Mobile Header Compressor

Description

The Mobile Header Compressor (M-HC) is a functional entity implemented within the Packet Data Convergence Protocol (PDCP) layer of the User Equipment (UE) in 3GPP UMTS and LTE networks. Its primary role is to apply header compression algorithms to uplink IP data packets before they are transmitted over the radio interface. The M-HC operates on a per-radio bearer basis, compressing the often large and repetitive headers of protocols like IPv4, IPv6, UDP, and RTP. By analyzing the static and dynamic fields within these headers, it replaces the full header with a much smaller context identifier and compressed header information for subsequent packets of the same flow. This process is governed by state machines defined in specifications like RFC 2507, RFC 3095 (ROHC), and RFC 5795, which the M-HC implements to maintain synchronization with the corresponding decompressor in the network (U-HD). The compressor maintains a compression context containing the static fields of the original header and the dynamic fields from the last transmitted packet. For each new packet, it compares the header to this context, generates a compressed packet containing only the changes (deltas) or a context identifier, and updates its internal state accordingly. The M-HC is a critical component for enabling efficient voice over IP (VoIP), video streaming, and other interactive services over cellular networks, as it directly mitigates the bandwidth constraints and packet overhead inherent in wireless links.

Purpose & Motivation

The M-HC was introduced to address the significant inefficiency of transmitting full IP/UDP/RTP headers over bandwidth-constrained and costly radio resources in 3G UMTS networks. Early IP-based services, especially VoIP, suffered from poor spectral efficiency because the payload (e.g., a voice frame) could be smaller than the 40-60 byte headers required. This overhead drastically reduced the number of simultaneous users a cell could support and increased latency. The creation of the M-HC, alongside its network-side counterpart, provided a standardized method for compressing these headers to as little as 1-4 bytes for subsequent packets in a flow, dramatically improving capacity. It solved the problem of enabling cost-effective, high-quality packet-switched voice and real-time multimedia over 3GPP systems, which was essential for competing with circuit-switched services and enabling the evolution to all-IP networks. Its integration into the PDCP layer ensured it was tightly coupled with radio bearer management and security functions.

Key Features

• Resides in the UE's PDCP layer for uplink processing
• Implements Robust Header Compression (ROHC) framework and profiles
• Maintains compression contexts per radio bearer and flow
• Reduces IP/UDP/RTP headers from ~40-60 bytes to 1-4 bytes for subsequent packets
• Operates with state machines (e.g., IR, FO, SO states) for robustness
• Synchronizes with the U-HD decompressor in the network using feedback channels

Evolution Across Releases

Defining Specifications