Design and implementation of CLASS: A Cross-Layer ASSociation scheme for wireless mesh networks

Abstract

This paper focuses on the design and implementation of CLASS, a Cross-Layer Association scheme for IEEE 802.11-based multi-hop wireless mesh networks. The widely-used association strategy in traditional IEEE 802.11 wireless LANs allows a Mobile Station (MS) to scan wireless access links and then associate with the Access Point (AP) that has the best Received Signal Strength Indication (RSSI) value. Unlike traditional wireless LANs, IEEE 802.11-based wireless mesh networks consist of a multi-hop wireless backhaul. As such, the performance experienced by an MS after association with a specific Mesh Access Point (MAP) depends heavily on the conditions of both the access link (e.g., traffic load of associated stations, the frame error rate between an MS and an MAP) and the mesh backhaul (e.g., end-to-end latency and asymmetric uplink/downlink transportation costs). That is, selecting the MAP that yields the “best” performance depends on several factors and cannot be determined solely on the RSSI of the MS-MAP access link. CLASS uses an end-to-end airtime cost metric to determine the MAP to which an MS should associate. The airtime cost metric is based on the IEEE 802.11s, and comprises the access link airtime cost and the backhaul airtime cost. The proposed association scheme considers the frame error rate for various packet sizes, the available bandwidth on the access link after the association of the new MS, and the asymmetric uplink and downlink transportation costs on the backhaul. All experimental results are based on actual Linux-base testbed implementation. We also implement a general Cross-Layer Service Middleware (CLSM) module that is used to monitor network conditions and gather relevant metrics and factor values. Experimental results show that the proposed association scheme is able to identify the MAP which yields the highest end-to-end network performance for the mobile stations after their associations.

Introduction

IEEE 802.11-based multi-hop wireless mesh networks (WMNs) comprise of Mesh Routers (MRs) that connect together using wireless radio links to form large-scale wireless broadband networks. Fig. 1 shows the typical architecture of 802.11-based wireless mesh networks. Mesh routers on the mesh backhaul communicate with each other and connect the client nodes (i.e., Mobile Stations (MSs)) to the mesh network, using 802.11a [1] and/or 802.11b/g [2] techniques. According to the functions of a mesh router, it can be a Mesh Access Point (MAP) providing Access Point (AP) services in addition to mesh services, a Mesh Point Portal (MPP) connecting a mesh network to external networks, or a Mesh Point (MP) only without AP and portal functions [3]. When an MPP connects the mesh network to the Internet, it is also called an Internet GateWay (IGW). Different from the traditional 802.11 WLANs, the backhaul of WMNs is fully wireless.

The IEEE 802.11 standard [4] leaves the association strategy open to the implementers. A widely-used association strategy in current implementations is to allow a Mobile Station (MS) to associate with the Access Point (AP) which has the best Received Signal Strength Indication (RSSI) value during its scanning. Previous works [5], [6], [7], [8] have revealed that such a strongest signal strength (SSS) [8] based association scheme is unable to provide MSs with the best network performance, and have tried to optimize the AP selection in WLANs based on the access link conditions. Recent research work [9] has argued that the backhaul transportation latency should also be considered for the association in WMNs, and has used the airtime cost defined in the 802.11s draft [3] to evaluate both the access link quality and the backhaul performance for the association. However, the traffic load of the candidate MRs, the effective access link bandwidth after the association of the new MS, the impact of various packet sizes, and the asymmetric backhaul transportation costs on uplink and downlink have not been considered in that backhaul-aware association scheme.

In this work, we propose a Cross-Layer Association scheme for wireless mesh networks, called CLASS. CLASS has two fundamental design and performance goals. The first goal is to identify the MAP which will yield the highest end-to-end throughput performance for a mobile station. The second goal is to achieve a total overall association latency that is tolerable to most user classes. The notion of tolerable latency is discussed in Section 5.4. The end-to-end airtime cost adapted from 802.11s [3] is used to determine the MAP to which the MS should associate, comprising the access link airtime cost and the backhaul airtime cost. The access link airtime cost is determined by the channel access overhead, protocol overhead, dominant packet size, frame error rate, and the expected available bandwidth after the new MS associates to the MAP. The expected available bandwidth is calculated based on both the current traffic load of an MAP and the expected traffic load generated by the new MS, which infers if the MAP will be saturated after accepting that MS. The backhaul airtime cost is a weighted average of the uplink backhaul airtime cost and the downlink backhaul airtime cost, depending on the application traffic pattern of the MS (e.g., dominant downlink traffic of Video-on-Demand or dominant uplink traffic of video surveillance).

Compared to the previous works [9], [10], [11], [12], [13], our proposed association scheme considers (1) the frame error rate and airtime cost for various packet size categories, (2) the impact of the traffic load generated by the new client on the saturation status and available bandwidth of the access link, and (3) the impact of the asymmetric backhaul uplink and downlink transportation costs on the end-to-end performance of the client. Previous works [9], [11], [12], [13] on association in WMNs are implemented and evaluated in simulations. In our work, the implementation of CLASS is based on the off-the-shelf Wi-Fi devices and the open-source Madwifi driver [14]. Each node (either an MR or an MS) in the WMN testbed is running a Cross-Layer Service Middleware (CLSM) module implemented in this work, which collects association-related metrics from the modified Madwifi driver and returns them to the association daemon running in the user space. Unlike the previous works, CLASS has no constraint with regard to the routing protocol used on the mesh backhaul, and is independent from the routing metric.

The remainder of this paper is organized as follows. Section 2 discusses the airtime cost introduced in the 802.11s draft and the related work. In Section 3, we describe the proposed association scheme. The implementation of CLASS is described in Section 4, followed by the performance evaluation in Section 5. Finally, we summarize our work in Section 6.