Rapid response backup system for telecommunications networks

An instant telecommunications network backup system includes several trunk diverter circuits and several responder circuits. Each trunk diverter circuit is installed proximate to an originating central office within the telecommunications network and monitors an outgoing trunk for faulty conditions. The trunk diverter circuit maintains a database that classifies each responder circuits as being in one of two classes: a maintain link class and a disabled class. One or more responder circuits are installed proximate to an emergency response facility. Whenever a faulty condition is detected, the trunk diverter circuit establishes an alternative telecommunications path through either a public switch telephone network or a mobile telephone switching network to one of the responder circuits in the maintain link class. The trunk diverter circuit and the responder circuit exchange communications with each other. When the trunk diverter circuit detects an incoming emergency telephone call, the trunk diverter circuit and responder circuit surrender the alternative path to the emergency telephone call to permit the telephone caller to access the emergency facility through the alternative path. If no alternative path has previously been established, then the trunk diverter circuit establishes an alternative communications path to any of the responder circuits, regardless of their class, and reroutes the emergency telephone call therethrough. Alternatively, the trunk diverter circuit is installed approximate to a remote switching office and monitors for an alarm signal from the remote switching office. The trunk diverter circuit establishes an alternative communications path to one of the responder circuits after detecting the alarm signal. Data identifying the location of the central office is supplied to the emergency response facility if specific data regarding the location of the emergency telephone call is unavailable.

TECHNICAL FIELD 
This invention relates to backup systems for communications networks, 
particularly backup systems for emergency telecommunications networks. 
BACKGROUND OF THE INVENTION 
Emergency telecommunications networks have been established to respond to 
emergency telephone calls from the public. These "911 networks" were 
established and added to the existing public switched telephone networks 
("PTN"). In such a 911 network, a person places an emergency telephone 
call (a "911 call") that is routed in through the PTN to an appropriate 
public service answering point ("PSAP"). The PSAP receives the 911 call 
and contacts the appropriate emergency agency, such as the fire department 
or police department. 
As shown in FIG. 1A, a 911 call is generally routed through the PTN as 
follows. First, the 911 call 114 is sent through the line connected to the 
calling phone to an originating central office ("C.O.") 112 that is 
connected thereto. Second, the call is routed on a trunk 107 from the 
originating C.O. to a tandem switch 110. Finally, the tandem switch routes 
the 911 call over a trunk 106 directly to the PSAP 102. In some 
applications, the 911 call is sent directly to the PSAP by the C.O. 
The tandem switch 110, or tandem office, is a complex software-controlled 
trunk-to-trunk switching system that routes the 911 call to the 
appropriate PSAP facility 102. The tandem switch 110 compares the number 
of the originating phone call to a database to determine which PSAP has 
jurisdiction over the 911 caller's location. The caller's phone number is 
transferred from the tandem switch to the PSAP facility 102 to be 
displayed on a terminal at the PSAP facility. 
Currently, most PSAPs have a dedicated trunk line from the originating 
central office to the PSAP, often via the tandem switch. At any point in 
this dedicated path, a failure could happen. A failure could be caused by 
a cable break or dig-up, equipment failure, tandem hardware or software 
failure, or catastrophic damage from storm activity, flood, earthquake, 
and the like. When the dedicated 911 network fails, the 911 caller 114 
often receives a false busy signal or ring and never contacts the PSAP 
facility 102 in an emergency. A malfunction in the 911 network during an 
emergency can be devastating. 
To overcome this problem, 911 network backup systems have been developed 
that detect when a 911 caller attempts to contact a PSAP but fails to do 
so because of a malfunction in the 911 network. In response to the 911 
call, these network backup systems reroute the call through other trunks 
or lines in the PTN, avoiding the severed cables, faulty tandem switches 
or malfunctioning C.O. 
One known network backup system uses a pair of transceivers, a trunk 
diverter circuit 116 positioned proximate to a central office and a 
responder circuit 122, having its own PTN number, positioned proximate to 
the PSAP facility 102 (and/or proximate to the tandem switch 110). When 
the trunk diverter circuit 116 detects that a 911 call cannot reach its 
appropriate PSAP facility, this transceiver reroutes the 911 call by 
dialing the number of the responder circuit 122 through a PTN or cellular 
network route, and reroutes the 911 call through the newly established 
route. 
After dialing a 911 call, the 911 caller 114 normally waits six to ten 
seconds from the last digit depressed on the telephone before the PSAP 
receives the call and the 911 caller hears a ringing signal. When the 
dedicated 911 network is malfunctioning and the network backup system is 
initiated, the 911 caller experiences this normal ten-second delay, plus 
additional setup time as the trunk diverter circuit 116 attempts to dial 
the responder circuit 122. This setup time can add up to nine seconds for 
a PTN route and up to forty seconds for a cellular route, even when the 
PTN/cellular trunks are not busy. If the trunks are busy, the network 
backup system would attempt to reroute the call through other 
preprogrammed routes, adding additional time to the 911 call. 
In an attempt to compensate for these delays, the known network backup 
system provides several voice messages to the 911 caller informing them of 
the progress of their call, and hopefully keeping them on the line. During 
an emergency, the 911 caller may not stay on the line, or, if they do not 
understand the voice messages (e.g., the 911 caller does not understand 
English), the caller may hang up and attempt to redial. Again, the 911 
call will be subject to the above delays as the network backup system 
attempts to reroute the new 911 call. 
While the network backup systems reroute 911 calls when the 911 network 
malfunctions, these backup systems are subject to delays. These delays are 
significant for panicked emergency callers. Even if these emergency backup 
systems improve the speed of their performance, they are nevertheless 
subject to the delays inherent in the PTN and cellular networks. 
Overall, the inventors are unaware of a 911 or other telecommunications 
network backup system that provides rapid response despite the delays 
inherent in PTN and cellular networks. 
SUMMARY OF THE INVENTION 
According to principles of the present invention, an instant network backup 
system reduces rerouting and call processing time after the dedicated 
communications network malfunctions. The present invention monitors and 
reroutes trunks to alternative paths after a network malfunctions, even 
before a caller places a call. The backup system of the present invention 
is particularly suited for dedicated 911 networks. 
The backup system of the present invention determines before a 911 call 
occurs that the dedicated route to the PSAP is unavailable. The backup 
system begins to search for an available backup route to the PSAP 
immediately after determining that the dedicated route to the PSAP is 
unavailable. If the tandem switch malfunction such that a route cannot be 
established to the appropriate PSAP, or if the PSAP is unavailable, a 
route to an alternative PSAP is established. The backup system uses two 
transceivers, a trunk diverter circuit and a responder circuit. The trunk 
diverter circuit is installed in series between an outgoing 911 trunk from 
each possible originating central office. Each responder unit is 
preferably installed as close to the PSAP as possible, and connected in 
series with each backup line to the PSAP (e.g., each ground start line). 
The backup system of the present invention locates and maintains 
communication with a responder circuit within the PTN or cellular networks 
whenever it determines that the dedicated 911 network is malfunctioning. 
In essence, the instant network backup system of the present invention 
locates and holds a PTN line between the trunk diverter circuit and the 
responder circuit; however, the responder circuit does not contact or 
"ring" the PSAP until the trunk diverter circuit receives a 911 call. 
