of DCMG can be extended over the
national section. Equipment used in the multi-destination mode will typically be
located at the earth station or cable head. The reason for this is that whereas in multi-
clique mode the number of receive bearer channels at the DCME terminal is approxi-
mately equal to the number of transmit bearer channels, in multi-destination mode the
number of receive bearer channels at the DCME terminal is the number of transmit
bearer channels multiplied by the number of destinations. It therefore may be uneco-
nomic to provide sufficient transmission capacity between earth station and ISC to per-
mit location of multi-destination DCME at an ISC.
3 Transmission requirements
DCMS are usually required to carry any traffic which can be carried on ordinary
General Switched Telephone Network (GSTN) connections. That includes voice-band
data using V-Series Recommendation GSTN modems, facsimile calls following Rec-
ommendations T.4 and T.30 and using V.29 modems. In addition, in the ISDN 64kbit/s
unrestricted on-demand digital data and alternate speech/64kbit/s unrestricted bearer
services must be carried.
DCMS are primarily designed to maximize the efficiency of speech transmis-
sion. Use with voice-band data, especially at high rates, presents problems. These
problems are mainly due to the difficulty for 32kbit/s ADPCM of encoding voice-band
data waveforms.
4 DCME gain (DCMG)
The gain of DCME is the input trunk channel transmission multiplication ratio,
which is achieved through application of DCME, including LRE and DSI (for a speci-
fied speech quality at a certain level of bearer channel activity). The maximum avail-
able gain depends on:
û number of trunk channels;
û number of bearer channels;
û trunk channel occupancy;
û speech activity;
û voice-band data traffic;
û ratio of half duplex to full duplex voice-band data;
û type of signalling;
û 64 kbit/s traffic;
û minimum acceptable speech quality;
û dynamic load control threshold.
Of these the factor which has the greatest significance is the percentage of 64 kbit/s
digital data traffic. This is because a trunk channel carrying 64kbit/s traffic requires
two 32kbit/s bearer channels to be removed from the pool of channels available to the
DSI process.
The peak percentage of voice-band data may vary between 5 and 30 per cent, depend-
ing on route. This is discussed in greater detail in SupplementNo.2.
The type of signalling system used on the route can significantly affect the gain. Con-
tinuously compelled signalling systems hold channels active for undesirably long peri-
ods. In the case of CCITT R2 digital signalling via a DCMS used on a satellite, the
channel might be active for 5 to 14seconds.
The measured speech activity depends on the characteristics of the activity detector. It
is usual to assume 35 to 40per cent. Channels with high ambient background noise can
increase this activity factor. Outside of the route busy hour the occupancy of the trunk
channels by traffic will be lower than in the route busy hour. The effect of this is to
reduce the ensemble activity measured by the activity detector to about 27per cent out-
side the route busy hour, whereas it will be close to the speech activity factor, i.e.about
40per cent during the route busy hour.
The speech quality is governed by two main factors; the LRE encoding rate, and the
amount of speech lost while a newly active trunk channel is awaiting connection to a
bearer channel. If there are a great many newly active trunk channels in competition
the beginning of a burst of speech is more likely to be clipped or frozen out than if rel-
atively few trunk channels are active.
The speech quality of a DCME in a network with external echo control devices may be
affected by clipping introduced by echo control devices and by a possible noise con-
trast effect. In particular when echo suppressors or echo cancellers are used on circuits
where the near end generated noise is high with respect to the noise generated in the
remainder of the link, suppression of the far end noise may be objectionable due to
noise contrast. Possible means of eliminating this problem are use of echo control
devices which insert idle line noise at the appropriate level during suppression periods,
or insertion of idle line noise at the DCME during the relevant period when the echo
control device is integrated in the DCME. Another approach is discussed in AnnexB, º
B.5 to RecommendationG.763.
When commissioning a new DCMS, observations should be made of the type and char-
acteristics of the traffic which will use it. It is unwise to rely solely on customer com-
plaints to indicate when a system is poorly dimensioned. This is because interactions
between the DCMS and echo control (note) may obscure the true problem. Further-
more the consequence of trying to concentrate too many trunk channels onto too few
bearers may be simply to increase the calling rate and to reduce the call holding time.
