Switched line modem interface system

A system is provided in a circuit switched network data transmission link between a user terminal and a host computer in instances where the terminal is connected to the network by a single telephone line and the terminal user has subscribed to a telephone company service feature such as call waiting. The system includes an interface at the terminal location between the telephone line and the terminal equipment. During a data session the interface detects a call waiting signal and generates a user signal. If the user desires to take the incoming call and suspend a data session, the interface responds to the telephone receiver being lifted off-hook and communicates through the circuit switched network with an interface at the host end of the communication. The host interface is programmed to maintain the data session at the host computer during a suspension of the communication by generating a signal simulating a terminal location signal. The terminal interface locally generates a signal indicating that the host is not ready and toggles the telephone company central office to switch the user's line to the incoming call. To resume the data session, the user places the telephone instrument on hook and the terminal interface toggles the central office to access the host interface for the purpose of resuming the data session.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to data transmissions utilizing a circuit 
switched network and more particularly to an interface system for 
preventing loss of a data session when a line is temporarily switched for 
an optional use. 
2. Background Art 
Among the many circuit switched network service features offered by 
telephone companies and some private exchanges which have been made 
available to a single line telephone subscriber have been the ability to 
switch between an existing call and a second incoming call which is 
"waiting" or placing an existing call on hold while making an outgoing 
call. A single line telephone subscriber was provided with a single 
telephone access number which supported a single voice grade circuit. The 
single line telephone instrument and/or equipment was connected to the 
telephone company central office and telephone company equipment over a 
local loop and supported analog transmissions on the local loop. In the 
"call waiting" situation, during the subscriber's initial conversation the 
telephone central office transmitted an audible "call waiting" signal to 
the subscriber. Upon receipt of such signal, the subscriber had the option 
of placing the existing call on hold and answering the incoming call. This 
was accomplished by toggling, i.e. pressing and releasing the hook switch 
in a predetermined sequence. The toggling thus constituted a communication 
with the central office instructing the telephone company equipment to 
alter the communications path. The central office then placed the existing 
call on hold and switched the incoming call to the subscriber's telephone 
line. Upon completion of the second call, the central office was again 
toggled to reinstate the initial call. 
To utilize the call hold, conference call or three way calling optional 
service feature, the subscriber placed an existing call on hold, by 
toggling the central office, obtained a dial tone to initiate a second 
call and if the two calls were to be conferenced, again toggled the 
central office to effect conferencing. If the calls were not to be 
conferenced the second call was terminated and the original call 
reinstated by toggling. 
Such telephone subscriber features, while convenient, presented problems 
when the initial call comprised a data session between a subscriber's 
personal computer terminal and a host computer. Often the incoming "call 
waiting" signal introduced data transmission errors at either the modem or 
the terminal. Not only did the "call waiting" signal interfere with the 
data transmission but, in addition, it was virtually impossible for the 
subscriber to access the incoming call waiting or even determine who the 
caller was without loss of the data session. 
If the subscriber toggled the central office, not only did the toggle 
signal itself provide an error source, but, in addition, since neither the 
host nor the terminal was programmed to recognize that the central office 
placed their connection on hold status on their existing lines, carrier 
signals were lost and the communication was terminated. Such loss of 
communication resulted in unnecessary expense to the subscriber and 
generally required the subsequent reestablishment of another data session 
where at least a portion of the prior data transmission had to be 
duplicated. The same problems existed if a subscriber wished to initiate a 
call hold feature while on a data session. 
When either feature was attempted to be utilized, the carrier signal 
between the terminal modem and the host modem was broken. Since the 
communications path through the telephone network was suspended, the 
carrier signal was lost causing the modems to disconnect the session. 
SUMMARY OF THE INVENTION 
In compendium, the invention comprises an interface system for use in data 
transmissions which utilize a circuit switched telephone network. An 
interface at the user terminal end of the transmission communicates with 
an interface at the host computer end of the transmission prior to and 
after suspension of a data session. During a data session, the interface 
detects a call waiting signal and rings the user's telephone instrument. 
If the user's telephone instrument goes offhook, the interface transmits 
appropriate status signals to the terminal for maintenance of the session 
while the user's telephone line is utilized for the incoming telephone 
call. The terminal end interface unit also communicates in-band with an 
interface at the host end which is programmed to maintain the data session 
at the host computer during the suspension period and keeps an offhook 
condition at the host central office. Thereafter, the terminal interface 
toggles the telephone company central office equipment to switch the 
user's telephone line to the incoming call. 
