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Cognitive Radio with Spectrum Sensing for Future Networks
Published in Mahmoud Elkhodr, Qusay F. Hassan, Seyed Shahrestani, Networks of the Future, 2017
Nabil Giweli, Seyed Shahrestani, Hon Cheung
The IEEE 802.11e standard is proposed to enhance QoS in IEEE 802.11 networks (IEEE, 2005). As applications have different requirements, in 802.11e, the frames belonging to different applications are prioritized with one of the eight user priority (UP) levels. In contrast, previous IEEE 802.11 standards use the distributed coordination function (DCF) mechanism at MAC layer where the best-effort service is provided equally to all traffic streams from different applications to access the medium. In IEEE 802.11e, the hybrid coordination function (HCF) is used for prioritizing traffic streams to enhance QoS on top of the DCF. The HCF function accommodates two medium access methods (i.e., a distributed contention-based channel access mechanism, called enhanced distributed channel access [EDCA], and a centralized polling-based channel access mechanism, called HCF controlled channel access [HCCA]). Based on the UP, the EDCA defines four access categories (AC); voice (AC_VO), video (AC_VI), best effort (AC_BE), and background (AC_BK). These categories are assigned different priorities, ranging from highest to lowest, respectively. The category AC_VO has top priority and is usually given to traffic carrying voice information. It is followed by the AC_VI category for video traffic and then the AC_BE category for data traffic. The category AC_BK has the lowest priority and is usually assigned to unnecessary data traffic. Each AC has a contention window (CW) which has a specified minimum size and maximum size (i.e., CWmin and CWmax). Also, an arbitration interframe space (AIFS) value and a transmit opportunity (TXOP) interval are used to support QoS prioritization (Inan et al., 2007). Instead of using fixed distributed interframe space (DIFS), also called DCF interframe space, the AIFS value is a variable value calculated based on the AC. The AIFS value determines the time at which a node defers access to the channel after a busy period and before starting or resuming the back-off duration. Hence, the time for a station to wait for the channel to become idle before it starts sending data is calculated based on the AC category of the data AC (Bianchi et al., 2005). However, the short interframe space (SIFS) value is used as the shortest interframe space (IFS) value for transmitting high-priority frames, such as data acknowledgment frames and CTS frames. The TXOP is a new concept introduced in IEEE 802.11e to limit the time of transmission for a given station. On one hand, if a frame to be transmitted requires more than the TXOP interval, the frame should be fragmented into smaller frames, each of which can be transmitted within a TXOP interval (Yang, 2004). On the other hand, the use of a TXOP interval limits the number of smaller frames to be aggregated.
Performance Evaluation of Co-Channel Interference on Wireless Networks
Published in Journal of Computer Information Systems, 2022
Joong-Lyul Lee, Joobum Kim, Myungjae Kwak
where DIFS is the minimum medium idle time, SIFS is the short interframe space which is used for the highest-priority transmissions, TDATA_1 is the time to send a frame composed that comprise of PHYhdr + MAChdr + MACpayload + FCS, and TACK is the time to send an ack frame composed that is comprised of PHYhdr + MAChdr + MACack + FCS. When the medium is busy, the node must wait during backoff time. Backoff time is × SlotTime. As shown in Equation (3), total time to send data in the application layer is DIFS, TDATA_1, SIFS, TACK and backoff time. Table 3 shows the parameter values used in this analysis as the IEEE 802.11 g standard.