Resource management and transport layer

The main challenges to be faced by transport protocols using a satellite link are the variability of the channel, due to weather conditions and the large propagation delay. Adaptive network management and control algorithms are therefore desirable to guarantee the Quality of Service (QoS) to data flows over Additive White Gaussian Noise (AWGN) channels (a baseline assumption in the following analysis). Many popular Internet applications including email, file transfer, remote access, and Web browsing require a reliable data delivery service. End-to-end reliability for Internet traffic is guaranteed by the Transmission Control Protocol (TCP) at the transport layer. TCP specification covers a wide family of implementations, some of them having traditionally very poor performance over satellite links [1],[2]. Furthermore, TCP performance actually depends on the adopted Radio Resource Management (RRM) techniques. The DVB-RCS standard [3],[4] (see Scenario 2 for GEO-based communications, detailed in Chapter 1, Section 1.4) defines a set of Demand Assignment Multiple Access (DAMA) schemes, based on control loops with time constants similar to those used by TCP. Specific design choices and circumstances can lead to unwanted interactions between the link layer, implementing DAMA, and TCP, with a significant degradation of the overall performance. In particular, the so-called access delay (DAMA loop) becomes an important factor that affects TCP performance (see the following Section 9.4 for further details). The variability of the satellite channel conditions also impacts Internet flows that do not utilize TCP (e.g., streaming multimedia utilizing User Datagram Protocol, UDP). This traffic typically is less demanding in terms of reliability, but more demanding in terms of jitter. Loss and/or corruption of multimedia packet payloads can be efficiently handled by the multimedia codec at the receiver and does not imply the need for retransmissions (as in TCP). The effect of propagation delay is also much less important, except for interactive services requiring low round trip delay (e.g., Internet gaming, which is not suited for satellite use). Adaptive RRM procedures can be optimized for multimedia traffic. In particular, cross-layer optimization approaches are becoming widespread for wireless networks in general, and their application to satellite links needs to be studied both from a performance and an architectural viewpoint. This Chapter addresses the design and the implementation of cross-layer interactions between RRM (layer 2) and transport layer (layer 4) protocols in satellite environments. In particular, Section 9.2 offers an overview of TCP over GEO satellite links; Section 9.3 proposes an interaction between TCP and lower layers aiming at maximizing the TCP connection throughput; Section 9.4 describes the design of a cross-layer interaction between TCP and Medium Access Control (MAC) layer; Section 9.5 focuses on UDP and the performance of multimedia applications over satellite links, and, finally, Section 9.6 provides conclusions.