Transceiver having collision detection capability for contention-formatted FDM local area networks

A transceiver provides data collision detection and avoidance in a contention-formatted, FDM communications network using FSK modulation. Locally-generated baseband data is compared to recovered baseband data received from the system bus. A failure to compare indicates that there has been a data collision. The wide disparities in amplitude between locally-generated FDM signals and signals received from the bus are accommodated by a signal splitter which provides greater attenuation of the local signal than the generally weaker bus signal, and by the use of logarithmic amplifiers which generate relatively constant output levels for a wide range of input signal amplitudes. Collision avoidance is provided by monitoring the bus signal demodulators for data activity, and by inhibiting the local transmitter when such activity is detected, subject to an override during transmission by the local station.

The present invention relates generally to the field of data communications 
and, more particularly, to an apparatus for data communications systems 
which allows detection of collisions of information signals within a 
communications medium. 
In multi-access communications systems, or networks, for interconnecting 
multiple devices or stations in a communications network, means must be 
provided for controlling access of the devices to the communications 
channel. Such a system may employ frequency division multiplexing (FDM) to 
provide a multiplicity of transmissions over a single coaxial cable 
interconnecting all stations. In an FDM system two or more stations are 
interconnected by the assignment of a single frequency band for exclusive 
use by those stations for the duration of their interconnect. In addition, 
each station must also have continuous communications access to the 
controlling device, i.e., that device which controls station interconnects 
by the assignment of frequency bands, for the purposes of sending status 
information and receiving its frequency assignments. 
If the control information between each station and the controller were to 
be assigned an exclusive frequency band, this would severely limit the 
frequency bands available for station-to-station interconnects. Therefore, 
the controller commands to each station and the status returns from each 
station are all provided within a single frequency band using contention 
formatting, that is, the several transmitters each "contend" for use of 
the control channel and, of course, only one device can transmit on the 
control channel at one time. Each station desiring use of the control 
channel monitors activity at the control channel frequency and, when it 
becomes idle, begins transmission on that channel. Contention formatting 
is further justified by the nature of the control channel traffic which 
tends to be bursty, that is, occasional and very brief, as contrasted with 
station-to-station messages which may be lengthy. 
As was mentioned above, each transmitting device monitors, or "listens to," 
the control channel to determine if any other station is using it, before 
beginning its own transmission. The real danger of data collision occurs 
when two devices begin to transmit at virtually the same time. This 
problem has been dealt with for baseband systems in a number of U.S. 
patents, including U.S. Pat. No. 4,063,220, issued to Metcalfe et al. on 
Dec. 13, 1977, and entitled "Multipoint Data Communications System With 
Collision Detection," and U.S. Pat. No. 4,099,024, issued to Boggs et al. 
on July 4, 1978, and entitled "Communications Network Repeater." The 
Metcalfe and Boggs patents show collision detection by utilizing a device 
which determines when a collision has actually occurred by measuring phase 
shift in the information signal received. 
FDM systems present an additional problem related to the variations of 
signal amplitude on the system bus. In a baseband data system all levels 
on the bus are held substantially constant and collision detection 
involves the relatively simple task of comparing the locally transmitted 
signal with the signal received from the bus. However, when data is 
transmitted using FDM, the inherent properties of frequency modulation 
prohibit the detection of a colliding signal that is not close in 
amplitude to the locally transmitted signal. Until now, there has been no 
adequate system to reliably detect FDM collisions because the signal 
variations present on an FDM bus are sometimes quite large. The higher 
amplitude signal swamps out and prevents detection of the smaller 
amplitude signal. 
One object of this invention is to provide an apparatus whereby a weak 
FDM-formatted signal on a data communications bus can be detected in the 
presence of a stronger signal. 
It is a further object of this invention to provide an improved FDM 
transceiver which detects collisions between data signals of widely 
disparate amplitudes. 
It is yet another object of this invention to provide an improved 
transmitter which will be prevented from transmitting on the system bus 
whenever a remotely-originated signal is present on the bus. 
In accordance with the principles of the present invention, an apparatus is 
disclosed for transmitting and receiving RF modulated signals. The 
apparatus includes means for frequency modulating a baseband signal and 
means for combining the output signal of the modulating means with a 
signal received by the apparatus. Means are provided for demodulating the 
output signal of the combining means. Finally, the apparatus includes 
means for comparing the output signal of the demodulating means with the 
baseband signal.

Referring to the block diagram of FIG. 1, the information to be transmitted 
by the local station, in the form of TTL data (that is, digital data 
signals generated by or compatible with TTL logic elements), is applied at 
input terminal 11 where it is coupled to modulator 21. Modulator 21 
translates the TTL data to an RF frequency and the modulated signal is 
coupled to transmitter 22. In the present example, a frequency shift 
keying (FSK) modulation scheme is used, which is similar to the type 
illustrated in FIG. 2. 
