But there are drawbacks, "[D]epending on the PSM implementation strategies used by the clients/Access Points (APs), [then] the presence of competing background traffic results in one or more of the following negative consequences: a significant increase, up to 300%, in a client’s energy consumption, a decrease in wireless network capacity due to unnecessary retransmissions, and unfairness.

This has led to development of improved power management via Network-Assisted Power Management or NAPman. "NAPman leverages AP virtualization and a new energy-aware fair scheduling algorithm to minimize client energy consumption and unnecessary retransmissions, while ensuring fairness among competing traffic." [You can find the 2009 paper ]

But, say the SleepWell authors, NAPman is most effective "where an isolated AP is connected to multiple clients." But that’s rarely the case in real-world networking. "In reality, multiple APs are within the wireless vicinity, and this strongly impacts the energy consumption of individual clients," the authors write. "Specifically, when a PSM client wakes up to download its own burst of packets, it has to share the channel with all other clients of all other APs in the vicinity. In homes or dense office areas, it is not unusual to overhear 5 to 10 other APs. Since the APs are likely to share the channel fairly between them, it is possible that a client remains awake almost 5 times longer than it would if there was no contention with other APs. Thus, the energy wastage during network activity can be 5 times, and even more if other APs have multiple clients associated to them."

The SleepWell protocol is designed to sidestep this contention.

Always-on APs monitor the ongoing wireless traffic of nearby APs. In a SleepWell network, each APs track the periodic bursts of traffic from surrounding access points. With that information, each one can dynamically re-schedule its own traffic burst to coincide with an open period, creating minimal overlap with other bursts.