Multiplex communications – Fault recovery
Reexamination Certificate
1998-03-13
2001-07-03
Kizou, Hassan (Department: 2662)
Multiplex communications
Fault recovery
C370S225000, C370S242000, C370S248000, C340S870030
Reexamination Certificate
active
06256291
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method of switching between redundant routes of a communication system not having redundant transmission lines. More particularly, the invention relates to a method of switching between redundant routes of a communication system in which terminal stations having a redundant structure are connected by two transmission lines, namely uplink and downlink transmission lines, not having a redundant structure.
In an optical transmission system, a 1+1 line switching point-to-point system is constructed by adding a switch controller to a 1+1 configuration having one working line and one protection line, or a 1:N line switching point-to-point system is constructed by adding a switch controller to a 1:N configuration having N working lines and one protection line. When a working line fails, a changeover is effected to a protection line so that communication can continue.
The sending and receiving of information relating to the switching of optical signal lines is stipulated by the international standard (SONET) regarding SDH (Synchronous Digital Hierarchy). It is so arranged that K1/K2 bytes are used in overhead bytes OHB.
FIG. 10A
is a diagram useful in describing a SONET STS-3 (OC-3) frame format. One frame is composed of 9×270 bytes. The first 9×9 bytes constitute section overhead (SOH), and the remaining bytes constitute path overhead (POH) and payload (PL). The section overhead SOH is the part of the frame that sends information (a frame synchronizing signal) representing the beginning of the frame, information specific to the transmission line (such as information which checks for error at the time of transmission and information for maintenance of the network), and a pointer which indicates the position of the path overhead POH. Further, path overhead POH is the part of the frame that sends end-to-end monitoring information in the network. The payload PL sends information at 150 Mbps.
The section overhead SOH is composed of repeater section overhead of 3×9 bytes, a pointer of 1×9 bytes and multiplex section overhead of 5×9 bytes. As shown in
FIG. 10B
, the repeater section overhead has bytes A
1
~A
2
, C
1
, B
1
, E
1
, F
1
, D
1
~D
3
, and the multiplex section overhead has bytes B
2
, K
1
~K
2
, D
4
D
12
, S
1
, Z
1
~Z
2
. The repeater section overhead and the multiplex section overhead each have a large number of undefined bytes the use of which is left to the particular communications concern.
The K
1
byte among the overhead bytes is used mainly to request switching and designates the level of the switch request and the switched line. The K
2
byte is used mainly to respond to the K
1
byte and is employed also to represent system architecture, switching mode and AIS (Alarm Indication Signal)/FERF (Far End Receive Failure). Switching requests include, in addition to a request for switching at the time of signal failure, a switching request based upon forced switching and manual switching.
FIGS. 11 and 12
illustrate the bits of the K
1
/K
2
bytes stipulated by the SONET standard, as well as the meanings thereof.
K
1
Byte
The first four bits b
1
~b
4
of the K
1
byte represent the switching request, and the remaining four bits b
5
~b
8
represent the switched line and are capable of specifying a maximum of 14 transmission lines. “Lockout of Protection” is a switch request that prohibits switching to a protection transmission line. “Forced Switching” is a request for switching of an artificially designated transmission line. If a switch has been made, a switch will not be made to another a line even if a fault has occurred there. “SF” (Signal Failure) is a switching request for when a transmission line loses it signal. This request has two priorities, namely high and low. “SD” (Signal Degradation) is a switch request based upon signal degradation of a transmission line and has two priorities, namely high and low. The SF switching request has a higher priority than that of the SD switching request. “Manual Switching” is an artificial switching request. If a fault occurs somewhere, priority is given to switching of this location. “Wait to Restore” is a request wherein, even if a switch-back request is issued following restoration of a faulty line, switch-back is performed upon elapse of a predetermined period of time. “Exerciser” performs an actual switching to self-diagnose whether switching will be performed normally. “No Request” is sent when the situation is normal or when bridging is canceled.
K
2
Byte
The first four bits b
1
~b
4
of the K
2
byte specify a transmission line number and are nulled (0000) in a case where the bits b
5
~b
8
of a received K
1
byte are null. In other cases these bits represent the number of the transmission line to which a changeover has been made. The b
5
bit indicates the network configuration; “1” indicates a 1+1 system and “0” a 1:N system. The b
6
~b
8
bits indicate the category of the switching mode, the specifics of the fault, etc. There are two types of switching modes, namely a unidirectional mode, in which only a signal in one direction is changed over, and a bidirectional mode, in which signals in both directions are changed over simultaneously.
Switching Sequence Using K
1
, K
2
Bytes
In the case of the unidirectional mode, the K
1
byte (switching request) is sent to a station A if a station B detects SF, as shown in FIG.
13
A. The station A performs bridging control in regard to the line specified by the K
1
byte (switching request) that has been received. Bridging control is control for sending identical signals to both working and protection lines. After performing bridging control, the station A transmits the K
2
byte (switching response), which is in response to the received K
1
byte, to the opposing station (station B). Upon receiving the K
2
byte, the station B performs switching control. Switching control is control for switching a designated line signal in the receiving direction to a protection line.
In the case of the bidirectional mode, the K
1
byte (switching request) is sent to station A if station B detects SF, as shown in FIG.
13
B. Station A performs bridging control in regard to the line specified by the K
1
byte (switching request) that has been received, sends back the K
2
byte (switching response) in the same manner as in the unidirectional mode and, at the same time, sends the K
1
byte designating “reverse request” (RR). Upon receiving RR, station B performs switching control and bridging control in regard to the line that was designated by the K
1
byte sent by the B station itself and sends the K
2
byte (switching response) to the opposing station (station A). Upon receiving the K
2
byte, station A performs switching control.
FIG. 14
is a diagram useful in describing the details of switching based upon line protection using the K
1
and K
2
bytes. Shown in
FIG. 14
are a terminal station (station A)
1
, a terminal station (station B)
2
opposing the station A, a working transmission line
3
comprising a working uplink line
3
a
and a working downlink line
3
b
, and a protection transmission line
4
comprising a protection uplink line
4
a
and a protection downlink line
4
b.
The terminal station (station A)
1
includes a multiplexer/demultiplexer
1
a,
working and protection transmitters (TX-W, TX-P)
1
b,
1
c
, respectively, which send exactly identical signals, and working and protection receivers (RX-W, RX-P)
1
d
,
1
e
, respectively, which receive exactly identical signals. The terminal station (station B)
2
includes a multiplexer/demultiplexer
2
a
, working and protection transmitters (TXW, TX-P)
2
b
,
2
c
, respectively, which send exactly identical signals, and working and protection receivers (RX-W, RX-P)
2
d
,
2
e
, respectively, which receive exactly identical signals.
The working transmitter
1
b
of terminal station
1
is connected to the working receiver
2
d
of the terminal station
2
via the working uplink line
3
a
, and the protection transmi
Fujitsu Limited
Kizou Hassan
Staas & Halsey , LLP
Stevens Roberta
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