無線網路環境下利用ROHC壓縮網路語音效能之研究

Abstract

網路電話（Voice over Internet Protocol，簡稱VoIP）是透過即時傳輸協定（Real-time Transport Protocol，簡稱RTP）及UDP(User Datagram Protocol)通訊協定將語音封裝成封包，再利用IP(Internet Protocol)網路來傳輸語音的技術。在802.11b無線網路環境下，語音封包標頭是由連結層標頭(MAC)及錯誤檢查碼、IP標頭、UDP標頭及RTP標頭所組成。以G.711為例，使用IPv4(Internet Protocol version 4)標頭格式的語音封包有234位元組，其中標頭欄位就佔74位元組(31%)，而使用IPv6(Internet Protocol version6)標頭格式的語音封包有254位元組，標頭欄位佔了94位元組(37%)，可知標頭若不經過壓縮，很容易佔用有限的無線網路傳輸頻寬，造成不必要的浪費。本論文研究如何在802.11b無線網路環境下，使用ROHC(Robust Header Compression)來壓縮語音封包的IPv4或IPv6封包標頭，並分析比較壓縮前後語音傳輸的效能。A Voice over Internet Protocol (VoIP) packet is encapsulated by Internet Protocol (IP), User Datagram Protocol (UDP) and Real-time Transport Protocol (RTP) headers. In IEEE 802.11b wireless environments, the 802.11 data-link layer (i.e. Media Access Control) header and a checksum field should be added to the VoIP packet. Take G.711 as an example, the total length of IPv4 voice packet is 234 bytes, including 74 bytes (31%) header fields, and the total length of IPv6 voice packet is 254 bytes, including 94 bytes (37%) header fields. The header overhead occupies more than 30% wireless bandwidth. To reduce header overhead of the IPv4 or IPv6 voice packets, we designs a VoIP compression method which integrates Robust Header Compression (ROHC) with Session Initiation Protocol (SIP). Then, we evaluate the improvement of the proposed method compared to the original header format for both IPv4 and IPv6.