Delay optimal queue-based CSMA

Abstract

In the past year or so, an exciting progress has led to throughput optimal design of CSMA-based algorithms for wireless networks. However, such an algorithm suffers from very poor delay performance. A recent work suggests that it is impossible to design a CSMA-like simple algorithm that is throughput optimal and induces low delay for any wireless network. However, wireless networks arising in practice are formed by nodes placed, possibly arbitrarily, in some geographic area. In this paper, we propose a CSMA algorithm with per-node average-delay bounded by a constant, independent of the network size, when the network has geometry (precisely, polynomial growth structure) that is present in any practical wireless network. Two novel features of our algorithm, crucial for its performance, are (a) choice of access probabilities as an appropriate function of queue-sizes, and (b) use of local network topological structures. Essentially, our algorithm is a queue-based CSMA with a minor difference that at each time instance a very small fraction of frozen nodes do not execute CSMA. Somewhat surprisingly, appropriate selection of such frozen nodes, in a distributed manner, lead to the delay optimal performance.

Improving the delay performance of CSMA algorithms: A Virtual Multi-Channel approach

“…From equations (9) and (10), we know that the numerator on the left hand side of equation (8) is equal to…”

“…However, it is difficult to modify the algorithm to optimize global system utility. In [9], the authors propose to effectively partition the network into (random) pieces with finite size and run CSMA in each piece. Hence, the expected packet delay can be bounded by a function of the size of each piece and is independent of the network size.…”

Effect of network density and size on the short-term fairness performance of CSMA systems

“…Degradation in the short-term fairness as the throughput optimality is achieved is investigated in [4]. Several bounds for delay are proposed [5,[15][16][17][18] and methods for minimizing the delay are devised [19][20][21]. To reduce delay, appropriate selection of the rate adaptation function is also investigated [22][23][24].…”

Achievable delay performance in CSMA networks

“…Unfortunately, simulation experiments demonstrate that such activity factors can induce excessive backlogs and delays, which has triggered a strong interest in developing approaches for improving the delay performance [16], [18], [20]. Motivated by this issue, Ghaderi & Srikant [9] recently showed that it is in fact sufficient for the logarithms of the activity factors to behave as logarithmic functions of the backlogs, divided by an arbitrarily slowly increasing, unbounded function.…”