This may result in greatly reduced quality, especially where continuously compelled
signalling systems are used, and levels of trunk channel activity occur far above what
was envisaged in the original system dimensioning.
Note û This highest speech quality is obtained when echo cancellers conforming to
Recommendation G.165 (Red Book) are used for echo control. However echo suppres-
sors conforming to Recommendations G.164 (Red Book) and G.161 (Yellow Book)
may be used.
Two possible criteria for acceptable speech performance are an average of 3.7 bits per
sample and less than 2.0% probability of clipping exceeding 50ms, or alternatively
that less than 0.5% of speech should be lost due to clipping.
Using the above criteria, approximations have been derived that relate the percentage
of voice-band data and the number of trunk channels to the gain of a DCME. Approxi-
mations intended for use in initial system dimensioning are given in SupplementNo.2
to RecommendationG.763.
If a more accurate representation is required, then it will be necessary to do the first
order Markov chain analyses referred to in the literature on DSI [1], [2], [3].
5 ISDN bearer services
DCMS are generally required to carry the full range of ISDN bearer services
which can be provided on a 64 kbit/s channel as specified in RecommendationI.230
(Blue Book). These are:
û Circuit mode 64 kbit/s unrestricted, 8 kHz structured bearer service category.
This may be used among other things for speech, multiple sub-rate informa-
tion streams multiplexed by the user, or for transparent access to an X.25
public network.
û Circuit mode 64 kbit/s, 8 kHz structured bearer service category, usable for
speech information transfer.
This is broadly similar to the preceding category, but with different access
protocols.
û Circuit mode 64 kbit/s, 8 kHz structured bearer service category, usable for 3.1
kHz audio information transfer.
This bearer service provides the transfer of 3.1 kHz bandwidth audio infor-
mation, such as for example voice-band data via modems, GroupI, II and III
facsimile information, and speech.
û Circuit mode alternate speech/64 kbit/s unrestricted 8kHz structured bearer
service category.
This service is similar to both the unrestricted and speech 64kbit/s circuit-
mode bearer services, but provides for the alternate transfer of either voice
or unrestricted digital information at 64kbit/s within the same call.
6 Restoration of services
For most applications the loss of traffic under failure conditions would be such
that it would be insufficient to install a single pair of terminals on a route without a
means of rapid changeover to spare equipment in the event of failure. This means that
DCME is often used in a cluster of N active DCMEs for onestandby. Automatic
changeover permits the standby to be loaded with the configuration and synchroniza-
tion information of the failed terminal. Other automatic fallback modes may be consid-
ered.
Failure of the transmission system between DCME terminals can be handled by
normal transmission system restoration procedures. Failure of the transmission systems
entering the DCME terminals from the exchanges may result in a wide range of differ-
ent alarm conditions being experienced particularly where a multi-destination DCME
terminal serves more than one exchange and more than one route. It is desirable to limit
the generation of alarm conditions to the channels which have actually failed.
7 Control of transmission overload
The reduction in the number of bearer channels available to the interpolation
process, due to the high activities of voice band and 64kbit/s data services or statistical
variations in the ensemble input speech activity can occur when the number of instan-
taneously active trunk channels exceeds the number of available bearer channels.
Either event requires action to be taken to safeguard speech quality. There are four pos-
sible solutions:
û The system can be dimensioned so that with the maximum anticipated short-
term trunk channel activities there is negligible probability of violating the
speech quality criteria. This employs the DCMS very inefficiently outside
the busy hour.
û A multi-destination system can be made to carry routes with widely different
busy hours, so that though the trunk channels might have relatively low non-
busy hour occupancy, the bearer channels would always be well loaded.
û Signals can be sent from the DCME to the exchange to busy out part of the
route when the quality criteria are violated. This is known as dynamic load
control (DLC), and can be an effective control method, but it cannot be ret-
rospective and it is slow to take effect. Furthermore care must be taken to
ensure that when circuits are returned to service the increase in bearer chan-
nel activity is not sufficient to result in the immediate reapplication of DLC.
