Note: Descriptions are shown in the official language in which they were submitted.
W094/18782 213 3 7 3 0 PCT~S94/0l199
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"P~ADIO TELEPHONE SYSTEM USING VOICEtDATA SIGNAL".
~ield of the Invention
The present invention relates to a method and an
apparatus for automatically discriminating between voice
and modulated data transmissions o~er a radiotelephone
system, particularly to facilitate control operations of
the radiotelephone system to provide differential
treatment of data and voice calls.
Back~round of the Invention
Over the last decade, cellular radiotelephone
systems have been introduced in most ma~or population
areas in the United States and cellular radiotelephone
service is now widely available. These cellular systems
were originally designed for voice communication, and in
many respects were not well suited to digital data
communications.
The wèll known early difficulti.es of transmitting
modulated data over a cellular radiotelephone were first
overcome when a practical oellulsr data modem was
developed, through research by the assignee of the
present application. An early cellular data modem is
described in U.S. Patent 4,697,281 to O'Sullivan (now
U.S. Patent RE 34,034), assigned to the assignee of the
present application Development of more ad~anced
cellular modem systems continues, as exemplified by the
systems disclosed in the original inventor's U.S. Patent
No. ~,127~041, also owned by the assignee of the present
application, and pending U.S. Patent application Serial
No. 07/863,568, similarly assigned to the assignee of
the present application.
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Cellular ready modems, as distinguished from
traditional landline modems, ha~e particular protocol
and operating characteristics that effectively
compensate for adverse characteristics of the cel~lular
S radio en~ironment. As examples, an extended delay
before hangup on loss of carrier, packetized data
transmission with dynamically adjustable pa~ket sizes,
and the use of forward error correction, have all been
found to improve link quality and data throughput in
cellular data communications. Such enhancements make it
practical to transmit modulated computer or facsimile
data through a cellular telephone.
Another radio data transmission system, the
Cellular Digital Packet Data (CDPD) system, is under
development by a number of large cellular carriers as a
further part of the cellula~ radiotelephone network.
This system is discussed in the "CDPD System Over~iew~
dated May 22, 1992. The CDPD system uses cellular radio
frequencies and operates with modems similar in most
important respects to the cellular radiotelephone modems
described above. It is expected that customers
transmitting data over this sys~em will be charged for
each packet of data transmitted. Since only data is
being transmitted, a sLmultaneously two-way
communication link is not required. The CDPD system is
less ad~antageous than the cellular telephone network
for mobile data communica$ions for several reasons.
First, the CDPD system requires esta~lishing a parallel
cellular network, including costly additional
transceiver and processing hardware to be installed at
each cellsite where coverage is desired. The CDPD
system requires a special subscription and special modem
and transmitting equipment for trans~erring data
packets; this special modem and transmitter are
dedicated to this purpose, and are thus not useful for
SUBSTITUTE SHEET (RULE 26
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WO94/1X782 21~ 3 7 3 ~ PCT~S94/01199
any kind of telephone communications. In particular,
the modem cannot be used with a standard telephone.
Thus, the equipment package needed by each user is a
relatively high-cost, limited market, dedicated purpose
product. In contrast, a sta~dardized, mass produced
modem installed in a portable computer, such as the
modems disclosed in U.S. Patent 5,127,041 and pending
application Serial No. 07/863,568, can be used both for
mobile and fixed data communications over a variety of
telephone and other data networks. Such a modem can be
used with the portable cellular transcei~er already
possessed by the user for making mobile voice calls, so
no dedicated transceiver is needed. Since the planned
CDPD packet radio sys~em is dedicated to data
lS transmission, it does not transmit voice signals, but is
instead designed to record the number of packets
transmitted for billing purposes.
Cellular radiotelephone systems are made up of a
number of geographically dispersed cellsite
transceivers, which are connected within a network,
usually using standard Tl telephone trunk lines, to a
mobile ~elephone switching office IMTSO) which is
similarly connected to the local telephone central
office (C0). These systems are generally designed to
establish and maintain an analog communications channel
between a cellular radiotelephone user in a cellsite
area, and another telephone user anywhere in the world.
Generally, radiotelephone service is provided on a
metered basis. Various ser~ice plans are available, bu~
typical cellular call charges during weekday business
hours may be between 19 and 65 cents per minute.
