When stations associate with an access point, one of the parameters specified is the listen interval, which is the number of Beacon intervals between instances when the station wakes up to received buffered traffic. Longer listen intervals enable a station to power down the transceiver for long periods. Long power-downs save a great deal of power and can dramatically extend battery life. Each station may have its own listen interval.
Lengthening the listen interval has two drawbacks. Access points must buffer frames for sleeping stations, so a long listen interval may require more packet buffer space on the access point. Large numbers of clients with long listen intervals may overwhelm the limited buffer space in access point hardware. Second, increasing the listen Interval delays frame delivery. If a station is sleeping when its access point receives a frame, the frame must be buffered until the sleeping station is awake. After powering up, the station must receive a Beacon frame advertising the buffered frame and send a PS-Poll to retrieve the frame. This buffering and retrieval process can delay the time the frame spends in transit. Whether this is acceptable depends on the traffic requirements. For asynchronous communications such as email, lengthening the listen Interval isn't likely to be a problem. But in other applications that require synchronous, time-sensitive communications (such as securities market data feeds today or an IP phone with an 802.11 interface in the future), a longer interval might not be acceptable. Certain applications may also have trouble with the increased latency. Database applications, in particular, are significantly affected by increased latency. A task group is working on MAC enhancements to provide quality of service for transmissions on 802.11 networks, but no standard has emerged yet.
The DTIM period is a parameter associated with an infrastructure network, shared by all nodes associated with an access point. It is configured by the access point administrator and advertised in Beacon frames. All Beacon frames include a traffic indication map (TIM) to describe any buffered frames. Unicast frames buffered for individual stations are delivered in response to a query from the station. This polled approach is not suitable for multicast and broadcast frames, though, because it takes too much capacity to transmit multicast and broadcast frames multiple times. Instead of the polled approach, broadcast and multicast frames are delivered after every Delivery TIM (DTIM).
Changing the DTIM has the same effect as changing the listen Interval. (That should not be a surprise, given that the DTIM acts like the listen Interval for broadcast and multicast frames.) Increasing the DTIM allows mobile stations to conserve power more effectively at the cost of buffer space in the access point and delays in the reception. Before increasing the DTIM, be sure that all applications can handle the increased delay and that broadcasts and multicasts are not used to distribute data to all stations synchronously. If the application uses broadcast or multicast frames to ensure that all mobile stations receive the same blob of data simultaneously, as would be the case with a real-time data feed, increasing the DTIM will likely have adverse effects.
In an infrastructure network, access points provide most of the power-saving support functions. In an independent or ad hoc 802.11 network, many of those functions move into the network interface driver. In ad hoc networks, stations are required to power up for every Beacon transmission and remain powered up for the duration of the Announcement TIM (ATIM) window, which is measured in time units (TUs).
Decreasing the ATIM window increases the power savings because the required poweron time for the mobile stations is reduced. Stations can power down quickly and are not required to be active during a large fraction of the time between Beacons. Increasing the ATIM window increases the probability a power-saving station will be awake when a second station has a frame. Service quality is increased, and the required buffer space is potentially smaller.
Decreasing or disabling the ATIM window would probably have the same effect on synchronous or real-time applications as increasing the DTIM timer on an infrastructure network—that is, it is likely to cause problems with less reliable communications or applications that depend on real-time data. One of the most obvious examples of a realtime application of ad hoc networking is gaming, but it is far more likely that ad hoc gaming networks would be tuned for low delay and high throughput than for low-power operation.
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