Patent Application
20080019264
The features and advantages of the invention will become more apparent from the following detailed description of the preferred embodiment(s) with reference to the attached figures, wherein:
It is noted that in the attached figures, like features bear similar labels.
Referring to
A near end box 100 services a bidirectional IMA virtual link 305 with a far end box (not shown). The IMA virtual link 305 includes an IMA group reception link 225 from the far end box to the NE box 100 and an IMA group transmission link 221 from the NE box 100 towards the far end box. Although not shown in
To service the IMA virtual link 305, the near end box 100 comprises a first line card 120 having a first IMA state machine 122, and a second line card 110 having a second IMA state machine 112. In a redundancy protection state, one of the line cards 120, 110 and its respective IMA state machine 122, 112 will be active and designated as the working line card and the working IMA state machine respectively, while the other line card 120, 110 and its respective IMA state machine 122, 112 will be inactive and designated as the protection line card and the protection IMA state machine respectively.
In
The second line card 110 is a protection line card 110 and the second IMA state machine 112 is a protection IMA state machine 112. The protection line card 110 and the protection IMA state machine 112 are coupled to the far end box via a protection IMA group reception link 117 which is coupled to a second output of the IMA reception splitter 126. Each of the working IMA group reception link 127 and the protection IMA group reception link 117 has a copy of signals incoming over the IMA group reception link 225 which were split by the IMA reception splitter 126. The working IMA state machine 122 and the protection IMA state machine 112 are coupled to each other by an IMA synchronization link 111.
A bidirectional OC3 link 307 is coupled via an OC3 reception link 121 to an input of an OC3 reception splitter 124. A first output of the OC3 reception splitter 124 is coupled to a working OC3 input link 123 which in turn is coupled to the working IMA state machine 122 of the working line card 120. A second output of the OC3 reception splitter 124 is coupled to a protection OC3 input link 113 which is coupled to the protection IMA state machine 112 of the protection line card 110.
The working IMA state machine 122 is coupled to a working OC3 output link 125, while the protection IMA state machine 112 is coupled to a protection OC3 output link 115, and a protection IMA output link 113b.
While functioning in a redundant protection configuration, the protection OC3 input link 113 is coupled to the protection line card 110 at the protection IMA state machine 112, which sends IMA traffic over protection IMA output link 113b. The protection IMA output link 113b terminates 114 in the protection line card 110, while the protection OC3 output link 115 also terminates 118 in the protection line card 110.
Referring now also to
For a hot redundant switch to succeed with minimal data loss and interruption as possible, ideally the IMA state machine at the far end will have no indication of the switchover, nor detect any change in state of the IMA state machine of the near end. This transparency can be obtained by synchronizing the state of the protection IMA state machine 112 with the state of the working IMA state machine. In other words, the state of the protection IMA state machine 112 is made to be a duplicate of the state of the working IMA state machine 122 before a hot redundant switch is executed. Synchronization is facilitated by splitting the signals arriving over the IMA group reception link 225 and distributing them to both the working and protection IMA state machines 122, 112, splitting the signals arriving over OC3 reception link 121 and distributing them to both the working and protection IMA state machines 122, 112, and by the exchange of information over the IMA synchronization link 111. During a steady state before any redundant switch, the protection IMA state machine 112 is synchronized with the working IMA state machine and functions as if it were actually participating in the IMA virtual link.
During a steady state of operation in protection mode, the near end box 100 operates as depicted in
Each ICP cell 422 contains information for IMA group and link management. Synchronization involves ensuring that the states of each of the working and the protection IMA state machines 122, 112 are the same. This state information is present in each ICP cell sent from the near end to the far end. To ensure a smooth switchover, the ICP cells received by the far end IMA state machine should have continuity of the state information they contain before and after the switchover. In
Values of the fields of the ICP cell 422 depicted in
For the purposes of synchronization for a hot redundant switch, the Link ID, IMA ID, and ICP cell offset for each link of the IMA group are easily coordinated between the working IMA state machine 122 and the protection IMA state machine 112 since in general they are static forced variables which can be set in both the working IMA state machine's 122 and the protection IMA state machine's 112 configuration. These values also can be reliably exchanged over IMA synchronization link 111. Due to their dynamic nature, the Frame sequence number and the SCCI are more challenging to synchronize.
In the receive direction, the protection IMA state machine 112 operates as though it were actually participating in the IMA virtual link except its transmission towards the far end and towards the OC3 link 307 get dropped. As such all of the information the working IMA state machine 122 is receiving in the ICP cells from the far end IMA state machine is also being received by the protection IMA state machine 112. These ICP cells contain values such as the frame sequence number and the SCCI of the far end IMA state machine. According to the IMA standard, to conserve processing resources, the ICP cell is only processed if the SCCI number changes or during a start-up of the IMA state machine. As such the protection IMA state machine will obtain the SCCI and frame sequence number from the first ICP cell received after startup, and obtain them again if the SCCI ever changes in any subsequent ICP cell. When an ICP cell is processed it is passed up into hardware/software. Since the IMA state machine of the far end is active and already in a session with the working IMA state machine 122, the Group Status and Control octet of the first ICP cell received by the protection IMA state machine 122 after start-up will indicate that the far end IMA state machine is “operational”. According to the standard start-up protocol of an IMA virtual link between two IMA state machines, both IMA state machines must participate in a handshake involving the request and acknowledgement of startup before they enter one of three working states, namely “blocked”, “insufficient-links”, and “operational”. Detection by a near end IMA state machine of “blocked”, “insufficient links”, or “operational” as a Group Status and Control field before actually performing the handshake with the far end IMA state machine would not be understood by the near end IMA state machine. This situation would not be resolved by a standard unmodified IMA state machine. According to a preferred embodiment of the invention, the protection IMA state machine 112 is a modified IMA state machine adapted to, upon detection of the first ICP cell received after startup with a Group Status and Control field of any of the three IMA working states, immediately switch itself to the appropriate IMA working state according to the state of the IMA virtual link. The SCCI and frame sequence numbers received by the protection IMA state machine 112 in the incoming ICP cell are stored and now that it is in a working state, on the receiving side, the protection IMA state machine 112 has been synchronized and can proceed in the event of a switchover, to function as expected in active mode, linked over IMA virtual link 305 with the far end IMA state machine.