When the subscriber wishes to resume the data session, he merely places his 
telephone instrument on hook. The terminal end interface then toggles the 
central office to reconnect the host communications path and communicates 
with the host interface to resume the data session. 
From the foregoing compendium, it will be appreciated that it is a 
consideration of the present invention to provide a data transmission 
interface system of the general character described which overcomes the 
disadvantages encountered with the background art aforementioned. 
An aspect of the present invention is to provide a data transmission 
interface system of the general character described which is easy to use. 
To provide a data transmission interface system of the general character 
described which is relatively low in cost is a further consideration of 
the present invention. 
A feature of the present invention is to provide a data transmission 
interface system of the general character described which permits a single 
telephone line subscriber utilizing the telephone network for a data 
transmission to employ a call waiting or call hold option during a data 
session without terminating the data session. 
A further feature of the present invention is to provide a data 
transmission interface system of the general character described which 
facilitates utilization of optional telephone company service features. 
Another aspect of the present invention is to provide a data transmission 
interface system of the general character described which does not require 
additional programming of a terminal or host computer and is transparent 
to such devices. 
A still further consideration of the present invention is to provide a data 
transmission interface system of the general character described which is 
user accessed by conventional telephone instrument operation. 
To provide a data transmission interface system of the general character 
described which is well suited for economical mass fabrication is another 
aspect of the present invention. 
Other aspects, features and considerations of the present invention in part 
will be obvious and in part will be pointed out hereinafter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
This invention relates to a single line supporting analog transmission on 
the subscribers "local loop" with telephone features allowing "toggling" 
the local telephone company equipment at the data terminal end to alter 
the endpoint of the single communications path without discontinuing the 
data session involuntarily. The interfaces that support the data session 
are at the terminal and host end, independent of the circuit switched 
network and are controlled by in-band signaling on the circuit between the 
interfaces. 
Referring now in detail to the drawings, the reference numeral 10 denotes 
generally a data communications link utilizing a circuit switched network 
12 for interconnecting a user terminal 14 with a host computer 16. The 
user terminal 14 is conventional in both construction and programming and 
typically includes a keyboard 18, a display and associated peripherals 
such as a printer. The terminal 14 is connected to a modem 20 through a 
connecting line 19. If the modem 20 comprises an internal modem, a 
separate connecting line is not required. 
The modem 20 is in turn connected to a user's telephone company central 
office 22 by a telephone line 24 (local loop). 
It should be understood that the user has a single telephone line and has 
subscribed with his telephone company for optional service features such 
as call waiting and/or call hold features, e.g. conference or three way 
calling that it actuated by toggling the network. 
In accordance with the invention, an interface 28 is connected between the 
modem 20 and the telephone line 24. It is of significance that the user is 
employing a single telephone line 24 to the central office 22. 
The user also has a conventional telephone instrument 26 which, however, is 
not connected directly to the telephone line 24 in a conventional manner 
but is connected to the interface 28 by a line 27. The communications link 
10 extends through the public switched telephone network 12 to a host 
computer central office 30 and from the central office 30 to the host 
location via a telephone line 32. 
At the host end of the communications link, the telephone line 32 is 
connected to an interface 34 and from the interface 34 to a host computer 
modem 36 which communicates with the host computer 16. The connection at 
the host end may also comprise a connection to a port on a network 
interface or entry into a protocol converter for purposes of compatibility 
and/or multiplexing multiple signals through a network. 
An indicator 33 at the terminal end signals that the complimentary 
interface 34 is at the host end when the communications link is 
established through a protocol exchange of data upon a data call set up. 
Thus the terminal user will know that the interface system is operable. 
In accordance with the invention, the interfaces 28, 34 of the system are 
totally transparent to both the user terminal 14 and the host 16. During a 
data session, data is transferred through the interfaces without 
interruption. In the event a third party wishes to place a telephone call, 
he utilizes a remotely located telephone instrument 38 which is connected 
to the public switch telephone network 12. 
The third party call is received and held by the telephone network 12 while 
a call waiting signal is transmitted to the user through the telephone 
line 24. 
With reference now to the subroutines indicated in FIGS. 3 through 5, the 
interface 28 detects the call waiting signal and does not pass the signal 
on through to the modem 20. The interface 28 rings the user's telephone 
instrument 26 using local power. If the user decides to suspend the data 
session and take the incoming call, he merely lifts the telephone receiver 
so that the instrument is "offhook". In response to the "offhook" 
condition, detected by an offhook detector 29, the interface 28 transmits 
an in-band signal through the circuit switched network 12 to the host 
interface 34 which signal indicates that the call is about to be 
interrupted. Upon receipt of an in-band host interface responsive 
acknowledgement, the user interface 28 maintains the carrier signal to the 
modem 20 while at the same time the interface 28 can generate a signal 
indicating that the host is not ready. Thereafter, the interface 28 
toggles the central office 22 and the central office switches the user's 
telephone line 24 to the telephone instrument 38 of the third party. 
