CRC Indicator

Description

The CRC Indicator (CRCI) is a critical control element within the Frame Protocol (FP) of the Iub and Iur interfaces in 3GPP UMTS/HSPA networks. It operates at the Data Link Layer (Layer 2) of the control plane, specifically within the transport channel processing chain between the Node B and the Radio Network Controller (RNC). The CRCI is embedded in the FP data frames that carry user data over the Iub (Node B-RNC) and Iur (RNC-RNC) interfaces, particularly for High-Speed Downlink Packet Access (HSDPA) and Enhanced Uplink (E-DCH) transport channels.

Architecturally, the CRCI is generated by the Node B after it receives and attempts to decode a transport block from the UE over the Uu air interface. The Node B performs a Cyclic Redundancy Check (CRC) on the received block. The result of this check—whether the block passed (CRC OK) or failed (CRC NOT OK)—is then encapsulated as the CRCI field within the corresponding FP data frame sent upstream to the RNC. This frame travels over the Iub interface via the Transport Network Layer, which typically uses ATM or IP transport. The RNC's MAC-hs or MAC-e/es entity extracts the CRCI to inform its Hybrid ARQ (HARQ) and radio link control (RLC) processes.

In operation, the CRCI directly influences the retransmission strategy. For HSDPA downlink, when the RNC receives an FP frame with CRCI indicating 'CRC OK', it considers the corresponding MAC-hs Protocol Data Unit (PDU) as successfully delivered to the UE and can acknowledge it to higher layers. If the CRCI indicates 'CRC NOT OK', the RNC knows the UE failed to decode the block. However, in HSDPA, the primary HARQ retransmissions occur between the Node B and UE. The RNC uses the negative CRCI primarily for statistics, radio link monitoring, and, in some configurations, to trigger outer-loop RLC retransmissions if the HARQ process at the Node B ultimately fails after its maximum attempts. Its role is more direct in the uplink for E-DCH, where the CRCI from the Node B informs the Serving RNC's (SRNC) selective combining and HARQ processes.

The CRCI's role is fundamental to the split-architecture of HSPA where HARQ is terminated at the Node B for low latency. It provides the RNC, which handles the RLC layer, with vital visibility into the performance at the air interface termination point without being in the direct HARQ loop. This enables coordinated layer 2 operation, efficient flow control on the Iub, and accurate radio quality measurements for functions like admission control and handover decisions. It is a key component in ensuring reliable data delivery while optimizing the use of both radio and transport network resources.

Purpose & Motivation

The CRC Indicator was introduced to address the specific challenges of the Iub/Iur interface architecture in UMTS and, crucially, to enable the high-performance operation of HSPA features. In pre-HSPA UMTS, all retransmission (RLC) and scheduling was centralized in the RNC. With the introduction of HSDPA in Release 5, HARQ and fast scheduling were moved to the Node B to reduce latency. This created a new problem: the RNC, still responsible for reliable end-to-end RLC delivery, was now blind to the success or failure of individual transmissions over the air interface, as these were handled locally by the Node B's HARQ entity.

Without the CRCI, the RNC would have no direct knowledge of whether a data block was successfully received by the UE. It would have to rely solely on RLC acknowledgments from the UE, which are slow, or on indirect indicators from the Node B. This would lead to inefficiencies. For example, the RNC might unnecessarily initiate an RLC retransmission for a block that the Node B's HARQ was still actively retrying, wasting radio and transport bandwidth. Conversely, it might not be aware of a persistent radio link failure in a timely manner. The CRCI solves this by providing a fast, in-band signaling mechanism that reports the result of the air interface CRC check directly to the RNC within the same user data flow.

This feedback mechanism was essential for enabling efficient flow control on the Iub interface, accurate congestion management, and effective radio quality monitoring. It allowed the RNC to perform selective processing—discarding or flagging corrupted data early in the chain—and to maintain accurate statistics for Operation and Maintenance (O&M). Fundamentally, the CRCI bridged the information gap created by decentralizing the HARQ function, ensuring that the higher-layer RLC protocol in the RNC could operate cohesively with the lower-layer HARQ in the Node B, which was critical for achieving the high throughput and low latency goals of HSPA.

Key Features

• In-band signaling of air interface CRC result within Iub/Iur FP frames
• Enables RNC awareness of Node B HARQ outcome for selective processing
• Supports efficient flow control and congestion management on Iub transport
• Provides critical input for outer-loop RLC retransmission decisions
• Facilitates accurate radio link quality measurement and monitoring
• Essential for HSDPA and E-DCH operation in split-architecture RAN

Evolution Across Releases

Defining Specifications