The trunk diverter circuit, after receiving a 911 call, contacts the 
responder circuit by instructing the responder circuit to ring the PSAP to 
which it is connected. When the PSAP answers, the responder circuit 
signals back with an "off hook" signal indicating that the 911 call can be 
connected through the responder circuit to the PSAP. Communications 
between the trunk diverter circuit and the responder circuit are 
discontinued and the route established therebetween is used by the 911 
caller to access the PSAP. 
The backup system establishes and maintains a signal path between the trunk 
diverter circuit and the responder circuit so that no additional setup 
time or rerouting time is necessary when a 911 call is received after the 
911 network malfunctions. The backup system of the present invention 
passes 911 calls through the route established between the trunk diverter 
circuit and the responder circuit such that the 911 call appears to the 
911 caller as a regular call with ringing, connect and disconnect common 
with most PTN telephone calls. The backup system essentially converts PTN 
and/or cellular networks to dedicated 911 trunks in the event of a 911 
dedicated trunk path outage. 
In a broad sense, the present invention embodies a system for maintaining a 
communications signal exchange between a first location and a 
telecommunications switching location over first or second 
telecommunication paths. The telecommunications switching location is 
coupled to a plurality of telephone transceivers and is capable of 
coupling one of the telephone transceivers to the first location upon 
receiving a predetermined request signal from the one telephone receiver. 
The system includes a first transceiver coupled to the second 
telecommunications path approximately at the first location and a second 
transceiver coupled to the first and second telecommunication paths at the 
telecommunication switching location. The second transceiver has a memory, 
a microprocessor or central processing unit ("CPU") coupled to the memory, 
and at least one coupling circuit intercoupling the CPU to the first and 
second telecommunication paths. The CPU monitors the first 
telecommunications path and exchanges signals with the first transceiver 
over the second telecommunications path when the CPU determines that the 
first telecommunications path is faulty. The CPU provides a second 
telecommunications path to the one telephone transceiver when the CPU 
receives the predetermined request signal. 
The present invention also embodies a method of maintaining a signal 
exchange between first and second locations having first and second 
transceivers associated therewith. The first and second transceivers are 
selectively coupled for signal exchange about a signal path upon an 
initiation by the first transceiver. The method includes the steps of: (1) 
monitoring a first signal path between the first and second locations, (2) 
determining if the first signal path is disrupted, (3) establishing a 
second signal path between the first and second locations before the 
initiation by the first transceiver, (4) maintaining the second signal 
path, and (5) providing a second signal path to the first and second 
transceivers upon the initiation by the first transceiver.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENT 
Referring to FIGS. 1A through 1C, a typical emergency telecommunications 
service network 101, or "911 network" for handling emergency telephone 
calls includes a PSAP 102. An automatic number identification ("ANI") 
controller 104 in the PSAP facility 102 is coupled between an incoming 911 
trunk 106 and an answering position 108. The ANI controller 104 receives 
and converts ANI signals to signals displayed on a visual display at the 
answering position 108 in order to identify the phone number of the party 
placing the 911 call. The answering position 108 includes a telephone 
transceiver and other facilities standard in PSAP facilities. 
An emergency 911 caller, represented in FIGS. 1A-1C as a telephone 114, 
places a 911 call to the PSAP facility 102. A central office switch 115 in 
an originating C.O. 112 receives the 911 call and routes it to a tandem 
office or switch 110 by means of a tandem trunk 107. The tandem switch 110 
in turn routes the 911 call to the PSAP facility 102 along the 911 trunk 
106. 
A trunk diverter circuit 116 is preferably coupled proximate to the 
originating C.O. 112 and in series with the tip and ring leads of the 
outgoing tandem trunk 107. This coupling configuration is used for loop 
reverse battery signaling trunks; however, the present invention is 
compatible with other configurations such as E&M signaling and four-wire 
trunks. The operation of these trunk types is well established in the 
prior art, and an explanation of their operation would not add to the 
understanding of the operation of the present invention. The present 
invention is described herein with respect to loop reverse battery 
signaling and two-wire trunks; those skilled in the relevant art will 
understand that the present invention may be readily adapted for other 
trunk signaling types based on the detailed description provided herein to 
produce the same results. 
As described more fully below with respect to FIGS. 2A-2C, the trunk 
diverter circuit 116 monitors the tandem trunk 107 and the tandem switch 
110 for malfunctions and reroutes 911 calls to the PSAP facility 102 
through alternate paths before a 911 call is received. Thus, in the event 
of a malfunction, an alternative route for the 911 call is ready and 
immediately available when the 911 call is placed. A responder circuit 122 
is coupled to the ANI controller 104 in the PSAP facility 102. When the 
trunk diverter circuit 116 detects a trunk 107 or tandem switch 110 
failure, the trunk diverter circuit 116 establishes an alternate path or 
route 118 through the PTN 119 to reroute 911 calls to the PSAP facility 
102. While the alternative path 118 is generally shown and described (and 
used interchangeably) herein as a single ground start line circuit 118 
established through the PTN 119, the path includes numerous connections 
along trunks and lines and may be established along other networks such as 
cellular networks as will be described more fully below. 
The trunk diverter circuit 116 contacts and then exchanges communications 
with the responder circuit 122 in the PSAP facility 102 through the 
alternative path 118 in the PTN 119, from the originating C.O. 112 to a 
terminating C.O. 124. Preferably, responder circuits 122 are installed in 
each PSAP facility 102 and at each tandem switch 110. If the preferred 
PSAP facility 102 is unavailable, the trunk diverter circuit 116 
establishes a second alternate path 118' from the terminating C.O. 124 to 
another responder circuit 122' installed at an alternate PSAP facility 
102'. Alternatively, the trunk diverter circuit 116 can establish a third 
alternate path 118" to another responder circuit 122" installed proximate 
to the tandem switch 110. Each additional responder circuit 122 installed 
within the telecommunications networks (including the 911 network 101 and 
the PTN 119) further ensures that an alternative path to an available PSAP 
facility can be established under the present invention. 
Referring to FIG. 2A, the trunk diverter circuit 116 includes a 
microprocessor or central processing unit ("CPU") 130 for performing most 
of the tasks of the trunk diverter circuit 116. A random access memory 
("RAM") 131 is coupled to and provides temporary storage for the CPU 130. 
An electrically programmable read-only memory ("EPROM") 132, coupled to 
the CPU 130, stores the instant backup routine performed by the CPU that 
is described more thoroughly below. A voice EPROM 134 and a voice 
amplifier 136, both coupled to the CPU 130, respectively store voice 
messages, and interpret, amplify, and provide these messages to the 
originating 911 caller 114 though the originating C.O. 112. A 
multi-frequency ("MF") generator 138 and an MF receiver 140 coupled 
between the CPU 130 and the tandem trunk 107 preferably receive MF signals 
from the trunk and provide MF signals to the ANI controller 104 via the 
responder circuit 122. Similarly, a dual-tone multi-frequency ("DTMF") 
generator 142 and a DTMF receiver 144 are coupled between the CPU 130 and 
the alternative path 118 (i.e., the ground start line). As is known by 
those skilled in the art, MF signaling is used for trunk-to-trunk 
communications (e.g., between the originating C.O. 112 and the terminating 
C.O. 124) while DTMF signaling is used for communications over lines in 
most PTNs (e.g., between the originating C.O. 112 and the 911 caller 114). 
A series of light emitting diodes LED 1-LED 5, coupled to the watchdog 
power monitor 148, provide an indication of the status of the trunk 
diverter circuit 116. The table below summarizes what each LED indicates 
when it is illuminated. 