Within their bandwidth and other limitations, the
cellular channels convey any signals generated by
parties to the link, without examining the particular
character of the signals. Thus, there is presently no
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wos~ll8782 213 3 7 3 ~ ; PcT~ss4/ollss
means by which a cellular carrier could vary operation
of the system depending on the sort of information
conveyed by the customer over the cellular link.
Apparatus for discriminating between voice and data
signals is ~nown, al~hough as far as the applicant is
aware, such apparatus has not been used in conjunction
with a cellular radiotelephone system to achieve the
advantages pro~ided by the present invention. U.S.
Patent 5,073,921 to Nomura et al. shows an apparatus
which distinguishes between voice and fax signals to
control the connection of a facsimile machine to a
telephone line. U.S. Patent 3,927,259 to ~rown shows a
system that identifies signals as either noise,
modulated data, voice, or no signal based on analysis of
repetition elements in the signals. This system may be
used to control circuit switching in a telephone system.
Similarly, U.S. Patents 3,851,112 to ~usan,
5,095,534 to Hiyama, 5,081,673 to Engelke et al.,
4,972,462 to Shibata, 4,95S,083 to Phillips et al.,
4,498,173 to Reudink, 4,403,322 to Kato et al.,
4,376,310 to Stackhouse et al., 4,330,862 to Smolik, and
3,939,431 to Cohlman show circuits that carry and/or
dete~t both ~oice and data signals.
U.S. Patent 4,654,867 to Labedz et al. discloses a
cellular voice and data communications system which
switches between data and Yoice formats in response to
requests transmitted by user equipment. However, this
system merely receives and responds to predetermined
commands and does not provide any automatic data
determination functions. In addition, Labedz does not
disclose control of billing computers to provide
differential service rates based on the type of
transmission.
Persons transmitting computer or facsimile image
data o~er cellular radiotelephones tend to be business
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users who are willing to pay for mobile data
communications services offering, alternatively, voice
or data transmission capabilities. The in~entor
believes that some cellular data carriers may wish to
charge a differential rate, or even a lower rate, for
data transmission services. In addition, the inventor
has determined that advantageous operation could be
obtained by controlling calls differently depending on
whether they are data or ~oice calls. For example,
actions that may result in degraded data transmission
performance could be delayed or avoided if it was known
- that the cellular channel was being used for data
transmission. For these re~sons, there is a need for a
system and method for identifying cellular
radiotelephone calls as modulated data transmission
calls.
Summary of the Inven~iQn
Therefore, it i5 a general object of the present
invention to provide a novel and unique method for
discriminating between voice and modulated data
transmissions over a radiotelephone system to facilitate
differential treatment of data and ~oice calls.
Another general object of the present invention to
psovide novel and unique apparatus for discriminating
between voice and modulated data transmissions over a
radiotelephone system to facilitate differential
treatment of data and ~oice calls.
Another object of the present invention is to
provide a system and method to facilitate cXarging a
differential rate for the transmission of data over a
cellular radiotelephone link.
It is another object of the present invention to
pro~ide a system and method for identifying cellular
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telephone calls as modulated data calls so that a
modified call handling procedure can be followed with
respect to such calls.
A further object of the present invention is to
pro~ide a system and method for detecting modulated data
transmission over cellular radiotelephone channels to
minimize cell handoffs of data calls which are not
strictly necessi~ated by travel between cellsites.
These objects and others which will be apparent
upon review of the specification and drawings, are
achieved in the present invention by pro~iding various
methods and apparatus for differentiating between voice
calls and calls in which aata is transmitted. In a
first preferred embodiment, data transmission detectors
installed at the cellsite monitor cellular channels
carrying calls and detects the presence of modulated
. data. The data transmission detectors are connected to
the command channel network of the cellular system and
indicate which channels are carrying modulated digital
data. This information is then used by the cellular
~ system for billing and call-control purposes.
-~ In a second embodiment, a cellular compatible modem
or an interface device connecting the modem to the
cellular telephone t~ansceiver generates predetermined
dialing string instructions during call placement to
infonm the cellular carrier that the call being placed
is a data call.
In a third embodiment, a cellular telephone user
desiring to transmit data over a cellular telephone
network can identify his call as a data call tO allow
the cellular system to acti~ate special handling for the
call.
In a fourth embodiment, periodic transmissions
readily detectable by cellsite or MTSO switching
3S equipment are inserted in the data stream transmitted by
'~
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a modem and their presence is used to activate special
handling for the call.