At the host end of the communication, the host interface receives the 
signal from the user interface 28 and, in response, maintains the carrier 
to the host modem 36 and drops the terminal ready signal going to the 
host. The host 16 is thus placed in a suspension state during the time 
that the user's telephone line 24 is switched to the third party call. In 
addition, the host interface 34 maintains an offhook signal to the central 
office 30 so that the telephone network line 32 is not disconnected. Also, 
the host interface communicates its status to the terminal interface. Upon 
receiving a subsequent in-band signal from the interface 28 through the 
network 12, the host interface 34 generates a terminal ready signal to the 
host and resumes its subroutine. Errors introduced due to suspension and 
reestablishment of the data session, network sign calling and the like are 
corrected by the higher level protocols contained in communications 
software when the data session is resumed. 
Signaling between the interfaces 28, 34 is accomplished in-band yet 
preferably outside the frequency range of the data transmit and receive 
signals for the purpose of assuring that the interface communications are 
totally transparent to any data transmissions. 
An optional switch 35 may be provided for the user to signal the interface 
28 to break the carrier through the network 12 and terminate rather than 
suspend the data session. The switch 35 is actuated prior to lifting the 
receiver. 
It should also be appreciated that during a data session, the interface 28 
can respond to a user utilizing a call hold option should the user merely 
put his telephone instrument 26 off hook, the interface 28 would initiate 
the subroutine commencing with the portion illustrated in FIG. 4. 
Referring now in detail to FIG. 2 wherein the various functional components 
of the terminal interface 28 are shown, it should be appreciated that the 
interface 28 includes a microprocessor 40 such as a Motorola 68XX series 
which is intergrated with a memory 44 and flip flops 46. The flip flops 46 
are employed to hold various logic states as may be necessary to generate 
signals including the host terminal not ready signal or signals 
transmitted to the network. 
A power supply 48 is also provided not only for the purpose of powering the 
components of the interface 28 but, in addition, for generating the ring 
signal for the telephone instrument 26 during a data session. The 
interface 28 also includes analog circuitry 50 and switch circuitry 52 
which are provided for the purpose of keeping carrier up to the modem 20 
and generating status signals or toggle signals to the central office. In 
addition, the interface 28 includes a suitable filter 54 for detecting 
carrier and optional feature signals generated by the central office 22. 
The host interface 34 includes similar components with its microprocessor 
programmed to effect the necessary subroutines in response to 
communications from the user interface 28 and also keeping an offhook 
condition to its central office. 
As illustrated in the processor subroutine depicted in FIG. 3, after 
receipt of an incoming call signal, the processor 40 makes an inquiry as 
to whether or not a data session is in progress and if the data session is 
in progress, proceeds to ring the telephone instrument 26. The telephone 
instrument 26 may be rung by utilizing the power supply 48 as heretofore 
mentioned. 
Additionally, the processor 40 makes an inquiry as to whether or not the 
telephone 26 is offhook. An offhook detector 29 may comprise any 
conventional sensor suitable for interrogation by the processor 40. For 
example, in FIG. 1, the offhook detector 29 is illustrated as comprising a 
lever switch, an optical sensor, an attitude sensor, e.g. mercury switch, 
or a proximity sensor placed on or adjacent the telephone instrument. In 
FIG. 2, the offhook detector is illustrated as a device monitoring loop 
current through the telephone instrument. 
Thus it will be seen that there is provided a data transmission interface 
system which achieves the various aspects, features and considerations of 
the present invention and which is well suited to meet the conditions of 
practical usage. 
Among possible variations without departing from the spirit of the 
invention are included the integration of the modem and interface in a 
single unit. Such integration could also include the latching of signal 
functions, especially in integrated equipment with multiple functions. It 
should be noted that the host computer can be interpreted to include a 
personal computer and similarly the user terminal should be interpreted to 
include a personal computer. 
In addition, while the host interface 34 has been illustrated as being 
positioned in front of the host computer 16, the interface may be located 
within the switched telephone network 12 prior to signal entry to network 
equipment that converts, stores and/or routes the communications to their 
final destinations. 
As various possible embodiments might be made of the present invention and 
as various changes might be made in the embodiment above set forth, it is 
to be understood that all matter herein described or shown in the 
accompanying drawings is to be interpreted as illustrative and not in a 
limiting sense.