______________________________________ 
LED Number Indication 
______________________________________ 
LED 1 Trunk diverter 116 is powered up 
LED 2 Trunk diverter 116 is disabled from 
rerouting calls 
LED 3 Data being transmitted from the 
CPU 130 to the alarm and data port 
card 
LED 4 Call diverted from trunk 
LED 5 911 call in progress 
______________________________________ 
A relay driver 146 is coupled to the CPU 130 drives relays K1-K7 in the 
trunk diverter circuit 116, as will be described more fully below. A 
watchdog power monitor 148, also coupled to the CPU 130, monitors and 
adjusts the power input to the CPU. The CPU 130 is coupled to an alarm and 
data port card (not shown) that provides alarm signals to the originating 
C.O. 112 and records various conditions within the 911 network 101 and the 
PTN 119. The alarm and data port card also provides inputs for an operator 
to adjust or modify the trunk diverter circuit 116, and is thus not 
necessary to the routine operation of the trunk diverter circuit 116. The 
trunk diverter circuit 116 also preferably includes a diverter switch (not 
shown) that allows the operator to disable the instant network backup 
system of the present invention if so desired by the operator. 
With reference to FIGS. 2B and 2C, coupling and switching circuitry 147, 
intercoupling the CPU 130 with the tandem trunk 107 includes optical 
couplers OC1 and OC2, and OC3 and OC4 that passively monitor the tip and 
ring leads of the incoming and outgoing tandem trunk, respectively, and 
provide signals to the CPU 130. Similarly, optical couplers OC5 and OC6 
are coupled to the outgoing ground start line 118 for the same purpose. 
The relays K1 through K7 (FIG. 2B) provide various switching features, 
including splitting the tandem trunk 107 and diverting it to the outgoing 
ground start line 118, as will be discussed more fully below. As shown in 
FIGS. 2B and 2C, each optical coupler preferably includes a light-emitting 
diode (e.g., OC1A), and a phototransistor (e.g., OC1B). Similarly, each 
relay generally provides two or more switching functions, for example, 
relay K1 provides switching functions to the tip end ring leads of the 
tandem trunk 107 at switch points K1A and K1B, respectively. A pair of 
transformers T1 and T2 inductively couple the outgoing 911 trunk 107 to 
the outgoing ground start line 118 and allow the CPU 130 to inductively 
transmit and receive voice and tone signals therefrom. A pair of 
transformers are used, rather than a single transformer, to allow the CPU 
130 to transmit voice messages to the originating C.O. 112 and thus to the 
911 caller 114 as the CPU simultaneously transmits tones to the C.O. or 
ANI controller 104 (via the responder circuit 122). 
Referring to FIG. 3A, the responder circuit 122 includes a CPU 150 for 
performing most of the tasks of the responder circuit 122. A memory 151 
provides storage for the instant network backup routine and other data for 
the CPU 150. A DTMF generator 152 and a DTMF receiver 154, coupled between 
the CPU 150 and the incoming ground start line 118 from the terminating 
C.O. 124 (FIGS. 1B and 1C), provide and receive DTMF signals from the 
ground start line 118. A relay driver 156 coupled to the CPU 150 drives 
relays K1'-K5' in the responder circuit 122. 
Referring to FIGS. 3B and 3C, coupling and switching circuitry 157, 
intercoupling the CPU 150 with the PSAP's 911 trunk 106 and the ground 
start line 118, includes optical couplers OC1 and OC2, and OC3 and OC4, 
that passively monitor the ground start line 118 and the PSAP 911 trunk 
106, respectively, and provide signals to the CPU 150. A transformer T1 
inductively couples the ground start line 118 to the PSAP's 911 trunk 106 
and allows the CPU 150 to inductively transmit and receive DTMF tones 
thereto and therefrom. 
Referring back to FIG. 1B, before a malfunction is detected in the 911 
network 101, the trunk diverter circuit 116 and the responder circuit 122 
are not intercoupled. However, as shown in FIG. 1C, after the trunk 
diverter circuit 116 detects a malfunction in the 911 network 101 (shown 
as a cable break 107'), the trunk diverter circuit and one of the 
responder circuits 122, 122' or 122" exchange communications therebetween 
and establish an alternate path 125 for a 911 caller 114, as described 
below. 
Referring now to FIGS. 4-10, an instant network backup routine 200 is shown 
that establishes and maintains a communication link between the trunk 
diverter circuit 116 and the responder circuit 122 when the 911 network 
101 malfunctions. As noted above, the appropriate portions of the routine 
200 are stored as software in non-volatile memory (EPROM 132 and memory 
151) of the trunk diverter circuit 116 and the responder circuit 122, to 
be executed by the CPUs 130 and 150, all respectively. After the trunk 
diverter circuit 116 initializes itself, the instant network backup 
routine 200 begins in FIG. 4, the "top chart", with a start step 202. 
Following the start step 202, the routine 200 proceeds to step 203 where a 
trunk monitor routine (shown in FIG. 5) is called. 
Referring to FIG. 5, the CPU 130 enters the trunk monitor routine at step 
204, and in step 205, monitors the tandem trunk 107 to determine if the 
trunk is acceptable for exchanging communication thereon. The CPU 130 
monitors the tip and ring leads of the tandem trunk 107 through optical 
couplers OC1, OC2, OC3 and OC4, and detects for normal polarity on the 
exemplary loop reverse battery signaling trunk. If the CPU 130 determines 
that the tandem trunk 107 is unacceptable for transmission of signals 
thereon, i.e., has improper or incorrect polarity, the CPU in step 206 
splits the tandem trunk from the tandem switch 110 by providing an 
appropriate signal to the relay driver 146 which in turn actuates relay 
K1. Concurrently, the CPU 130 provides an appropriate signal to the relay 
driver 146 which in turn operates the relay K7 to provide an alarm signal 
to the originating C.O. 112, notifying personnel at the C.O. that a 
malfunction is occurring. The CPU 130 in step 206 also provides 
appropriate data signals to the alarm and data port card. Following step 
206, the routine 200 returns to exit step 203 in FIG. 4. 
If, in step 204, the tandem trunk 107 is acceptable for data 
communications, the CPU 130 determines in step 208 whether a 911 call is 
in progress by detecting a normal current flow through the optical coupler 
OC3. If a 911 call is not in progress in step 208, the routine 200 returns 
back to the steps in FIG. 4. 
If the CPU 130 determines in step 208 that a 911 call is in progress, then 
in step 210, the CPU 130 begins a protocol timer and waits for a response 
from the tandem switch 110. In step 212, the CPU 130 determines if the 
tandem switch 110 responds with an appropriate switching protocol, and if 
so, then the routine 200 returns to the steps in FIG. 4, because the 911 
call is being processed normally. The CPU 130 determines in step 212 that 
the protocol is satisfied and that the tandem switch 110 responded 
properly by detecting, through optical coupler OC4, that a reversed 
current is flowing through the tandem trunk 118. 
If the CPU 130 determines at step 212 that the appropriate switching 
protocol has not yet been satisfied, it checks at step 214 to determine if 
the protocol timer started at step 210 has expired. If so, the CPU 130 
splits the tandem trunk 107 from the tandem switch 110 and provides an 
alarm by activating relays K1 and K7, respectively, in step 206, as 
described above. Concurrently, the CPU 130 sets an 911 alarm timer in step 
216. 