FIG. 2 depicts a portion of the frequency spectrum for a data rate of 1 MHz 
and a 5 MHz separation between frequencies, at a center frequency of 402.5 
MHz. A low level of the TTL-data signal is converted to a 400 MHz signal 
and a high level is converted to a 405 MHz signal. 
Modulator 21 may be, for example, a varactor oscillator which generates an 
output signal whose frequency is dependent on the voltage level of the 
input signal, thus providing a coherent FSK signal. Alternatively, 
modulator 21 may cause switching between two RF signals in response to the 
level changes of the TTL data input signal, thus providing a noncoherent 
FSK signal. 
The output signal from transmitter 22 is applied to terminal 24a of 
splitter 24. Splitter 24 may be a hybrid junction with small signal 
attenuation between terminals 24a and 24b and between terminals 24b and 
24c, but with considerable isolation between terminals 24a and 24c. In the 
present example splitter 24 will attenuate a signal by 3 db between 
terminals 24a and 24b, and also 3 db between terminals 24b and 24c, but a 
signal applied at terminal 24a will appear at terminal 24c attenuated by 
30 db. Terminal 24b is coupled to tap 25 which interfaces the local 
transceiver with system bus 26. 
The signal at terminal 24c of splitter 24, which may have originated on bus 
26 (and is slightly attenuated) or which may have originated in 
transmitter 22 (and is greatly attenuated), is applied to receiver 27 
which inter alia down-converts the incoming RF signal to a more manageable 
IF signal having, perhaps, a center frequency of 10.7 MHz. Thus a TTL low 
level signal would be represented at the output terminal of receiver 27 as 
an 8.2 MHz signal (10.7-2.5), and a TTL high level signal would be 
represented as a 13.2 MHz signal (10.7+2.5). 
The receiver 27 output signal is applied to the input terminals of 
demodulators 28 and 29. Low frequency demodulator 28 comprises band-pass 
filter 30, which, in the present example, is tuned to a center frequency 
of 8.2 MHz, logarithmic amplifier 31, AM detector 32, and threshold 
detector 33. High frequency demodulator 29 comprises band-pass filter 34, 
which is tuned to a center frequency of 13.2 MHz, logarithmic amplifier 
35, AM detector 36, and threshold detector 37. With the exception of the 
pass-band frequencies of filters 30 and 34, the balance of the 
corresponding components of demodulators 28 and 29 are identical. 
Considering first low frequency demodulator 28, filter 30 passes only the 
portion of the IF output signal from receiver 27 which corresponds to a 
low level TTL signal. This signal is applied to logarithmic amplifier 31 
which provides a limited range of output signals for wide variations in 
input signal. Logamps are also advantageous for their fast response time. 
The output signal of amplifier 31 is applied to AM detector 32 which 
removes the IF component, leaving only the envelope corresponding to the 
low level elements of a TTL signal, essentially a digital signal. 
Threshold detector 33 shifts the output signal of AM detector 32 so as to 
be compatible with TTL levels, while additionally providing noise 
immunity. The output signal of threshold detector 33 is applied to a first 
input of TTL NAND gate 40. 
High frequency demodulator 29 performs a similar function on the IF output 
signal from receiver 27 corresponding to a high level TTL signal. Thus, 
the signal emitted by threshold detector 37 is a TTL-compatible signal 
which, when inverted by TTL logic inverter 39, is essentially identical 
with the output signal of threshold detector 33. Thus the output signal of 
NAND gate 40 will be a logic low when there is a TTL-low-modulated signal 
entering receiver 27, and a logic high when there is a TTL-high-modulated 
signal at receiver 27. The output signal of NAND gate 40 represents, in 
digital form, the signal recovered from system data bus 26 and is coupled 
to the local station utilization means (not shown) via output terminal 13 
as the RECOVERED DATA signal. This signal is also applied to a first input 
terminal of TTL exclusive OR gate 41. 
The locally-generated TTL data signal, transmitted via modulator 21, 
transmitter 22, splitter 24 and tap 25, onto bus 26 is also coupled 
through delay circuit 42 to the second input terminal of exclusive OR gate 
41. The delay period of delay circuit 42 is equivalent to the delay of the 
TTL data signal through modulator 21, transmitter 22, splitter 24, 
receiver 27, demodulators 28 or 29, and gate 40. Thus, if there is no 
signal being received from bus 26, identical signals are presented to the 
two input terminals of exclusive OR gate 41. Delay circuit 42 may 
comprise, for example, a monostable multivibrator configured to delay the 
propagation of a signal applied to its input terminal for a presettable 
period of time. Fine tuning of the delay timing may be accomplished by a 
potentiometer adjustment. 
