Fault tolerant data transmission network

A data transmission system, in which a plurality of repeater nodes are interconnected by optical fiber or electrical transmission links, which has a predetermined bypass path split ratio and does not require an automatic gain control circuit. Each node includes an electrical splitter at its input, an active branch including a receiver and a transmitter, and a passive branch which bypasses the active path. The active and passive branches are joined at the output of the node by an electrical combiner. By choosing the splitting ratio of the electrical splitters to have approximately a 70/30 split, with 70% going to the active branch and 30% going to the passive branch, a 10.sup.-9 bit error rate is readily achievable.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a data transmission system in which a plurality 
of repeater nodes are interconnected by transmission links and, more 
particularly, to a system that has a predetermined bypass path split ratio 
and does not require an automatic gain control circuit. 
2. Description of the Related Art 
Fault tolerant systems for data transmission systems are known in the art. 
Conventional methods for providing fault tolerant systems include methods 
which bypass the main transmission path utilizing a bypass switch which, 
upon failure at a node, will allow a bypass around the failed node. 
Another method utilizes parallel redundant transmission equipment, thereby 
allowing use of a secondary system when a primary system fails. Chown et 
al., U.S. Pat. No. 4,166,946, teaches a data transmission system utilizing 
optical fibers in which an optical fiber bypass arrangement is provided in 
the event of a repeater node failure. In each of the above systems, the 
main signal input into the repeater is boosted and sent further down the 
line. This boosting is needed to cure the effect of signal degradation as 
the signal travels over the transmission line. 
FIG. 1 is a graph conceptually showing how a repeater of the prior art 
affects a signal applied to its input. As shown in FIG. 1, a digital 
signal (line A), which has degraded due to propagation losses, is applied 
to the input. In the repeater, the signal is boosted by adding to the 
input signal (portion C in FIG. 1), thus raising the amplitude of the 
signal so that it is output as shown by dotted line B. A major problem in 
implementing such a boosting scheme is the accuracy by which the boosted 
signal is added to the original signal. Also, an optically incoherent 
boost scheme cannot improve the accumulation of pulse dispersion. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a data transmission 
system having a plurality of repeater nodes interconnected by optical 
fiber or electrical transmission links, which can, even in the event of 
failure of one or more repeater nodes, still transmit data between the 
remaining functioning repeater nodes. 
Another object of the present invention is to provide a data transmission 
system having a plurality of repeater nodes interconnected by optical 
fiber or electrical transmission links, in which each repeater node 
recreates or re-transmits a signal based upon the signal applied to its 
input. 
An additional object of the present invention is to provide a data 
transmission system in which a plurality of repeater nodes are 
interconnected by optical fiber or electrical transmission links and in 
which the repeater nodes do not require an automatic gain control circuit. 
A further object of the present invention is to provide a repeater in which 
a signal applied to the repeater's input is retransmitted at an improved 
level. 
According to the present invention, there is provided a data transmission 
system comprising a plurality of repeater nodes coupled to each other via 
signal transmission cables. Each node includes a signal splitter at its 
input, an active branch including a receiver and a transmitter, and a 
passive branch including a signal transmission cable coupled to the signal 
splitter. The passive branch is terminated at a signal combiner located at 
the output of the repeater node. The transmitter of the active branch is 
also coupled to the signal combiner. The splitters are chosen so that a 
signal across the passive branch is equal to 1/12 of the signal on the 
main or active line. By so choosing the splitting ratios, a 10.sup.-9 bit 
error rate is readily achievable. An automatic gain control circuit is not 
needed and at least two successive nodes can fail without system 
degradation. 
These together with other objects and advantages which will be subsequently 
apparent, reside in the details of construction and operation as more 
fully hereinafter described and claimed, reference being had to the 
accompanying drawings forming a part hereof, wherein like numerals refer 
to like parts throughout.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 2 is a graph conceptually showing the effect of a repeater according 
to the present invention on an input signal. According to the present 
invention, the repeater receives a signal (line D) at its input. Rather 
than adding to the input signal to increase its amplitude, the present 
invention "retransmits" a new signal (dotted line E) which is the input 
signal delayed in time and at the transmit level for the system. 
