Umeå universitets logga

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 adapt to the contention situation by backing off. This results in an unreliable service for multicast transmissions. We propose early multicast collision detection (EMCD), an algorithm with the purpose of increasing the reliability of multicast transmissions in the MAC layer of an IEEE 802.11 network. A multicast sender using it will introduce an early pause in a transmission, perform a clear channel assessment (CCA), and if a collision is detected abort the transmission after a fixed time and schedule a retransmission. This allows for detecting collisions with both multicast and unicast transmissions but also adapting to the contention situation. A probabilistic analysis is provided showing that EMCD is more efficient than ordinary multicast and can be made even more efficient by tuning parameters.

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.

We present a novel MAC scheme for solving the QoS parameter adjustment problem in IEEE 802.11e EDCA. The default values of the QoS parameters in EDCA, contention window (CW), arbitration Interframe Space (IFS) etc only yield good performance for few scenarios. We first propose a simple adaptation scheme, called a-EDCA, where the access point adapts the CWs based on the network conditions. The main contribution of this work is the second approach, called i-EDCA, where we make modifications to the backoff phase of EDCA and introduce a random IFS scheme. Simulation results show that both approaches result in stable capacity ratios between priority classes and very high channel utilization, compared to EDCA, when the number of stations is increased. Moreover, i-EDCA yields close to optimal channel utilization for a large number of scenarios without any need of adapting the parameters. In i-EDCA, the capacity for each class is directly proportional to its CW. Results also show that i-EDCA has improved fairness and prevents low priority classes to be starved under higher loads, a problem found in EDCA.

In this paper, we study the characteristics of two different backoff schemes: one that maximizes the channel utilization and one that maximizes the probability of a successful transmission. Our results indicate that while the latter provides slightly lower channel utilization, its shape is much less sensitive to the number of contending nodes. That is, the channel utilization is kept almost constant for a very wide range of node densities if the backoff distribution has increasing shape. This motivates us to propose a simple heuristic backoff scheme called the Truncated Geometric Backoff Distribution (TGBD). We provide simple analytical expressions for the probability of a successful transmission and the channel utilization. We also show that the TGBD can easily be extended to support service differentiation by adjusting the window lengths, and we provide a simple approximation that gives the relative share of the capacity for a node in a priority class compared to nodes in other classes. This extended backoff scheme easily outperforms the much more complex Quality of Service (QoS) standard, IEEE802.11e EDCA. Finally, a two-stage backoff model, based on the TGBD, is proposed that further increases the probability of a successful transmission. Results indicate that the channel utilization is almost independent of the number of contending nodes.