Dual multipoint data transmission system modem

A modem is provided which allows two independent sets of multipoint users to share a common communication media such as a telephone channel. At each site there is provided a modem which transmits and receives two independent frequency division multiplexed data streams simultaneously over a common telephone channel. Each data stream is encoded into an eight phase PSK or QAM signal. One data stream signal is transmitted in a low frequency range and the second is transmitted in a high frequency range. Means are provided at each site to combine the data streams for transmission over a single communication line and to separate the received combined signal from the communication line.

BACKGROUND OF THE INVENTION 
The present invention relates to a high speed modem used for data 
transmission and in particular to a modem which allows two independent 
multipoint networks to share the same communications lines or media. 
Multiplexing in frequency or in-time permits multiple users to share a 
common communication media simultaneously. In Communication System 
Engineering Handbook by D. H. Hamsher, McGraw-Hill, 1967, various data 
transmission multiplexing techniques are discussed in detail. These 
techniques assume that the multiple users and hence the multiplexing 
equipment are connected in a point-to-point fashion by the communication 
media. 
Recently many communication systems have been developed which employ a 
multi-point or multi-drop configuration. In these systems, many users 
share a common party line to a central site. While the central site may 
broadcast to all users, at any one time only one user may transmit to the 
central site. If two users transmit to the central site simultaneously, 
they mutually interefer and both transmissions are lost. The protocol 
employed to insure proper user response is referred to as polled. A 
typical multi-point communication system is depicted in FIG. 1 in which 
the communications line is a telephone line. The operation of the depicted 
multi-point or, as it is sometimes called, multi-drop system is discussed 
in detail in Systems Analysis for Data Transmission by James Martin, 
Prentice-Hall, Inc., 1972. It suffices for the present discussion to note 
that multiple terminals and associated modems (1, 2, and 3, for example) 
are connected through a telephone party line to a master modem which in 
turn is connected to the system front end processor. 
With the advent of large, complex data communication systems, many users 
have found a need for two independent multi-point communication systems 
which serve, or are located at common sites. Previously, this would 
require twice the hardware and components of a one multipoint system. That 
is, a dual system requires two sets of communication lines and two modems 
at each site, one for each of the terminals. FIG. 2 illustrates such a 
dual system in which it should be noted that at each location 1, 2 and 3 
there is an "A" modem and a "B" modem and a connection with an "A" and a 
"B" party line. 
The obvious disadvantage of the dual system of FIG. 2 is that it requires 
double the outlay for equipment as would be required for the system of 
FIG. 1. In addition, double the rental fees must be paid for the two lines 
required as compared to the single line of FIG. 1. 
In view of the above, it is the principal object of the present invention 
to provide a multi-point system which permits the virtual independent 
operation of two multipoint communication systems through a single modem 
at each location. 
A further object of this invention is to provide a transmitter/receiver 
structure which allows virtual independent operation of each of the 
multi-point systems over the same communication line. 
Other objects and advantages will be self-evident from the description of 
the preferred embodiment of the invention. 
SUMMARY OF THE INVENTION 
The above and other beneficial objects and advantages are attained in 
accordance with the present invention by providing dual multipoint data 
transmission networks sharing common sites wherein at each site there is 
provided a modem which transmits and receives two independent frequency 
division multiplexed 2400 BPS data streams simultaneously over a common 3 
KHz telephone channel. Each data stream is encoded into an eight phase PSK 
or QAM signal. One data stream signal is transmitted in the frequency 
range of 500 to 1600 Hz and the second is transmitted in the frequency 
range of 1800 to 2900 Hz. Means are also provided at each site to combine 
the data streams for transmission over a single communication line and to 
separate the received combined signal from the communication line.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference is now made to the drawings and to FIG. 3 in particular wherein a 
dual multipoint data transmission system serving common sites 1, 2 and 3 
is depicted. It should be noted at the outset that while three common 
sites are depicted, the present invention is by no means limited to any 
particular number of sites. At each of the sites there is an A terminal 
and a B terminal with the A terminals A1, A2 and A3 making up the "A" 
network and the B terminals B1, B2 and B3 making up the B network. At each 
of the sites 1, 2 and 3 there is also located a modem connected to both 
terminals in a manner to the described. Thus, at site 1 modem 10 is 
connected to terminals A1 and B1; at site 2 modem 12 is connected to 
terminals A2 and B2; and at site 3 modem 14 is connected to terminals A3 
and B3. Each of modems 10, 12 and 14 is connected to a common 
communication line such as a voice grade telephone party line 16. The 
telephone line 16 connects the remote sites with a master modem 18 which 
in turn is connected with a central front end processor 20 usually located 
at the master site. 
