Mode signalling method and apparatus

A method for identifying that a data communications device is operable in a predetermined mode. The first step of the method provides for generating a frequency division modulation capabilities signal. The generated signal is limited to a predetermined frequency range. The method further provides for superimposing mode identification information upon the generated signal to form a modified signal. Like the generated signal, the modified signal falls within the predetermined range. The superimposed mode identification information may be applied by frequency modulating the generated signal.

BACKGROUND OF THE INVENTION 
The present invention relates to data communication systems and, more 
particularly, to data communication systems that are operable in more than 
one mode. Data communication systems of this sort frequently utilize the 
public switched telephone network for at least a portion of the 
communication channel. 
At the endpoints of a data communication system, one will find data 
communication equipment. The primary element of such equipment is 
typically some sort of modem. A communication channel connects the 
endpoints so that information may be communicated between the endpoints. 
The communication channel may be analog, digital or, commonly, a 
combination of digital and analog links. The modems typically modulate 
information for transmission over the communication channel and demodulate 
signals received from the communication channel to recover information. 
A schematic representation of a data communication system is shown in FIG. 
1. A first modem 10 is connected to a second modem 20 by a communication 
channel 30. The first modem 10 modulates input data 40 for transmission 
over the channel 30 to the second modem 20. The second modem 20 recovers 
the input data 40 by demodulating the signal received from the first modem 
20. In like manner, input data 50 may be modulated by the second modem 20 
and transmitted over the channel 30 to the first modem 10. 
Implicit in the foregoing are: 1) there are different types of modems; and 
2) different types of modems should be capable of communicating with each 
other. With respect to the different types of modems, modems may be 
classified, for example, by manufacturer, by structure (e.g. internal, 
external, PC card etc . . . ) or by the communication protocols that they 
support. For purposes of the present invention, the third criterion is the 
most pertinent. 
A communication protocol, generally speaking, is a set of rules that 
defines how data communication devices interact. Numerous communication 
protocols have been developed to enable data communication between a wide 
range of data communication devices. For accurate information transfer 
between devices, the devices must operate under, or support, the same 
protocol. 
It is becoming increasingly common for data communication devices to 
support several communication protocols. An appropriate communication 
protocol may be selected from among the available protocols depending upon 
the capabilities of the remote device and the communication channel. 
Unfortunately, one typically does not know, before a connection is made 
over the public switched telephone network, what type of modem will be 
encountered at the remote end of a switched connection. It is therefore 
desirable, at an early stage in the connection process, to identify the 
communication protocols that are supported. In addition, it is usually 
desirable to communicate information at the highest rate that the modems 
and the communication channel can reliably support. 
The International Telecommunication Union-Telecommunications 
Standardization Bureau ("ITU-T") in Geneva, Switzerland, develops and 
publishes "Recommendations" that relate to communication protocols. The 
ITU-T Recommendations are non-binding international standards whose 
objective is to ensure compatibility of international telecommunications 
on a world-wide basis. The Recommendations referred to herein are publicly 
available from the ITU-T. 
Protocols are known that identify the capabilities of a call modem that is 
connected to an answer modem by a communication channel. For example, 
Recommendation V.34, which is incorporated herein by reference and which 
is entitled A Modem Operating at Data Signalling Rates of up to 28,800 
bit/s for use on the General Switched Telephone Network and on Leased 
Point-to-Point 2-Wire Telephone-Type Circuits, describes a network 
interaction protocol for identifying V.34 capability. 
The V.34 protocol includes a preliminary data exchange according to ITU-T 
Recommendation V.8, Procedures for Starting Sessions of Data Transmission 
over the General Switched Telephone Network. Recommendation V.8 is 
incorporated herein by reference. During the preliminary data exchange, a 
call modem initially conditions its receiver to detect an answer tone, 
such as ANS or ANSam, from an answer modem. The call modem then transmits 
a call indicator or call tone, such as CI, CNG or CT, as defined by 
Recommendation V.8. 
The ANS signal is a 2100 Hz tone, and is the typical tone produced by an 
answering modem that is not V.8 compatible. However, if the answering 
modem is V.8compatible, it provides the ANSam signal, which is defined as 
a 2100 Hz tone that has been amplitude modulated with a 15 Hz tone. 
