Transparent technique for Mu-law modems to detect an all-digital circuit connection

A transmitting Mu-law modem creates a probe signal by modifying the 7th bit of each of a number of PCM samples during the "hand-shaking" phase of a data connection. Furthermore, the transmitting Mu-law modem only makes this modification on those PCM samples that fall into the range of segment 1 as defined in CCITT Recommendation G.711. Similarly, a receiving Mu-law modem searches for this known pattern. Upon detection of this pattern, the receiving Mu-law modem provides an acknowledgment signal and both modems subsequently switch to a digital-mode of operation. On the other hand, if, in fact, there is a least one analog link in the data connection, the probe signal becomes distorted and, since the receiving Mu-law modem does not detect the probe signal, handshaking is completed in the standard fashion.

BACKGROUND OF THE INVENTION 
The present invention relates to data communications equipment, e.g., 
modems, and, more particularly, to Mu-law modems. 
A "Mu-law modem" is identical to an analog modem with the exception that 
the Mu-law modem does not have an analog interface to the public switched 
telephone network (PSTN). Instead, the Mu-law modem couples to the PSTN 
via a digital interface. Typically, an end user utilizes a Mu-law modem 
behind a customer-premises private branch exchange (PBX), which itself is 
connected to the PSTN via wideband digital facilities like T1, etc. In 
such an arrangement, the Mu-law modem can be coupled to the PBX over 
in-house wiring or the modem can be physically resident in the PBX itself. 
Whether coupled to the PBX or physically in the PBX, the Mu-law modem 
generates a 64 thousand bit per second (kbps) DS0 data stream for 
transmission through the PSTN to a far-end, or remote, data endpoint of a 
data connection. As known in the art, this DS0 data stream is a sequence 
of pulse-code modulated (PCM) samples using the same analog-to-digital 
sampling and encoding technique used by the PSTN for transmission of 
voice-band signals through the PSTN. Specifically, the digital signal 
processor of the Mu-law modem uses either of the standard companding 
encoding schemes, Mu-law or A-law, as defined in CCITT Recommendation 
G.711, to produce the DS0 data stream. This DS0 data stream transits the 
PSTN exactly like those DS0 streams created at the central office/local 
loop interface of the PSTN. 
The obvious benefits of using a Mu-law modem are (1) superior performance 
since echoes and other impairments are eliminated from one end of the data 
connection and (2) lower cost, smaller size, etc. due to the elimination 
of the analog interface from the modem. 
In addition, another benefit of using a Mu-law modem is the possibility of 
data transmission at speeds approaching the DS0 data rate. Unfortunately, 
this requires a completely digital connection between the two data 
endpoints--which cannot be guaranteed. For example, the PSTN itself might 
include an analog facility as part of the data connection. In addition, 
the other data endpoint is typically coupled to the PSTN via an analog 
local loop, which results in the DS0 data stream being converted back to a 
voice-band analog signal. As a result, the actual data rate of the data 
connection is limited to standard analog-based data transmission rates. 
However, with the increased deployment of all-digital facilities both in 
the PSTN and into customer premises, it can be expected that there will be 
an increasing frequency of data connections in which there are no analog 
links between the data endpoints. These "Mu-law modem to Mu-law modem" 
data connections offer the opportunity to greatly increase the effective 
data transmission rate by taking advantage of this all-digital connection. 
Unfortunately, the PSTN does not provide notification to a Mu-law modem 
when the data connection is entirely digital. As a result, a Mu-law modem 
cannot determine when an all-digital connection exists. Consequently, a 
Mu-law modem is limited to standard analog-based data transmission unless 
a priori a particular data connection is known to be all-digital. Such a 
situation might exist in a dedicated "point-to-point" data connection. In 
this instance, the Mu-law modem can be manually administered to a digital 
mode of operation. 
Indeed, even if the modem endpoints could identify themselves to each 
other, this still does not solve the problem of whether the connection is 
all-digital. For example, U.S. Pat. No. 5,311,578, issued May 10, 1994, to 
Bremer et al., entitled "Technique for Automatic Identification of a 
Remote Modem," describes the use of a "low-level" identification signal 
within an industry standard answer tone to identify the answering modem. 
This technique would allow a Mu-law modem to recognize what type of modem 
was at the opposite end -but identifying the modem at the opposite end 
does not guarantee that intervening data connection is completely digital. 
