Fair packet scheduling and bandwidth management in wireless networks

Our study focused on fair packet scheduling and bandwidth management in CSMA/CA based wireless networks. We address the fairness problem for MAC-layer links and study end-to-end service differentiation and rate assurance for multihop flows. Fine-level rate control, particularly meeting rate requirements and differentiating various types of end-to-end traffic, remains an open problem for multihop wireless networks. Traditionally, rate assurance in wired networks is achieved through resource reservation and admission control, which can be efficiently implemented since the bandwidth capacity of each communication link is known and the sender of a link has the information of all flows that compete for the bandwidth of the link. In a wireless network, however, the capacity of each wireless link can change unpredictably over time due to contention from nearby links and dynamic channel conditions. An end-to-end flow consumes available bandwidth not only at links on its route but also at all nearby contending links, which makes resource reservation extremely complicated. We propose a new adaptive rate control function based on two novel techniques, called proportional packet scheduling (PPS) and dynamic weight adaptation with floor and ceiling (DWA). PPS distributes channel bandwidth among MAC (one-hop) flows in proportion to their weights. DWA adapts flows' weight according to their rate requirements and priorities. End-to-end traffic is classified into two categories: best-effort flows and QoS flows with rate requirements. The QoS flows are assigned with different priorities. PPS and DWA together achieve three important objectives without resource reservation and admission control. First, when bandwidth contention arises, the rate requirements of the QoS flows are satisfied in the order of priorities. Second, beyond the rate requirements, the rest bandwidth is allocated to the flows in a differentiated manner, taking both bandwidth demand and priority into consideration. Third, no flow is starved and all bandwidth is effectively utilized. Another important problem is on how to achieve fairness for the MAC-layer links in multiple contending WLANs or multihop networks, where the carrier sensing range and the interference range are much larger than the transmission range. We demonstrate that CSMA/CA networks, including IEEE 802.11 networks, exhibit severe fairness problem in many scenarios. Most existing solutions require nodes to overhear transmissions made by contending nodes and, based on the overheard information, adjust local rates to achieve fairness among all contending links. Their underlying assumption is that transmissions made by contending nodes can be overheard. However, this assumption holds only when the transmission range is equal to the carrier sensing range, which is not true in most real networks. As our study reveals, the overhearing-based solutions, as well as several non-overhearing AIMD solutions, cannot achieve MAC-layer fairness in various settings. We propose a new rate control protocol, called PISD (Proportional Increase Synchronized multiplicative Decrease). Without relying on overhearing, it provides fairness in CSMA/CA networks, particularly IEEE 802.11 networks, by using only local information and performing localized operations. It combines several novel rate control mechanisms, including synchronized multiplicative decrease, proportional increase, and background transmission. PISD works precisely for scenarios where all MAC flows mutually contend but has limitations when applied to networks consisting multiple contention groups. We develop PISD further and propose two new schemes to overcome the limitations. We prove that flows' rates attained under the two new schemes approximate proportional fairness.