FIG. 3 shows a network for transmission of digital information which can 
bypass field repeater nodes without the use of optical or mechanical 
switching, according to a preferred embodiment of the present invention. 
The system includes a fiber optic data transmission line 6. Along the 
optical fiber transmission line 1, several repeater nodes are dispersed. 
FIG. 3 shows four such repeater nodes, node 1, node 2, node 3 and node 4. 
Each node includes a directional fiber optic splitter 8 at its input, an 
optical receiver 10 which receives approximately 71% of the input signal, 
an optical transmitter 12 which retransmits the input signal, and a 
directional fiber optic combiner 14 on the output side that combines the 
bypass path signal and the retransmitted signal. The optical splitter 8 
and conbiner 14 are preferably identical. 
FIG. 4 is a graph of the Probability of Error vs Signal to Bypass ratio for 
the optical receiver of each of the nodes in the system. As indicated by 
the curve, to achieve a preferred 10.sup.-9 bit error rate, which is 
normally considered an error free transmission, a signal to bypass ratio 
of 10.7 dB is required. When a coupling ratio is chosen so that the bypass 
signal is 1/12 of the main signal, the above 10.sup.-9 bit error rate is 
readily achievable. As can be seen from the graph bypass ratios, less than 
the preferred ratio will produce an acceptable signal if a higher error 
rate is permitted. The preferred range of bypass to signal ratios is from 
3 dB to 13 dB. 
FIG. 5 is a detailed drawing of one of the nodes. An optical fiber 16 
couples the receiver 10 to the fixed split ratio directional fiber optic 
splitter 8, available from Gould, Inc. as part number 1300-SM-71/29. The 
signal processing electronics 18 are electrically coupled to the receiver 
10 and the transmitter 12 via wires 20 and 22. The signal processing 
electronics are used to add local data input to the transmission line, if 
desired. If the node is only to be used as a repeater, the signal 
processing electronics can be omitted, and receiver 10 can be directly 
coupled to transmitter 12 via wire 20 or wire 22. The transmitter 12 is 
coupled via optical fiber 24 to the directional fiber optic combiner 14 
also, available from Gould, Inc. as part number 1300-SM-71/29. This path, 
which flows from the directional fiber optic splitter 8 through the 
receiver 10, is the active branch. The active branch carries the main 
signal when the fault tolerant fiber optic network is functioning 
normally. An optical fiber 26 coupled between the directional fiber optic 
splitter 8 and the directional fiber optic combiner 14 provides the 
passive branch which carries the bypass signal which is preferably at 1/12 
the amplitude of the signal received by the receiver 10. 
FIG. 6 is a block diagram of receiver 10 for the repeaters of the present 
invention. In FIG. 6, a photodetector 28 is coupled to a transimpedance 
amplifier 30. The photodetector 28 and the transimpedance amplifier 30 are 
matched. For example, if a PIN detector is used for the photodetector, a 
very high gain transimpedance amplifier should be used to compensate for 
the low gain of the PIN detector. The type of photodetector used does not 
matter as long as the photodetector and the transimpedance amplifier are 
matched. It is very important that the dynamic range of the detector 
28/amplifier 30 be at least 30 dB, so that at least two nodes can be 
bypassed before the signal degrades past the point of detection. A dynamic 
range of 40 dB is preferred. An RCA CA30902E avalanche photodetector used 
with a Signetics NE 5212 transimpedance amplifier is an appropriate 
matched pair. 
Following the transimpedance amplifier 30, a coupling capacitance 32 is 
inserted, providing AC coupling, and is followed by a high gain post 
amplifier 34 such as a Signetics SE592. The coupling capacitance 32 is 
used so that the post amplifier 34 does not amplify any DC drift 
associated with any of the previous stages, for example, from the 
transimpedance amplifier. By using this coupling capacitance, along with 
the signal to bypass ratio discussed herein, is not necessary to use an 
automatic gain control. 
