Multiplex communications – Fault recovery – Bypass an inoperative switch or inoperative element of a...
Reexamination Certificate
2000-09-22
2003-09-23
Chin, Wellington (Department: 2664)
Multiplex communications
Fault recovery
Bypass an inoperative switch or inoperative element of a...
C370S351000
Reexamination Certificate
active
06625115
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a switching method of transmission lines in a transmission network. Specifically, the present invention relates to a method of transmission line switching, transmission equipment, and network architecture suitable to SONET (Synchronous Optical Network) or SDH (Synchronous Digital Hierarchy) network.
Recent years, there are proposed many transmission line switching methods to protect signals against line failure (for example, inadvent disconnection or degradation of line failure of repeaters) in order to improve the reliability of transmission services.
These methods comprise for example: (1) 1 to N type NPS (Nested Protection Switching) network in which a plurality of working lines and protection lines are installed in a same path, and line switching method thereof, (2) 4-Fiber BLSR (Bidirectional Line Switching Ring) and transmission line switching method thereof, in which a plurality of transmission equipment are connected by the working line and protection line in a ring form. Examples of the former method include ““Nested Protection Switching” T1X1.5/90-132,1992” and Fiber Network Service Survivability, and the examples of the latter include “Bellcore “SONET BLSR Genetic Criteria” GR-1230-CORE,1993”.
FIG. 9
shows an example of N-type NPS network. In this figure,
101
through
104
designate transmission equipment. The network example of
FIG. 9
is connected as follows: its working line
105
is terminated by the transmission equipment
101
and
102
. A working line
106
is terminated, on the other hand, by the transmission equipment
102
and
103
. These working lines
105
and
106
are connected by using an Add-Drop Multiplexing equipment in the transmission equipment
102
. A working line
107
is terminated by the transmission equipment
102
and
104
. And the working line
107
is routed by the transmission equipment
103
.
On the other hand; the protection lines
109
through
111
are depicted in
FIG. 9
by dotted lines. The protection lines
109
through
111
are all connected to every transmission equipment
101
through
104
, being connected by using Add-Drop Multiplexing equipment
114
within respective transmission equipment. Each of transmission equipment has ability of switching between transmission lines, and therein the working lines and protection lines transmit signals bidirectionally.
One to N type NPS network as shown in
FIG. 9
may select Add-Drop Multiplexing equipment or repeater for a transmission equipment when required for a working line. This allows the flexibility of the N-type network service to be improved. Also its economical efficiency may be improved, since N working lines share the protection line in this network. Furthermore, this network is predominant in the addition of working lines. For example, when traffics between the transmission equipment
101
and
103
are required to be newly added, it is possible to add working lines of the required capacity easily. Thus, as in the example of working line
108
, the line addition may be realized by terminating by the transmission equipment
101
and
103
, and by repeating by the transmission equipment
102
.
At this point, how to switch when a failure occurs in such an architecture will be described with reference to FIG.
9
. The switching method is dependent on following three factors: (1) the position in the transmission equipment at the point where the failure has been occurred; (2) the level of importance of the failure; and (3) the order of the occurrence of failures.
If the first failure of the importance level
3
has been occurred in the working line
105
, the working line will be protected by using the protection line
109
. In this case the larger the importance level, the faster the protection of the failure will be realized.
If the second failure of the importance level
1
has been occurred in the working line
106
, the working line will be protected by using the protection line
110
.
If the third failure of the importance level
2
has been occurred in the working line
108
, the protection lines
109
and
110
will be required for the protection. However, in this case, the protection lines
109
and
110
are already in use. By comparing the importance level between failures in the protection lines, the importance level of the protection line
109
is three and that of the protection line
110
is one. As the importance level of the protection line
109
is higher than the importance level of failure of the working line
108
, the working line
108
will not be protected. In this case the working line
106
will remain protected. Thus the transmission equipment which has detected the failure of the working line
108
should know the working line
108
is denied being protected.
If the fourth failure of the importance level
4
has been occurred in the working line
107
, the protection lines
110
and
111
are required for the protection while the protection line
110
are already in use. When referring to the importance level of that protection line, the importance level of the protection line
110
is one, which importance level is lower than the importance level of the working line
107
. Thus the protection line
110
will be used for the protection of the working line
107
. At this time the fourth failure will be protected, whereas the second and third failures will not.
As described above, the switching decision and switching operation between transmission lines in an NPS network will be done in the transmission equipment which terminates the working line. This means that the transmission equipment should know the information on other transmission lines that the working line requests as a protection line simultaneously. Therefore, whether or not the switching operation is proper should be determined correctly based on the communication of switch control information among respective transmission equipment.
There are proposed such methods as follows, in which the switching operation is to be performed by exchanging the control information in the transmission equipment based on the overhead of SOMET/SDH. These include: (1) a method using Automatic Protection Switching bytes (APS bytes) and DCC bytes (e.g., ITU-T(International Telecommunication Union-Telecommunication Standardization Sector), T1X1.5/90-132,1990); and (2) a method using APS bytes and a timer (Tsong-Ho Wu,“Fiber Network Service Survivability”,Aretec house,1992 ). In this context the APS bytes indicates the bytes defined in the SONET/SDH for the use of exchanging of control information for transmission line switching on the SOMET/SDH. APS bytes are comprised of so-called K1 byte and K2 byte. The use of APS bytes on a Point-to-Point basis may be found in the section 5 of “Bellcore GR-253-CORE,” issue Dec. 1, 1993.
Now, SONET, SDH and a network of the present invention conduct digital transmission by using an overhead of transmission frames for digital transmission and by using performing frame phase alignment and stuff control by swapping pointers in the digital transmission, as known well.
The above described first switching method “T1X1.5/90-132” is a method for an appropriate switching of working lines on the basis of comparison of the importance level by transmitting the importance level of the working line using a plurality of DCC bytes.
The above described second switching method “Fiber Network Service Survivability” is a method as follows. The transmission equipment having detected a failure transmits K1 bytes of APS bytes to wait for the response with K2 bytes. The destination node transmits K2 bytes indicating the response when K1 bytes are received, on the assumption that a protection line has been allocated. The source node receiving the K2 bytes indicating this response starts the switching operation. If there exists a request of higher importance level on the route to the destination node, the K1 bytes will not be arrived at the destination node, so that the K2 bytes indicating the response will not be transmitted. Ther
Ikeda Hiroki
Nakano Yukio
Sugawara Toshiki
Chin Wellington
Hitachi , Ltd.
Pham Brenda
Sofer & Haroun LLP
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