Technique for automatic identification of a remote modem

In the public switched telephone network, an originating modem can identify an answering modem, e.g., as to the type of modem it is, by detection of a low-level identification signal sent from the answering modem. The low-level identification signal is hidden within an industry standard "answer tone," e.g., a CCITT V.25 answer tone of 2100 Hz. If the low-level identification signal is detected by the originating modem, the industry standard "handshaking" procedure is terminated and a non-standard handshaking procedure is implemented. If the identification signal is not detected by the originating modem, the industry standard handshaking procedure is simply completed.

BACKGROUND OF THE INVENTION 
This invention relates to data communications techniques used in modems. 
More particularly, this invention relates to a method and apparatus for an 
originating modem to identify an answering modem. 
In a dial-up communications network, e.g., the public switched telephone 
network (PSTN), an originating modem does not necessarily know what type 
of modem will answer the data call. As a result, the originating modem 
typically assumes that all data calls are made to a "generic" modem and 
that all "initial handshaking" and initial communications must be done 
according to established industry standards like International Telegraph 
and Telephone Consultative Committee (CCITT) V.25. For example, in order 
to establish a data connection, the originating modem dials the telephone 
number of the location where the answering modem is located. The answering 
modem detects the ringing signals on the telephone line and answers the 
telephone call. At this point the answering modem places an "answer tone" 
on the telephone line. The originating modem detects the answer tone and 
then places an "originating carrier" on the line. When the answering modem 
detects the originating carrier, the answering modem places an "answering 
carrier" on the line and the initial handshaking process is completed, 
i.e., a physical data connection is established between the originating 
modem and the answering modem. From this point forward, the originating 
modem and the answering modem can execute additional handshaking and 
protocol procedures (like CCITT V.32) to further establish the desired 
level of connection, e.g. the desired baud rate, communications protocol, 
etc. 
As described above, a data communications industry standard, like CCITT 
V.25, provides a common reference point whereby a modem manufacturer can 
ensure its modem will be able to communicate with modems produced by other 
manufacturers. However, in offering data communications services to a 
customer, the data communications industry standards may not provide all 
of the features that a customer may desire, or may not provide the 
features in a manner that suits a particular modem manufacturer. For 
example, a modem manufacturer may be of the view that a network management 
feature properly requires that a secondary communications channel also be 
established between the originating modem and the answering 
modem--however, available industry standards may not provide for this type 
of a secondary channel. Another example is to allow faster connection 
between the modems by shortening the time required to determine modem 
types. 
As a result, after the establishment of the switched data connection 
between an originating modem and an answering modem, it may be desired to 
switch to a proprietary form of operation in order to offer non-industry 
standard features to a customer. Consequently, this requires that the 
originating modem and the answering modem be the same type of modem and 
requires these modems to perform some additional handshaking process in 
order to identify each other. 
Various techniques have been disclosed in a number of U.S. Patents which 
provide for an originating modem and an answering modem to identify each 
other. U.S. Pat. No. 4,215,243, issued to Maxwell on Jul. 29, 1980, 
provides an ability to generally identify a modem as to the type of 
industry standard protocol it supports (as opposed to identifying a 
particular manufacturer's modem) by identifying the frequency of the 
originating carrier after the answer tone has been provided from the 
answering modem. U.S. Pat. No. 4,680,773, issued to Amundson on Jul. 14, 
1987, discloses a technique for sending special characters after the 
physical data connection is established. These special characters, when 
detected, allow the originating modem and answering modems to identify 
each other for changing to a proprietary form of operation. Finally, U.S. 
Pat. No. 4,782,498, issued to Copeland, III on Nov. 1, 1988, establishes a 
special mode for proprietary use by means of a special handshake 
procedure. For example, upon answering the telephone call, the answering 
modem will not provide the industry standard answering tone but, instead, 
provide a special sequence of characters. If the originating modem 
recognizes the special sequence of characters it will signal to the 
answering modem that it is of the same type, and both modems can then 
switch to a proprietary form of operation. However, if the originating 
modem does not recognize the special sequence of characters it will 
"time-out" and either assume a default operation or drop the line assuming 
that the no modem has answered. 
As can be seen from the above prior art, there are basically two ways to 
provide identification between modems. One method first establishes the 
physical data connection before the originating modem and the answering 
modem attempt to identify each other through an additional handshaking 
procedure. Unfortunately, this only adds to the delay that already exists 
in establishing the physical data connection--a delay which is already on 
the order of 3 to 8 seconds depending on network delays. The second method 
uses a proprietary handshaking process before establishing the physical 
data connection, with the result that an incompatible originating modem 
may become confused and drop the connection. Consequently, it may be 
necessary to avoid any attempt at identification between the modems in 
order to eliminate this possibility--with the result that any non-standard 
industry features have to be manually administered when both the 
originating modem type and answering modem type are known a priori. 