The present invention is able to detect both malfunctions in the 911 
network 101 caused by a severing of a trunk line or a malfunction in 
various communication equipment. For example, if the tandem trunk 107 is 
severed, the CPU 130 detects abnormal polarity on the tandem trunk 107 in 
step 204. The trunk diverter circuit 116 establishes the alternative path 
118 until the tandem trunk 107 is repaired and the CPU 130 detects normal 
polarity on the trunk. 
If, however, the tandem trunk 107 appears to be operating normally (i.e., 
normal polarity is detected), but other telecommunications equipment is 
malfunctioning, then the CPU 130 fails to receive the proper protocol 
signals within the specified protocol time period in step 214. The trunk 
diverter circuit 116 cannot determine when the malfunctioning 
telecommunications equipment is repaired, and therefore, the trunk 
diverter circuit establishes the alternative path 118 for a predetermined 
time period established by the 911 alarm timer in step 216 (e.g., 20 to 60 
minutes). 
If the CPU 130 receives the protocol within the specified time period, the 
911 call is coupled to the PSAP facility 102, and the call is processed in 
its usual fashion in step 218. Following steps 206 or 218, the routine 200 
returns, via step 219, to exit step 203 in FIG. 4, and then determines if 
the 911 alarm is active in step 220. If the 911 call is being processed 
normally and thus, the 911 alarm is not active, the routine 200 calls a 
retest responder routine, shown in FIG. 10, at step 221, as will be 
described below. 
If, however, the 911 alarm was found to be active in step 206, then the CPU 
130 determines in step 222 whether a 911 call is in progress. If a 911 
call is in progress, then the routine 200 calls an ANI routine, shown in 
FIG. 6, at step 223. Concurrently, since the 911 alarm was found to be 
active in step 220, the routine 200 also calls a connect responder 
routine, shown in FIG. 7, at step 225. While the two processes shown in 
FIGS. 6 and 7 are discussed herein separately, they are instead performed 
concurrently by the CPUs 130 and 150 of the trunk diverter circuit 116 and 
responder circuit 122. 
Referring now to FIG. 6, the CPU 130 enters the ANI routine at step 224 
and, in step 225, provides an appropriate signal to the relay driver 146 
which in turn momentarily operates the relay K2. This momentary operation 
of the relay K2 provides a request signal to the originating C.O. 112 over 
the outgoing 911 trunk 107 instructing the originating C.O. to transmit 
the 911 caller's ANI data. Concurrently, at step 225, the CPU 130 starts 
an ANI timer. The CPU 130 receives the ANI data through the transformer T1 
and the MF receiver 140 and stores it in the RAM 131 for subsequent 
processing. If the instant network backup system of the present invention 
is installed in a multiple numbering plan area and the backup route 118 is 
to a responder circuit 122 located in a PSAP facility 102, then the CPU 
130 replaces the "info digit" received with the ANI data with a "numbering 
plan digit." The ANI data with either the info digit or the numbering plan 
digit are supplied to the PSAP facility 102 as described below. 
In step 226, the CPU 130 determines if the ANI timer has expired, and if 
not, in step 228 determines whether the complete ANI signal has been 
received. If the timer has not expired and the ANI signal has not been 
completely received, the routine 200 loops back; however, if the ANI timer 
is found to have expired in 226, the CPU 130 in step 230 replaces the ANI 
data with appropriate data indicating that a request for ANI information 
had failed. Following the ANI request steps in FIG. 6, the routine 200 
returns via step 229 to exit step 223 in FIG. 4. 
Referring now to FIG. 7, the CPU 130 enters the Connect Responder routine 
at step 231 and determines in step 232 whether one of several responder 
circuits 122 installed in the 911 network 101 and PTN 119 are available. 
The CPU 130 preferably maintains a "TDC's responder database" in the RAM 
131. The TDC's responder database includes a list of those responder 
circuits 122 to which the trunk diverter circuit 116 presently has access 
and can communicate. If the CPU determines in step 232 that all the 
responder circuits 122 in the TDC's responder database are inaccessible, 
the routine 200 sets a Responder Not Available flag at step 233 and then 
returns to the steps in FIG. 4 via step 235. 
If at least one responder circuit 122 listed in the TDC's responder 
database is found to be available at step 232, then in step 234, the CPU 
130 determines whether a responder circuit 122 is already connected to the 
outgoing ground start line 118. If a responder circuit 122 is already 
connected to the trunk diverter circuit 116, the routine 200 sets a 
Responder Connected flag at 256 before exiting through step 235 and 
returning to the steps in FIG. 4. 
If one of the responder circuits 122 is available but currently 
unconnected, the CPU 130 executes step 236 by causing the relay driver 146 
to actuate the relay K5 and seize the outgoing ground start line 118 in 
preparation of dialing the PTN number of an available responder circuit 
122 selected from the TDC's responder database. The PTN numbers of all of 
the responder circuits 122 in the TDC's responder database are stored in 
the RAM 131. 
The CPU 130 monitors the outgoing ground start line 118 through the optical 
couplers OC5 and OC6 to determine when the outgoing ground start line 
circuit has been seized. When the CPU 130 monitors the proper response 
from the outgoing ground start line 118, the CPU actuates the relay K6 and 
releases the relay K5 through the relay driver 146. The CPU 130 then 
transmits the PTN number of the selected responder circuit 122 to the 
ground start line 118 by providing an appropriate signal to the DTMF 
generator 142. The DTMF generator 142 also provides the appropriate tones 
of the PTN number to the outgoing ground start line 118 through the 
transformer T1 in step 236. 
Concurrently with the execution of step 236 as described above, the CPU 130 
starts a Network Timer in step 238. Thereafter, the CPU 130 checks at step 
240 to determine if the responder circuit 122 is connected. If the 
responder circuit 122 is found at step 240 to be not connected, the CPU 
130 checks at step 242 to determine if the network timer started at step 
238 has expired. If not, the CPU 130 checks at step 244 to determine if 
the called responder circuit 122 is busy. If the called responder circuit 
122 is not busy, the CPU returns to step 240 to once again determine if 
the called responder circuit has connected. By looping through steps 240, 
242, and 244, the CPU 130 waits until the time out of the Network Timer 
started at 238 for the called responder circuit to answer. If the called 
responder circuit 122 is busy, then the CPU 130 branches from step 244 to 
step 246 where a Responder Not Connected flag is set before exiting 
through step 235. If the called responder circuit 122 does not answer 
before the Network Timer expires, the CPU 130 branches from Step 242 to 
execute step 248 by indicating in the TDC's responder database that the 
selected responder circuit 122 is unavailable and is to be retested later. 
Thereafter, the CPU 130 releases that particular PTN ground start line 
118, and sets the Responder Not Connected flat at step 246 before 
returning back to the steps in FIG. 4. 
