TCP / IP Enhancements for Satellite Networks

ecently, the interest in broadband satellite networks has grown rapidly. Advances in transmission technology have led to the availability of low-cost earth terminals with interfaces to standard terrestrial networks. The relative ease of deployment and superior remote access capabilities of satellite networks are envisioned to provide bandwidth on demand to geographically diverse user groups. This is clearly evidenced by the ongoing development of several large-scale space-based networks, such as Teledesic, Galaxy/Spaceway, and Astrolink. Most notably, the Teledesic system aims to provide access rates of up to 2 Mb/s per user and 150 Mb/s per gateway [1]. Overall, these trends represent a significant departure from traditional fixed-circuit broadcast satellite systems (i.e., for telephony and television). The desire to support a diverse range of services in satellite networks implies that many features inherent in broadband networks, such as asyn-chronous transfer mode (ATM) and Internet Protocol (IP), will also emerge in satellite networks. These include packet switching, buffer management, and call admission control techniques (see [1, 2] for good overviews). Despite these emerging similarities with terrestrial networks, however, satellite networks still pose many additional challenges. A crucial requirement for any broadband network is the reliable transport of data traffic, namely non-real-time loss-sensitive flows (as opposed to real-time voice/video streams). Examples include e-mail and file transfers, Web messages, and facsimile transmissions. Since reliable transmission is mandatory , the ubiquitous Transmission Control Protocol (TCP)/IP has emerged as the workhorse protocol for most data applications. TCP uses a sliding window mechanism coupled with reactive congestion control techniques to gate source emissions. The protocol modulates a sender's transmission behavior contingent on returning acknowledgment (ACK) packets from the remote receiver's end, and two variants of TCP are in wide use today: Tahoe and Reno. A simple overview of a satellite TCP/IP network is shown in Fig. 1, where land-based networks are connected to satellite modems via interworking units (IWUs), such as the ATM satellite interworking unit (ASIU) [1]. These IWUs (possibly collocated with modems) perform protocol conversion between WAN protocols (e.g., IP, ATM) and satellite link-layer protocols. The latter are necessary due to satellite channel requirements, and most such protocols use time-division multiplexing (TDM) techniques, although frequency-division multiplexing (FDM) techniques and even hybrid schemes are possible [1]. To accommodate dynamic usage patterns, demand assignment approaches can be incorporated with various access schemes [2]. Most IWU systems use ATM and broad-band integrated services digital network (B-ISDN) interfaces, although Ethernet …