û The signal to quantization performance can be traded against the clipping of
speech bursts. By using variable rate ADPCM algorithms it is possible to
quantize to three or optionally two rather than four bits on individual speech
channels on a pseudo-cyclic basis for a given number of samples. In this
way the system can be given a gradual degradation characteristic, rather than
suddenly overloading.
In a DCME conforming to Recommendation G.763 all of these techniques may be
used.
8 Transmission link performance monitoring
Experience with DCMEs has shown the value of using cyclic redundancy check
information in the detection and tracing of certain faults. In order to provide a compre-
hensive set of long-term and short-term indicators the DCME should provide the fol-
lowing means of monitoring the performance of any digital path(s) terminated upon it:
û cyclic redundancy check (CRC);
û frame alignment signal (FAS);
û other primary rate alarms;
û far end block error information of distant CRC (FEBE);
û DCME control channel FAS;
û violations of the Golay FEC of the control channel(s).
References
[1] KOU (K.Y.), O'NEAL (J.B.), NILSON (A.A.): Computations of DSI (TASI)
overload as a function of the traffic offered, IEEE Trans. on Communications,
Vol.COM-33, No. 2, February1985.
[2] BRADY (P.T.): A model for generating on-off speech patterns in 2-way conver-
sation, Bell System Technical Journal, page2445 etseq, September1969.
[3] Special issue on bit rate reduction and speech interpolation, Guest Editors M.R.
Aaron and N.S. Tayant, IEEE Trans. on communications, Vol.COM-30, No.4,
April1982.
Supplement No. 2
DCME DIMENSIONING METHODS FOR DIFFERENT ROUTE CHARAC-
TERISTICS
(to Recommendation G.763)
1 Introduction
This supplement draws attention to the implications of the measurements of
channel occupancy and voice-band data levels which have been done on particular
routes for which the number of voice-band data calls is either large in absolute terms,
or large compared to the total number of calls.
2 Route profiles
Figure 1 shows the kind of profile which has been obtained from measurements
on an FDM route between the United Kingdom and a country for which the proportion
of voice-band data calls was suspected to be high. It can be seen from this that there are
two peaks which are of interest in DCME dimensioningûone (the voice peak) where
voice is the dominant feature with a relatively small amount of voice-band data, and
another (the data peak) where voice-band data dominates voice.
Note û The data profile is not symmetric in each direction of transmission.
Voice-band data requires more bearer capacity than voice in a DCME system
incorporating digital speech interpolation (DSI) and low rate encoding (LRE) and
therefore it is not immediately obvious which of these peaks is the limiting factor when
calculating the achievable gain of a DCME on a particular route. Each route has to be
examined carefully to determine the achievable gain. The limiting value of the gain
does not necessarily occur at either of the peaks and in practice a scan across several
daysÆ profiles is necessary to determine the achievable gain.
FIG. 1
Figure 2 shows a typical profile obtained from the TDMA route for the same country.
Due to different traffic origins and loading distributions the voice and data peaks are
coincident, and the transmit and receive profiles are more nearly symmetrical in this
case.
FIGURE 2
3 DCME operation
Figure 3 shows a DCME consisting of a DSI stage and an LRE stage. Voice and
voice-band data have to be treated separately in each of these stages when trying to
access the achievable gain of a particular DCME faced with a particular route profile.
3.1 DSI gain for voice
This is dependent upon the number of input trunks carrying voice and it is not a
linear relationship.
Fig. 3 = 6 cm
3.2 DSI gain for data
Facsimile is the dominant data service and can be considered as half duplex, i.e.
on a particular call if data is flowing in one direction of transmission at a particular
time, then the opposite direction is silent. If the total amount of facsimile traffic in one
direction of transmission is balanced by an equivalent amount in the opposite direction
of transmission then a technique known as silence elimination can be employed to free
the opposite channel when data is flowing in one direction. This leads to a theoretical
DSI gain of 2. However, if the total facsimile traffic on a route is not balanced in each
direction of transmission, making silence elimination difficult to implement (or if
silence elimination has not been built into a particular DCME) then the DSI gain for
voice-band data is1.