If the selected responder circuit 122 is found to be connected in step 240, 
then in step 250, the CPU 130 determines if the responder circuit is "on 
hook" in step 250. As is currently known by those skilled in the relevant 
art, the term "off hook" refers to a PTN connection or path that is 
unavailable because it is in use, while the term "on hook" refers to an 
available connection or path within the PTN. Referring to FIGS. 3A through 
3C, the responder circuit's CPU 150 monitors the tip and ring leads of the 
PSAP's 911 trunk 106 through the optical couplers OC3' and OC4'. As is 
known by those skilled in the art, a loop reverse battery signaling trunk, 
such as the exemplary 911 trunk 107, is determined to be on hook or off 
hook from the side attempting to seize the trunk when either a high or low 
impedance is measured on the trunk, respectively. Similarly, as measured 
from the side providing power to the trunk, an off hook condition is 
determined by a reversal of the normal polarity. Therefore, if the CPU 150 
determines that the polarity of the tip and ring leads for the PSAP 911 
trunk 106 is reversed (i.e., off hook), the CPU actuates the relay K1' by 
means of the relay driver 156 to thereby provide the PSAP facility 102 
with an appropriate alarm signal. 
The CPU 150 detects the incoming telephone call from the trunk diverter 
circuit 116 by monitoring the ground start line 118 from the terminating 
C.O. 124 through the optical couplers OC1' and OC2'. Once the CPU 150 
detects ringing on the ground start line 118, the CPU seizes the ground 
start line by operating the relay K3' through the relay driver 156. 
Following actuation of the relay K3', the PTN 119 makes an audio 
connection between the responder circuit 122 and the trunk diverter 
circuit 116. The CPU 150 determines that the audio connection has been 
made by monitoring current flow through the ground start line 118, and 
operates the relay K5', through the relay driver 156. 
Referring back to FIG. 7, if the CPU 150 determines at step 250 that the 
PSAP is "off hook," the responder circuit 122 then releases the relay K3' 
to restore the C.O. ground start line 118 to an idle state in step 252. 
Additionally, in step 252, the CPU 150 of the responder circuit 122 
transmits a "PSAP Off Hook" message signal to the trunk diverter circuit 
116, by means of the DTMF generator 152 through the transformer T1', to 
the trunk diverter circuit 116 over the ground start line 118. In response 
thereto, the CPU 130 of the trunk diverter circuit 116 in step 248 
indicates in the TDC's responder database that the selected responder 
circuit 122 is unavailable and is to be retested, as explained above. In 
step 246 the CPU 130 then sets the Responder Not Connected flag at step 
246, and the routine 200 returns back to the steps in FIG. 4. 
Alternatively, if the PSAP is found at step 250 to be on hook, the CPU 150 
of the responder circuit 122 provides a "PSAP On Hook" message signal to 
the trunk diverter circuit 116. In response thereto, the CPU 130, in step 
256, sets the Responder Connected flag and exits through step 235 to 
return back to the steps in FIG. 4. 
Referring back to FIG. 4, the CPU 130 determines in step 258 whether a 911 
call is in progress. If so, the CPU 130 determines again if any responder 
circuits 122 are available in step 260. The responder circuit 122 called 
in the steps of FIG. 7 could have been the only available responder 
circuit in the TDC's responder database and if so, no other responder 
circuits are available. If no responder circuits 122 are found to be 
available in step 260, then the CPU 130 retrieves an appropriate voice 
message from the voice EPROM 134 in step 262. The voice message is 
transmitted through the voice amplifier 136, the transformer T2 and the 
outgoing 911 trunk 118 back to the 911 caller 114. An appropriate message 
to the 911 caller 114 would be a voice message informing the caller that 
his or her call cannot be connected to a PSAP facility followed by a busy 
signal. The CPU 130 in step 262 also waits for the 911 caller 114 to 
disconnect and makes the outgoing 911 trunk 107 idle (as discussed above). 
Thereafter, the routine 200 returns to the steps in FIG. 5. 
If the CPU 130 determines in step 260 that a responder circuit 122 is 
available, then the CPU retrieves another appropriate message from the 
voice EPROM 134 and provides this message to the 911 caller 114 indicating 
the progress of his or her call, in step 264 (e.g., a message that the 
call is being rerouted and that he or she is to remain on the line). In 
step 266, the CPU 130 determines whether a responder circuit 122 is 
connected to the trunk diverter circuit 116. If no responder circuit 122 
is currently connected, then the routine 200 loops back to monitoring the 
tandem trunk 107 under the steps shown in FIG. 5 and continues through the 
steps described above until either (1) a responder circuit 122 is 
connected in step 256 (FIG. 7), (2) no responder circuit 122 listed in the 
TDC's responder database can be connected (steps 232 and 260), or (3) the 
911 caller 114 disconnects and the 911 call is no longer in progress. 
If the CPU 130 determines in step 266 that the selected responder circuit 
122 is connected, then the routine 200 exits via step 267 to a process 
call routine shown in FIG. 8. Referring to FIG. 8, the CPU 130 enters the 
routine at 268 and, at step 269, sends the responder circuit 122 a seize 
PSAP message by providing an appropriate signal to the DTMF generator 142, 
which in turn provides the seize PSAP message to the outgoing ground start 
line 118 via the transformer T1. Concurrently, at step 269, the CPU 130 
starts an ANI timer. In step 270, the CPU 150 of the responder circuit 122 
receives the seize PSAP message via its transformer T1' and DTMF receiver 
154. In response thereto, the CPU 150 actuates the relay K2', through the 
relay driver 156 to thereby seize the 911 trunk 106. In step 272, the CPU 
150 monitors the 911 trunk 106 through the optical couplers OC3' and OC4' 
to determine if the PSAP facility 102 provides a request for an ANI 
signal. 
When the CPU 150 receives the request for an ANI signal from the PSAP 
facility 102, the CPU 150 sends the PSAP off hook message to the trunk 
diverter circuit 116 via its DTMF generator 152 and transformer T 1' at 
step 274. The CPU 150 also activates the relay K5' to connect its input to 
its output and allow the trunk diverter circuit 116 to communicate with 
the PSAP facility 102 in step 274. In step 276, the CPU 130 retrieves the 
ANI data stored in RAM 131 and transmits it to the PSAP facility 102 by 
means of the MF generator 138. Also in step 276, the trunk diverter 
circuit 116 restores the relay K3, thereby connecting the trunk diverter 
circuit's inputs to its outputs, connecting the 911 caller 114 to the PSAP 
facility 102 so that an operator at the PSAP facility may speak with the 
911 caller. 
The CPU 150 of the responder circuit 122 monitors the progress of the 911 
call in step 278 by determining whether it detects a network disconnect 
signal via the optical couplers OC1' and OC2', and OC3' and OC4', 
indicating that the 911 caller 114 or the PSAP facility 102 has 
disconnected, respectively. Similarly, during the 911 call, the CPU 130 of 
the trunk diverter circuit 116 monitors for the PSAP on hook message from 
the responder circuit 122 on the outgoing ground start line 118 or a 
disconnect signal from the outgoing 911 trunk 107 from the originating 
C.O. 112 (indicating that the 911 caller 114 has disconnected), through 
the optical couplers OC3 and OC4, and OC5 and OC6, respectively. If the 
responder circuit 122 detects a disconnect signal, then in step 280 it 
sends the trunk diverter circuit 116 a disconnect message over the ground 
start line 118. The responder circuit 122 also restores its inputs and 
outputs to idle at step 280 by releasing the appropriate relays (i.e., 
releases its PTN line). Alternatively, if the trunk diverter circuit 116 
detects a disconnect signal, then in step 282 it sends the responder 
circuit 122 a disconnect message. Thereafter, the trunk diverter circuit 
116 restores its inputs and outputs to idle. After the trunk diverter 
circuit 116 and the responder circuit 122 release their PTN lines 
(represented herein as the ground start line 118 shown in FIG. 1), the 
routine 200 exits through step 283 to the steps in FIGS. 4 and 5, and the 
circuits wait for the next 911 call. 
