Understanding TDM CES Pseudowire Alarms

The CES pseudowires transport the alarm events detected on the service interface and the alarm events triggered on the PSN transport using the LRM bits in the pseudowire control word. The significance and the usage of the LRM bits are covered by RFC4553 for SAToP pseudowires and RFC5086 for CESoP pseudowires. The end-to-end alarm handling between two E4G units for SAToP and CESoP pseudowires are discussed below. The alarms generated and the alarm events detected in the CES pseudowires are logged.

CES Alarms in SAToP Pseudowires

TDM Service LOS Alarm

The Loss of Signal alarm event in the TDM service attached to the SAToP pseudowire is handled end-to-end as shown in the following figure.
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SAToP Alarm Handling: TDM Service LOS Alarm
../Graphics/EX_ports_TDMServiceLOSAlarm.svg
  1. The Loss of Signal alarm event from the TDM service is detected by the local end E4G node.
  2. The local end E4G node notifies the alarm condition to the remote end of the CES pseudowire by setting the L-bit in the TDM pseudowire control word.
  3. The remote E4G node, upon receiving the CES pseudowire with L-bit, ignores the TDM payload carried in the packet and plays out Alarm Indication Signal to the remote TDM CE node.
  4. The remote TDM CE node sends a response to the Alarm Indication Signal, which could be a specific pattern in case of unframed services, for example, an all ones pattern.
  5. The remote E4G node sends the alarm response with R-bit set, indicating the packet loss caused due to dropping of packets with L-bit set.
  6. The local E4G node receives the alarm response packets with R-bit set and forwards the alarm response data to the local TDM CE node.

TDM Service AIS Alarm

The Alarm Indication Signal alarm from the TDM service is not detected by the E4G switch. This alarm is carried transparently to the remote TDM CE node and the alarm response is carried back transparently to the local TDM CE node as pictured. The CES pseudowire control word is not updated to reflect the presence of this alarm condition.

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SAToP Alarm Handling: TDM Service AIS Alarm
../Graphics/EX_ports_TDMServiceAISAlarm.svg

PSN Loss of Packet State

The CES pseudowire packets carry the TDM service payload at a constant rate depending on the payload size. The replay of TDM service payload at the remote end of the CES pseudowire is done based on the sequence number in the CES pseudowire control word. Due to the variable nature of the packet switched network, the CES pseudowire streams get dropped in the intermediate nodes. Under this scenario, the remote end of the CES pseudowire is said to be in LOSY state. The LOSY state of the CES pseudowire is indicated to the peer by setting the R-bit in the CES pseudowire control word.

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SAToP Alarm Handling: PSN Loss of Packet State
../Graphics/EX_port_PSNLossOfOPacketState.svg

The R-bit in the control word is set on the CES pseudowire packets from remote E4G node in LOSY state to the local E4G node, regardless of the RAI pattern received from its local TDM CE node.

CES Alarms in CESoP Pseudowires

The handling of CES alarms in CESoP pseudowires are more involved due to association of one or more timeslots to a TDM service and hence multiple services originating from a single TDM port with disjoint timeslots. On alarm conditions, the configured trunk condition code for data channels is played out. For signaling channels, the configured seized code pattern is played out.

TDM Service LOS/LOF/AIS Alarm

The Loss of Signal, Loss of Frame and Alarm Indication Signal events in the TDM service attached to the CESoP pseudowire is handled end-to-end as shown in the following figure.
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CESoP Alarm Handling: TDM Service LOS/LOF/AIS Alarm
../Graphics/EX_ports_TDMServiceLOS_LOF_AIS_Alarm.svg

The alarm handling sequence is similar to SAToP pseudowires, with an exception that the alarm is indicated only on the specific TDM service bound to the CES pseudowires. For instance, if the TDM service has 10 timeslots bound to the CES pseudowire, the alarm is indicated by the remote E4G node by playing out the configurable trunk conditioning pattern on those 10 timeslots in the TDM service. If signaling multiframe mode is configured on the TDM port, the configurable seized code pattern is played on the signaling bits.

TDM Service RAI Alarm

The CESoP pseudowires indicates the remote E4G node of the Remote Alarm Indication (or Remote Defect Identifier) alarms detected on the TDM service attached to local TDM CE node. The M-bit in the CES pseudowire control word is set to indicate the detected alarm. The remote E4G node sets the RAI indication on the TDM port attached to its local TDM CE node in addition to playing out the TDM payload received. The following figure shows the alarm handling sequence.

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CESoP Alarm Handling: TDM Service RAI Alarm
../Graphics/EX_ports_TDMServiceRAIAlarm.svg

PSN Loss of Packet State

The CESoP pseudowires handle the LOSY state due to loss of CES pseudowire packets in the PSN, in the similar way as handled by SAToP pseudowires. The configured trunk conditioning code and seized code is played on the timeslots connected to the TDM service instead of AIS. The following figure shows the alarm handling sequence.

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CESoP Alarm Handling: PSN Loss of Packet State
../Graphics/EX_port_PSNLossOfOPacketState2nd.svg