If the signal response ANSam is detected by the call modem, then the call 
modem transmits silence for a period of time, T.sub.e as specified by 
Recommendation V.8. If, on the other hand, the signal response ANS is 
detected, indicating that the answer modem is incapable of performing the 
CM/JM exchange, the call modem proceeds under a different protocol, for 
example as provided in Annex A of Recommendation V.32bis or Recommendation 
T.30. 
Next, the call modem conditions its receiver to detect a joint menu signal, 
JM, and transmits a call menu signal, CM. The call menu signal, CM, 
includes the appropriate bits set in the modulation modes category, as 
provided in Table 4/V.8, to indicate that operation in accordance with 
Recommendation V.34 is desired. The CM sequence includes eight information 
bits (an octet) plus a start bit and a stop bit, for a total of ten bits 
per word. 
When a minimum of two identical JM sequences have been received, the call 
modem completes the current CM octet and then sends a call menu terminator 
signal, CJ, to acknowledge JM and terminates the call menu signal. After 
sending CJ, the call modem transmits silence for 75.+-.5 ms and proceeds 
with the remaining steps of ranging/probing, equalizer and echo-canceller 
training, and final training as described in Recommendation V.34. 
During the Recommendation V.8 interchange described above, the CM and JM 
words are transmitted between modems using the signaling format specified 
in Recommendation V.21, which is entitled 300 Bits Per Second Duplex Modem 
Standardized for Use in the General Switched Telephone Network. 
Recommendation V.21, which is incorporated herein by reference, defines a 
type of Frequency Shift Keying (FSK) that is referred to herein as 
Frequency Division Modulation (FDM). The modulation format operates at 300 
bits per second (bps), where a logic one bit (also referred to as a 
"mark") is transmitted from the call modem to the answer modem (channel 
no. 1) as a 980 Hz nominal tone, and a logic zero bit (also known as a 
"space") is transmitted from the call modem to the answer modem (channel 
no. 1) as a 1180 Hz nominal tone. Recommendation V.21 specifies that the 
transmitter frequencies are to be within .+-.6 Hz of their nominal values, 
and that the receiver must be able to correctly interpret the mark and 
space frequencies if they are within 12 Hz of their nominal values. 
A disadvantage of the network interaction protocol described in 
Recommendation V.34 is that it does not identify the availability of 
non-standard or later devised data signalling modes or protocols. It is 
desirable to identify the availability of non-standard or later devised 
capabilities. In addition, it is desirable to do so without interfering 
with normal training and operation under Recommendation V.34. 
Accordingly it would be desirable to have an improved mode signalling 
method and apparatus. 
SUMMARY OF THE INVENTION 
In accordance with a first aspect of the present invention, a method for 
identifying that a data communications device is operable in a 
predetermined mode, is provided. The first step of the method provides for 
generating a frequency division modulated signal. The generated signal is 
limited to a predetermined frequency range. The method further provides 
for superimposing mode identification information upon the generated 
signal to form a modified signal. Like the generated signal, the modified 
signal falls within the predetermined range. 
It is an object of the present invention to provide a generalized escape 
sequence for changing operational modes in accordance with identified 
capabilities. 
It is a further object of the present invention to provide a mode 
signalling method that does not interfere with standardized interaction 
protocols. 
It is a still further object of the present invention to provide a mode 
signalling method that is capable of identifying operational capabilities 
without altering the procedures of Recommendations V.8 and V.34.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
The presently preferred embodiments of the invention will now be described 
with reference to FIGS. 2 through 4, wherein like elements are referred to 
by like numerals. FIG. 2 shows a data communication system. A first data 
communications device 100 receives information 102 from a data source 104. 
The information 102 may be in either analog or digital form, although 
digital information is preferred for high speed data transfer 
applications. For instances in which the data communications device 100 
receives information 102 from the data source 104 that is in analog form, 
the data communications device 100 may include an analog-to-digital 
converter that converts analog information 102 into digital form. Thus, 
the input to the communications device 100 may be in either analog or 
digital form, and the output of the communications device 100 is digital. 