The systems and methods disclosed permit charging
- a differential rate for the transmission of data over
the cellular link. In addition, the provision of data
transmission detection permits modifications in call
handling. For example, a call identified by the
cellular carrier as a data call can be given priority in
avo.ding cell handoffs not strictly necessitated by
1~ tra~el between cellsites.
~rief Description of the Drawin~s
.
Figure 1 is a flowchart of an embodiment of the
process of the present invention in which modulated data
is detected by analyzing signals on a channel within the
cellular carrier system.
Figure 2 is a block schematic diagram of an
apparatus for performing the process of Figure 1.
Figure 3 is a graphical depiction of the criteria
used to select a reference level signal for use in
performing the process of Figure 1.
Figure 4 is a block schematic diagram of the
installation of the apparatus of Figure 2 at a cellular
radiotelephone system cell site transceiver.
Figure S is a flowchart showing ano~her embodiment
of a process for identifying a call as a data call, in
which a special predetermined dialing sequence is
generated by user equipment and detected by the cellular
! carrier equipment.
Figure 6 is a block schematic diagram of a digital
trunk-monitoring embodiment of the data/voice
discrimination apparatus of the present invention.
Detailed Description of the Preferred Embodiments
.
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Referring to Figures lA and l~, a flowchart of one
embodiment of the process of the present in~ention in
which modulated data is detected by analyzing signals on
a channel within the cellular carrier system is shown.
The process is initiated at bl~ck 102. The first
operation in this process is selecting a cellular
channel to be analyzed, as shown in block 104. The
channels to be analyzed are selected from the available
channels at the location where the process is performed,
and prefexably include all the channels a~ailable at
that location in which there is a possibility of data
transmission. Typically, when ~he method is performed
at a cellsite location, the channels analyzed include
all of the voice channels operative at the cellsite.
lS Preferably, a data detector may be provided for each
channel, although in some instances a plurality of
channels may be monitored in sequence by a single data
detector. If a plurality of channels are to be
analyzed, the channels may be selected sequentially or
by some other suitable criteria. For example, if
certain cell channels were frequently found to be used
for data transmission, these channels may be analyzed on
a priority basis.
Following the selection of a channel to be
analyzed, if a separate apparatus is not pro~ided for
each channel, the selected channel is filtered in block
106. Specifically, the selected channel is high-pass
filtered so that only frequencies above approximately
800 Hertz remain. This remo~es any telephone company
call progress signals such as dial tones, busy, and
ringback signals, as well as the most powerful voice
frequency fundamentals from the monitored signal.
The filtered channel is then rectified in block
108. This rectification con~erts the signal from an
alternating current, or AC, signal to a rectified or
SuBsTlTu-l-csH~L7~ LE26)
WO94/18782 ~13 3 7 3 J PCT~S94/01199
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g
chopped signal. The rectified signal is then filtered
in block 110 by an integrating circuit to produce an
integrated DC signal. The level of this slowly varying
DC signal is a measure of the high frequency power level
of the selected channel being monitored.
The slowly varying DC signal, or channel level
signal, is then compared with a reference level signal
in bloc~ 112. The reference level signal is selected
based on empirical testing. The level is selected such
that when a data transmission is present on the selected
channel, the channel level associated with that data
transmission will be greater than the reference level.
Similarly, when the selected~channel is carrying a voice
signal, the channel level associated with the voice
transmission will be lo~er than the reference level.
The selection of this reference level is discussed in
more detail below with reference to Figure 3.
If the comparison in block 114 between the channel
level and the reference level indicates that the channel
level is below the reference level, then control will
return to block 104 so that the process will immediately
select another channel to be analyzed. If the
comparison in block 114 hetween the channel le~el and
the reference level indicates that the channel level is
above the reference level, then control is transferred
to block 116.
Once it has been de~ermined that the channel level
is abo~e the reference level, a timer is started in
block 116. The process then continuously compares the
channel level to the reference level as shown in bloc~
118 and checks the elapsed time in block 120. If the
channel level remains above the reference level for a
predetermined time, for example se~en seconds, the
process alerts the cellular command network in block
122. In this situation, it has been determined that the
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channel being analyzed contains a data transmission.
If, however, the channel level drops below the reference
level prior to the tim:er~indicating the predetermined
time period has passed, the process will return to
select another channel to be analyzed in block 104. In
this case, it has been determined that the cellular
channel does not contain a data transmission.