After passing through the coupling capacitance 32, and the post amplifier 
34, the signal is then sent into a comparator 36 that has a hysteresis 
characteristic, such as a National LMb 360. The hysteresis feature is 
important in optical receiver design because the resistors associated with 
a transimpedance amplifier produce inherent noise which can be amplified 
hundreds of times. Accordingly, the comparator 36 must receive a signal 
with positive and negative threshold values. The comparator threshold has 
to be above the intrinsic noise of the system created by the 
transimpedance amplifier and any noise created by the photodetector. FIG. 
7 is a more detailed schematic diagram of the above-described receiver 10. 
FIG. 8 is a block diagram of a transmitter 12 for the repeaters of the 
present invention. The transmitter is a high output type transmitter. 
Preferably, the transmitter 12 will have an optical power level of 
approximately -8 dBm. In order to operate with two contiguous node 
failures, the transmitter's output requirement must be at least 30 dB 
higher than the dynamic range of the receiver associated with it. A 
transmitter having the above specifications is available from Advance 
Fiber Optics. 
Referring to FIG. 8, a digital input, such as a TTL level signal, is input, 
and the transmitter 12 creates an optical replica of the digital input 
signal. If it receives a digital 1, it optically creates, or retransmits, 
a digital 1. An optically created digital 1 is transmitted by lighting an 
LED 42, while an optically created digital 0 is transmitted when the LED 
42 is not lit. In the transmitter 12, a level detector 40 gets rid of any 
input noise on the digital input signal. The output of the level detector 
40 is applied to a driving network 41 which scales the electrical digital 
1's and 0's applied to a controlled current source 43. The driving network 
41 and controlled current source 43 form an analog stage, in which a 
variable output current is produced. A current above the threshold level 
of the LED 42 indicates a digital 1, or ON state, and a current level 
below the threshold level of the LED 42 indicates a digital 0, or OFF 
state. The output of the LED 42 is applied to the optical fiber 24. 
The directional fiber optic splitters 8 allow a portion of the received 
signal to bypass the node via the passive branch 26. By building the 
splitter 8 with a split ratio such that 71% of the energy goes to the 
active branch while 29% of the energy goes to the passive branch and the 
combiner 14 with a corresponding combining ratio, a signal to bypass ratio 
of 12 to 1 can be achieved. When operating normally, any bypass signal 
going across the passive branch of node 1 is treated by the next receiver 
in the string (the receiver of node 2 in this example) as noise. This is 
because the data stream from the optical transmitter of node 1 overwhelms 
the bypass signal. 
FIG. 9 is a signal diagram of signals passing through a node of the present 
invention. FIG. 9a shows a data stream as input to the splitter and 
receiver of a properly functioning node. Line 51 represents a reference 
level for the data. For a fiber optic system, the references level 51 
represents the level of light intensity at which a transition between a 
digital 1 and digital 0 occurs. For an electrical data transmission 
system, line 51 represents the level of the electric signal at which a 
transition between a digital 1 and digital 0 occurs. Line 50 is the signal 
applied from a previous repeater or from a data input terminal. FIG. 9B 
shows thc signal carried by the passive or bypass path, identified by line 
52. The signal across the bypass path is, in this example, approximately 
1/12 the value of the signal input to the repeater. FIG. 9C shows the 
outputs of the combiner 14, which represents the two signals of 9A and 9B 
superimposed upon each other (line 53) with a retransmit delay F caused by 
the retransmission electronics. The delay has been exaggerated for ease of 
explanation. The signal across the bypass path is ahead of the signal 
across the active branch due to the delay caused by the components of the 
active branch. Thus, the portion labeled x in FIG. 9C consists of the 
portion x' of FIG. 9a combined with the portion x" of FIG. 9b. The slight 
value added to or subtracted from the retransmitted signal due to the 
bypass signal is so small that it is perceived as noise by subsequent 
repeaters. 
When the active path of a repeater node of the present invention is 
experiencing a fault, for example, a failure of the receiver and/or 
transmitter, the 1/12 signal shown in FIG. 9b is output at the combiner 14 
of the failed node and continues on to the next repeater in line. When the 
dynamic range of the receivers 10 of the repeaters is at least 30 dB, the 
receivers 10 are able to receive a signal across at least two failed 
nodes, and the properly functioning repeater retransmits the signal at the 
correct level. 
