Philipp Hofmann

E-MAC: Self-Organizing 802.11-Compatible MAC with Elastic Real-time Scheduling

To overcome the lack of strict QoS guarantees in existing hotspots, in this paper we present a system for realtime traffic support in 802.11 networks that works in either infrastructure or ad-hoc mode. The proposed mechanism, called elastic MAC (E-MAC) protocol, helps stations with real-time traffic to organize and establish a transmission schedule in a distributed manner, while coexisting with standard 802.11 stations. This distributed scheduling guarantees very short delays and a minimum reserved data rate for real-time stations, while protecting best-effort 802.11 traffic from starvation. Another feature of our mechanism is time-slot reuse, which improves the network efficiency by allowing other real-time stations to take over unused slots (e.g., in case of using voice codecs with silence suppression). We evaluate the performance of our system using a testbed implementation, ns-2 simulations and a mathematical model, and show how it outperforms other QoS schemes (e.g., 802.11e) in terms of throughput, delay, and jitter.

Performance impact of multihop handovers in an IP-based multihop radio access network

This paper presents and evaluates an IPv6-based protocol for the interconnection of ad hoc networks to the Internet. Its main novel feature is the support for different types of multihop handovers between two access routers. This topic represents an essential functionality of multihop radio access networks, but it has not been investigated so far in the context of a testbed. We measure and analyze the performance of different handover schemes in a testbed with IEEE 802.11b and ad hoc routing. We find that a proactive access router discovery scheme yields the lowest handover rate in our setup. Furthermore, it is advantageous to introduce optimizing multihop handovers, which gives mobile devices the opportunity to choose a new, better suited access router in case the hop distance to its current access router increases. Although this additional handover type increases the total number of handovers, it improves the system behavior in terms of handover delay and packet delivery fraction.

E-MAC: an elastic MAC layer for IEEE 802.11 networks

We present a system for real-time traffic support in infrastructure and ad hoc IEEE 802.11 networks. The proposed elastic MAC (E-MAC) protocol provides a distributed transmission schedule for stations with real-time traffic requirements, while allowing a seamless coexistence with standard IEEE 802.11 clients, protecting best-effort 802.11 traffic from starvation by means of admission control policies. Our scheduling decisions are based on an ‘elastic’ transmission opportunity (TXOP) assignment which allows for efficient wireless resource usage: whenever a real-time station does not use the assigned TXOP, the other real-time stations can take over the unused access opportunity, thus preventing the well-known inefficiencies of static time division multiple access (TDMA) schemes. Unlike other TDMA-based solutions for 802.11, E-MAC does not require a tight synchronization among the participating clients, thus allowing its implementation on commodity WLAN hardware via minor software changes at the client side, and no changes at the access points (APs). We studied the performance of our mechanism via ns-2 simulations and a mathematical model, showing that it outperforms IEEE 802.11e in terms of throughput, delay, and jitter. We finally provide a proof of concept through the results obtained in a real testbed where we implemented the E-MAC protocol.

Performance Impact of Mobility in an Emulated IP-based Multihop Radio Access Network

This paper investigates the performance of a multihop radio access network. In our testbed, nodes communicate to one access point using IEEE 802.11b and AODV routing. We measure the average packet delay and delivery ratio, if the node movement is emulated employing the random waypoint and random direction model, respectively. We find that random waypoint mobility yields up to 100% better results. This shows that the testbed performance is highly sensitive to the mobility model, even if comparable mobility behavior is assumed.