Method of restoring transmission line

When a trouble occurs on a transmission line, each station transmits a predetermined frame to the adjacent station. The station which does not receive the frame outputs to the adjacent downstream station a frame instructing that its primary output be looped back to its secondary input. The information transmission trouble is removed by arranging the responses to the frames by the respective stations and the transmission procedure.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of restoring a dual transmission 
line which connects respective stations to make a local-area network. 
2. Description of the Prior Art 
FIG. 6 shows a conventional ring local area network having three stations 
S1, S2, and S3 connected to each other with a primary transmission line 1 
and a secondary transmission line 2 at respective primary and secondary 
input and output terminals PI, PO, SI, and SO. Each station has a computer 
and other equipment, and these machines are able to communicate each other 
through the stations. The data are circulated along the ring transmission 
line in the direction of an arrow. Normally, only the primary line 1 is 
used, with the secondary line 2 standing by for use when the primary line 
has a fault. 
FIG. 7 shows the fields of a frame which flows in a ring local-area network 
such as shown in FIG. 6. This frame has a start delimiter SD indicating 
the beginning of the frame, a destination station address DA, a source 
station address SA, a command C indicating the frame type, a user data I 
to be added if necessary, a frame check sequence FCS for detecting a frame 
error, an end delimiter ED indicating the end of the frame, and a frame 
status FS indicating the response. 
For example, the station 2 transmits a frame of respective fields SD 
through ED at the primary output PO on the primary transmission line 1. 
The frame is circulated along the primary line 1 and returned to the same 
station 2 at the primary input PI, where it is discarded. The other 
station S1 or S3 constantly monitors the frame on the primary line 1 while 
repeating it and captures the contents of the frame addressed to the 
station and transmits the result of the capture in the FS field. The 
transmitting station S2 checks the FS field to see whether the 
transmission is completed. The transmitting station must obtain the right 
to transmit according to the procedure determined by the system, but this 
procedure has nothing to do with the present invention and its description 
will be omitted. 
A conventional method of restoring a transmission line will be described 
with reference to FIG. 8 in which the primary line 1 is broken between the 
stations S1 and S2. When the primary line 1 is broken between the stations 
S1 and S2 causing a fault and the station S2 detects the carrier breakdown 
at the primary input PI on the primary line 1, it connects the primary 
input PI to the secondary output SO and transmits without interruption 
through the secondary line 2 to the adjacent station S1 a special symbol 
instructing that the primary output PO be connected to the secondary input 
SI. The station S1 receives the special symbol at the secondary input SI 
on the secondary line 2 and connects its primary output PO to its 
secondary input SI as instructed. In this way, the transmission line of 
the ring local-area network is restored to continue communications. 
The condition that the secondary output is connected to the primary input 
is hereinafter called a "WRAPA" state, that the primary output is 
connected to the secondary input, a "WRAPB" state, both WRAPA and WRAPB 
states, generally a "wrap" or "loopback" state, and the other or 
non-loopback condition, a "THRU" state. 
In the above example, the transmission line is completely broken to cause a 
carrier breakdown, but there is another kind of fault that only the 
contents of information are broken without causing a carrier breakdown 
(hereinafter called "information transmission fault"). At present, there 
is no method of restoring a transmission line from such a fault, but only 
the fault location is able to evaluate. 
A conventional method of evaluating a fault location will be decribed with 
reference to FIG. 9. Each station S1, S2, or S3 has a nomal mode 11 and a 
beacon mode 12. In the normal mode, when there is no information 
transmission trouble on the transmission line, each station performs a 
normal transmission reception operation while monitoring a control symbol 
which is generated on the transmission line in a predetermined period of 
time. In the beacon mode, when a fault occurs on the transmission line, a 
station detects the fault by monitoring the control symbol and transmits 
to the adjacent downstream station a beacon frame indicating the 
transmission fault. When the station in the beacon mode receives the 
beacon frame, it returns to the normal mode. 
A method of restoring a transmission line from an information transmission 
fault will be described with reference to FIG. 10. When a fault occurs on 
the primary line 1 between the stations S1 and S2, the respective stations 
in a normal mode are unable to detect a control symbol within a 
predetermined period of time and go to a beacon mode about the same time. 
The respective stations S1, S2, and S3 in the beacon mode transmits a 
beacon frame to the adjacent downstream station as indicated by a broken 
line 100. The station S3 receives the beacon frame from the station S2 to 
return to the normal mode, and the station S1 receives the beacon frame 
from the station S3 to return to the normal mode, but the station S2 is 
unable to receive the beacon frame from the station S1 because of the 
information transmission fault on the transmission line 1 and remains in 
the beacon mode. Hence, the system is brokendown, but it is only possible 
to judge that the fault location lies between the stations S2 in the 
beacon mode and its upstream station S1. In the above beacon frame, the DA 
and FS fields are insignificant and the I field does not exist. 