3.3 LRE gain for voice
Studies have indicated that the minimum acceptable average number of bits per
sample is of the order of 3.6, which will be the threshold for operation of dynamic load
control. Therefore the LRE gain for voice is unlikely to exceed 8/3.6.
3.4 LRE gain for data
The LRE gain for data depends on how many bits/sample a particular system
allocates to a data call.
In this supplement all calculations assume the use of the 40kbit/s encoding rate
for voice-band data, in conformity with RecommendationG.763, therefore the LRE
gain for data=8/5.
Examples for facsimile compression are not presented.
4 Calculation of DCME gain
Table 1 gives some approximate non-analytical formulas for calculation of the
voice part of the DCME gain. It should be noted that these approximations are strictly
valid only for DCMEs conforming to RecommendationG.763 and having ideal speech
detection (i.e.the activity indicated by the speech detector is the same as the actual
speech activity).
4.1 Limitations
Ideally the calculation of the DCME gain would be done by a comprehensive
computer modelling of the system in the way which has already been demonstrated
with great success by Swedish Telecom Radio. Given an intimate knowledge of the
route, in terms of its hourly, daily and seasonal variations in voice and voice-band data
traffic flow, signalling systems, call holding times and effective/ineffective ratios over
a period of time it may be possible to model the route with a high degree of accuracy, at
least retrospectively. However the major limitation is the quality of the information fed
into the model. To overcome this limitation the digital channel occupancy analyser
(DCOA) has been developed. If the DCOA is used on a group of circuits which previ-
ous sampling or other information has shown to be typical then very useful dimension-
ing information results. The limitation then is the total permissible measuring time. In
most cases, for operational reasons, greater than twoweeks is unlikely to be feasible.
This represents a severe limitation on the attempt to create an accurate model, such that
for dimensioning (as opposed to the verification of the operation of the equipment)
Monte Carlo type simulations do not appear to be necessary.
4.2 Example gain calculations using simplified techniques
The following examples illustrate the concepts outlined in º 2, and demonstrate
the use of a simplified technique for DCME dimensioning using DCOA route profiles.
4.2.1 DCME dimensioning using the profile of a route without silence elimination
Assumptions:
Number of trunk channels at service date=240.
Figure 4 is the applicable DCOA route profile.
FIG. 4
Remark:
From experience or from rough calculations it can be seen that for the given number of
trunk channels and quantity of voice-band data traffic at least three DCMEs each using
30 bearer channels are likely to be required, but let us assume that four DCMEs are to
be used on the route in order to calculate the gain for the voice traffic (this gain is
dependent upon how many DCMEs the voice traffic is spread over). This is to ensure
that the DCMEs are not overloaded and may also allow for growth on the route. In
practice an iterative procedure would have to be used to determine the optimum num-
ber of DCMEs for each route.
From Figure 4 there are two peaks to be considered. One is dominated by the amount
of data (data peak) and the other is dominated by the amount of voice (voice peak):
Data peak
59% data:
number of data trunks =240 ┤ 0.59
=142 trunks,
=036
DSI gain =001(no silence elimination advantage to be gained because
almost all
=001the data is in one direction of transmission)
LRE gain =
17% voice:
number of voice trunks =240 ┤ 0.17
=041 trunks total
number of voice trunks
per DCME =010
DSI gain (for 10 trunks) =1.25 (from tables)
LRE gain =
Hence the 64 kbit/s bearer channel requirement is:
=23 (data) + 4 (voice)
=27 bearer channels.
The total bearer requirement is therefore:
27┤4
=108 bearer channels.
Voice peak
13% data:
number of data trunks =240 ┤ 0.13
=032 trunks total,
=8
DSI gain =001(no silence elimination advantage to be gained because
almost all t =001tthe data is in one direction of transmission),
LRE gain =
83% voice:
number of voice trunks =240 ┤ 0.83
=200 trunks total
number of voice trunks
per DCME =050
DSI gain (for 50 trunks) =1.92 (from tables)
LRE gain =
Hence the 64 kbit/s bearer channel requirement per DCME is:
=5 (data) + 12 (voice)
=17 bearer channels.
