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Wireless Networking Standards (WLAN, WPAN, WMAN, WWAN)
Published in K.R. Rao, Zoran S. Bojkovic, Dragorad A. Milovanovic, Wireless Multimedia Communications, 2018
K.R. Rao, Zoran S. Bojkovic, Dragorad A. Milovanovic
PCF. The PCF was introduced to support multimedia transmission and it can only be used if a WLAN operates in an infrastructure mode. It is an optional MAC function because the hardware implementation of PCF was thought to be complicated at the time the standard was finalized. PCF is a polling-based contention-free access scheme, which uses an access point as a point coordinator. When a WLAN system is set up with PCF enabled, the channel access time is divided into periodic intervals called beacon intervals. A beacon interval is composed of a contention-free period (CFP) and a contention period (CP). During a CFP, the access point maintains a list of registered stations and pools them according to the list. The size of each data frame is bounded by the maximum MAC frame size (23,004 bytes). If the PHY data rate of every station is fixed, the maximum CFP duration for all stations, CFP max duration, can then be decided by the access point. The time used by an access point to generate beacon frames is called the target beacon transmission time (TBTT). The next TBTT is announced by the access point within the current beacon frame. To give PCF higher access priority than DCF, the access point waits for a starter interval called PCF interframe space (PIFS), before starting PCF.
Contemporary Wireless Technologies
Published in G. S. V. Radha Krishna Rao, G. Radhamani, WiMAX, 2007
G. S. V. Radha Krishna Rao, G. Radhamani
The DLL within 802.11 consists of two sublayers: LLC and MAC. The 802.11 standard uses the same 802.2 LLC and 48-bit addressing as other 802.x LANs, allowing for very simple bridging from wireless to wired networks, but the MAC is unique to WLANs. Of particular interest in the specification is the support for two fundamentally different MAC schemes to transport asynchronous and time-bounded services. The first scheme, distributed coordination function (DCF), is similar to traditional legacy packet networks. The DCF is designed for asynchronous data transport, where all users with data to transmit have an equally fair chance of accessing the network. The point coordination function (PCF) is the second MAC scheme. The PCF is based on polling that is controlled by an access point.
Adaptive Techniques in Wireless Networks
Published in Mohamed Ibnkahla, Adaptation and Cross Layer Design in Wireless Networks, 2018
The 802.11 MAC layer initially defined two access mechanisms: the distributed coordination function (DCF) and point coordination function (PCF). DCF provides a contention-based access scheme where each station contends for channel access based on the carrier sense multiple access with collision avoidance (CSMA/CA) protocol. PCF can be used on a WLAN with an access point (AP) that acts as a point coordinator to schedule the transmission in the network and provide contention-free access. The IEEE 802.11e standard extends the basic DCF/PCF access schemes to include more sophisticated priority control such that traffic with different priorities may receive different QoS. In this chapter, we will focus on the adaptive issues mainly related to the MAC layer in 802.11 WLANs.
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
PCF is different from the distributed medium access of DCF, where all STAs communicate with each other via a centralized STA called a point coordinator (PC), which usually resides in the AP. The PC controls the access period of the medium by splitting the resource airtime into super-frames of contention-free periods (CFPs). The controlled access period still follows the polling-based contention process. In the CFP, each STA initially sets its NAV to the maximum duration (CFPMaxDuration) at the beginning of each CFP. This NAV duration is reset if the STA receives a CF-End or a CF-End plus ACK frame from the AP, indicating the end of the CFP. Although the PCF is a contention-free medium access in the CFP, there are still several issues with providing efficiency like QoS for applications, because it does not support AC differentiation for priority applications. There is no prediction for the occupancy of the resource by the polled STA. Therefore, the fairness issue persists in the network for each STA, because the transmission time is not bounded. With densely deployed networks, this problem can cause more severe unfairness [34].