If the CPU 130 determines in step 272 that the PSAP facility 102 has not 
requested the ANI data, and the ANI timer is found to have expired in step 
284, the CPU 130 recognizes that communications with the PSAP facility are 
disrupted. Consequently, the CPU 130 sends the responder circuit 122 a 
disconnect message in step 286. The CPU 130 also indicates in the TDC's 
responder database that that particular responder circuit 122 is 
unavailable, in step 286, and the CPU releases its outgoing ground start 
line 118 by deactivating the relay K6. The responder circuit 122, in 
response to the disconnect message from the trunk diverter circuit 116, 
releases its ground start line 118 and the 911 trunk 106 by deactivating 
the relays K3' and K2' in step 288. The Responder Not Connected flag is 
set at 290 and the routine 200 returns via step 283 to the steps in FIGS. 
4 and 5. The routine 200 continues through the preceding steps until 
either the 911 call is processed through a responder circuit 122, the 911 
call terminates (e.g., the 911 caller 114 disconnects) or the trunk 
diverter circuit 116 cannot connect to any responder circuits (i.e., no 
responder circuits are available). 
Referring back to FIG. 4, the routine is entered if the 911 alarm is found 
to be still active in step 220 because the 911 tandem trunk 118 is 
unacceptable for signal transmission or because the 911 alarm timer has 
not expired since the last 911 call has been diverted, and the CPU 130 
determines in step 258 that a 911 call is not in progress, the routine 200 
calls a link maintenance routine shown in FIG. 9 at step 259. 
The link maintenance routine shown in FIG. 9 is entered at step 291, and, 
if a 911 call is found to be not in progress in step 292 and the 911 alarm 
set in step 206 (FIG. 5) is found to be still active in step 294, the CPU 
130 of the trunk diverter circuit 116 begins a communication timer and 
transmits a maintain link message to the responder circuit 122 in step 
296. The maintain link message indicates to the responder circuit 122 that 
it should maintain its connection to the PTN 119 but not seize the PSAP 
facility 102. In response to the maintain link message, the responder 
circuit 122 transmits the PSAP on hook message to the trunk diverter 
circuit 116 if the PSAP facility 102 is available. The responder circuit 
122 and the trunk diverter circuit 116 continue to exchange their 
respective PSAP on hook and link maintenance messages in a "ping pong" 
signaling manner along the alternate path 118 until: (1) the trunk 
diverter circuit 116 receives a 911 call, (2) the responder circuit 122 
monitors an off hook signal from the PSAP facility 102, (3) the CPU 130 no 
longer recognizes a 911 alarm because it monitors an acceptable condition 
on trunk 118, or (4) the PTN connection between the trunk diverter circuit 
116 and the responder circuit 122 is disrupted. 
The communication timer sets a fixed period of time in which the responder 
circuit 122 has to transmit either the PSAP on hook or off hook message to 
the trunk diverter circuit 116. If the trunk diverter circuit 118 has not 
received either the PSAP on hook or PSAP off hook messages within this set 
time period, the PTN connection between the trunk diverter circuit and the 
responder circuit 122 could be disrupted, the responder circuit could be 
malfunctioning, or some other malfunction could be occurring. Therefore, 
until the communication timer has expired in step 298, the CPU 130 checks 
at step 304 to determine if the PSAP is on hook and, until the PSAP is off 
hook, then the responder circuit 122 sends the trunk diverter circuit 116 
the PSAP on hook message in step 306 and the routine 200 loops back to 
determining if a 911 call is in progress in step 292 and if the 911 alarm 
is active in step 294. The 911 alarm was previously set in either steps 
204 and 206 or 214, 216 and 206 of FIG. 5. If a 911 call is not in 
progress in step 292 and the 911 alarm is active in 294, the trunk 
diverter circuit 116 sends the responder circuit 122 the maintain link 
message and the routine 200 continues to loop through steps 298, 304, 306, 
292, 294, and 296, in the ping pong signaling exchange described above. 
Once the Communication Timer has been found to have expired at step 298, 
the trunk diverter circuit 116 releases its PTN line (i.e., the ground 
start line 118) in step 300. The CPU 130 then sets a Responder Not 
Connected flat at 302 before returning to the steps of FIG. 4 via step 
303. If the communications timer has not expired in step 298, the CPU 150 
of the responder circuit 122 determines if the PSAP facility 102 is on 
hook in step 304, as explained above. 
The loop of the routine 200, where the trunk diverter circuit 116 and the 
responder circuit 122 exchange messages, is terminated in step 292 
whenever a 911 call is received. The routine 200 thereafter returns via 
step 303 to the steps shown in FIG. 4, and the 911 call is processed as 
described above. Importantly, the 911 call is routed to the PSAP facility 
102 without the normal delay inherent in the PTN 118 (i.e., up to ten 
seconds) or other delays. No voice messages indicating the status of the 
911 call are necessary because the 911 caller 114 perceives a standard 911 
call, rather than a call that has been rerouted to the PSAP facility 102. 
If the 911 alarm is found to be no longer active in step 294, or after the 
responder circuit 122 sends the trunk diverter circuit 116 a PSAP off hook 
message in step 308, the trunk diverter circuit sends the responder 
circuit the disconnect message and the trunk diverter circuit releases its 
PTN line in step 310. The responder circuit 122 then releases its PTN line 
in step 312. The CPU 130 thereafter determines that the responder circuit 
122 is no longer connected in step 302 (and logs an appropriate an 
availability flag in the TDC's responder database), and the routine 200 
returns through step 303 to the steps shown in FIG. 4. 
Referring back to FIG. 4, if the 911 alarm is found to be no longer active 
in step 220, the routine 200 calls at step 221 the retest responder 
routine shown in FIG. 10. Referring to FIG. 10, after starting the routine 
at step 313, the CPU 130 determines in step 314 if a retest list is empty. 
The retest list is a list compiled from the TDC's responder database that 
indicates those responder circuits 122 that previously were indicated or 
flagged as being unavailable. If the retest list is not empty, the CPU 130 
selects the first responder circuit 122 on the list to test and retrieves 
its particular PTN number from the EPROM 132 in step 316. The trunk 
diverter circuit 116 seizes its outgoing ground start line 118 as 
described above and dials the PTN number of the selected responder circuit 
122 by means of its DTMF generator 142 in step 316. Concurrently, the CPU 
130 starts a network timer. 
In step 318, the CPU 130 determines if the selected responder circuit 122 
is connected. If the responder is not connected and the CPU 130 does not 
receive a busy signal from the selected signal 122 in step 320, the CPU 
then determines whether the network timer has expired yet in step 322. If 
the network timer has not yet expired in step 322, the CPU 130 continues 
to wait until the responder circuit 122 is connected in step 318. 
If the responder circuit is connected in step 318, the CPU 150 of the 
responder circuit 122 determines if the PSAP facility 102 to which it is 
connected is on hook in step 324. If the responder circuit 122 determines 
that the PSAP facility 102 is on hook, the responder circuit sends the 
trunk diverter circuit 116 a PSAP on hook message in 326. In response to 
the PSAP on hook message, the trunk diverter circuit 116 sends the 
responder circuit 122 a disconnect message in step 328. The CPU 130 of the 
trunk diverter circuit 116 also removes that particular responder circuit 
122 from the retest list and indicates in the TDC's responder database 
that it is an available responder circuit. In response to the disconnect 
message, the responder circuit 122 releases its PTN line in step 330. 