The total bearer requirement is therefore:
17 ┤ 4
=68 bearer channels.
Inference:
It seems therefore that in this case the DCME dimensioning is determined by
the number of trunk channels required to cope with the speech peak, and by the number
of bearer channels required to handle the data peak. Since the number of channels
shown as active by the DCOA is an average over the measurement interval, it is rea-
sonable to assume that all 240trunk channels, rather than only 132 were active for
some brief duration. Assuming that only the wanted bearer channels are used, and
neglecting the assignment channel, the achievable gain will be:
.
4.2.2 DCME dimensioning using the profile of a route with silence elimination
Assumptions:
Number of trunk channels at service date=347.
Figure 5 is the applicable DCOA route profile.
Fig. 5 = 13,5 cm
Remark:
On this route it appears that use of silence elimination will give some benefits.
Other DCOA measurements have indicated that there is approximately twice as much
voice-band data activity in the transmit direction as in the receive direction. Therefore
the achievable DSI gain on voice-band data due to silence elimination is of the order of
1.5. This assumes that there are as many transmit as receive bearer channels on each
DCME terminal. Rough calculations and experience indicate that because of the rela-
tively low voice-band data percentage of this example three DCMEs will probably be
sufficient.
From Figure 5 there is only one peak to be considered:
15% data:
number of data trunks =347┤0.15
=052 trunks
=018
DSI gain =1.5(due to silence elimination)
LRE gain =
72% voice:
number of voice trunks =347┤0.72
=250 trunks total
number of voice trunks
per DCME =083
DSI gain (for 83 trunks) =2.08(from tables).
Hence the 64 kbit/s bearer channel requirement per DCME is:
=8 (data) + 19 (voice)
=27 bearer channels.
The total bearer requirement is therefore:
27 ┤ 3
=81 bearer channels.
Inference:
In this case, assuming that only the wanted bearer channels are used, the DCME
can achieve a gain of:
.
However, as was shown by the previous example, it would be very unwise to
assume that a DCME gain as high as four will be achievable for all types of DCME,
without careful consideration of the route conditions. A corollary to this is that when a
DCME has been installed on a route its performance must be continually monitored to
ensure that changes in the traffic distribution on the route do not impact seriously upon
the transmission quality.
4.3 Two pitfalls for the unwary
Figure 6 shows a plausible example of a DCOA record, covering a typical two
hour period. On the basis of the trunk occupancy percentage for the route it might be
thought that the maximum bearer occupancy would be coincident with the peak in
voice traffic, however this is not so. The actual maximum occurs immediately before,
as Figure7 shows, during period2. The reason for this is that the voice-band data traf-
fic peaks before the voice traffic. Administrations may wish to consider whether this is
a likely state of affairs; whether for example the facsimile transmission of financial
results at close of business on any particular day is likely to result in follow-up tele-
phone conversations. The relevant information for each period is summarized in
Table2.
FIG. 6
FIG 7
Care must be taken when the short-term characteristics of a measured route are
not known. This may be especially significant when the route is small, since the pre-
sentation of voice-band data traffic may not be very uniform. Over a five minute
period2:1 variations in the short-term voice-band data activity level are not unusual
events. It might therefore be prudent to repeat any dimensioning exercises which use a
DCOA profile, but doubling all the voice-band data occupancies, for comparison
against the absolute maximum number of channels available when all voice activity is
allocated 3bits. If that comparison shows that clipping would be experienced under
those conditions then a lower gain setting should be chosen, based on whichever is
believed to be the limiting period.
5 Conclusion
An approach to dimensioning DCME systems has been demonstrated, which
though not statistically rigorous, is nevertheless capable of giving reasonable estimates
of system capabilities, given adequate input data. A number of potential dimensioning
problems have been described, and the solutions outlined. These methods have been
used successfully in the introduction of DCMEs on a number of routes.