The requirement that the high frequency power level
be high throughout a predetermined time period before
identifying the call as a data call provides a
significant advantage, because this test tends to reduce
false data call identifications which might otherwise
result, for example, from~a happened coincidence of
monitoring and a brief high pitched voice transmission
such as laughter or a shriek.
Figure 2 shows a block diagram of a data detector
~- sccording to one embodiment of the present invention.
As shown in Figure 2, a data detector 202 comprises high
pass filter 204, rectifier and filter 206, comparator
208, and timer 210.
High pass filter 204 is preferably constructed as
a multi-order operational amplifier circuit, with a
cutoff freguency of 800 Hz. High pass filter 204
filters out any telephone company call progress signals
such as dial tones, busy, and ring~ac~ signals, as well
as voice frequency fundamentals~ The input 212 of high
pass filter 204 may be connected through a differential
input interface to tip and ring lines of the circuit to
be monitored. Alternatively, the input 212 of high pass
filter 204 may be connected directly to an output (no~
shown) of a demodulator located at a cell processing
location. Preferably, a separate data detector 202 may
be provided for each communications channel to be
monitored. However, the input 212 of high pass filter
3~ 204 can be optionally connected to a selecting device
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213, which may be an electronic multiplexing circuit
designed to sequentially provide inputs to data detector
202 from each of a selected set of communications
channels at the site which are to be monitored.
SRectifier and filter 206 operates to con~ert an AC
signal received from high pass filter 204 to a slowly
varying DC signal representing the high frequency power
level of the telephone signal. As shown, rectifier and
filter 206 may consist in its simplest form of a diode
10207 or other rectifying element(s) placed in series with
the signal passing through rectifier and filter 206, and
a capacitor 209 or equivalent integrating element(s)
connected between the signal and ground.
Comparator 208 receives this power level signal and
15compares it to a fixed reference voltage v+ provided at
comparator input 214. Comparator 208 is conventional
and the output 216 of the comparator is a logic level
ndie~ting the relationship between the power le~el
signal and the reference voltage V+. If the power level
signal exceeds the reference voltage V+, the logic level
-~ output of the comparator goes high to indicate that the
monitored signal appears to be a data signal rather than
a voice signal. The voltage V+ at reference ~.ltage
input 214 to comparator 208 is adjusted based on
empirical testing to produce reliable switching of
comparator 208 at high frequency power signal levels
experienced when a data modem is used to transmit
information through the telephone line, while providing
a low le~el output when ~oice signals are transmitted.
! - 30 Timer 210 measures the time that the output of the
compara~or 208 has remained high, indicating a possible
data signal. If the comparator output is continuously
high for more than, for example, 6 to 7 seconds, the
timer 210 times out and the timer output signal 218 can
then be used as a definitive indication that a data call
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is taking place. Data detector 202 is provided with an
output 220 from timer 210 to indicate the presence of a
data call on the monitored channel.
In response to output 220, the cellular carrier can
take desired steps in response to the use of the line
for data transmission. For example, the indication of
a data call may be fed to a billing and call tracking
computer of the cellular carrier to produce differential
billing, either at a higher or a lower rate than is used
for voice calls. Also, call handling functions may be
~aried for detected data calls as opposed to ~oice
calls. For example, the cellular carrier may prioritize
channels within a cell bas~d on their current usage so
that channels being used for voice are handed off in
lS preference to channels used for data. Data calls may
thus be given a higher priority to avoid handoffs unless
absolutely necessary for system operation or because the
incoming signal has weakened because of mo~ement into an
adjacent cell. In metropolitan areas, there are a large
number of cellular telephone users and significant cell
o~erlap exists. With the method of the present
in~ention, once a data call is detected, the cellular
telephone system can reduce handoffs of data calls which
occur not because of distance from the transmitter, but
because of the movement of other users into the area and
resulting overloading of the cell. Where cells overlap
significantly, the system may also be able to reduce the
a~solute number of handoffs experienced by a moving
vehicle conducting a da~a call. The call could be
handed off from a cell only when the user is in range of
a more distant cell that is not adjacent to the first
cell, producing one handoff, rather than handing off to
the adjacent cell and then to the more distant cell for
a total of two handoffs. Furthermore, cer~ain cellular
channels that may be more resistant to noise and other
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outside interference could be assigned to calls that are
known to be transmitting data. For example, if another
radio network was generating an interfering signal at a
frequency that overlapped with a cellular channel, that
channel, although suitable for voice transmission, may
be less suitable, and thus avoided, for data
transmission. Also, channels could be established with
differential bandwidth capabilities, with wider
bandwidth channels being assigned based on the detection
of a data call according to the present invention.