SUMMARY OF THE INVENTION 
I have realized a technique that allows a pair of Mu-law modems to 
determine whether they have established a data connection that consists 
entirely of digital transmission links. In particular, a probe signal is 
transmitted from one Mu-law modem to the other Mu-law modem. This probe 
signal is selected such that the presence of an analog link distorts the 
probe signal--with the result that the probe signal can no longer be 
reliably detected by the receiving DCE. As a result, reception and 
detection of this probe signal by the other Mu-law modem ensures to a 
degree the existence of an all-digital connection. Once the probe signal 
is detected, the Mu-law modems can then switch to transmission of data in 
purely digital form thereby increasing the maximum possible data rate to 
56 kbps or more. 
In an embodiment of the invention, the transmitting Mu-law modem creates 
this probe signal by modifying the 7th bit of each of a number of PCM 
samples during the "hand-shaking" phase of the data connection. 
Furthermore, the transmitting Mu-law modem only makes this modification on 
those PCM samples that fall into the range of segment one as defined in 
CCITT Recommendation G.711. Similarly, the receiving Mu-law modem searches 
for this known pattern. Upon detection of this pattern, the receiving 
Mu-law modem provides an acknowledgment signal and both modems 
subsequently switch to a digital-mode of operation. On the other hand, if, 
in fact, there is a least one analog link in the data connection, the 
probe signal becomes distorted and, since the receiving Mu-law modem does 
not detect the probe signal, handshaking is completed in the standard 
fashion. 
In accordance with a feature of the invention, the above-described 
technique is "transparent." The term "transparent" means that if a modem 
using this technique is connected via a data connection with one or more 
analog spans or is connected to a modem which has not implemented this 
technique, normal operation in the standard manner of a dial modem is not 
impaired. For example, if the receiving modem is a standard analog modem 
or is not a compatible Mu-law modem, the contents of this low significance 
bit simply resembles noise and, again, handshaking is completed in the 
standard fashion.

DETAILED DESCRIPTION 
A switched point-to-point data communications system embodying the 
principles of the invention is shown in FIG. 1. In the following example, 
it is assumed that a calling party located at data terminal equipment 
(DTE) 50 initiates a data call to DTE 60 via data communications equipment 
(DCE) 200, T1 Interface 15, PSTN 10, T1 interface 25, and DCE 100. For the 
purposes of this example, both DCE 200 and DCE 100 are Mu-law modems. 
Further, although the type of DTE is not relevant to the inventive 
concept, each DTE is a personal computer. The communications channel 
includes PSTN 10, and T1 facilities 101 and 201, which are representative 
of digital facilities. In this example, T1 facility 201 terminates in T1 
interface 15, which is assumed to be co-located with DCE 200. T1 interface 
15 can be a stand-alone piece of equipment or part of a digital PBX. T1 
facility 201 conveys a T1 signal from the respective terminating central 
office (not show) within PSTN 10. A T1 signal includes a plurality of 
channels, only one of which is assigned to DCE 200. (Although not shown, 
T1 facility 201 is actually shared among a plurality of DCEs, of which DCE 
200 is one. Like DCE 200, described below, these other DCEs access T1 
facility 201 via time-division-multiplexed (TDM) bus 261 and T1 interface 
15.) A similar description applies to T1 facility 101 which terminates in 
T1 interface 25. Finally, it is assumed that any data call through PSTN 10 
takes one of two possible types of paths. The first path is represented by 
path 11, which is representative of a path that only includes digital 
facilities. The second path is represented by path 12, which includes not 
only digital facilities but at least one analog facility as well. 
In this example, both DCE 200 and DCE 100 embody the principles of the 
invention. As such, only DCE 200 will be described in detail. As noted 
above, DCE 200 is a Mu-law modem, which is shown in block diagram form in 
FIG. 2. Other than the inventive concept, the components of DCE 200 are 
well-known. DCE 200 comprises memory 220, CPU 210, Digital Signal 
Processor (DSP) 230, time-slot access device 260, and data terminal 
equipment interface 270. In accordance with the principles of the 
invention, DCE 200 also includes transmit element 250 and receiver element 
240. CPU 210 is a microprocessor-based central processing unit, which 
operates on, or executes, program data stored in memory 220, via path 213. 
