Prepared Reconfiguration

Description

Prepared Reconfiguration is a state defined in 3GPP TS 25.423 (UTRAN Iur interface RNSAP) and TS 25.433 (UTRAN Iub interface NBAP) for the UMTS Terrestrial Radio Access Network (UTRAN). It represents an intermediate state in the Synchronised Radio Link Reconfiguration procedure, which is used to modify the configuration of one or more radio links in a Node B. When the Controlling RNC (CRNC) initiates a Synchronised Radio Link Reconfiguration Preparation, it sends a request to the Node B with the proposed new configuration parameters. If the Node B can accept the request, it reserves the necessary resources and enters the Prepared Reconfiguration state. The actual switch to the new configuration does not occur until a subsequent Commit procedure is executed, or the prepared state is discarded via a Cancellation procedure.

The Prepared Reconfiguration state is crucial for maintaining service continuity during reconfigurations. It allows the network to verify that the target Node B can support the requested configuration before tearing down the existing one. This is particularly important for hard handover and for reconfigurations that involve adding or modifying HS-DSCH (High-Speed Downlink Shared Channel) resources. The state persists even if the radio link object itself is removed, as long as the Node B Communication Context remains. This ensures that the prepared resources are not accidentally released and can still be committed or cancelled as needed.

From an architectural perspective, the Prepared Reconfiguration exists within the Node B's context for a specific UE. The CRNC manages the procedure, and the Node B maintains the prepared configuration until it receives a Commit or Cancellation request. The Synchronised Radio Link Reconfiguration Preparation message includes the necessary radio link parameters, such as channelization codes, power offsets, and transport format sets. The Node B checks these parameters against its capabilities and current load. If successful, it stores the prepared configuration and responds with a Preparation Complete message. The CRNC can then decide to commit the reconfiguration, which activates the new configuration, or cancel it, which releases the reserved resources.

The state machine for Prepared Reconfiguration is defined in the NBAP and RNSAP specifications. The Prepared Reconfiguration is created only after a successful Preparation procedure. It is destroyed only after a Commit or Cancellation procedure completes. This strict lifecycle ensures that the network can rely on the prepared resources being available until explicitly released. The state is also maintained across other procedures; for example, if a radio link is deleted while a Prepared Reconfiguration exists for that link, the prepared state remains if the Node B Communication Context is still active. This prevents race conditions and ensures that the reconfiguration can still be completed or aborted cleanly.

In practice, Prepared Reconfiguration is used extensively in UMTS networks for mobility and resource management. It enables synchronized reconfigurations that minimize disruption to the user's data flow. For HSDPA, it allows the addition or modification of HS-DSCH resources without dropping the connection. The procedure is also used for serving HS-DSCH cell change, where the prepared state ensures that the target cell can accept the UE before the source cell releases it. Overall, Prepared Reconfiguration is a fundamental mechanism for reliable and efficient radio resource management in UTRAN.

Purpose & Motivation

The Prepared Reconfiguration state was introduced to solve the problem of synchronizing radio link reconfigurations between the RNC and Node B in UMTS networks. Before its introduction, reconfigurations were performed in a single step, which could lead to service interruption if the Node B could not support the new configuration. By splitting the procedure into Preparation, Commit, and Cancellation phases, the network can first verify that the target configuration is feasible and reserve resources, and only then switch to it. This two-phase approach minimizes the risk of dropped calls and ensures that the network can gracefully handle failures during reconfiguration.

Another motivation was the need for atomic reconfigurations across multiple radio links. In soft handover, a UE may be connected to multiple Node Bs. Changing the configuration of all these links simultaneously requires coordination. The Prepared Reconfiguration allows the CRNC to prepare the new configuration on all involved Node Bs, and then commit it only when all have successfully prepared. If any Node B fails to prepare, the CRNC can cancel the procedure without affecting the existing configuration. This atomicity is essential for maintaining the integrity of the radio connection.

Furthermore, the Prepared Reconfiguration state provides a clear separation between resource reservation and resource activation. This is particularly important for HSDPA, where the addition of HS-DSCH resources involves complex allocation of codes, power, and HARQ processes. By preparing the reconfiguration, the Node B can ensure that the necessary resources are available and reserved, preventing other users from taking them. The Commit then activates the resources, and the Cancellation releases them if the reconfiguration is aborted. This mechanism has been a cornerstone of UMTS radio resource management since its introduction in Release 8 and continues to be used in later releases.