Mobile Header Compressor/Decompressor

Description

The Mobile Header Compressor/Decompressor (M-HCD) represents the unified functional entity within the User Equipment's Packet Data Convergence Protocol (PDCP) layer responsible for all header compression and decompression operations. In practice, it encompasses the separate M-HC (compressor) and M-HD (decompressor) functions into a single logical block for implementation efficiency. For uplink traffic, the M-HCD acts as the compressor (M-HC), applying algorithms like Robust Header Compression (ROHC) to IP, UDP, and RTP headers before packets are sent over the radio interface to the network. It maintains compression contexts, tracks packet flows, and uses state machines to ensure robust operation even in lossy radio conditions. For downlink traffic, it acts as the decompressor (M-HD), receiving compressed packets from the network-side compressor (U-HC), using the shared context to reconstruct the full original headers before passing the packets up the protocol stack to the IP layer. The M-HCD manages synchronization between its compressor and decompressor sides, handles feedback messages (if supported by the profile), and executes error recovery procedures when context damage or desynchronization is detected. Its integration into PDCP allows it to leverage security functions (like ciphering and integrity protection) and radio bearer management, ensuring compressed packets are correctly associated with their respective QoS flows and security contexts.

Purpose & Motivation

The M-HCD concept was standardized to provide a complete, bidirectional header processing solution within the UE, addressing the need for efficient IP packet handling in both transmission and reception directions over the radio link. Prior to header compression, symmetric real-time services like VoIP required full headers in both uplink and downlink, doubling the bandwidth waste. By integrating compression and decompression into a single entity, 3GPP ensured a coherent implementation that could manage the stateful contexts required for ROHC in both directions simultaneously. This solved the problem of inconsistent implementation and potential interoperability issues between separate compressor and decompressor modules. It also simplified the UE design and testing, as the M-HCD could be treated as a unified protocol feature within PDCP. The creation of the M-HCD was motivated by the all-IP vision of 3GPP networks, where maximizing spectral efficiency for packet-switched voice, video, and data was paramount for commercial viability and user experience.