Selecting one of these various forms of "premium"
channels for data transmission would result in fewer
data transmission errors~ and thus greater data
transmission efficiency. ~herefore, o~erall connect
time and cost could be reduced.
If users are expected to end data use and continue
a call as a ~oice call, timer 210 can also used to
identify the end of the data use phase of the call. If
sfter indicating a data call, timer 210 indicates that
the comparator has subseguently produced a n low output
for more than, for example, 6-7 seconds, this could be
~; used as an indication of the end of the data use phase.
An extended time in a non-data mode is required to
identîfy the end of a data phase since cellular handoffs
can be expected to interrupt the data flow and cause the
comparator output ~o go temporarily low, even though
data transmission will resume after the handoff is
completed. Thus, timer 210 operates to compensate for
effects of handoffs in this manner.
I The apparatus illustrated in Figure 2 has been
described and illustrated in analog form for simplicity
of understanding. However, it should be recognized that
such apparatus may also be constructed using digital
processing equipment. The circuit shown in Figure 2
~ 35 could also be appropriately constructed using a digital
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signal processing chip programmed to pro~ide the
described operations. In general, with the analog
version of this circuit, it is desirable to minimize the
number of channels to be monitored by each circuit since
the time required for detecting data on each channel (up
to seven seconds) means that a single circuit miqht miss
some data transmissions if required to monitor a large
number of channels. In a digital Lmplementation of the
circuit, it would be possible to increase the number of
channels monitored with a single data detector.
In addition, it should be recognized that the
apparatus disclosed could be implemented in software
associated with a digital ~elephone switch, such as a
Harris switch, used in the cellular network for
lS digitizing and transmitting multiplexed telephone
signals over a Tl or other trunk line. Such switches
operate on a digitized, pulse code modulated, time
di~ision multiplexed data stream in which each time slot
carries a different call. It would be possible to
perform the functions disclosed with reference to
Figures l and 2 by monitoring the series of numbers
passing through the Tl compatible switch, representinq
digitally sampled signal levels, and to similarly
determine the frequency content of the signal in each
~all by examining the Tl trunk line's PCM signal level
sample numbers in one or more successive ~ime divisions
associated with that call. A large concentration of
sLmilar numbers could be used to indicate a data call.
Fourier transform algorithms could also be used to
! 30 determine the frequency content of the signal based on
the changes in signal level noted, and a data call could
be identified by a large high-frequency level content in
a manner similar to that described above.
One method for selecting a reference level for use
with the process and apparatus discussed above is
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depicted in Figure 3. Figure 3 shows a graph of channel
power le-vel Yersus frequency. The graph contains a
typical power level curve for a channel containing a
data transmission 302, a typical power level curve for
a channel containing a voice transmission 304, and one
possible reference level 306 that could be used. As
discussed above, the reference le~el is selected based
on empirical testing. The level is selected such that
when a data transmission is present on the selected
channel, the channel level 302 associated with that data
transmission will be greater than the reference level
306. 5Lmilarly, when the selected channel is carrying
a voice signal, the channeI level 304 associated with
the voice transmission will be lower than the reference
le~el 306. As can be seen from Figure 3, there are a
number of acceptable reference levels that could be
used. In the most preferred embodiment of the
invention, however, the reference level would lie
approxLmately half-way between the typical output
voltage of the rectifier and filter 206 (shown in Figure
2)- for a data transmission and the typical output
~oltage of this rectifier and filter 206 for a voice
transmission.
The data detector 20~ may be installed at any
desired location in a cellular radiotelephone system,
for example at a cell site as shown in Figure 4. Figure
4 contains an antenna 402, cellular transcei~er and
demodulator circuitry 404, channel transmission lines
406, channel selection circuitry 408, channel analysis
apparatus 202, cellular command network 410, and billing
compu~er 412.
The antenna 402 is connected to the cellular
transceiver and demodulator circuitry 404. Circuitry
404 is connected to the channel selection circuitry 408
through one or more channel transmission lines 406.
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Channel selection circuitry 408 is connected to channel
analysis apparatus 202, cellular command network 410, ~-
and billing compute~ 412. Channel analysis apparatus
202 is connected to cellular command network 410 and
billing computer 412.
