Network Header Decompressor

Description

The Network Header Decompressor (N-HD) is a functional entity within the UMTS Radio Access Network, implemented in the Packet Data Convergence Protocol (PDCP) layer of the Radio Network Controller (RNC). It operates on the uplink path, processing data packets received from the User Equipment (UE). The UE's header compressor (U-HC) sends packets with compressed headers over the radio interface to conserve uplink bandwidth. The N-HD receives these packets and performs the inverse operation: it uses the shared Robust Header Compression (ROHC) context, which it maintains in synchronization with the UE's compressor, to reconstruct the full original IP, UDP, RTP, or ESP headers. The decompression process involves interpreting the compressed header format, applying the context (which contains static field values and interpretation rules for changing fields), and outputting a packet identical to the one before compression at the UE. The N-HD is responsible for context management on the network side for the uplink direction, including context verification using checksums, handling context damage due to packet loss, and initiating repair procedures via ROHC feedback mechanisms. It must robustly handle out-of-order delivery and errors intrinsic to the wireless link. After successful decompression, the PDCP layer forwards the complete IP packet to the core network via the Iu-PS interface. The N-HD works in tandem with the N-HC (for the downlink) but operates independently, managing separate context state machines for each uplink flow from the UE.

Purpose & Motivation

The N-HD was created to enable efficient uplink transmission from mobile devices, which often have more limited transmit power and bandwidth allocation compared to the downlink. Just as downlink compression saves radio resources, uplink compression reduces the amount of data the UE must transmit, conserving its battery life and improving overall uplink capacity. The N-HD's role is critical because it allows the network to benefit from compression gains in both directions, making the radio resource savings symmetric. By placing the decompression complexity in the network (RNC) rather than the core network gateways, the system localizes the decompression function close to the radio interface, minimizing latency and simplifying the end-to-end architecture. This design ensures that the core network always receives standard, uncompressed IP packets, maintaining compatibility with existing IP infrastructure and simplifying charging, deep packet inspection, and security functions. The N-HD, as part of the ROHC framework, addresses the challenge of maintaining compression context synchronization over a lossy link, using robust algorithms to prevent context desynchronization, which could lead to persistent packet loss.

Key Features

• Located in the RNC's PDCP layer for uplink traffic decompression
• Reconstructs full IP headers from ROHC-compressed packets sent by the UE
• Maintains and synchronizes decompression contexts for each uplink flow from the UE
• Implements error detection and context repair mechanisms using ROHC feedback
• Handles packet reordering and loss resilience specific to the radio link
• Outputs standard IP packets to the core network, ensuring network transparency

Evolution Across Releases

Defining Specifications