We consider a system of several collocated nodes sharing a time slotted wireless channel, and seek a MAC (medium access control) that (i) provides low mean delay, (ii) has distributed control (i.e., there is no central scheduler), and (iii) does not require explicit exchange of state information or control signals. The design of such MAC protocols must keep in mind the need for contention access at light traffic, and scheduled access in heavy traffic, leading to the long-standing interest in hybrid, adaptive MACs. Working in the discrete time setting, for the distributed MAC design, we consider a practical information structure where each node has local information and some common information obtained from overhearing. In this setting, "ZMAC" is an existing protocol that is hybrid and adaptive. We approach the problem via two steps (1) We show that it is sufficient for the policy to be "greedy" and "exhaustive". Limiting the policy to this class reduces the problem to obtaining a queue switching policy at queue emptiness instants. (2) Formulating the delay optimal scheduling as a POMDP (partially observed Markov decision process), we show that the optimal switching rule is Stochastic Largest Queue (SLQ). Using this theory as the basis, we then develop a practical distributed scheduler, QZMAC, which is also tunable. We implement QZMAC on standard off-the-shelf TelosB motes and also use simulations to compare QZMAC with the full-knowledge centralized scheduler, and with ZMAC. We use our implementation to study the impact of false detection while overhearing the common information, and the efficiency of QZMAC. Our simulation results show that the mean delay with QZMAC is close that of the full-knowledge centralized scheduler.

翻译：我们考虑一个由多个共置节点共享时隙无线信道的系统，旨在设计一种MAC（介质访问控制）协议，要求其满足：(i) 提供低平均延迟，(ii) 采用分布式控制（即无集中调度器），(iii) 无需显式交换状态信息或控制信号。此类MAC协议的设计需兼顾轻流量下的竞争接入与重流量下的调度接入需求，因而催生了长期受关注的混合自适应MAC方案。在离散时间框架下，我们针对分布式MAC设计，考虑一种实际信息结构：每个节点拥有本地信息以及通过窃听获得的公共信息。在此背景下，现有协议"ZMAC"即属于混合自适应协议。我们通过两步法解决该问题：(1) 证明策略只需满足"贪婪性"与"穷尽性"即可。将策略限定为此类可简化为在队列空置时刻执行队列切换策略的问题。(2) 将延迟最优调度问题建模为POMDP（部分可观测马尔可夫决策过程），并证明最优切换规则为"随机最大队列"（SLQ）。基于该理论，我们进一步开发了可调优的实用分布式调度器QZMAC。我们在标准商用TelosB节点上实现QZMAC，并通过仿真将其与全知集中式调度器及ZMAC进行对比。利用实际实现，我们研究了窃听公共信息时错误检测的影响以及QZMAC的效率。仿真结果表明，QZMAC的平均延迟与全知集中式调度器接近。