As noted above, the dynamic range of the receiver 10 is chosen so that each 
receiver 10 can receive the bypass signal bypassed across several nodes. 
In the failure mode, that is, in the event of receiver or transmitter 
failure in Node 1 of FIG. 3, the bypass signal is still within the dynamic 
range of the receiver of the second node 2, and thus the receiver of node 
2 accepts the bypass signal. In this manner, the continuity of the network 
is retained. 
During normal operation, receiver 10 of node 2 receives an optical signal 
from both the unamplified or bypass path of node 1 and the active 
retransmitted signal; however, because node 1 is functioning normally, the 
retransmitted signal from node 1 is twelve times as large as the bypass 
signal of node 1. Thus, the receiver of node 2 will recognize the bypass 
signal as noise and node 2 reproduces the signal from node 1 with a 
10.sup.-9 error rate. A bit error rate at this level is considered 
virtually error free transmission. 
In a conventional optical receiver, an optical signal representing a 
digital 1 must be of sufficient intensity to produce a photo-current level 
from the detector and the preceding amplifiers above a predetermined 
reference level. Light pulses with insufficient intensity to trip the 
comparator are assumed to be zeroes. To accomplish this task over a wide 
range of optical input intensities, an automatic gain control circuit 
raises the signal level, whether it is noise or the true signal, up to an 
absolute D.C. reference level proportional to the average value of the 
input optical intensity. The data signal then transitions around this D.C. 
level. Because of the inter-stage coupling capacitance 32 in the present 
invention the reference level 51 (see FIGS. 9a-9b) at the input to 
amplifier 34 is zero volts. 
According to the present invention, signals above the reference or zero 
level represent a high state or digital 1. Signals below the reference 
level indicate a low state or digital 0. If the main signal is removed, as 
in the event of a repeater failure, the bypass signal, shown in FIG. 9b, 
is applied to the input of the next repeater in line. The bypass signal 
still transitions above and below the reference zero volt level. As shown 
in FIG. 9b, the automatic gain control is not needed because the bypass 
signal is transitioning across the zero level. Because the bypass signal 
is transitioning about the zero level, the bypass signal can be received 
by the receiver of a repeater several nodes down the line, as long as the 
signal is within the dynamic range of the receiver. In a four-node system, 
if the middle two nodes fail, the receiver in the last node in the line 
will receive the smaller bypass signal. The signal will be retransmitted 
by the repeater and continue down the line. 
The system of the present invention can function as long as the signal to 
bypass ratio is at least 2. The preferred range for the absolute signal to 
bypass ratio is from 2 to 12, depending on the maximum dynamic range of 
the receiver. The preferred signal to bypass ratio, based on relative 
signal level, is from 3 dB to 13 dB, as illustrated in FIG. 4, with 10.7 
dB being the ideal preferred ratio. 10.7 dB provides a signal to bypass 
ratio of 12 to 1. As the signal to bypass ratio gets larger, the system 
can bypass more and more repeaters. However, from a practical standpoint, 
the greater the signal to bypass ratio, the more optical energy is wasted 
and the amplifiers have to have a greater dynamic range. Large dynamic 
range amplifiers are very expensive. When a bypass ratio of 2 is used, the 
system neecs a separate parallel path for the clock signal or a pnase 
locked loop to remove the clock signal. Such a system could function with 
amplifiers of lower dynamic range and also bypass a greater number of 
failed nodes. Additionally, the distance between repeaters could be 
increased. 
As noted above, the present invention is not limited to a fiber optic 
network. The system of the present invention can be implemented equally 
well utilizing electrical means, such as standard ooaxial cable, 
directional electrical splitters, and electrical receivers and 
transmitters. 
The many features and advantages of the invention are apparent from the 
detailed specification and thus it is intended by the appended claims to 
cover all such features and advantages of the invention which fall within 
the true spirit and scope thereof. Further, since numerous modifications 
and changes will readily occur to those skilled in the art, it is not 
desired to limit the invention to the exact construction and operation 
illustrated and described, and accordingly all suitable modifications and 
equivalents may be resorted to, falling within the scope of the invention.