SUMMARY OF THE INVENTION 
According to the principles of this invention, an improved modem is 
constructed in which an identification signal is combined with, or hidden 
within, an industry standard answering signal. 
In an embodiment of the invention, an originating modem initiates a data 
call by dialing a telephone number of a remote modem. The remote modem, 
constructed in accordance with this invention, provides a signal 
comprising a standard CCITT V.25 answer tone of 2100 Hz and an 
identification signal. In this illustrative embodiment the identification 
signal is represented by an identification tone "A," the identification 
tone being hidden within the answer tone. The energy level of this 
identification tone is below the CCITT defined energy levels of the V.25 
answer tone. In particular, the identification tone energy level is set 
low enough to appear as a part of the background noise to a conventional 
modem. In other words, the conventional modem would not detect the 
identification tone. The originating modem detects identification tone "A" 
and sends an identification tone "B" while still receiving the V.25 answer 
tone. The answering modem receives identification tone "B" and provides a 
confirmation tone "C" while still sending the V.25 answer tone. Finally, 
the originating modem detects the confirmation tone "C," and sends back 
confirmation tone "D" while still receiving the V.25 answer tone. As a 
result, both the originating modem and the answering modem have identified 
each other. Alternatively, if the answering modem is a conventional modem, 
the originating modem only detects the CCITT V.25 answer tone without 
identification tone "A." Consequently, the originating modem merely 
completes the call establishment process. 
One feature of the invention is that no additional time is added to the 
prior art call establishment process and in fact, the process can be 
shortened. As a result, upon the receipt of tones "C" and "D" by the 
respective modems, the CCITT V.25 call establishment procedure is 
terminated by both the originating and the answering modem. This early 
termination of the CCITT V.25 call establishment procedure not only avoids 
any subsequent delays in the originating modem and the answering modem 
identifying each other, but also establishes the physical data connection 
faster than the CCITT V.25 call establishment procedure. 
Another feature of the invention is that it allows an improved modem to 
query another conventional modem with a proprietary handshaking process 
that does not interfere with the conventional modem's operation. As a 
result, since the CCITT V.25 call establishment procedure is not 
interfered with, there is no danger of a conventional modem becoming 
confused and dropping the connection.

DETAILED DESCRIPTION 
FIG. 1 shows a modem that embodies the inventive concept of this invention. 
The individual components of the data communications system are well-known 
and are not described in detail. 
As shown, modem 100 is connected to telephone network 200 via telephone 
line 101. Similarly, modem 300 is connected to telephone network 200 via 
telephone line 301. Either modem 100 or modem 300 can place a telephone 
call to another modem by going "off-hook" and following standard dialing 
procedures. Modem 100 comprises data transmitter 160, answering tone 
generator 150, identification signal generator 140, adders 155 and 165, 
answering tone detector 120, identification signal detector 180, telephone 
line interface 170, data receiver 130, and controller 110. 
In the following first example, it is assumed that modem 300 is identical 
to modem 100, i.e., that modem 300 also embodies the principles of the 
invention. Modem 300 is the originating modem and places a telephone call 
to modem 100 through telephone network 200. Telephone line interface 170 
of modem 100 answers the telephone call (e.g., by going "off-hook") and 
signals controller 110, via lead 171, that a telephone call has been 
answered. Controller 110, via lead 112, turns on answering tone generator 
150, which provides a V.25 compatible answer tone (V.25 answer tone) to 
adder 155. At the same time, controller 110 turns on identification signal 
generator 140 to provide an identification tone "A" for 500 milliseconds 
on lead 141, which is applied to adder 155. Identification signal 
generator 140 is capable of providing a plurality of different tones which 
are different from the V.25 answer tone. The type and duration of the 
identification tone is controlled by controller 110 via lead 113, which is 
representative of a plurality of control signals. Adder 155 provides the 
sum of the V.25 answer tone and identification tone "A" on lead 156, which 
is applied to adder 165. Since the data connection has not yet been 
established, there is no signal present on lead 161. As a result, the 
output of adder 165 is the sum of the V.25 answer tone and identification 
tone "A." This output signal is applied to telephone line interface 170 
for transmission to modem 300 via telephone line 101, telephone network 
200 and telephone line 301. 
