Switched permanent virtual circuit
A recent enhancement to the NNI is the switched (aka soft) permanent virtual circuit (SPVC). SPVCs, along with SVCs, are documented in FRF.10, the NNI SVC Implementation Agreement.
SPVCs are designed to minimize the burden of building multisegment PVCs for which the various networks must administer and coordinate parameters, such as the originating and terminating DLCI, Bc, Be, maximum frame size, etc. In a PVC environment these parameters must be manually configured and coordinated between the networks involved.
With SPVCs, the networks use SVC signaling to dynamically establish virtual circuit segments at the NNI, negating the need for manual intervention and coordination. Increased reliability can also be a benefit of SPVCs. Should an outage occur resulting in the SVC portion of the SPVC being cleared, the calling endpoint will attempt to reestablish the connection. As the network(s) process the “new” call request, malfunctioning links and equipment will be avoided, thereby yielding a new connection that bypasses the problem that caused the first connection to be cleared.
As shown on the visual, the customer views the service as a PVC offering. The CPE uses PVC management protocols, such as ANSI T1.617 Annex D, and does not necessarily support Q.933 SVC signaling. The service provider configures calling and called endpoint entities within the networks that act as proxy SVC signaling entities on behalf of the customer. When the network senses that the customer’s access link is up (via PVC management polling), the calling endpoint will attempt to establish a connection to the called endpoint using SVC signaling. Multiple NNIs may be crossed in the establishment of this connection. The calling and called endpoints are preconfigured by the service provider with the required call establishment parameters such as E.164/X.121 addresses, CIR, maximum frame size, etc.
The calling endpoint determines the DLCI value that will be used at the called end through the use of the Called Party SPVC Information Element, which is part of the call request (SETUP) message. This Information Element can specify the use of a specific DLCI, any DLCI, or a specific SPVC correlator. The specific SPVC correlator allows the called endpoint to choose a DLCI based on information found in the User-to-User Information Element, which is included in the SETUP message. Use of the SPVC correlator requires bilateral agreement between the calling and called endpoints.