Memory 220 is representative of random access memory (RAM), and comprises 
a number of representative storage locations, of which a subset is shown 
in FIG. 2. For example, storage location 505 corresponds to that portion 
of memory that stores instructions and data associated with implementing 
step 505 of FIG. 3, described below. Time slot access device 260 provides 
information to, and receives information from, T1 interface 15, via TDM 
bus 261. In addition, as known in the art, time slot access device 260 
receives signaling information from T1 interface 15, via TDM bus 261. This 
signaling information includes the identification of the particular time 
slot of TDM bus 261 that is assigned to DCE 200 and includes other 
information like "Automatic Number Identification" (ANI), which represents 
the calling party's telephone number. Time slot access device 260 both 
removes and inserts data into the assigned time slot of TDM bus 261. This 
data is the above-described PCM signal. In particular, data extracted by 
time slot access device 260 from the assigned time slot is provided as a 
received signal, RX, to DSP 230 via line 241, block 240 and line 242. The 
received signal, RX, is a received form of the data signal transmitted by 
far-end DCE 100. Conversely, DSP 230 applies transmit signal TX to time 
slot access device 260, via line 232, transmit element 250 (described 
below), and line 251 for transmission to the far end. 
Before describing further the elements of DCE 200, the inventive concept 
will be generally described. In particular, in accordance with the 
inventive concept, a probe signal is transmitted from one Mu-law modem to 
the other Mu-law modem. Reception and detection of this probe signal by 
the other Mu-law modem ensures to a degree the existence of an all-digital 
connection. As described further below, the probe signal is selected such 
that the presence of an analog link distorts the probe signal--with the 
result that the probe signal can no longer be reliably detected by the 
receiving DCE. This inventive concept allows a Mu-law modem to "test" for 
an all-digital connection at the same time it is communicating using 
standard analog techniques. This is done in a way which does not interfere 
with the standard communication process. At the same time, it provides a 
positive indication to both Mu-law modems (provided they both have 
implemented this technique) to confirm not only that the data connection 
should be all-digital, but that the opposite modem also has recognized 
this condition. Once this state is reached, a transition to digital 
communications can be quickly made with little or no interruption, or 
corruption, of any data stream. Reference should now be made to FIGS. 3 
and 4, which show illustrative methods for use in originating and 
answering DCEs in accordance with the principles of the invention. 
The illustrative methods of FIG. 3 and FIG. 4 represent a simple 
interlocked handshake to provide positive confirmation of mutual 
recognition of an all-digital connection. Other methods are also possible. 
Since FIG. 4 is similar to FIG. 3, except for use in an answering DCE, 
only FIG. 3 is described in detail below. 
DCE 200 begins in an idle mode in step 505. In step 510, DCE 200 initiates 
a data call to DTE 60. As is known in the art, this data call can be 
initiated in a number of ways, e.g., a user at DTE 50 can keyboard dial 
using the well-known "AT-command set." In step 515, DCE 200 waits for the 
standard answer tone, which DCE 100 provides upon answering the data call. 
If no answer tone is detected, DCE 200 disconnects in step 520, e.g., 
after a suitable time-out. 
However, upon detection of an answer tone, DCE 200 begins handshaking in 
accordance with any one of a number of analog modem standards, e.g., CCITT 
V.32 bis, and, in accordance with the inventive concept, adds a probe 
signal in step 525 to any handshaking signals. In this example, the probe 
signal is a sequence of scrambled marks for a period of time T.sub.1 
(described further below). The transmission of the probe signal should 
start immediately after completion of the standard answer tone sequence to 
allow PSTN 10 to recognize that the telephone connection is being used for 
modem rather than voice signals. (It should be noted that the probe signal 
could be sent by the originating modem starting immediately after 
completion of dialing, and by the answering modem immediately after going 
off hook, to help minimize the time to detect the all digital circuit.) 
After step 525, DCE 200 continues the handshaking process, and the 
transmission of the probe signal, and further monitors for reception of a 
corresponding probe signal representing scrambled marks transmitted from 
DCE 100 in step 530. 
If no received probe signal is detected for a period of time T.sub.2 and 
before the handshaking process completes, DCE 200 simply establishes the 
data connection in accordance with the respective analog standard, e.g., 
V.32 bis, in step 535. Since, in this example, both DCE 100 and DCE 200 
embody the inventive concept, the lack of detection of a probe signal by 
DCE 200 is representative of the data connection taking path 12, which 
contains at least one analog link, through PSTN 10. (Also, if DCE 100 did 
not embody the inventive concept, the lack of a detected probe signal 
could be due to DCE 100 simply being incompatible with the inventive 
technique, e.g., DCE 100 is a standard analog modem or a Mu-law modem that 
does not embody the inventive concept.) 
