Services in wireless networks must be capable of receiving information about the network and adaptively tune their transport parameters to the underlying networking conditions and technologies. A central problem in wireless transports is obtaining reliable metrics of congestion. How can lower layers assist transports and what is the performance tradeoff with pure peer-to-peer end-to-end solutions? We design and evaluate a lower layer assistance architecture. We focus on adding minimal intelligence to lower layers, according to the end-to-end principle. We find that we can adequately solve the measurement problem by minimal medium access control (MAC) assistance and describe an architecture that can aid transports over wireless links. The MAC-assisted solution is scalable to large number of flows, where the performance of pure end-to-end transports deteriorates rapidly. An improvement factor of 30-50 is exhibited in our experiments. We argue that including this minimal additional functionality in the MAC is sufficient for transports. It is also necessary when compared with end-to-end techniques.

Bluetooth scatternets are formed by interconnecting Bluetooth piconets in an ad-hoc fashion and consists typically of the handheld electronic devices from one or more user(s). Thus, scatternets may be used to form and interconnect ad-hoc Bluetooth personal area networks (PANs). The inter-piconet Bluetooth units, i.e. the gateways interconnecting the piconets in a scatternet, need to time division multiplex their presence in each of their piconets. This requires an inter-piconet scheduling (IPS) algorithm, operating in each inter-piconet unit in order to efficiently coordinate its presence with other Bluetooth units. In this paper an IPS algorithm based on periodic rendezvous points is proposed and analyzed with simulations, The algorithm is called maximum distance rendezvous point (MDRP) and utilizes the Bluetooth SNIFF functionality to establish the periodic rendezvous points between gateways and their peer nodes. Simulation results show that TCP works well with MDRP, but the large round trip delays caused by the inter-piconet gateway nodes give a slowly growing flow control window for TCP. The latter will in particular have an impact on the TCP performance in scatternets hosting "thin" (embedded) clients with limited dynamic memory capacity.

Higher layer protocols in wireless networks need to dynamically adapt to observed network response. The common approach is that each session employs end-to-end monitoring to estimate quantities of interest, like delay, delay jitter and available bandwidth. A less conventional approach is to employ lower layer explicit feedback mechanisms in place or in aid of end-to-end efforts. Available bandwidth measurements are known to follow multi-modal distributions and therefore are especially difficult to measure and filter, even in wired networks. In 802.11-based multi-hop networks obtaining usable end-to-end measurements is questionable. They are affected by a combination of a large number of transient variables due to the virtual carrier sense, head of line problems on each link and mobility. Motivated by this, we are developing a network explicit feedback mechanism. Our study of this accurate network feedback architecture aids in the cost/benefit analysis of an important trade-off: deployment of network support mechanisms for transports and QoS, versus the simple, scalable and easily deployable end-to-end solution. We test our solution in: (i) multimedia adaptation and (ii) measurement based call admission. Loss rates of end-to-end adaptive video and audio connections have been more than 4 times higher than in the network feedback case. A simple call admission strategy has also proved very effective using the feedback. In our experiments it led the network to a maximal performance and stable operating point.

Higher layer protocols in wireless networks need to dynamically adapt to observed network response. A common approach is for each protocol to employ an end-to-end monitoring and measuring mechanism and estimate quantities of interest, usually related to delay, latency or bandwidth. A less conventional approach is to employ network layer feedback mechanisms in place or in aid of end-to-end efforts. In this paper we use an 802.11-based link throughput and permissible throughput measurement. In our experiments these source-destination pair permissible throughput measurements are propagated in a multi-hop network using the AODV algorithm. In this setting, we are investigating how source rate adaptive multimedia applications can make use of this network feedback. We find that the investigated network feedback approach provides a strong network control feature, an accurate and timely available measurement at the sources, while the overhead of supporting and maintaining it is minimal. Finally, we argue that network based feedback approaches are particularly promising for higher layer protocol support and soft-QoS support in wireless networks.