In operation, antenna 402 receives a radiotelephone
transmission. The transmission is then processed by the
cellular transceiver and demodulator circuitry 404.
Circuitry 404 demodulates the radiotelephone
transmission and produces a plurality of single cellular
channel outputs. The output of circuitry 404 is
connected to transmission lines 406 which carry the
single cellular channels to ~hannel selection circuitry
408. Preferably, a separate data detection circuit may
be provided for each channel. However, if the data
detection circuits are constructed such that a plurality
of channels may be effectively monitored by a single
channel analysis unit 202, channel selection circuitry
408 may be pro~ided to select one of the cellular
channels for analysis and provide that channel on output
409. The channel to be analyzed is provided to the
channel analysis unit 202. Additionally, identification
of the channel to be analyzed is pro~ided to both the
cellular command network 4lO and the billing computer
412. This operation is desirable so that if the channel
is found to contain a data transmission, the command
network 410 and billing computer 412 will be able to
determine which channel was analyzed and to take
appropriate action. This may include raising or
lowering the charges for that channel, or marking the
channel for modified call processing. Alternatively,
the command network 4lO and billing computer 412 could
determine the channel directly from the output of the
channel analysis apparatus 202, e.g. based on the timing
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of the monitoring operations of channel analysis
apparatus 202.
The output 411 of the channel analysis apparatus
202 indicates whether that channel contains a data
transmission. This output is connected to cellular
command network 410 and billing computer 412. ~s
described abo~e, this allows these devices to handle the
call differently if a data transmission is detected.
Figure S shows a ~lowchart of a second preferred
embodiment of the present invention in which a special
predetermined dialing sequence is generated by user
equipment and detected by the cellular carrier
equipment. Figure SA shows a flowchart of a process
that could be used in mobile cellular equipment for
automatically inserting the predetermined dialing
sequence into a number dialed by a user. Fi~ure SB
shows a flowchart for a process that could be used at a
- cellular receiving site by a cellular carrier for
detecting the predetermined dialing sequence. Although
this dialing sequence may be automatically inserted
- during dialing by cellular equipment, it may also be
manually inserted by a user desiring to make a cellular
data call. In such a case, a user might append a * or
# ~ey, or another predetermined signal qenerated from
the cellular telephone or an attached device. For
example, such signals could include numeric or any other
keypad generated signals depending on the capabilities
of the keyboard ox its circuitry. A standard DTMF
keypad is capable of generating 16 different dialing 30 signals, including the numerals 0-9, #, ~, and four
additional signals A, B, C, and D for which keys are not
typically provided on landline telephones. Any of these
signals, or combinations thereof, may be appended at the
end of the dialed number to indicate to the cellular
carrier that the call is a data call. The signals used
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may be signals which can be generated by the cellular
telephone keypad or may be other signals which cannot
normally be generated by the keypad. A sLmilar code
imbedded in the dialed telephone number or preceding the
dialed telephone number could also be used, depending on
the con~ention established by the cellular carrier.
Typically, it is anticipated that the dialing
signals used to identify the call to the cellular
carrier as a data call will be generated by a cellular-
capable computer modem of the type disclosed in U.S.Patent 5,127,041 or pending application Serial No.
07/963,568, the disclosures of which are incorporated by
reference, or an interface associated with such a modem
where applicable. In this case, the computer modem or
its associated interface will generate the dialing
si~nal instructions to the cellular radiotelephone being
used to place the call, and these dialing signal
instructions will include the required dialing sequences
according to a convention established by the celiular
carrier(s) to identify the call as a data call. The
communications software of the computer associated with
the computer modem can be provided with the telephone
number to be dialed in a known manner, with such
programming including in this case any additional
2S dialing sequences reguired to identify the call as a
data call. For example, if the telephone number of the
call receiving party is 555-1234, and the c~llular
carrier recognizes the dialing sequence #~ as indicating
a data call, the "phone book~l of the communications
! 30 software can be programmed with the number 5551234#*.
In one particularly preferred em~odiment, the
correct telephone number to be dialed can be entered in
the ~phone bookl~ of the communications software in the
computer, such as "5551234", and the cellular-capable
~: 35 modem is designed to automatically add the required
'~ '
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additional dialing sequences to identify the call as a
data call, but does so only when the modem is connected
to a cellular telephone system. This avoids the
potential need to change the programming of the computer
communications software or to select a different "phone
book entry or scripted connection sequence depending on
the type of telephone network being used (e.g. landline
vs. cellular).