As described above, the identification tone is combined with the V.25 
answer tone by adder 155. A feature of this invention is that any 
identification tone, e.g., identification tone "A," is "hidden" within the 
V.25 answer tone. In particular, the energy level of the identification 
tone is below the minimum energy level specified for the V.25 answer tone. 
In other words, any identification tone is a "low-level" tone. In this 
example, the V.25 answer tone, as defined in the CCITT standard, comprises 
a single frequency of 2100 Hz, with a minimum energy level of -9 dBm. The 
energy level of an identification tone, e.g., identification tone "A," is 
below this minimum energy level of the V.25 answer tone. This feature is 
shown in FIG. 2, which shows two call establishment sequences. Sequence 10 
represents the call establishment sequence followed by originating modem 
300, while sequence 20 represents the call establishment sequence followed 
by answering modem 100. The time when telephone interface 170 answers the 
telephone call is represented by vertical bar 21, which is labeled 
connect. A time .tau. later, the sum of the V.25 answer tone and 
identification tone "A" is applied to telephone line interface 170 (as 
described above). The V.25 answer tone as provided by the answering modem 
is represented by block 25. The vertical height of block 25 represents the 
variation in the energy level in dBm of this combined answer tone signal. 
As mentioned above, the V.25 answer tone has a defined maximum energy 
level of -9 dBm. Below this maximum level of the V.25 answer tone is the 
permissible range of identification tone "A." As shown in FIG. 2, the 
energy of any identification tone can vary between the limits of -9 dBm to 
-55 dBm. Therefore, during time period t.sub.A, DTMF identification tone 
"A" is hidden within the V.25 answer tone. As can be further seen, if a 
conventional modem was the originating modem, it would only detect the 
V.25 answer tone and would not detect the presence of any hidden 
identification tone. As a result, these hidden identification signals do 
not interfere with the defined call establishment process and signal 
levels of CCITT V.25. 
As shown in FIG. 2, any identification tone is generated or detected within 
a particular time period. Continuing with the above first example, 
controller 110 turns on identification signal generator 140 to generate 
identification tone "A" for 500 milliseconds, which is the duration of 
time interval t.sub.A. At the end of time period t.sub.A, controller 110 
turns off identification signal generator 140 and turns on signal detector 
180, via lead 117, to look for identification tone "B" from the 
originating modem for the next 500 milliseconds, which is shown in FIG. 2 
as time period t.sub.B. Modem 300 detects identification tone "A" during 
time period t.sub.A and sends identification tone "B" (described below) 
during time period t.sub.B to modem 100 via telephone line 301, telephone 
network 200 and telephone line 101. Telephone interface 170, of modem 100, 
receives identification tone "B" from telephone line 101 and provides 
identification tone "B" to identification signal detector 180. The latter 
detects identification tone "B" and signals controller 110 that this tone 
has been detected during time period t.sub.B. As a result, controller 110 
then turns on identification signal generator 140 to generate 
identification tone "C" for 500 milliseconds, which occurs during time 
period t.sub.C. Similar to the description above, identification tone "C" 
is hidden within the V.25 answer tone and is transmitted to modem 300. At 
the end of time period t.sub.C, controller 110 then turns off 
identification signal generator 140 and turns on signal detector 180 to 
look for identification tone "D" from the originating modem for the next 
500 milliseconds, which is shown in FIG. 2 as time period t.sub.D. Modem 
300 detects identification tone "C" during time period t.sub.C and sends 
identification tone "D" (described below) to modem 100. Telephone 
interface 170 receives identification tone "D" from telephone line 110 and 
provides identification tone "D" to identification signal detector 180. 
The latter detects identification tone "D" and signals controller 110. At 
this point, modem 300 and modem 100 have completed executing an 
illustrative "handshaking" call establishment sequence by using hidden 
signals within a standard V.25 answer tone. The successful completion of 
this call establishment sequence allows modem 300 and modem 100 to 
identify each other as a particular type of modem. That is, the low-level 
identification tones provided by each modem characterizes that modem as a 
particular type of modem to the far, or remote, modem. This enables the 
modems to then establish non-standard, or proprietary, operation that is 
transparent to any particular users of these modems. 
From FIG. 2, another feature of the invention is illustrated. In 
particular, the handshaking process provided by the exchange of these 
hidden identification tones is faster than the required time interval for 
just the V.25 answer tone. The V.25 answer tone lasts for at least 3.3 
seconds, while, as described above, the handshaking process between modem 
100 and 300 completed after only 2 seconds. It should also be noted that 
there are further steps not illustrated herein in the V.25 call 
establishment sequence that provide additional delay and which are 
advantageously avoided by this invention. 