One shot circuit 43, which may be, for example, a monostable multivibrator, 
acts as a filter of the output signal from gate 41, removing any high 
frequency pulses which may result from switching transients at the input 
terminals of gates 40 and 41. The signal at the output of one shot circuit 
43 is applied to output terminal 14 and when true (high logic level), 
indicates to the controlling means (not shown), that a collision has 
occurred between locally-generated and remotely-generated data. 
Summing circuit 38, whose inputs are coupled respectively to the output 
terminals of AM detectors 32 and 36, provides an indication of signal 
activity on bus 26 to inhibit circuit 23. If such signal activity exists, 
circuit 23 provides an inhibit signal to transmitter 22 and to output 
terminal 15, where it is received by a utilization means (not shown) as a 
CHANNEL BUSY indication. If, however, the signal activity detected in 
summing circuit 38 is a result of locally-generated data traffic, the 
LOCAL KEY signal applied at input terminal 12 overrides the inhibit signal 
provided at transmitter 22 and output terminal 15. 
The apparatus of the present invention performs two functions. It detects 
data collisions by comparing the baseband data being transmitted by the 
local station with the baseband data on the system bus, and it avoids data 
collisions by monitoring the system bus and inhibiting the local 
transmitter when there is signal activity on the bus. 
When operating as a collision detector, baseband digital data, applied at 
input terminal 11 is FSK-modulated by modulator 21, and is transmitted 
onto the system bus 26 by transmitter 22 via splitter 24 and tap 25. A 
small fraction of the locally-transmitted signal, applied at port 24a, 
reaches port 24c. This small fraction of signal is combined with the 
signal received at port 24b from bus 26 via tap 25, which arrives at port 
24c with much less attenuation. The combined signal is down-converted in 
frequency by receiver 27 and the resulting signal is applied to 
demodulators 28 and 29. 
The lower frequency component, corresponding, in the present example, to a 
low logic level baseband signal, is passed by filter 30 to logarithmic 
amplifier 31, which tends to provide a uniform output signal level for 
wide variations in input signal level. AM detector 32 extracts the 
envelope of the output signal from amplifier 31, and threshold detector 33 
converts the envelope signal to logic-compatable levels while filtering 
out noise. Thus, the output of demodulator 28 is a logic high level 
corresponding to a logic low level baseband signal originating locally or 
received from bus 26. 
Similarly, demodulator 29 receives the higher frequency component of the 
output signal from receiver 27, corresponding to a logic high level 
baseband signal. The signal output provided by demodulator 29 is thus a 
logic high level corresponding to a logic high level baseband signal 
originating locally or received from bus 26. 
The output signal from demodulator 29 is logically inverted by inverter 39, 
and the resulting signal applied to one input of NAND gate 40. With the 
second input of gate 40 coupled to the output of demodulator 28, it can be 
seen that the signals at the inputs to gate 40 will both be high for a low 
level baseband signal and will both be low for a high level baseband 
signal. The signal at the output terminal of gate 40, coupled to output 
terminal 13, has the same polarity as the corresponding baseband signal 
and therefore represents RECOVERED DATA. 
If there is no signal received from bus 26, through splitter 24, receiver 
27, demodulators 28 and 29, and gate 40, the signal at the output terminal 
of gate 40, and applied to a first input terminal of exclusive OR gate 41, 
will be the same signal applied at input terminal 11, delayed through 
delay circuit 42, and coupled to a second input terminal of gate 41. This 
being the case, the signal at the output terminal of gate 41, and applied 
via one shot circuit 43 to output terminal 14 will be a steady low level, 
and the COLLISION DETECTED signal will remain false. 
If, however, there is signal activity on bus 26, the signal at the output 
terminal of gate 40 will fail to track the data applied at input terminal 
11 (including its delay through circuit 42), and the COLLISION DETECTED 
signal will exhibit high and low levels, indicating to the controller (not 
shown) the presence of a data collision. 
When acting as an apparatus for avoiding collisions, the present invention 
monitors the signal activity on bus 26. Bus signals are applied to 
receiver 27 via tap 25 and splitter 24. Receiver 27 down-converts the 
frequency of the received signal and applies the down-converted signal to 
demodulators 28 and 29. Detectors 32 and 36 extract the baseband data 
signals which, if present, are represented as high level signals and which 
are applied to summing circuit 38. Therefore, if there is signal activity 
on bus 26, a high-level output signal will be coupled to inhibit circuit 
23 from summing circuit 38, thereby causing the operation of transmitter 
22 to be inhibited during such signal activity. When, however, the signal 
activity detected in demodulators 28 and 29 is a result of a 
locally-generated signal, the LOCAL KEY signal applied to inhibit 
circuitry 23 overrides the inhibiting function, permitting transmitter 22 
to continue its operation. 
Other embodiments of the present invention will be apparent to those 
skilled in the art to which it pertains. The scope of this invention is 
not intended to be limited to the embodiment disclosed herein but should 
instead be gauged by the breadth of the claims which follow.