As has been described above, the conventional method of restoring a 
transmission line is able to correct a carrier breakdown but unable to 
correct an information transmission fault without carrier breakdown except 
for providing an approximate fault location. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a method of restoring a 
transmission line from an information transmission fault. 
It is another object of the invention to provide a method of restoring a 
transmission line from an information transmission fault by the minimum 
procedure. 
It is still another object of the invention to provide a method of 
restoring a transmission line from an information transmission fault 
resulting from a trouble in the station. 
According to the invention, respective stations in respective modes 
exchange with the adjacent upstream or downstream station a frame 
corresponding to the mode so as to restore the transmission line from an 
information transmission fault. 
Other objects, features, and advantages of the invention will be apparent 
from the following description taken in conjunction with the accompanying 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now FIG. 1 there is shown a system embodying the present 
invention, which has three stations S1, S2, and S3. Each station has a 
mode switching means 1A, a control circuit 1B for controlling the mode 
switching means 1A and effecting loopback or release of the loopback and a 
timer 1C for measuring time. In this embodiment, each station has five 
modes; normal, beacon, test, and configuration instruction modes 1 and 2. 
In the normal mode, a station performs a normal transmission reception 
operation while monitoring a control symbol. In the beacon mode, a station 
outputs a beacon frame indicating that the transmission line has a fault. 
In the test mode, a station loops back the primary output to the secondary 
input and outputs to the adjacent upstream station a test frame indicating 
that the transmission line is now in order. In the configuration 
instruction mode 1, a station loops back the secondary output to the 
primary input and outputs to the adjacent downstream station a 
configuration instruction frame instructing that the primary output be 
looped back to the secondary input. In the configuration instruction mode 
2, a station releases the loopback of the primary output to the secondary 
input for making a THRU sate and outputs the above configuration 
instruction frame. 
How a station chages its operational mode will be described with reference 
to FIG. 2. When a control symbol is unable to detect in a predetermined 
period of time in the normal mode (control symbol time-out), each station 
goes to a beacon mode. When a station in the beacon mode receives no 
beacon frame from other stations in the beacon mode in a predetermined 
period of time (beacon mode time-out), the station goes to a configuration 
instruction mode 1 in a WRAPA state and outputs a configuration 
instruction frame. When a station in the beacon mode receives a beacon 
frame from another station in the predetermined time, the station returns 
to the normal mode. 
When a station in the normal mode receives a configuration instruction 
frame after a fault has been detected, the station goes to a test mode in 
a WRAPB state. When the station in the test mode completes the 
transmission of a test frame, it releases the WRAPB state into the THRU 
state going to the configuration instruction mode 2. 
When the station in the configuration instruction mode 2 receives a test 
frame, it returns to the normal mode as it is in the THRU state. When the 
configuration instruction mode 2 continues for the predetermined time 
(configuration mode 2 time-out), the station returns to the normal mode in 
a WRAPB state. When the configuration instruction mode 1 continues for a 
predetermined time (configuration mode 1 time-out), the station releases 
itself from the local-area network. 
The control means 1B controls the switching means 1A to effect the above 
mode transition. The timer 1C measures time for the time-outs. When a new 
station is added to the local area network or a station is bypassed and 
removed from the local area network, the transmission line is disturbed 
temporarily and the recovery takes some time so that the period of 
time-out has been determined taking it into consideration. 
FIG. 3 shows a process of correcting an information transmission fault 
between the stations S1 and S2 while the mode is changes as described 
above. In the figure, M1 stands for the normal mode, M2 the beacon mode, 
M3 the test mode, M41 the configuration instruction mode 1, and M42 the 
configuration instruction mode 2. 
(1) When a control symbol time-out takes place, all the stations go to a 
beacon mode M2. 
(2) Only the station S2, which is downstream of the location of an 
information transmission fault, remains in the beacon mode M2, while the 
other stations S1 and S3 receive a beacon frame from their upstream 
station S3 or S2 to return to the normal mode M1. 
(3) The station S2, now in a beacon mode time-out, goes to a configuration 
instruction mode 1 M41 and transmits in the WRAPA state a configuration 
instruction frame to the adjacent downstream station S3. 
(4) The station S3 receives the configuration instruction frame and goes to 
a test mode M3 and transmits in the WRAPB state a test frame to the 
adjacent upstream station S2. 
(5) The station S2 receives the test frame and goes to the normal mode M1 
in the WRAPA state. 
(6) When the test frame transmission is completed, the station S3 goes to 
the configuration instruction mode 2 M42 and transmits in the THRU state a 
configuration instruction frame to the adjacent downstream station S1. 
(7) The station S1 receives the configuration instruction frame and goes to 
a test mode M3 and transmits in the WRAPB state a test frame to the 
adjacent upstream station S3. 
(8) The station S3 receives the test frame and goes to the normal mode M1 
in the THRU state. 
(9) When the test frame transmission is completed, the station S1 goes to a 
configuration instruction mode 2 M42 and transmits in the THRU state a 
configuration instruction frame to the downstream station S2. 