However, if a received probe signal is detected for a period of time 
T.sub.2, DCE 200 continues the standard handshaking and changes the probe 
signal to a sequence of scrambled spaces for a time period T.sub.3 in step 
540. After step 540, DCE 200 continues the handshaking process, and the 
transmission of the probe signal representing now scrambled spaces, and 
further monitors for reception of a corresponding probe signal 
representing scrambled spaces transmitted from DCE 100 in step 545. If no 
received probe signal is detected for a period of time T.sub.4, DCE 200 
simply establishes the data connection in accordance with the respective 
analog standard, e.g., V.32 bis, in step 535. 
However, if a received probe signal is detected for a period of time 
T.sub.4, DCE 200 terminates the standard handshaking procedure and 
switches to an all-digital mode of operation in step 550. Detection of 
this sequence of probe signals by DCE 200 is representative of the data 
connection taking path 11 (shown in FIG. 1), which is an all-digital 
connection to DCE 100. After switching to an all-digital mode of 
operation, transmission of data occurs in purely digital form as known in 
the art thereby increasing the maximum possible data rate to 56 kbps or 
more. 
In accordance with the above-described inventive concept, transmit element 
250 of DCE 200 (shown in FIG. 1) inserts the probe signal into the PCM 
data stream. An illustrative embodiment of transmit element 250 is shown 
in block diagram form in FIG. 5. Transmit element 250 includes selector 
715, segment one detector 710, and sequence generator 705. The basic idea 
is simply for the transmitter of DCE 200 to modify the PCM data stream in 
a manner which results in the probe signal being interpreted as just noise 
by a receiving modem that is not "looking" for this pattern. This is done 
by imposing the modification on the bits in the PCM data stream that have 
the least significance when the data from the PCM signal is interpreted as 
a representation of an analog signal. However, since the modification is 
deterministic and has a bit error rate which is no worse than any of the 
other data, it can be recognized as a known pattern by a receiving Mu-law 
modem that embodies the inventive concept. 
More specifically, transmit element 250 modifies only the 7th bit of the 
PCM samples (based on bit numbering given in G.711). (It should be noted 
that bit 8 is not used because this is sometime used for inter-office 
signaling and framing, etc.) A means of identification of this bit is 
implicit in the required PCM framing. Furthermore, transmit element 250 
only makes this modification on the samples which fall into the range of 
segment one (as defined in G.711) where the modifications have the least 
impact on the analog interpretation. (As defined in G.711, the higher the 
segment number of a PCM sample, the higher the signal level). Since the 
communication via these modifications requires very low bandwidth, the DCE 
200 can use the opportunistic approach and wait for segment one samples to 
occur before applying the modification. 
As described above in the context of FIGS. 3 and 4, it is proposed that the 
probe signal, or pattern, which is imposed on the 7th bit of the segment 
one samples consists of marks or spaces scrambled by a polynomial such as 
is typically used in modem scramblers and inter-DCE signaling techniques. 
The benefit of using this type of probe signal is that it resembles random 
data and can be easily detected with an almost arbitrarily low probability 
of false detection. If, in fact, the modem at the other end of the data 
connection is not sending the pattern, the contents of this low 
significance bit resembles a random data stream. Further, in the situation 
where one or more analog links exist, the probe signal is corrupted by the 
analog conversion process so that again the contents of this low 
significance bit resembles a random data stream. The likelihood of false 
detection of the all-digital connection can be reduced to a suitable level 
by extending the length of the pattern required for detection. 
