Backpressure scheduling in IEEE 802.11 wireless mesh networks: Gap between theory and practice

Abstract

Achieving efficient bandwidth utilization in wireless networks requires solving two important problems: (1) which packets to send (i.e., packet scheduling) and (2) which links to concurrently activate (i.e., link scheduling). To address these scheduling problems, many algorithms have been proposed and their throughput optimality and stability are proven in theory. One of the most well-known scheduling algorithms is backpressure scheduling which performs both link and packet scheduling assuming a TDMA (Time Division Multiple Access) MAC (Medium Access Control) layer. However, there has been limited work on realizing backpressure scheduling with a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) MAC layer (e.g., IEEE 802.11). In IEEE 802.11 networks, it is expected that the throughput optimality will not be achieved. In this paper, we investigate the extent of this throughput gap between theoretical TDMA-based backpressure scheduling and an approximation of it for IEEE 802.11 WMNs (Wireless Mesh Networks). Through extensive testbed measurements, we verify that there is indeed a non-negligible throughput gap. We present two main reasons behind this gap: Control inaccuracy that results from approximation of link scheduling and information inaccuracy that results from late or incorrect information, for instance, about queue lengths or network topology. Our results show that losses by MAC-layer collisions and backoff, which mainly occur due to control inaccuracy plays a major role for the throughput gap. On the other hand, while losses by queue drops, typically due to information inaccuracy, do occur, their effect can be tolerated. Nevertheless, both types of inaccuracies need to be mitigated in order to improve throughput.

Analyzing Backpressure routing interaction with TCP for Wireless Mesh Networks

“…This is principally due to the differences between the Backpressure in theory and in practice [31]. This issue is the subject of our next section.…”

“…The "link scheduling" is the second stage of the classical Backpressure algorithms. It consists of determining the link activation sequence among multiple links composing the network [31]. Let consider the following first:…”

“…This requirement could be a prohibitive overhead for a large network [2]. We refer to [31] for an in-depth review of the subtle gap between Backpressure assumptions and the practical reality.…”

“…However, a large amount of these experiments are based upon simplified scenarios. The same propositions are unable to perform in a satisfactory manner when combined with a number of protocols that operate at adjacent layers [31]. TCP at transport layer, and Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) at Data Link Layer, are examples of such widely deployed complex mechanism with which the Backpressure-based routing protocols are incapable to interact in an effective way [24] [28] [22].…”

Lifetime-Aware Backpressure—A New Delay-Enhanced Backpressure-Based Routing Protocol

“…As stated in the original paper [3], the Backpressure algorithm assumes the existence of a central controller with a global view of the whole network, to perform complex computations at each time slot. Such requirements in addition to the computational complexity are too prohibitive in practice [4]. Moreover, this algorithm requires maintaining a queue for each potential destination at each node, which may limit its scalability to large networks due to the induced excessive overhead.…”