In the transmit direction, the protection IMA state machine 112 does not have the frame sequence number or the SCCI being transmitted by the working IMA state machine 122. Since the SCCI remains constant until there is a change of state of the IMA state machine, the likelihood of the value of the SCCI of the working IMA state machine 122 to change while it is in transit to the protection IMA state machine 112 over the IMA synchronization link 111 is very rare. Hence, the SCCI value is simply transmitted by the working IMA state machine 122 to the protection IMA state machine 112 once, upon a startup of the protection IMA state machine 112, and again each time the SCCI value changes. The frame sequence number however can and does change very rapidly, the value when it is received by the protection IMA state machine 112 could be quite different from what it was when it was transmitted by the working IMA state machine 122. In order to ensure synchronization, the simultaneous or near simultaneous receipt of duplicate IMA frames by the working and protection IMA state machines 122, 112 over the working IMA reception link 127 and protection IMA reception link 117 respectively is utilized. It is safe to say that the frame sequence numbers of frames received by the working IMA state machine 122 and the protection IMA state machine 112 at the same time are the same. The working IMA state machine 122 determines a frame sequence number delta by calculating, upon receipt of an IMA frame over working IMA reception link 127, the frame sequence number of the IMA frame minus the frame sequence number of an IMA frame simultaneously being output from the working IMA state machine 122 over the working IMA transmission link 221. This frame sequence number delta is sent to the protection IMA state machine 112 over the IMA synchronization link 111. Upon receiving the frame sequence number delta from the working IMA state machine 122 the protection IMA state machine 112 calculates the frame sequence number to be output by subtracting the frame sequence number delta from the frame sequence number of a frame arriving at the protection IMA state machine 112 from the far end IMA state machine. Preferably, the frame sequence number delta is transmitted from the working IMA state machine 122 to the protection IMA state machine 112 once, upon startup of the protection IMA state machine 112, and transmitted again each time the value of the frame sequence number delta changes. In order to avoid retransmission of the frame sequence number delta in times of minor timing or frame number misalignment, the value is only retransmitted when an average value of the frame sequence number delta changes. If an event, such as a group enable/disable, causes the value of the frame sequence number delta to change, it is recalculated and retransmitted from the working IMA state machine 122 to the protection IMA state machine 112.
In some embodiments the SCCI value is repeatedly transmitted by the working IMA state machine 122 to the protection IMA state machine 112 on a timed periodic basis.
Since the states of the working IMA state machine 122 and the protection IMA state machine 112 are the same, and since the working OC3 reception link 123 and the protection OC3 reception link 113 are carrying the same OC3 data stream, and since the working IMA reception link 127 and the protection IMA reception link 117 are carrying the same IMA data stream, the hot redundant switch should not cause any interruption or change in state visible to the far end IMA state machine and hence would not cause it to reactivate or otherwise change its activity.
As shown in
Referring now to
The LCR +APS system of
During redundancy protection, the simplex IMA switch 244 is configured so that the working IMA group link 227 is active. While the first I/O card 222 is active, the second I/O card 212 is inactive and hence serves as a protection I/O card 212. When an APS switch occurs, the simplex IMA switch 244 switches to couple the second IMA group link 245 with the protection IMA group link 217. The second I/O card 212 would become the active and hence working I/O card 212 while the first I/O card 222 would become inactive and hence the protection I/O card 222.
Referring now to
In step 500 the working IMA state machine and the protection IMA state machine are preconfigured to have similar state variables as described above including LinkIDs, IMA group ID, and ICP cell offsets.
During the steady state of the system in which the working IMA state machine functions as an active IMA state machine in a session with the far end IMA state machine, duplicates of both the incoming IMA data stream over the DS1 links and the incoming ATM data stream over the OC3 link, are delivered to both the working IMA state machine and the protection IMA state machine in steps 510 and 520. Exposing both IMA state machines to the same input streams helps to ensure synchronicity for the hot redundant switch.
In step 530 upon a startup of the protection IMA state machine, it enters into a working state when it receives the first ICP cell from the far end IMA state machine, without the need for the activity startup handshake of the IMA standard.
Once the protection IMA state machine is in a working state, in step 540, the frame sequence number delta, and the SCCI are transmitted from the working IMA state machine to the protection IMA state machine. As described above the frame sequence number delta is used by the protection IMA state machine to choose its own frame sequence number for participating with the far end IMA state machine in the IMA virtual link.
The embodiments presented are exemplary only and persons skilled in the art would appreciate that variations to the embodiments described above may be made without departing from the spirit of the invention. The scope of the invention is solely defined by the appended claims.