Thus, when such a modem is connected to a cellular
telephone system, and a dialing instruction is received,
the modem may add data call identifying dialing
sequences, such as appending a #~ sequence to a
received number '5551234`. When the modem is connected
to a landline telephone system, the modem will not
15~ generate the additional appended dialing sequences since
such signals are not necessary in landline
communications. Of course, it would also be possible,
although less preferred, to program the modem to
automatically generate the additional signals for both
landline and cellular communications since telephone
company landline central offices normally ignore
additional dialing sequences after a full telephone
number is received. However, if the cellular carrier
adopts a convention which involves additional dialing
sequences that precede or are intermixed within the
telephone number to be dialed, selective transmission of
such dialing sequences with only cellular connections
would be essential since such additional dialing
sequences would interfere with normal landline call
placement.
Referring now to Figure 5A, a flowchart of a~
process that may be used to insert a predetermined
dialing sequence into a dialed number is shown. In
block 502, the modem or its assDciated interface
receives dialing instructions from the associated
, .
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computer. The receipt of these signals continues until
it is determined th~-~ the dialing is complete. Dialing
completion may be determined by a number of methods,
including counting the number of di~its dialed, failure
S to receive a further dialed digit within a predeterminad
time, or the receipt of an indication signal indicating
that the full telephone number has been transmitted and
that dialing should commence.
When the full telephone number is recei~ed, the
modem or its interface operates in block 504 to insert
the predetermined additional dialing sequence to
identify the call to the cellular carrier as a data
call. In the example given~above, the dialing sequence
#~ would be appended to the telephone number received,
e.g. 5551234 to form the full dialing sequence
55~1234#*. This dialing request would then be
transmitted as shown in bloc~ 505 through the cellular
telephone by the modem or the interface, typically by
transmitting signals to the cellular bus which cause the
cellular telephone to emulate keypr~ss signals from its
own control keypad. The modem or the interface will
then cause the cellular telephone to emulate the
pressing of the ~send~ key to transmit the dialing
sequence to the cellular carrier, thus completing the
~ransmission of the modified dialing sequence.
While the predetermined dialing sequence has been
described in terms of transmission during call setup, it
would also be possible to transmit such dialing
information over the command channel of the cellular
30 system during the progress of a call or following the
completion of a call. Signals could be transmitted
periodically, if desired, during data transmission in a
data call operation.
Figure SB is a flowchart for a process useful at a
cellular receiving site by a cellular carrier for
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detecting the predetermined dialing sequence generated ;
according to the process disclosed above with reference
to Figure SA. The cellular telephone network command
system operates in block 508 to receive the cellular
S dialing request over a command channel from a cellular
radiotelephone which is in communication with a cellsite
connected to the command system. The receipt of this
dialing sequence occurs in a conventional manner, except
that the dialing sequence may selectively include
information specifying that a data call is to be placed.
In block SlO, the command system examines the dialing
sequence to determine whether this dialing sequence
includes the predetermined signals indicating that a
data call is being placed. If so, control passes to
~5 block 512 and the cellular command system will flag the
call as a data call and process it accordingly. As
descsibed previously, various differential processing of
data calls is possible as compared to ~oice calls,
including differential rates used in billing,
differential handoff operation, and differential channel
assignment .
In a further embodiment illustrated in Figure 6,
predetermined detectable information may be embedded in
the data and sent over the voice channel of the
cellular telephone system, where such information can be
detected by monitoring. For example, in the data
transfer protocol disclosed in U.S. Patent RE 34,034,
the information is fo~med into packets and each packe~
is transmitted with associated packet identification and
cyclic redundancy chec~ information. The system could
be designed to detect the packet identification or
packet separation information. It would also be
possible to add to any data stream to be transmitted
over a cellular telephone any desired periodic
predetermined bit pattern. For example, a pattern such
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as OOFFOOFFOOFF (hexadecimal) could be transmitted after
every N packets, where N is an integer. The modems used
would in any case be programmed to eliminate such bit
patterns upon receipt as being` part of the protocol
S overhead, so that this operat~ion would be transparent to
those transmitting data. The repeating bit pattern
provided in the protocol co~ld be detected by
demodulating the data stream at any point and monitoring
the resulting data to identify the protocol in use. In
this way, differential operation could be achie~ed
depending not only upon the transmission of data, but
upon the type of protocol used. More preferably, the
signal pattern of the data stream can be monitored as
the data stream passes through cellular telephone office
switching equipment in digital time-divisîon-multiplexed
form.
