Abstract. This paper studies the impact of variable transmission delays on the Transmission Control Protocol (TCP). Sudden delay variations, which are not unemurnon in mobile networks, may degrade the performance since they may cause spurious TCP timeouts. The rnost irnportant pararneter in this context is the TCP retransmission timer. In this paper, we analyze TCP's round-trip time estimation for bulk data traffic over wireless links. The rnain contribution is a new analytical rnodel that accurately predicts the timeout duration from given network pararneters. As a first result, the rnodel shows that the round-trip time sampling rate has a significant impact on the timer characteristics. Therefore, the standardized estimation algorithm does not harmonize weil with timestamp-based rneasurement. Second, we quantify the risk of spurious TCP timeouts triggered by changing round-trip tirnes, in particular long off periods. We conclude that delay variations are only critical when they are on the order of seconds.
Mobility models are widely used in simulation-based performance analyses of mobile networks. However, there is a trade-off between simplicity and realistic movement patterns. Synthetic models like the random waypoint and random direction model are simple to implement, but only provide unrealistic simple user sojourn densities and traffic flows. In contrast, graph and trip-based mobility models are complex to parameterize and their results are difficult to compare. In this paper, we propose the location-dependent parameterization of the random direction model to fill this gap. This model extension allows to setup nonhomogeneous mobility scenarios, in particular based on realworld traces, while it still belongs to the class of synthetic random walk mobility models. We show that the location-dependent parametrization can accurately model arbitrary mobility patterns with very limited implementation complexity.
Many scenarios of beyond 3G mobile communications describe the integration of various access technologies into one system. Being always best connected under certain optimization criteria will be a crucial point for network operators and mobile users and requires network changes of mobile devices at runtime, the so-called vertical handovers. For those, bandwidth fluctuations up to the order of one or two magnitudes, e. g., when changing from an IEEE 802.11 to a GPRS System, have to be expected and applications have to cope with them in an user friendly way. Multi-modality and flexible data representations exploiting weights and semantic of transmitted data as means for making applications resource adaptive are currently under investigation. On top of that, device and system-wide adaptation control instances are needed to solve cross-layer and inter-application issues. This requires a rethinking of the classical communication paradigm of OSI-like protocol layering. With this article, an overview on adaptation in communications is presented and an experimental framework providing system support for application adaptation and adaptation control is introduced as part of that discussion.
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