If the responder circuit 122 determines in step 324 that the PSAP facility 
102 is off hook, then in step 332, the responder circuit sends the trunk 
diverter circuit 116 a PSAP off hook message. The responder circuit 122 
also releases its PTN line. In response to the PSAP off hook message, the 
trunk diverter circuit 116 sends the responder circuit 122 a disconnect 
message in step 334 and the CPU 130 keeps that particular responder 
circuit on the retest list because the PSAP facility 102 is unavailable. 
The trunk diverter circuit 116 releases its PTN line in step 336 and the 
routine 200 returns via step 337 to the steps in FIG. 4 after: (1) the 
responder circuit 122 determines that the PSAP is off hook and the CPU 130 
maintains that responder circuit on the retest list in step 334, (2) the 
responder circuit successfully connects and transmits the PSAP on hook 
message in step 330, (3) the trunk diverter circuit receives a busy signal 
from the responder in step 320, or (4) the network timer expires in step 
322. 
As noted above, the instant network backup system of the present invention 
preferably uses several responder circuits 122 placed at various locations 
within the 911 network 101 and the PTN 119. As shown in FIG. 1, responder 
circuits 122 and 122' are preferably installed proximate to PSAP 
facilities 102 and 102' because the likelihood of a cable or line 
disconnect between the responder circuit and the PSAP facility is 
negligent. At least one responder circuit 122 is preferably installed 
proximate to each PSAP facility 102 in a given geographic region. Most 
PSAP facilities 102 have many incoming 911 trunks 106. To provide the most 
secure network, one responder circuit 122 is preferably installed at the 
PSAP facility for each incoming 911 trunk 106, however, often only a few 
responder circuits are installed for a few of the trunks. To further 
improve the security of the 911 network 101, responder circuits 122 are 
preferably also installed in the tandem switch 110. 
If a regional disaster occurs, such as an earthquake, numerous disruptions 
in the 911 network 101 occur. In this scenario, the trunk diverter 
circuits 116 distributed within this region will quickly establish 
alternate paths 118 to any available responder circuits 122. Generally, 
more trunk diverter circuits 116 will be installed within a given region 
than responder circuits 122 because fewer PSAP facilities 102 exist within 
a given region than C.O.s. If only a few responder circuits 122 and PSAP 
facilities 102 are accessible, then all of the trunk diverter circuits 116 
within the region would tie up and maintain all of those responder 
circuits. Consequently, any trunk diverter circuit 116 which failed to 
establish an alternate path 118 to one of the available PSAP facilities 
102 via the few responder circuits 122 would be locked out or prohibited 
from establishing such a path and any incoming 911 calls 114 could not be 
connected to a PSAP facility. 
To compensate for this potential problem caused by a regional disaster, a 
first alternative embodiment of the present invention assigns each 
responder circuit 122 in a geographic region to one of two classes: an 
express class and a non-express class. This first alternative embodiment, 
and all alternative embodiments described herein, are substantially 
similar to the first described embodiment and only the differences in 
construction and/or operation are described in detail. A trunk diverter 
circuit 116 may maintain an alternative path 118 under the steps of FIG. 9 
with a responder circuit 122 in the express or active class, but may not 
maintain such a path with a responder circuit 122 in the non-express 
class. A system manager or technician identifies and assigns each 
responder circuit 122 within a given region to one of the two classes in 
the TDC's responder database when a given trunk diverter circuit 116 is 
installed. 
In the first alternative embodiment, when the trunk diverter circuit 116 
detects a malfunction in the 911 network 101 under the steps of FIG. 5, 
the CPU 130 determines if a 911 call is in progress in step 222. If the 
CPU 130 determines that a 911 call is not in progress, then in step 236, 
the trunk diverter circuit 116 dials the PTN number of only those 
responder circuits 122 that are indicated in the TDC's responder database 
as being express. Thereafter, the trunk diverter circuit 116 and selected 
responder circuit 122 maintain the alternate path 118 under the link 
maintenance routine shown in FIG. 9. 
If the trunk diverter circuit 116 detects that a 911 call is in progress in 
step 236, then in step 236 the trunk divert circuit dials the PTN number 
of either class of responder circuits 122, i.e., either the express or 
non-express class of responder circuits. As a result, in the event of a 
regional disaster, only a portion of the trunk diverter circuits 116 
within the region (e.g., 50%) lock up and maintain communications paths 
with the available PSAP facilities 102. The remaining trunk diverter 
circuits 116 which fail to establish an alternate path 118 to a PSAP 
facility 102 can nevertheless still reroute a received 911 call 114 to an 
available PSAP facility 102. For example, a trunk diverter circuit 116 
installed at a particular C.O. 112 may fail to establish an alternative 
path 118 with an available PSAP 102. The potential 911 callers 114 linked 
to the particular originating C.O. 112 can still contact the available 
PSAP facility 102 through either class of responder circuits 122 installed 
therein, even though many of the express class responder circuits are 
maintained by other trunk diverter circuits 116 in the region. 
While the present invention has been generally described herein with a 
trunk diverter circuit 116 located proximate to the originating C.O. 112 
and a responder circuit 122 located proximate to the PSAP facility 102, 
the trunk diverter and responder circuits may be located at various points 
within the 911 network 101 and the PTN 119. As shown in FIG. 11, a trunk 
diverter circuit 116 can also be installed proximate to the tandem switch 
110. As a result, the trunk diverter circuit 116 may detect for a failure 
of the PSAP's 911 trunk 106. By installing trunk diverter switches 116 at 
both the originating C.O. 112 and the tandem switch 110, failures along 
both tandem trunk 107 and the 911 trunk 106 may be monitored and backed 
up. 
Additionally, while the present invention has been generally described 
herein as establishing an alternative path 118 for a 911 call using the 
PTN 119 land lines, the present invention can also establish an 
alternative route using available cellular or mobile telephone switching 
networks 127. As shown in FIG. 11, the trunk diverter circuit 116 and the 
responder circuit 122 may each be coupled to a cellular transceiver 123 so 
that the trunk diverter circuit can reroute a 911 call through either the 
alternative land line network path 118 or an alternate cellular network 
path 125. 
Referring to FIG. 12, some geographic regions use remote switch units 117. 
Remote switch units 117 are connected to an end office or host 121 via a 
proprietary data link that is commonly referred to as an "umbilical" 109. 
Together these remote switch units 117 and their hosts 121 are logically 
considered as a single C.O. If the umbilical 109 is disrupted (shown as a 
cable break 109' in FIG. 12), the remote switching unit 117 enters into an 
alarm state referred to as "emergency stand alone" and provides an alarm 
signal to associated circuitry at the remote switch unit. During the 
emergency stand alone state, the remote switch unit 117 directs all 911 
calls to a ground start line circuit as if the 911 caller 114 had dialed a 
local PTN telephone number. 
In a second alternative embodiment of the present invention; a trunk 
diverter circuit 116 is installed at the remote switch unit 117. The trunk 
diverter circuit 116 receives the alarm signal through the relay K7. The 
CPU 130 in step 205 detects this alarm signal and the trunk diverter 
circuit 116 performs the steps of the routine 200 described above, 
including the link maintenance routine shown in FIG. 9. As a result, the 
second alternative embodiment of the present invention monitors for 
disruptions in the umbilical 109 and establishes the alternate path with 
an available responder circuit 122 after detecting the disruption. 