In the embodiment of Figure 6, a cellular telephone
~ystem cell site is shown generally at 602. The
cellsite includes antenna 604, cellsite transceiver 606,
digital/analog converter 608, trunk switch 610, and
command system circuits 612. Command system circuits
612 are connected appropriately to control the other
system elements, and command system circuits 612 are
slso connected to the remainder of the cellular
telephone system 614, which inc~udes mobile switching
offices, other cellsites, billing and recording
equipment, landline telephone central office interfaces,
and other parts of a conventional cellular telephone
system. In operation, the conventional cellsite
tran~ceiver 606 transmits and receives calls and
commands between the cellsite 602 and mobile users
served by cellsite 602 o~er both voice and command
channels, under the control of the command system 612~
A large number of radio channels are pro~ided at each
cellsite 602 to support numerous mobile users within
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range of cellsite 602. Typically, at least the voice
channel information transmitted to and from these users
o~er the radio channels is converted to a multiplexed
digital form by analog/digital converter 608 which can
S then be processed on a standard telephonic ~runk line
616, such as by trunk switch 610.
In the present invention, the trunk switch 510 is
provided with special software, either operating within -~
trunk switch 610 or in a connected computer (not shown).
This ~oftware monitors the data stream o~er the trunk
line 616 to identify patterns associated with data
transmission rather than ~oice transmission. For
example, the software may be programmed to recognize the
passage of particular voltage patterns reflecting the
transmission of a particular bit pattern, such as the
OOFFOOFFOOFF pattern given as an example above. This
pattern could be identified in the digital T1
transmission stream as a series of alternations between
two substantially identical voltage levels, which would
be reflected as a series of alternations between two
substantially identical pulse code modulated codes. If
such a pattern is recognized during the passage of one
or more tLme divisions associated with a par~icular
channel, it can be deduced that ~he channel is being
used for data transmission~ Confirmation of this
dedu tion by more detailed analysis of the signal
carried on the channel can be pro~ided, if desired, in
any of the manners described herein to avoid incorrect
identification of data calls. If a channel is
identified as carrying a data call by the trunk switch
610 or an associated computer monitoring system, the
switch or computer can transmit a signal to the command
system 612 or to other components of the cellular
telephone system 614, such as a ~illing computer, to
cause differential operation of the cellular telephone
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system based on the presence of a data call rather than
a voice call. Thus, a method for detecting the use of
particular data protocols over a cellul~r telephone
network has been disclose~.
,
Simila~ly, as discu`ssed previcusly in conjunction
with Figure 2, a bandwidth usage analysis can be
performed if desired by the trunk switch 610 and its
associated equipmen~. By monitoring the dispersion over
a predetermined period of time of the digital number~
carried over each channel of trunk line 616, which
represent the original analog signal levels, the system
according to the present invention can discriminate
between data signals and Yo;ce signals. This could be
accomplished by counting the frequency of appearance of
each possible digital number over a brief, defined time
;~ period. Then, the collected frequency data can be
analyzed to discriminate between ~oice and data signals.
For example, a voice signal will typically have periods
of low or no signal level, and a broader ~ariety of
signal levels. A data signal can be identified by
highly concentrated signal levels.
Also, data signals could be identified by
maintaining either a count of the sample-to-sample
magnitude changes, or a running a~erage of the magnitude
2S changes in the digitized representation of the signal.
Larger and faster changes in the digitized signal levels
can be used as an indicator of data transmission. A
cutoff level of signal change magnitude usable to
effectively discriminate between voice and data signals
can be determined experimentally using methods simi~ar
to those described previously for determining the
reference voltage V+ used in the discrete analog
monitoring circuit of Figure 2.
The more abrupt voltage changes in a data signal
are closely associated with the greater level of high-
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frequency signal components present in the signal when
data is transmitted. That is, more substantial high
frequency components are required to produce a more
abruptly changing signal.
Thus, a series of methods and mechanisms ha~e been
disclosed for effectively discriminating between data `
and voice signals transmitted over a channel of a
cellular radiotelephone system. The methods disclosed
also make it possible to Yary the operation of the
10cellular radiotelephone system, including channel
assignment, billing rates and methods, and handoff
control based on the determination of current channel
usage (voice or data).
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