An illustrative method for an answering modem embodying the principles of 
the invention is shown in FIG. 3. In particular, modem 100 is the 
answering modem for a telephone call that has been placed by modem 300. As 
shown in block 400, modem 100 answers the telephone call and then turns on 
a V.25 answer tone (block 405), and generates identification tone "A" for 
500 milliseconds (block 410). The V.25 answer tone and identification tone 
"A" are combined and sent to modem 100 (block 412). Modem 100 then waits 
for the detection of identification tone "B" (block 415). If 
identification tone "B" is not detected within 500 milliseconds, modem 100 
merely completes the remaining V.25 call establishment sequence with modem 
300 (block 440). In this particular case, it would be assumed by modem 100 
that modem 300 is a conventional modem. 
However, if modem 100 detects identification tone "B" within the 500 
milliseconds, modem 100 then generates identification tone "C" for 500 
milliseconds (block 420). At this point, modem 100 will then wait 500 
milliseconds and then begin to look for identification tone "D" for 500 
milliseconds (block 425). If identification tone "D" is not detected 
within 500 milliseconds, modem 100 merely completes the remaining V.25 
call establishment sequence with modem 300 (block 440). Again, in this 
particular case, modem 100 assumes that modem 300 is a conventional modem. 
However, in accordance with the principles of this invention, if modem 100 
detects identification tone "D," modem 100 has identified modem 300 as an 
improved modem and turns off the V.25 answer tone even though the V.25 
call establishment sequence has not been completed (block 430). 
Turning now to FIG. 4, a representative method for use when modem 100 is 
the originating modem is shown. In particular, modem 100 initiates a 
telephone call to modem 300 (block 500). Specifically, controller 110 
initiates a telephone call to modem 300 by instructing telephone line 
interface 170 to go off-hook and apply a sequence of DTMF tones that are 
representative of the telephone number of modem 300 to telephone line 101. 
Modem 100 then searches for a V.25 answer tone from the far modem (modem 
300) by turning on answering tone detector 120. At this point, controller 
110 has disabled answering tone generator 150 and identification signal 
generator 140. If modem 100 does not receive the V.25 answer tone, modem 
100 drops the telephone call, e.g., goes on-hook, (block 515). However, if 
modem 100 receives the V.25 answer tone, modem 100 then searches for 
identification tone "A" (block 510). In particular, controller 110 turns 
on, via lead 117, identification signal detector 180 to search for 
identification tone "A" for the next 500 milliseconds. If identification 
tone "A" is not detected within 500 milliseconds, modem 100 completes the 
standard V.25 call establishment sequence (block 525). On the other hand, 
if modem 100 detects identification tone "A," modem 100 then sends 
identification tone "B" for 500 milliseconds (block 520) by turning on 
identification signal generator 140 to sent tone "B." After sending 
identification tone "B" for 500 milliseconds, modem 100 then searches for 
identification tone "C" (block 530). This is accomplished by controller 
110 turning on identification signal detector 180 to search for 
identification tone "C" for the next 500 milliseconds. If identification 
tone "C" is not detected within 500 milliseconds, modem 100 completes the 
standard V.25 call establishment process (block 525). However, if modem 
100 detects identification tone "C," modem 100 then sends identification 
tone "D" for 500 milliseconds (block 535). As similarly described above, 
controller 110 turns on identification signal generator 140 to send tone 
"D." After sending identification tone "D," modem 100 aborts the V.25 call 
establishment process (block 540). As a result, modem 100 and modem 300 
have successfully completed, in accordance with the principles of the 
invention, a non-standard call establishment process that does not 
interfere with a standard call establishment process like V.25. 
The foregoing merely illustrates the principles of the invention and it 
will thus be appreciated that those skilled in the art will be able to 
devise numerous alternative arrangements which, although not explicitly 
described herein, embody the principles of the invention and are within 
its spirit and scope. 
For example, although the invention is illustrated herein as being 
implemented with discrete functional building blocks, e.g., detectors, 
tone generators, etc., the functions of any one or more of those building 
blocks can be carried out using one or more appropriate programmed 
processors. In addition, the handshaking process can be shorter, e.g., the 
identification process can end after the exchange of identification tones 
"A" and "B." 
Also, other forms of hidden signals may be used as long as the signaling 
characteristics do not interfere with the standard call establishment 
procedure. For example, multiple tones, spread spectrum techniques may 
also be used.