(10) The configuration instruction frame is unable to reach the downstream 
station S2 because of the information transmission fault, and the station 
S1 experiences a time-out in the configuration instruction mode 2 M42. 
Consequently, it forms a WRAPB state and goes to the normal mode M1. 
If there is an information transmission fault at the receiver of the 
station S2, the station S2 transmits in the configuration instruction mode 
1 M41 a configuration instruction frame. However, the information is 
destroyed at its own receiver so that the transmission is unable to 
complete. Consequently, the station S2 experiences a test mode time-out 
and removes itself from the local area network. Thus, the fault is removed 
to restore the system. 
Next, another system embodying the present invention will be described. In 
this embodiment, each station has four modes; normal and beacon modes, and 
two test modes 1 and 2. In the normal mode, the station performs a normal 
transmission reception operation while monitoring a control symbol. In the 
beacon mode, a station transmits to the adjacent downstream station a 
beacon frame informing that there is a fault on the transmission line. In 
the test mode 1, a station transmits, with its secondary line 2 looped 
back to its primary line 2, to the adjacent downstream station a test 
frame instructing that the primary output be connected to the secondary 
input and, after the test frame transmission is completed, transmits to 
the same station a test mode instruction frame instructing that the 
station transmit in a non-loopback state a test frame to the adjacent 
downstream station. In the test mode 2, a station transmits in a 
non-loopback state the above test frame and test mode instruction frame to 
the adjacent downstream station. 
How a station changes its mode will be described with reference to FIG. 4. 
When a control symbol is unable to detect within the predetermined time in 
the normal mode (control symbol time-out), each station goes to a beacon 
mode. When a station in the beacon mode receives no beacon frame from 
other stations in the predetermined time period (beacon mode time-out), 
the station goes to a test mode 1 and transmits a test frame in the WRAPA 
state. 
When a station in the normal mode receives a test frame after a fault has 
been detected, the station effects loopback forming a WRAPB state in the 
same mode. 
When a station in the normal mode receives a test mode instruction frame 
after a fault has been detected, the station goes to a test mode 2 and 
releases the loopback forming a THRU state. 
When the test mode 1 continues for the predetermined time, bringing about a 
test mode 1 time-out, the station removes itself from the LAN. 
When the test mode 2 continues for the predetermined time, bringing about a 
test mode 2 time-out, the station effects the above loopback forming a 
WRAPB state and goes to the normal mode. 
The control means 1B controls the switching means 1A to change the above 
modes. The timer 1C measures time for the time-outs. When a new station is 
added to the LAN or a station is bypassed and removed from the LAN, the 
transmission line is disturbed temporarily and it takes some time for it 
to restore so that the interval of a time-out is determined by taking 
these facts into consideration. 
FIG. 5 illustrates a process of restoring a transmission line from an 
information transmission fault which has occurred between the stations S1 
and S2. 
(1) All the stations experience a control symbol time-out and go to a 
beacon mode M2. 
(2) Only the station S2, which is downstream of the location of the 
information transmission fault, holds the beacon mode while the other 
stations S1 and S3 receive a beacon frame from their upstream station, 
going to the normal mode M1. 
(3) The station S2, now in a beacon time-out, goes to a test mode 1 M31 and 
transmits in the WRAPA state a test frame to the adjacent downstream 
station S3. 
(4) The station S3 receives the test frame and goes to the WRAPB state 
without changing the mode. 
(5) Upon completion of the test frame transmission, the station S2 
transmits in the WRAPA state a test instruction frame and goes to the 
normal mode M1. 
(6) The station S3 receives the test instruction frame, going to the test 
mode 2 M32 and transmits in the THRU state a test frame to the adjacent 
downstream station S1. 
(7) The station S1 receives the test frame and goes to the WRAPB state 
without changing the mode. 
(8) Upon completion of the test frame transmission, the station S3 
transmits in the THRU state a test instruction frame and goes to the 
normal mode 1 M1. 
(9) The station 1 receives the test instruction frame and goes to a test 
mode 2 M32 and transmits in the THRU state a test frame to the adjacent 
downstream station S2. 
(10) Since the test frame is unable to reach the downstream station S2 
because of the information transmission fault, the station S1 experiences 
a time-out in the test mode 1 and forms a WRAPB state, going to the normal 
mode M1. 
If there is a similar fault at the receiver of the station S2, the station 
S2 transmits a test frame in the test mode 1 but cannot complete the 
transmission because the information is destroyed in its own station. 
Consequently, the station S2 experiences a test mode time-out and removes 
itself from the LAN. Thus, the trouble is removed and the system is 
restored. 
In the above embodiments, only three stations are provided for simplicity, 
but the principle of the invention is applicable to a large number of 
stations. The above method is carried out be using the switching means, 
control means, and timer in each station, but it may be carried out with 
the use of a program, too. 
While a preferred embodiment of the invention has been described using 
specific terms, such description is for illustrative purpose only, and it 
is to be understood that changes and variations may be made without 
departing from the spirit and scope of the invetion as defined by the 
following claims.