Mobile Header Decompressor

Description

The Mobile Header Decompressor (M-HD) is a functional entity within the Packet Data Convergence Protocol (PDCP) layer of the User Equipment (UE) in 3GPP systems. Its specific role is to process downlink packets received over the radio interface from the network-side header compressor (U-HC). These packets arrive with compressed headers, often reduced to a few bytes containing only differential changes or context identifiers. The M-HD uses a locally maintained decompression context, which must be synchronized with the compressor's context, to reconstruct the original full IP, UDP, RTP, and potentially other protocol headers. The decompression context contains the static fields and the last known dynamic field values for a given packet flow. Upon receiving a compressed packet, the M-HD interprets the header format, applies the delta updates or references the context, and rebuilds the complete header as it existed before compression. This process is governed by the same ROHC state machines (e.g., Initialization and Refresh, First Order, Second Order) as the compressor, ensuring robustness. The M-HD may also generate feedback packets (acknowledgments, negative acknowledgments, or static context updates) to send back to the U-HC to request retransmissions or signal context damage, depending on the ROHC profile and mode in use. Once decompression is successful, the packet with its full header is passed up to the IP layer in the UE for further processing. The M-HD is essential for maintaining the end-to-end transparency of IP-based services while leveraging the bandwidth savings of header compression on the downlink.

Purpose & Motivation

The M-HD was created to complement the uplink compressor (M-HC), enabling full bidirectional efficiency for header compression over the radio link. In early 3G packet data services, downlink traffic (e.g., web browsing, video streaming) often constituted the majority of bandwidth usage. Transmitting full headers for every downlink packet was a significant waste of scarce radio resources. The M-HD solved this by allowing the network (RNC in UMTS, eNB in LTE) to compress downlink headers, with the UE efficiently decompressing them. This was critical for making high-bandwidth services like video streaming and large file downloads economically feasible over cellular networks. It addressed the asymmetry of early mobile internet use, where downlink capacity was a key bottleneck. By standardizing the decompression function within the UE's PDCP layer, 3GPP ensured interoperability and consistent performance, allowing network operators to deploy header compression transparently, improving user data rates and network capacity without requiring changes to end-user applications.