The first data communications device 100 is connected by a communication 
channel 106 to a second data communications device 108. The communication 
channel 106 includes a digital telephone network ("DTN") link 110 that is 
coupled to an analog subscriber loop 112 by a local switch 114. The DTN 
110 carries digital information in the form of pulse code modulation 
("PCM") codewords, which are typically eight bits in length. The loop 112 
carries an analog representation of digital information. The local switch 
114 acts as an interface between the DTN 110 and the loop 112 by 
converting digital information supplied by the DTN 110 into analog form 
for transmission over the loop 112 and converting analog signals supplied 
by the loop 112 into digital form for transmission over the DTN 110. The 
analog-to-digital and digital-to-analog conversions performed by the local 
switch 114 are well known conversions, such as the .mu.-law conversion 
that is utilized in North America and Japan or the European A-law 
conversion. 
The preferred embodiments of the present invention relate to the ability of 
devices in the communication system, such as the data communications 
devices 100 and 108, to signal the availability of added capabilities. 
These added capabilities may include, for example, an alternative 
signalling mode, protocol compliance, a particular feature set or the 
like. The added capabilities may, for example, be capabilities that are 
proprietary to a particular manufacturer of data communications devices 
or, as a further example, the added capabilities may relate to 
non-standard data signalling modes or protocols, or to modes or protocols 
that are devised after a standard is put in place. Of course, it is 
envisioned that the added capabilities may be incorporated into later 
standards. 
In accordance with a preferred added capability that may be supported by 
the data communication system shown in FIG. 2, the data communication 
device 100 maps received information 102 into PCM codewords that are 
selected from a subset of the complete set of PCM codewords that are 
utilized by the DTN 110. Preferably, the information 102 is digital data 
and the data communication device 100 encodes the data directly into PCM 
codewords. As used in the previous sentence, "directly" means that the PCM 
codewords represent the data itself, rather than representing a modulated 
analog waveform that corresponds to the data. This preferred added mapping 
capability may be referred to herein as high speed signalling mode, or 
simply, high speed mode. 
In high speed signalling mode, the PCM codewords are transmitted to the DTN 
110 in digital form at a rate that is compatible with a clock rate 
utilized by the DTN 110, such as 8000 codewords per second. It is 
important to note that, in this embodiment, the information is not 
converted to analog form between the first data communications device 100 
and the DTN 110. 
When the PCM codewords transmitted by the first data communications device 
100 arrive at the local switch 114, the PCM codewords are converted into a 
corresponding sequence of analog voltages for transmission to the second 
data communications device 108 over the loop 112. The second data 
communications device 108 recovers the information 102, from the sequence 
of analog voltages that the device 108 receives from the loop 112, 
utilizing knowledge of: 1) the subset of codewords that are available to 
the first data communications device 100; and 2) the characteristics of 
the conversion between codewords and analog voltages that the local switch 
114 performs. 
Preferably, the subset of codewords that is utilized by the first data 
communications device 100 is determined in accordance with the 
characteristics of the communication channel 106. Typically, the 
digital-to-analog conversion characteristic of the local switch 114 
includes output analog voltages that are unequally spaced, which is a 
result of the design of the communication channel for the transmission of 
voiceband signals. Depending upon the characteristics of the communication 
channel 106, the second data communication device 108 may be unable to 
distinguish between closely spaced output analog voltages. In order to 
avoid such ambiguities, the subset of codewords, therefore, preferably 
includes only codewords that are associated with output analog voltages 
that may be uniquely identified by the data communications device 108. A 
training sequence may be utilized to identify codewords that are 
appropriate for inclusion, or exclusion, from the subset. 
By utilizing the high speed signalling mode described above, high speed 
transfer of the information 102 from the first data communications device 
100 to the second data communications device 108 may be obtained. For 
example, if the communication channel 106 is such that the second data 
communications device 108 is capable of distinguishing all of the possible 
PCM codewords, and each codeword is utilized to carry eight bits of 
information 102, then data signalling rates of up to approximately 64 
kilobits per second ("kbps") may be obtained. As the subset of codewords 
decreases in size from the set of all possible codewords, the data 
signalling rate decreases. Nonetheless, for a typical connection in the 
form shown in FIG. 2, data signalling rates in excess of 28,800 bits/s may 
be obtained in the high speed signalling mode. 
