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Ad Hoc Networks
Published in Jerry C. Whitaker, Microelectronics, 2018
Michel D. Yacoub, Paulo Cardieri, Élvio João Leonardo, Álvaro Augusto Machado Medeiros
The IEEE 802.11e is an emerging MAC protocol, which defines a set of QoS features to be added to the 802.11 family of wireless LAN standards. Currently there is a draft version of the specifications (IEEE, 2003). The aim is to better serve delay-sensitive applications, such as voice and multi-media. In 802.11e, the contention-based medium access is referred to as enhanced distributed channel access (EDCA). In order to accommodate different traffic priorities, four access categories (AC) have been introduced. To each AC corresponds a backoff entity. The four distinct parallel backoff entities present in each 802.11e terminal are called (from highest to lowest priority): voice, video, best effort, and background. For the sake of comparison, existing 802.11/a/b standards define only one backoff entity per terminal. Each backoff entity has a distinct set of parameters, such as CWmin, Cwmax, and the arbitration interframe space (AIFS). AIFS is at least equal to DIFS and can be enlarged if desired. Another feature added to 802.11e is referred to as transmission opportunity (TxOP). A TxOP defines a time interval, which a back off entity can use to transmit data. It is specified by its starting time and duration, and the maximum length is AC dependent. The protocol also defines the maximum lifetime of each MAC service data unit (MSDU), which is also AC dependent. Once the maximum lifetime has elapsed, the MSDU is discarded. Finally, the protocol allows for the optional block acknowledgement in which a number of consecutive MSDUs are acknowledged with a single ACK frame.
Evaluation of an alarm management system for an ATC centre
Published in Don Harris, Engineering Psychology and Cognitive Ergonomics Volume Five, 2017
Steven T. Shorrock, Richard Scaife
Acceptance is the act of acknowledging the receipt of an alarm. At this stage, operator acknowledgement should be reflected in other elements of the system providing alarm information. Alarm systems should aim to reduce operator workload to manageable levels – excessive demands for acknowledgement increase workload and operator error. ‘Ring-back’ design philosophies double the engineer’s acceptance workload; careful consideration is required to determine whether cleared alarms really need to be acknowledged. The issue of single versus multiple alarm acknowledgement does not yet seem to have been resolved (Stanton, 1994). Group acknowledgement may lead to unrelated alarms being masked in a block of related alarms. Single acknowledgement will increase workload and frustration, and may begin to resemble group acknowledgement as task demands increase. Two alternatives may be suggested. First, operators may select individual alarms with the mouse with a ‘Control’ key depressed. This would form a half-way house between individual and block acknowledgement, countering the effects of a block selection using ‘click-and-drag’ or a ‘shift’ key, whilst reducing workload. A more preferable solution may be to allow acknowledgement for alarms pertaining to the same system.
Design of MAC Layer Resource Allocation Schemes for IEEE 802.11ax: Future Directions
Published in IETE Technical Review, 2018
Rashid Ali, Sung Won Kim, Byung-Seo Kim, Yongwan Park
In 2005, IEEE developed a new extension of 802.11 to support QoS provisioning in WLAN. The extension was to overcome the problem tempted by the MAC layer techniques like point coordination function (PCF) and DCF in the earlier extensions. With PCF, a centralized device allocates the resources to the STA in the network [3]. Since such centralized devices are not available in certain cases, a distributed scheduling scheme is expected to be implemented. Due to the best effort service nature of DCF, it is unable to provide efficient performance for voice and video applications in WLANs. A hybrid coordination function (HCF) is introduced, which combines the features of PCF and DCF for resource allocation in 802.11. IEEE 802.11e is one of the WLAN standards providing QoS for voice and video applications using HCF, along with enhanced DCF (EDCF). In addition to these, two more MAC enhancements were also introduced in 802.11e to improve the MAC layer throughput; the Block Acknowledgement which enabled sending of a single acknowledgment (ACK) for a block of frames; and Direct Link Protocol, which enabled direct link between two STAs in a single WLAN network.