Error detection/correction and fault detection/recovery – Data processing system error or fault handling – Reliability and availability
Reexamination Certificate
1999-11-19
2003-07-01
Iqbal, Nadeem (Department: 2184)
Error detection/correction and fault detection/recovery
Data processing system error or fault handling
Reliability and availability
C370S253000
Reexamination Certificate
active
06587974
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates generally to signal transmission systems constructed of physical cables, and more particularly to a method for protection switching in case of detecting faults in such signal transmission systems.
2. Description of the Related Art
Signal faults in signal transmission systems that use a physical cable for signal transmission include reduced signal strength that results in signal degradation and complete loss of signal (e.g., break in the cable). In digital transmissions systems requiring high reliability, rapid signal fault detection is often used to enable the traffic to be re-routed so that there is little discernible loss of signal to the user when a fault occurs. In a conventional signal transmission system using signal transmission cables, one method of signal fault detection is to monitor and detect a signal fault at a receiving end of a first transmission signal cable and if a fault is detected a fault notification message is sent back to the originating (transmitting) end via a different (e.g., second) cable so that any fault in the first transmission cable will not keep the fault notification message from arriving at the originating end. Once a fault notification message is received by the originating end via the second cable, the signal transmission system re-routes the signal traffic from the cable on which a fault has been detected to another transmission cable. This type of fault detection protocol is in many circumstances slow to provide the desired level of user indiscernible signal reception when a fault occurs along a cable transmission signal path.
For example, the greater the length of the transmission path the longer time it will take a signal to propagate from one point to another on the transmission path, and thus the longer the time delay between the time when a signal fault occurs and the time when the signal is re-routed and recovered. This delay is of particular concern in, for example, transoceanic and transcontinental transmission because of the great lengths a signal (both the original signal and error notification message signals) must travel before it reaches its destination. The longer this distance and related signal propagation delay, the more likely that an end user will sense that the transmission has been dropped when a fault occurs in a transoceanic or transcontinental transmission cable path (e.g., a degradation or break in the transmission signal). A specific example of this conventional fault detection and restoration method is provided below.
FIG. 1
illustrates one signal transmission system that would use the fault detection method described above. In this example, the signal transmission system may be a telephone communication system having a first node
10
which includes a transmitter connected to cable
13
. The transmitter in node
10
sends voice signal transmission on cable
13
to node
12
, node
12
having a receiver. In this case, node
10
may be referred to as the originating or upstream cable station and node
12
may be referred to as the receiving or downstream cable station. The voice signals in this case may be transmitted on either of two frequency bands, X and Y bands
14
which accommodate a respective signal and operate in a unidirectional manner from node
10
to node
12
. If cable
13
is a fiber optic cable (single fiber line) that includes double band Erbium doped fiber amplifier (EDFA), then the X band may be for example a conventional amplification band (C band) which has a typical range of amplification wavelength band between 1525 nm to about 1570 nm, and the Y band may be for example a long wavelength amplification band (L band) which has a typical range of amplification wavelength band between 1560 nm to about 1610 nm. The signal transmission system also includes a second cable
15
over which voice and/or data signals may be transmitted from node
12
to node
10
. Cable
15
may likewise be a fiber optic cable that includes dual band EDFAs with X and Y bands
16
. The X
18
on cable
13
illustrates the location of a signal fault, for example an unacceptable signal degradation or a break in a signal being transmitted from node
10
to node
12
.
The conventional method for fault detection in the transmission system of
FIG. 1
will now be explained with reference to FIG.
1
and the process flow diagram of FIG.
2
. First, at step
20
a signal (e.g., voice or data signal) is transmitted on a first cable A, cable
13
, from the upstream originating node, node
10
, to a downstream receiving node, node
12
. This signal may travel on either X band, Y band, or both X and Y bands
14
. Next in step
22
, the signal is received at the downstream receiving node
12
. At step
24
, the receiver of node
12
connected to cable
13
monitors the incoming signal for a fault. If a fault is not detected in the received signal at node
12
, the receiver continues to monitor the received signal at step
22
. However, if an error is detected at step
24
, for example a break occurs at X
18
(see FIG.
1
), a transmitter of node
12
(downstream node) connected to a second cable, cable B (cable
15
), sends a fault notification message on the second cable B (
15
) to a receiver of the upstream originating node
10
. Finally, at step
28
, the transmission system activates re-routing of the data signal traffic transmitted from the upstream originating node
10
to the downstream receiving node
12
on another cable (or fiber) so as to restore error free data traffic from node
10
to node
12
.
Using the conventional error detection method, the time it takes the transmission system to restore error free signals includes (1) signal propagation delay time, (2) failure detection time, (3) hold-off time, (4) switching time, and (5) frame synchronization time. The signal propagation delay time includes a first propagation delay time T
2
along cable
13
from X
18
to node
12
, which represents the time it takes for a lost or degraded signal that occurs at fault X
18
to reach node
12
, and a propagation delay time T
3
from node
12
to node
10
, which represents the propagation time it takes the error notification to travel from node
12
to node
10
. These time delays can be considerable in transoceanic and transcontinental signal transmission where the cable lengths can span thousands of miles or kilometers. This translates into typical propagation delays (T
2
+T
3
) on the order of, for example, 50-100 milliseconds (ms) for a fiber optic transoceanic cable which may be too long in some systems and leads to total delays on the order of 300 ms from the time an error occurs until an error free signal is restored, such as a telephone voice signal transmission. This restoration time can result in providing a system user unacceptable service, for example, if a fault occurs along a long length telephone signal cable the cumulative signal recovery time my result in the telephone user concluding that the telephone call has been dropped. Therefore, there is a need for a fault detection method in signal transmission systems which reduces the delay associated with protection switching protocol and reduces the time span between the time a fault occurs and the time the transmission system restores error free signal transmission from one node to another node.
SUMMARY OF THE INVENTION
The present invention provides a method of detecting the loss or degradation of a transmitted signal in a signal transmission system. The method for detecting and correcting signal transmission faults in a signal transmission system includes bi-directional signaling on the same fiber, cable, or signal line. A fault in a transmitted signal from a first upstream node to a second downstream node is determined by the loss or degradation of a signal simultaneously transmitted from the second downstream node to the first upstream node transmitted in the opposite direction on the same fiber, cable, signal line. As a result, the propagation delay of the
Majd Casem
Nissov Morten
Iqbal Nadeem
Tyco Telecommunications (US) Inc.
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