We propose a process algebra for link layer protocols, featuring a unique mechanism for modelling frame collisions. We also formalise suitable liveness properties for link layer protocols specified in this framework. To show applicability we model and analyse two versions of the Carrier-Sense Multiple Access with Collision Avoidance (CSMA CA) protocol. Our analysis confirms the hidden station problem for the version without virtual carrier sensing. However, we show that the version with virtual carrier sensing not only overcomes this problem, but also the exposed station problem with probability 1. Yet the protocol cannot guarantee packet delivery, not even with probability 1.
There are far fewer formal analyses techniques available when it comes to CSMA CA (with and without virtual medium sensing). Traditional approaches to the analysis of network protocols are simulation and test-bed experiments. This is also the case for CSMA CA (e.g. @cite_11 ). While these are important and valid methods for protocol evaluation, in particular for quantitative performance evaluation, they have limitations in regards to the evaluation of basic protocol correctness properties.
@cite_11
2001480730 2154233760 2144812583 2166319816
CSMA with Enhanced Collision Avoidance (CSMA ECA) uses a deterministic backoff after successful transmissions to significantly reduce the number of collisions. This paper assesses by means of simulations the throughput and conditional collision probability obtained from a single-hop ad-hoc network using CSMA ECA. A comparison with the legacy CSMA CA reveals that the proposed protocol outperforms the legacy one in all considered scenarios. Specifically, it is shown that CSMA ECA presents advantages for both rigid and elastic flows. In this paper, the performance of the ALOHA and CSMA MAC protocols are analyzed in spatially distributed wireless networks. The main system objective is correct reception of packets, and thus the analysis is performed in terms of outage probability. In our network model, packets belonging to specific transmitters arrive randomly in space and time according to a 3-D Poisson point process, and are then transmitted to their intended destinations using a fully-distributed MAC protocol. A packet transmission is considered successful if the received SINR is above a predefined threshold for the duration of the packet. Accurate bounds on the outage probabilities are derived as a function of the transmitter density, the number of backoffs and retransmissions, and in the case of CSMA, also the sensing threshold. The analytical expressions are validated with simulation results. For continuous-time transmissions, CSMA with receiver sensing (which involves adding a feedback channel to the conventional CSMA protocol) is shown to yield the best performance. Moreover, the sensing threshold of CSMA is optimized. It is shown that introducing sensing for lower densities (i.e., in sparse networks) is not beneficial, while for higher densities (i.e., in dense networks), using an optimized sensing threshold provides significant gain. It is known that CSMA CA channel access schemes are not well suited to meet the high traffic demand of wireless mesh networks. One possible way to increase traffic carrying capacity is to use a spatial TDMA (STDMA) approach in conjunction with the physical interference model, which allows more aggressive scheduling than the protocol interference model on which CSMA CA is based. While an efficient centralized solution for STDMA with physical interference has been recently proposed, no satisfactory distributed approaches have been introduced so far. In this paper, we first prove that no localized distributed algorithm can solve the problem of building a feasible schedule under the physical interference model. Motivated by this, we design a global primitive, called SCREAM, which is used to verify the feasibility of a schedule during an iterative distributed scheduling procedure. Based on this primitive, we present two distributed protocols for efficient, distributed scheduling under the physical interference model, and we prove an approximation bound for one of the protocols. We also present extensive packet-level simulation results, which show that our protocols achieve schedule lengths very close to those of the centralized algorithm and have running times that are practical for mesh networks. Decentralized medium access control schemes for wireless networks based on CSMA CA, such as the IEEE 802.11 protocol, are known to be unfair. In multihop networks, they can even favor some links to such an extent that the others suffer from virtually complete starvation. This observation has been reported in quite a few works, but the factors causing it are still not well understood. We find that the capture effect and the relative values of the receive and carrier sensing ranges play a crucial role in the performance of these protocols. Using a simple Markovian model, we show that an idealized CSMA CA protocol suffers from starvation when the receiving and sensing ranges are equal, but quite surprisingly that this unfairness is reduced or even disappears when these two ranges are sufficiently different. We also show that starvation has a positive counterpart, namely organization. When its access intensity is large the protocol organizes the transmissions in space in such a way that it maximizes the number of concurrent successful transmissions. We obtain exact formula for the so-called spatial reuse of the protocol on large line networks.