Note: Descriptions are shown in the official language in which they were submitted.
~19795~
GR 96 P 1058
Method for fax transmission in diqital networks
The invention relates to a method for connection
of at least three terminals to a line of a digital
network, the transmitted voice data being compressed if
necessary.
One widely used digital network is the ISDN
network (Integrated Services Digital Network). The
transmission technigue which is used in the ISDN com-
prises two voice c~Annels being transmitted on a double
copper wire from an ~YchAnge to the subscriber' 8 buil-
ding. The two voice chAnnels are also called B-chAnnel 8
and have a transmission capacity of 64 kbit/s each. In
addition to the two B_C~Anne18, a control ch~nnel is also
transmitted which is also called the D-channel and has a
transmission capacity of 16 kbit/s. The two 64 kbit/s B-
chAnnels which are available in the ISDN are connected
via an appropriate adapter in the eYc~Ange to two ports
and, at the subscriber end, to two analogue telepho~e~.
One 64 kbit/s c~nnel and thus, if fast fax units are
connected, a best-possible transmission rate as well are
available to each of the two subscribers. The network
operator has the advantage that no additional wires need
be laid.
It is also possible to compress the voice data to
32 kbit/s. Transmission from two analogue subscribers per
chAnnel, and thus from a total of four subscribers via
one connecting line, is then possible on the ISDN line.
Compression to 16 kbit/s allows eight terminals to be
connected. Rnown algorithm~, such as the ADPCM algorithm
in accordance with CCITT StAn~rd (Comité Con6ultatif
International Telégraphique et Télépho~;gue), for
example, are used for compression and decompression,
without the voice quality being noticeably reduced.
Howe~er, transmission from $ast fax units and modems
suffers in this case, 80 that the full service is no
longer available to the subscriber. If fast fax units are
used, the compression system does not fully simulate four
individual analogue connecting lines. The following
- ~1979~6
GR 95 P 1058 D~ - 2 -
statements relating to fax signal transmissions apply to
the same extent to modem signal transmissions.
The invention is based cn the cbject of providing
a method of the type mentioned initially which makes fax
transmission possible with a transmission capacity that
is as high as possible.
The object is achieved according to the invention
in that fax identification is carried, and in that trans-
mission capacity is assigned for a fast fax transmission.
Such a method makes it possible to transmit a
multiplicity of analogue conversations using a trans-
mission capacity which is sufficient for the human voice,
of 16, 24 or 32 kbit/s. If a fax signal is identified,
then as much transmission capacity as possible is
assigned to the fax, in order to achieve transmission
that is as fast as possible, in the ideal case at
64 kbit/s. To this end, the fax transmission can be
assigned all the free transmission capacity which is not
yet occupied by other con~ersations.
The compression and the fax signal identification
in conjunction with the assignment of the transmission
capacity controlled by it can advantageously be inte-
grated in a single electronic module.
The data compression is preferably cancelled out
dynamically in this case. In the example of the ISDN
line, 8 bits are transmitted in parallel per ch~nnel, and
a transmission capacity of 64 kbit/s is thus achieved. If
split between four terminals, there are thus initially 4
bits available to each terminal in the case of data
compression. If one of the units is identified as a fax
unit, or a signal which is present for transmission is
identified as a fax signal, then, for example, one bit
can be withdrawn from a voice transmission and the
compression there can be increased to 3 bits, and this
additional bit can be assigned to the fax signal, 80 that
5 bits are available to it, with a total transmission
~ capacity of 40 kbit/s. In principle, compression can be
used which is the same for all conversations. Howe~e~, it
may be expedient to compress one voice signal to a
2197956
GR 95 P 1058 DB - 3 -
greater extent than the other voice signals, in favour of
the fax transmission. The fax transmis~ion can also be
distributed between both channel~ cf the line. In the
example just quoted, with four terminals distributed
between two c~Annel~, 1 bit could be withdrawn in each
ca~e from the three normal conversations, 80 that a total
of 7 bits, and thus 56 kbit/s, are available to the fax
unit.
If it is found during the fax transmission that
additional transmission capacity has become free as a
result of another conversation ~nA; ng, then, in a pre-
ferred development of the invention, this capacity which
has become free is likewise assigned to the fax trans-
mission .
In another embodiment of the invention, it is
possible to activate an additional, parallel modem. Such
a solution is particularly advantageous in the case of
lines which are loaded continuously and heavily. The
modem is switched off as long as only voice data are
being transmitted. It is switched on only if a faxtransmission is detected, 80 that less energy is consumed
overall.