In accordance with the present invention, as illustrated in FIG. 4, two 
users may share a common communication media such a telephone line 16, by 
the known technique of frequency division multiplexing. In telephone 
systems the usual available bandwidth is from 300 to 3000 Hz. Accordingly, 
the A network is allocated a bandwidth from approximately 500 to 1600 Hz 
and the B network is allocated the bandwidth from approximately 1800 to 
2900 Hz. A guard band of 200 Hz separates the two networks. 
In a Nyquist bandwidth of 800 Hz a symbol rate of 800 symbols per second 
may be achieved by the well known technique of QAM as is discussed in U.S. 
Pat. No. 3,887,768. By encoding 3 bits per symbol or equivalently 
selecting one of eight possible phases or points in the signal space, a 
data rate of 3 times 800 symbols per second or 2400 BPS may be achieved. 
In practice, the 800 Hz Nyquist bandwidth requires an effective bandwidth 
of approximately 1100 Hz due to practical restrictions on spectral 
filtering. Hence, in a nominal 300 to 3000 Hz bandwidth two independent 
2400 BPS data streams may be transmitted. 
The dual 2400 BPS transmitter and receiver structure employed in the modems 
of the present invention is illustrated in FIG. 5. At each site modem 10, 
12, 14 and the master modem 18 there is provided a dual transmitter 
structure 32 and a dual receiver structure 34. Thus, two independent 
transmitters, 22 and 24 for the A and B networks respectively are employed 
in transmitter 32. The A transmitter 22 employs a carrier frequency of 
1050 Hz while the B transmitters 24 employs a carrier frequency of 2350 
Hz. Each transmitter 22 and 24 utilizes the respective bandwidths 
mentioned above (i.e., 500 Hz-1600 Hz for A transmitter 22 and 1800 
Hz-2900 Hz for B transmitter 24). The two transmitter outputs are added 
through adder 26 before being outputted to the telephone line 16 or other 
transmission media. 
The dual receiver 34 is similarly composed of two separate receivers, the 
first receiver 28 for the A network employing a 1050 Hz carrier and the 
second receiver 30 for the B network employing a 2350 Hz carrier. Thus 
data supplied to the A transmitter 22 is outputted by an A receiver 28 and 
vice versa. Similarly data supplied to the B transmitter 24 is outputted 
by a B receiver 30 and vice versa. Data traffic through the A and B 
networks are thus mutually independent. 
A specific implementation of the dual 2400 BPS transmitter 32 is shown in 
FIG. 6. Data for the A network is supplied to a modulator 36 the data 
being clocked into the modulator under the control of a clock. The carrier 
frequency employed by the A modulator 36 is 1050 Hz. Similarly data for 
channel B is clocked into the B modulator 38 under control of a clock. The 
B modulator 38 employs a 2350 Hz carrier frequency. The construction of 
modulators 36 and 38 is conventional and well known to those versed in the 
art. 
When RTS of channel A goes from an OFF to ON state, modulator 36 turns ON 
and after a time period of 25 ms, CTS to the customer or user goes ON. 
Simultaneously, modulator A begins accepting data from channel A. The 
RTS/CTS control unit for channel B works in an identical manner. 
The output of modulator 36 is passed through a low pass filter 40 to remove 
any high frequency components that may interfere with channel B. The 
output of modulator 38 is passed through a high pass filter 42 to remove 
any low frequency components that may interfere with channel A. The 
filtered outputs are then added together in adder 44 and passed through a 
band pass filter 46 before being fed to the telephone line. The band pass 
filter 46 serves to remove unwanted frequencies outside the telephone 
channel bandwidth. 
Either one, both or neither channel may transmit at any specific instant in 
time. The transmission of each channel is independent of the other 
channel. 
Details of the dual receiver 34 are illustrated in FIG. 7. As shown, the 
received signal from the telephone line 16 or communication media is fed 
to a band pass filter 48 to remove unwanted noise outside the bandwidth of 
the two channels. The BPF 48 has a nominal pass band of 400 to 3000 Hz. 
After filtering the composite signal is passed through an automatic gain 
control amplifier 50 or AGC to restore the signal level to approximately 0 
dBm. The signal is then supplied to a low pass filter 52 and high pass 
filter 54. The low pass filter removes the channel B signal and leaves the 
channel A signal intact. The channel A signal which remains is fed to a 
demodulator 56 which recovers timing, carrier frequency and detects the 
channel A data. The construction of demodulator 56 is conventional and 
well known to those versed in the art. A nominal carrier frequency of 1050 
Hz is employed by demodulator 56. Channel A clock or timing and channel A 
data are supplied as output to a user or the A network. Demodulator 56 
also outputs a line signal detect signal (LSD) which when ON indicates to 
the user that the channel A modulator is ON and that valid channel A data 
is present. The channel A LSD signal functions independently of the fact 
that energy may or may not be present on channel B. 
The high pass filter 54 and channel B demodulator 58 function in a similar 
manner except that the high pass filter removes the channel A signal and 
leaves the channel B signal intact. 
It is apparent from the foregoing that the system appears to the A and B 
network users as two independent digital communication channels 
notwithstanding the fact that both share a common telephone line.