Greger Wikstrand

Early multicast collision detection in CSMA/CA networks

Reliable multicast is especially difficult to achieve in IEEE 802.11 CSMA/CA networks when a large number of unicast stations are contending with the multicast transmissions. Motivated by capacity problems in a sports infotainment product the authors developed early multicast collision detection (EMCD) specifically tailored to accommodate a large number of legacy terminals in multiple overlapping cells. The algorithm is based on the fact that in the absence of hidden terminals two stations must start their transmissions simultaneously to cause a collision. A station using EMCD introduces an early pause in the multicast transmission, performs a clear channel assessment (CCA), and if another sender is detected, schedules a retransmission. We have evaluated our method using simulations. Results show that EMCD increases the multicast reliability even at relatively moderate network loads.

Success Factors for Team Project Courses

Team project courses are important elements in most computer science and software engineering programs. For many students, the team project course represents the only non-trivial software development experience before graduation. The team project should be used to introduce them to important project and process issues that otherwise are very difficult to teach. Sloppy documentation, poor project planning and tracking or ineffective communication will eventually affect the teams and teach the students a important lessons for their future work. In this paper, we investigate how students select, carry out and complete their projects. The results show that students tend to select mainstream projects with good specifications and that certain project types are less suitable for the course. The results also show that process related deliverables are crucial to the final outcome of the projects. Among the hardest, but also most important, deliverables we find the project plan

Football animations for mobile phones

Animations, based on a streamed model, are a cost and bandwidth efficient way to transmit data to an end-user on the mobile network. They might also be enlightening and enjoyable to view. An experiment was performed to contrast different animations and video codings in terms of their cognitive and emotional effectiveness for spectating a football game on the mobile phone. Analysis shows that different renderings of the same model give differential results for understandability and enjoyment of a game. The conclusion is that more advanced renderings might be able to give an individually optimized blend between emotional and cognitive effectiveness.

Prioritized Repeated Eliminations Multiple Access: A Novel Protocol for Wireless Networks

A new and simple MAC protocol is proposed. Each node transmits a burst with length sampled from a geometric distribution with parameter q followed by a carrier sense slot. A node repeats the previous operations until it senses a busy slot. A node will access the channel and transmit its payload when it has sensed h idle slots. The parameter q can be adjusted for each node to achieve desired levels of relative priority with little impact on channel utilization and without a need for explicit knowledge about other traffic. Comparison to existing tree-splitting, carrier sensing and bursting protocols through analysis and simulations shows that our algorithm scales very well to the number of nodes. It has very high success probability, channel utilization and fairness. Extensions are provided for hidden terminal scenarios and are shown in simulations to provide adequate performance.

A collision detection method for multicast transmissions in CSMA/CA networks

Compared to unicast traffic, multicast is not protected by any ARQ mechanism in 802.11 networks: collisions with other multicast and unicast transmissions are not detected and senders will not adap ...

Untruncated Eliminations in the EY-NPMA MAC Protocol: Performance and Optimality

We present an analytical performance model for the channel access control mechanism in Hiperlan/1, elimination yield non-preemptive priority multiple access (EY-NPMA), based on an assumption of untruncated eliminations. Because of that, the model is briefer and powerful approximations are available. It predicts performance and optimal parameters to within a few percent, even for the standard case with truncated eliminations. In other words, the truncation limit has little effect. Untruncated eliminations lead to improved performance for large number of nodes with high elimination bursting probability