The fax identification is preferably carried out
by detection of a 2.1 kHz tone. To this end, a function
is implemented directly in the processor of the compres-
sion controller, which function comprises a narrowband
bAnArA~s filter having a mid-frequency of 2.1 kHz, and
having a notch filter which runs in a precisely inverse
manner. The tone is detected by comparison of the energy
in the 2.1 kHz band with the energy outside the band.
Such identification must be provided for both directions.
In addition to a number of logic gates, it require~ about
1 MIPS (Nillion Instructions Per Second) of computation
power per c~Ann~l and direction, that is to ~ay a total
of 2 MIPS for separate identification in each direction,
since two algorithms are required for this purpose.
However, a first step is preferably used to determine
whether a fax signal is present, and a second step is
used to determine whether this is an incoming or outgoing
~1~7~
GR 95 P 1058 D~ - 4 -
fax signal. The second step can be implemented by chec-
king both directions simultaneougly. This achieves a
~aving of about 1 MIPS per ~nn~l, with a correspon~;ng
reduction in the power consumption. The algorithm first
of all considers the sum of the energies received from
both directions. If a tone is identified, that is to say
it is found in the first step that a fax signal is
actually present, then any desired one, but a specific
one, of the two directions is separated from the identi-
fication. Depen~;ng on which direction continues to bemeasured in the second step, the tone i8 still identi-
fied, or is not. The direction of the fax signal is given
directly by this.
The identification of the tone lasts for a
relatively long time because of the required identifica-
tion reliability. This may last for up to 0.4 8, since
the tone must be identified reliably for a certain time
duration. After disconnection or suppression of one of
the two directions, however, it is possible to deduce the
direction of the tone very quickly, since ab~ence of the
energy when the tone iB switched off is governed only by
the time constant of the filters. The direction identifi-
cation thus does not last significantly longer than the
identification itself. The fax identification tone is
available for at least about 0.5 8, and can be identified
reliably within this time.
The invention will be explained further in the
following text with reference to an exemplary embodiment
which i8 illustrated in the drawing. In detail, in the
schematic illustrations:
Fig. 1 shows a block diagram of subscriber con-
nections to a digital network, and
Fig. 2 shows one possible split of the existing
transmission capacities.
Fig. 1 shows four a/b terminal adapters which are
designated 1 and by means of which units which are
2197~56
GR 95 P 1058 D~ - 5 -
operated in a con~entional analogue manner are connected
directly to the ISDN. The terminal adapter in this case
carries out the conversion of the analogue sig~als into
digital signals, and the control of the protocols in the
D-channel. The module, which is designated 2 in the
following text, is used to control this system, which is
designated PCM-4 (PCM = Pulse Code Modulation). The
compressor 3 which carries out the ~oice compression is
connected downstream from this system. The fax identifi-
cation is integrated in this compressor and can identify,on the one hand, whether a fax signal is present and, on
the other hand, likewise identifies the direction in
which thi~ fax signal is being transmitted. The identifi-
cation is also used to control a modem (not illustrated)
which is connected in parallel with the compressor 3. The
transmission via the modem which is connected in parallel
with the compressor may take place alternatively or in
addition to the dynamic data compression. The module 4
represents the interface into the digital network and, in
particular, an ISDN-U interface.
Fig. 2 shows schematically the splitting of the
signals from four terminals between two channels of an
ISDN line. The rhAnnels which are designated 5 and 6 each
ha~e a width of 8 bits, which are represented as boxes.
In this case, each channel has a transmisgion capacity of
64 kbit/s. If four terminals El to E4 are connected, 4
bits are assigned to each. The voice data must be com-
pressed to 32 kbit/s. If the controller now determines
that one of the terminals E1 to E4 is a fax F, then
additional transmission capacity is assigned to it, and
the voice data of the other terminals are further com-
pressed. The assignment of the respectively appropriate
bits illustrated in the figure has been chosen
arbitrarily and is recorded in each case by the control
3S channel, which is designated the D-channel. In the illus-
trated example, the fax signal is thus also split between
two channels 5 and 6. Two of the three conversations E1
and E2 have been compressed to 3 bits, and thus
24 kbit/s, and one, E3, has been compressed to 2 bits,
21979Sli
GR 95 P 1058 D~ - 6 -
and thus 16 kbit/s. The full transmission capacity o$
64 kbit/s is thus available for the fax.