For communication in the reverse direction, input information 116 is 
modulated by the second data communications device 108 and modulated 
analog signals are transmitted from the device 108 to the local switch 114 
over the loop 112. At the local switch 114, the modulated analog signals 
are converted into PCM codewords for transmission over the DTN 110. The 
first data communications device 100 receives the PCM codewords from the 
DTN 110 and recovers the information 116 by demodulating the digital input 
from the DTN 110. 
Preferably, communication in the reverse direction is performed in 
accordance with Recommendation V.34. Other protocols, such as 
Recommendation V.32bis may alternatively be used, although with a 
resulting decrease in data signalling rate. 
The data communications devices 100 and 108 preferably include a digital 
signal processor and associated memory structures. The devices 100 and 108 
may be modems. In addition, either or both of the data communications 
devices may be a personal computer, a server, a hub or the like, having 
modem capabilities, such as may be provided by a digital signal processor, 
or a microprocessor, and its associated memory structures. Moreover, an 
electronic device, such as a microprocessor, may perform the functions of 
both the data source 104 and the data communications device 100. 
In a preferred embodiment, the data communications device 100 is a hub or 
server and the data communications device 108 is a modem. In accordance 
with a preferred signalling mode for such a communication system, the data 
communications device 100 is capable of transmitting data in high speed 
mode as well as other signalling modes, such as in accordance with 
Recommendation V.34. Likewise, the data communications device 108 is 
capable of receiving data in high speed mode as well as other signalling 
modes, such as in accordance with Recommendation V.34. In addition, the 
data communications device 100 is preferably capable of receiving data 
transmitted from the data communications device 108 in accordance with 
Recommendation V.34. 
Referring now to FIG. 3, the preferred embodiment of a mode signalling 
method will be described. A call modem initially conditions its receiver 
to detect an answer tone, such as ANS or ANSam, transmitted from an answer 
modem. The call modem then transmits a call indicator or call tone 120, 
such as CI (shown), CNG or CT. If the signal response ANSam 122 is 
detected by the call modem, then the call modem transmits silence for a 
period of time, T.sub.e, 124. If, on the other hand, the signal response 
ANS is detected, indicating that the answer modem is incapable of 
performing the CM/JM exchange and that high speed mode is unavailable, the 
call modem proceeds under a different protocol, for example as provided in 
Annex A of Recommendation V.32bis or Recommendation T.30. 
Next, the call modem conditions its receiver to detect a joint menu signal, 
JM.sub.u, and transmits a call menu signal, CM.sub.u, 126. The call menu 
signal, CM.sub.u, 126 includes the appropriate bits set in the modulation 
modes category to indicate that V.34 is desired. Table 4 of Recommendation 
V.8 shows the coding over two octets to indicate availability of General 
Switched Telephone Network V-series modulation modes, including the 
availability of V.34. 
When a minimum of two identical JM or JM.sub.u sequences have been 
received, the call modem shall complete the current CM.sub.u octet and 
then send a call menu terminator signal, CJ or CJ.sub.u (shown), 128 to 
acknowledge JM or JM.sub.u, respectively, and terminate the call menu 
signal 126. After sending CJ or CJ.sub.u, the call modem transmits silence 
130 for 75.+-.5 ms and may then, for example, escape normal V.34 operation 
and proceed with the steps of ranging/probing, equalizer and 
echo-canceller training, and final training as appropriate for high speed 
signalling mode. The phrase "identical sequences" as used herein refers to 
sequences containing the same information. 
The preferred embodiment of the mode signalling method as it pertains to 
the answer modem will now be described with reference to FIG. 3. Upon 
connection to the line, the answer modem initially remains silent for a 
period of time, T.sub.a, 132. Preferably, T.sub.a is at least 200 ms. 
After waiting T.sub.a, the answer modem transmits ANSam 122 in accordance 
with the procedure set forth in Recommendation V.8, .sctn. 7.2, including 
phase reversals. Next, the answer modem conditions its receiver to detect 
CM and CM.sub.u. The answer modem may also be conditioned to detect call 
modem responses from other appropriate Recommendations. 