As shown in FIG. 12, the trunk diverter circuit 116 is connected to and 
housed together with the cellular transceiver 123, and thus the trunk 
diverter circuit establishes the alternate cellular network path 125 over 
the mobile telephone switching network 127. Thereafter, the trunk diverter 
circuit 116 reroutes any received 911 calls over this established 
alternate path 125 under the routine 200 described herein. 
Few remote switch units have standard analog trunks that provide 
conventional ANI data. Therefore, the remote switch units 117 must 
transfer 911 calls 114 to ground start line circuits. Due to the ground 
start line protocols used by these remote switch units 117, conventional 
ANI data is unavailable. Therefore, the remote switch unit 117 cannot 
transmit a 911 caller's ANI data to the trunk diverter circuit 116 in the 
conventional MF format. As a result, when a 911 call 114 is diverted to a 
PSAP facility 102 from these remote switch units 117, the personnel at the 
PSAP facility do not receive any ANI data and therefore cannot tell from 
where the call came. 
To compensate for the lack of conventional ANI data from remote switching 
units 117, in the second alternative embodiment, the RAM 131 stores pseudo 
ANI data. The pseudo ANI data is particular to each remote switch unit 117 
within a region and identifies the geographic region within which the 
remote switch unit is located. The system manager installing the trunk 
diverter circuit 116 within the particular remote switch unit 117 selects 
or inputs the appropriate pseudo ANI data into the RAM 131 using, for 
example, the alarm and data port card. The pseudo ANI data also provides a 
message indicating that specific data is unavailable. In the second 
alterative embodiment, the trunk diverter circuit 116 does not perform the 
ANI routine shown in FIG. 6. Instead, the trunk diverter circuit 116, 
after detecting that a 911 call is in progress in steps 222 or 258, 
provides the pseudo ANI data to the PSAP facility in step 276 (FIG. 8). As 
a result, the pseudo ANI data provides answering personnel at the PSAP 
facility 102 with an indication as to which geographic region the 911 call 
originated. 
In a third alternative embodiment of the present invention, the trunk 
diverter circuit 116 not only operates as described above in what may be 
referred to as a "trunk monitor mode," but also operates in a 
"non-monitor" mode upon appropriate setting of a software switch in the 
routine 200. In the non-monitor mode, the trunk diverter circuit 116 does 
not monitor the outgoing tandem trunk 107. Instead, the originating C.O. 
112, after receiving a 911 call 114, attempts to route the call to the 
PSAP facility 102. After failing to successfully route the 911 call 114, 
the originating C.O. 112 switches the call to the trunk diverter circuit 
116, which in turn establishes and reroutes the call to the PSAP facility 
102 over the alternate path 118. The alternate path 118 is thereafter 
maintained. 
In the non-monitor mode, the relay contacts K1A and K1B in FIG. 2A are 
continuously actuated so that all calls received over the tip and ring 
leads TO and RO of the tandem trunk 107 are diverted to the trunk diverter 
circuit 116. The trunk diverter circuit 116 emulates the protocols of the 
tandem switch 110, and thus, the trunk diverter circuit appears to the 
originating C.O. 112 as another tandem switch. When the 911 network 101 is 
suffering a malfunction, the originating C.O. 112 attempts to route the 
first received 911 call 114 to the PSAP facility 102 through all available 
tandem trunks 107 before the call is diverted to the trunk diverter 
circuit 116 over the leads TO and RO. 
The CPU 130 does not perform the trunk monitoring steps of the trunk 
monitor routine shown in FIG. 5 (including steps 205, 212, 210 and 214). 
Instead, the CPU 130 detects for the 911 call 114 in step 208 by detecting 
current flow through the tip and ring leads TO and RO by means of the 
optical couplers OC3 and OC4. After detecting the 911 call 114 in step 
208, the CPU 130 sets the 911 alarm timer in step 216. 
The CPU 130 thereafter finds the alternate path 118 to an available 
responder circuit 122 and connects the 911 call 114 to the PSAP facility 
102 therethrough as described above. After either the CPU 130 or the CPU 
150 detect in step 278 (FIG. 8) that the 911 call 114 is no longer in 
progress, the trunk diverter circuit 116 and the responder circuit 122 
continue to maintain the alternate path 118 under the steps of the link 
maintenance routine shown in FIG. 9. The trunk diverter and responder 
circuits 116 and 122 maintain the alternate path 118 for the duration of 
the 911 alarm timer set in step 216. 
The first 911 caller 114 suffers the delays caused by (i) the originating 
C.O. 112 attempting to route the call through the malfunctioning 911 
network 101, and (ii) the trunk diverter circuit 116 establishing and 
rerouting the call along the alternate path 118 to the responder circuit 
122. Consequently, this first 911 caller experiences a substantial delay 
before their call is received by the PSAP facility 102. However, since the 
trunk diverter circuit 116 and the responder circuit 122 maintain the 
alternate path 118 after this first call, any subsequent 911 callers 114 
do not experience such a delay. 
In a given PTN, several known trunk type signaling protocols may be used, 
including tip/ring loop (reverse battery) E&M types 1 or 2 (2-wire or 
4-wire), or ground start ring down. Therefore, the trunk diverter circuit 
116 preferably provides protocol conversion where necessary between 
differing protocols such as conversion between ring down ground start 
circuit protocols to loop reverse battery or E&M trunk signaling. 
The trunk diverter circuit 116 and responder circuit 122 of the instant 
network backup system of the present invention locate and maintain 
communication paths between each other whenever a dedicated communication 
path such as the trunks 106 or 107 fail. The present invention establishes 
the alternative path 118 or 125 within the PTN 119 or the mobile telephone 
switching network 127, respectively. When the trunk diverter circuit 116 
receives a 911 call, the trunk diverter circuit and responder circuit 122 
surrender the established alternative path 118 or 125 to the 911 call. The 
911 caller does not experience a delay beyond the normal delay in the 
emergency service network 101. The trunk diverter circuit 116 and 
responder circuit 122 maintain the alternative path 118 or 125 after the 
911 call disconnects if the trunks 106 or 107 are still malfunctioning, 
until the malfunction is corrected. 
While the present invention has generally been described herein as backing 
up a communication path within an emergency response telecommunications 
network, the present invention may also be used in a variety of 
communication environments. For example, the present invention may be used 
to back up communication paths between two facilities or locations which 
regularly exchange data over trunk lines or other dedicated communication 
paths. The present invention may be used to ensure that electronic funds 
transfer ("EFT") within the financial and banking environments are not 
disrupted. Furthermore, the present invention may be used to establish 
alternative communications paths within computer network environments. 
Consequently, as used herein, the terms telephone and telephone 
transceiver can include all types of transceivers including modems, 
cellular phones, fax machines, and so forth, and the "911 call" may 
include any predetermined request signal by such transceivers that 
instruct the present invention to surrender the alternative communications 
path thereto. 
Although specific embodiments of the invention have been described for 
purposes of illustration, various modifications may be made without 
departing from the spirit and scope of the invention as will be recognized 
in the relevant art based on the detailed description provided herein. 
Accordingly, the invention is limited by the disclosure, but instead its 
scope is to be determined entirely by reference to the following claims.