DSP 230 generates a sequence of 8-bit PCM samples, at the standard 8 kHz 
rate, on line 232, which provides each PCM sample in parallel, i.e., each 
bit of the 8-bit sample is provided on a separate signal path. Seven of 
these signal paths are represented by line 704, which couples to time-slot 
access device 260. The remaining bit of information, conveyed by line 706, 
is applied to selector 715. The latter either selects the PCM bit 
generated by DSP 230, via line 706, or, in accordance with the inventive 
concept, selects the data generated by sequence generator 705, via line 
711, for application to time-slot access device 260, via line 708. The 
selection of the signal to convey to time-slot access device 260 is 
performed by segment one detector 710, which is also under the control of 
DSP 230. As will be appreciated by one skilled in the art from a review of 
FIG. 5, the segment one detector 710 generates a control signal 712 which 
controls the operation of the selector 715. Namely, the value of control 
signal 712 determines whether the selector 715 "selects" the value on line 
711 (i.e., the probe signal) or the value on line 706 as the bit 7 value 
of the 8 bit PCM value that is sent to time-slot access device 260. 
In accordance with the methods described above, DCE 200 initially disables, 
via line 248, segment one detector 710, e.g., while in the idle mode. When 
disabled, signal one detector 710 causes selector 715 to convey the signal 
on line 706 to time-slot access device 260. In other words, there is no 
modification to the 8-bit PCM samples provided by DSP 230 when segment one 
detector 710 is disabled. However, in step 525 of FIG. 3, DSP 230 enables, 
via line 248, segment one detector 710, which then monitors for the 
occurrence of segment one samples on signal lines 707. When a segment one 
sample is detected, segment one detector 710 controls selector 715 so that 
the signal on line 711 is conveyed to time-slot access device 260. Segment 
one detector 710 also provides a clock signal, via line 713, to sequence 
generator 705 to generate the signal on line 711 (ie., the probe signal), 
which now replaces the current bit 7 value of the 8-bit PCM value provided 
by DSP 230. The latter also controls, via line 249, sequence generator 705 
to generate a scrambled marks or scrambled spaces signal based on the 
phase of the inter-modem exchange as described above. Once the transition 
to either all-digital mode or a standard analog mode, DSP 230 disables 
segment one detector 710. 
Similarly, the receiver of a Mu-law modem is modified to search for the 
known pattern. More specifically, receiver element 240 of DCE 200 includes 
segment one detector 810 and sequence detector 805 as shown in FIG. 6. 
Time-slot access device 260 provides a sequence of received 8-bit PCM 
samples in parallel form on line 241. Seven of the eight PCM bits are 
represented by line 807 and are applied to DSP 230 and to segment one 
detector 810. Bit 7 of the eight PCM bits is conveyed by line 806, which 
is applied to DSP 230 and to sequence detector 805. In accordance with the 
methods described above, at the appropriate phase in the connection 
process, DSP 230 enables segment one detector 810 via line 238. When 
segment one detector 810 detects a segment one sample, it clocks sequence 
detector 805 via line 813. DSP 230 controls sequence detector 805 to 
search for either scrambled marks or spaces via line 239. When sequence 
detector 805 detects the required number of bits of either sequence, it 
informs the DSP 230 via control line 242. One illustrative value for the 
required number of bits is at least 30 scrambled marks or at least 30 
scrambled spaces. DSP 230 then responds to this signal as described 
previously in accordance with FIGS. 3 or 4, appropriately. For example, in 
step 530, DSP 230 determines whether a probe signal representing scrambled 
marks has been detected by sequence detector 805, via line 242, and keeps 
track, via a counter (not shown), whether scrambled marks have been 
detected for the required time intervals (if any). 
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. transmit 
element 250, etc., the functions of any one or more of those building 
blocks can be carried out using one or more appropriate programmed 
processors, e.g., a digital signal processor. 
In addition, the inventive technique can be used with any modulation scheme 
that provides duplex communication, and can also be used with voice 
communications as well, e.g., such as switching to voice plus video upon 
detection of the all-digital connection. Also, although illustrated in the 
context of a probe signal represented by a scrambled marks or a scrambled 
spaces signal, other forms of probe signals are possible. Further, other 
modifications to the above-described handshaking method are possible. For 
example, even after the transition to a digital mode of operation, the 
probe signal could still be transmitted as part of a preamble for the all 
digital data stream. This preamble could simply be a stream of continuous 
samples of the same value to which the probe signal is added for some 
brief period of time. 
Finally, although described in the context of modifying the seventh bit of 
a PCM sample, it should be realized that the probe signal could be 
inserted into other bits of a PCM sample. For example, in those 
all-digital circuits that do not use the eighth bit for signaling, the 
probe signal could be put into the seventh and eighth bits of a PCM 
sample. These all-digital circuits without in-band signaling allow digital 
communication at 64 kbps instead of 56 kbps.