At this point, if the answer modem detects a minimum of two identical CM 
sequences and does not detect CM.sub.u, then the answer modem proceeds in 
accordance with Recommendation V.34, .sctn. 11.1.2.2. On the other hand, 
if the answer modem detects a minimum of two identical CM.sub.u sequences, 
then the answer modem transmits JM.sub.u 134 and conditions its receiver 
to detect CJ or CJ.sub.u. Once three octets of CJ or CJ.sub.u are detected 
by the answer modem, the answer modem transmits silence 136 for 75.+-.5 ms 
and may then, for example, escape normal V.34 operation and proceed with 
the steps of ranging/probing, equalizer and echo-canceller training, and 
final training as appropriate for the high speed signalling mode. 
The mode signalling method described herein is preferably utilized to 
identify the availability of the high speed signalling mode of data 
transfer described above. However, the described method includes a 
generalized escape sequence that may be utilized to allow any two 
compatible communications devices to "escape" from one mode of operation 
into a different mode that both devices support. 
In this description, the subscript "u" associated with the call menu (CM), 
joint menu (JM) and call menu terminator (CJ) signals represents signals 
that identify an added capability, for example the capability to operate 
in high speed mode. In the case wher the added capability is the 
capability to operate in high speed mode, CM.sub.u and JM.sub.u preferably 
conform to Recommendation V.8 but also signal a desire to escape normal 
V.34 operation and establish high speed signalling mode training and 
operation. 
As shown in FIG. 3, data communications devices that are capable of high 
speed mode data transfer preferably transmit and recognize CM.sub.u and 
JM.sub.u instead of the CM and JM signals described in Recommendation V.8. 
It is also preferable that CM.sub.u and JM.sub.u be recognized as CM and 
JM, respectively, as described in Recommendation V.8, by data 
communications devices that are incapable of operating in high speed 
signalling mode. 
In accordance with a signalling method that satisfies these preferences, 
CM.sub.u 126 and JM.sub.u 134 are transmitted in the modulation format 
specified in Recommendation V.21 and within the frequency tolerances 
specified in Recommendation V.21, but with a superimposed modulation that 
identifies high speed mode capability. In general, the signalling method 
of the preferred embodiment communicates one additional bit of 
information, i.e. capable or not capable of high speed signalling mode, 
through the superimposed modulation. Other than the superimposed 
modulation, CM.sub.u 126 and JM.sub.u 134 preferably include the same 
information content as the CM and JM signals described in Recommendation 
V.8. CM and JM may be referred to herein as frequency division modulation 
(FDM) capabilities signals. 
The superimposed modulation, for example, may be a slight change in the 
Recommendation V.21 frequencies representing a mark and a space, such as 
.+-.1 Hz, following a predetermined pattern that may be detected by a 
receiver in a data communications device. Specifically, the call modem may 
transmit a first CM.sub.u sequence having Recommendation V.21 mark and 
space frequencies that are 1 Hz above nominal, e.g. a mark is transmitted 
as a 981 Hz signal and the space is transmitted as an 1181 Hz signal. A 
second CM.sub.u sequence, having the same information content as the first 
sequence, is then transmitted having Recommendation V.21 mark and space 
frequencies that are 1 Hz below nominal, e.g. a mark is transmitted as a 
979 Hz signal and a space is transmitted as an 1179 Hz signal. As the 
pattern continues, mark and space frequencies in subsequent CM.sub.u 
sequences alternate between 1 Hz above nominal and 1 Hz below nominal. 
In the same manner, the answer modem may confirm its high speed mode 
capability by transmitting a pattern of JM.sub.u sequences in which the 
mark and space frequencies in successive sequences alternate between 1 Hz 
above nominal and 1 Hz below nominal. Of course, other space and mark 
frequencies that fall within the .+-.6 Hz range of the nominal 
Recommendation V.21 frequencies may alternatively be used. For example, 
CM.sub.u, JM.sub.u and CJ.sub.u may be transmitted in sequences in which 
the space and mark frequencies differ from the nominal frequencies by 
alternating between .+-.0.5 Hz from nominal for successive sequences. 
