Structure agnostic TDM over PSN (SATop) emulates an end-to-end TDM circuit over MPLS. This feature creates a pseudowire between two end points over MPLS as defined in RFC 4447.

This procedure show how to establish the pseudowire between provider edge routers. The configuration assumes that you are running the ospfd, ldpd, mstpd, nsm, and imi daemons.

Verify on Rtr1 or Rtr3 that the LDP session is operating and that the PW is set-up successfully.

Once a pseudowire has been set up, the CE-bound IWF begins to receive SAToP packets and to store their payload in the jitter buffer but continues to transmit the “all ones” pattern to its TDM attachment circuit. This intermediate state persists until a preconfigured amount of TDM data (usually half of the jitter buffer) has been received in consecutive SAToP packets or until a preconfigured intermediate state timer (started when the PW setup is completed) expires.

If the CE-bound SAToP IWF detects the loss of a preconfigured number of consecutive packets or if the intermediate state timer expires before the required amount of TDM data has been received, it enters its packet loss state. While in this state, the local PSN-bound SAToP IWF should mark every packet it transmits with the R bit set. The CE-bound SAToP IWF leaves this state and transitions to the normal one once a preconfigured number of consecutive valid SAToP packets have been received. (Successfully reordered packets contribute to the count of consecutive packets.)

In addition to the packet loss state of the CE-bound SAToP IWF, it may detect the following defects:

Stray packets may be detected by the PSN and PW demultiplexing layers. When RTP is used, the SSRC field in the RTP header may be used for this purpose as well. Stray packets must be discarded by the CE-bound IWF, and their detection must not affect mechanisms for detection of packet loss.

Malformed packets are detected by mismatch between the expected packet size (taking the value of the L bit into account) and the actual packet size inferred from the PSN and PW demultiplexing layers. When RTP is used, lack of correspondence between the PT value and that allocated for this direction of the PW MAY also be used for this purpose. Malformed in-order packets MUST be discarded by the CE-bound IWF and replacement data generated as with lost packets.

Excessive packet loss rate is detected by computing the average packet loss rate over a configurable amount of times and comparing it with a preconfigured threshold. Buffer overrun is detected in the normal operation state when the jitter buffer of the CE-bound IWF cannot accommodate newly arrived SAToP packets.

Remote packet loss is indicated by reception of packets with their Rbit set.

See Figure 15-1.