It is important, for the embodiment described above, that the modified 
signal frequencies fall within the range prescribed by Recommendation 
V.21. In this manner, a V.34 modem (or V.21 receiver) that does not have 
the added capability (in the example above, high speed mode) will not 
detect the modified V.21 signalling. On the other hand, a device that has 
the added capability will include a V.21 receiver that is modified to 
detect the superimposed signalling. 
FIG. 4a shows the mark frequencies f.sub.M 300, 302 and space frequencies 
f.sub.S 304, 306 of the V.21 FDM modulation scheme. Note that there are 
two independent channels, each having mark and space frequencies. Channel 
number 1, 308, is typically used for transmission of data by the calling 
modem, while channel number 2, 310, is typically used by the answer modem, 
in accordance with the V.21 specification. 
FIG. 4b shows the set of presently preferred variations of the mark and 
space frequencies for channel number 1, 308, that may be utilized to 
convey additional information between data communications devices. The 
same variations may be used with the channel 2 frequencies. The 
frequencies f.sub.M0, 312, and f.sub.S0, 314, are 981 Hz and 1181 Hz, 
respectively, and the frequencies f.sub.M1, 316, and f.sub.S1, 318, are 
979 Hz and 1179 Hz, respectively. As stated above, the frequency variation 
of the superimposed FDM signals may be larger, but should be such that 
they fall within the range, 320, specified by Recommendation V.21. 
Thus, the preferred signalling method may communicate high speed mode 
capability, or some other added capability, by substituting a four 
frequency FDM signalling method for the two frequency FDM signalling 
method described in Recommendation V.21. In effect, the preferred 
embodiment creates an independent signalling channel that is superimposed 
upon the V.21 signalling format. 
In alternative embodiments, the predetermined pattern of frequency shifts 
may be altered in any detectable manner and may occur at a higher baud 
rate. In addition, different frequencies may be utilized in the FDM 
signalling method as long as the frequencies remain within the transmitter 
tolerance of the nominal frequency .+-.6 Hz specified in Recommendation 
V.21. Furthermore, additional information may be transmitted using the 
superimposed modulation by defining additional signalling patterns or by 
utilizing more than four frequencies in the FDM signalling method. In a 
still further alternative, the superimposed modulation may be phase 
modulation of the mark and space frequencies specified in Recommendation 
V.21, rather than a multiple frequency FDM signalling method. 
Referring back to the communication system shown in FIG. 2, if the data 
communications device 100 is a hub or server having modem capabilities, 
then the data communications device 108 will typically operate as the call 
modem in FIG. 3 and the device 100 will operate as the answer modem in 
FIG. 3. If the data communications devices 100 and 108 are capable of 
operation in the high speed signalling mode, the data communications 
devices 100 and 108 detect the superimposed modulation carried by the 
CM.sub.u 126 and JM.sub.u 134 sequences, respectively. In response to 
detecting the superimposed modulation, the data communications devices 100 
and 108 are configured for high speed signalling mode. 
The mode signalling methods described herein provide the advantages of 
uniquely identifying an added capability, for example high speed mode 
capability, without interfering with normal training and operation under a 
predetermined procedure, such as set forth in Recommendation V.34, for 
instances in which the answer modem is not capable of operation in 
acordance with the added capability (high speed mode in this case). In 
addition, the mode signalling methods identify added capabilities without 
using codes that are already established in ITU standards or reserved for 
future ITU use. 
Although the foregoing discussion is addressed to the case wherein the 
added capability is a high speed signalling mode, the present invention is 
generally applicable to the identification of added capabilities. More 
specifically, a generalized escape sequence has been described herein. The 
preferred embodiments of the present invention relate to superimposing 
mode identification information upon a generated signal in a way that is: 
1) transparent to devices that are incapable of operating in accordance 
with the identified mode; and 2) detectable by devices that are capable of 
operating in accordance with the identified mode. In this manner, added 
capabilities may be communicated and utilized where appropriate. 
It is intended that the foregoing detailed description be regarded as 
illustrative rather than limiting and that it is understood that the 
following claims, including all equivalents, are intended to define the 
scope of the invention.