In IEEE 1609, it uses IEEE 802.11 Enhanced Distributed Channel Access (EDCA) mechanism to access the channel. IEEE 802.11 EDCA is a new wireless technology for wireless access in the vehicular environment (WAVE). It defines a new supplement to the existing IEEE 802.11 MAC protocol. In IEEE 802.11 EDCA, the aim is providing a QoS support. While the system serves different access categories (ACs), EDCA does not perform well under high load conditions. In order to improve the efficiency, we pay attention to the EDCA with transmit opportunity (TXOP) mechanism. We first proposed a Markov chain model and studied the behavior. We extend the model to support IEEE 802.11 EDCA and presented a more accurate analysis under nonideal channel environment. We also compared it with that without TXOP mechanism under channel error environment.
In recent years, the WLANs market is experiencing an explosive growth. The medium access control (MAC) protocol is the key element that provides the efficiency in accessing the channel, while satisfying the quality of service (QoS) requirements. IEEE 802.11 EDCA is a new wireless technology which is an enhanced version of IEEE 802.11 distributed coordination function (DCF). The IEEE 802.11 EDCA aims at improving the capabilities and efficiency of the IEEE 802.11 MAC protocol by defining a new mechanism to support the QoS services. While the system serves different ACs, EDCA does not perform well under high load conditions. In order to improve the efficiency, EDCA provides two mechanisms named as transmission opportunity (TXOP) and Block_ACK. These two mechanisms are allowed to offer new data transmission services that include the multiple frame delivery [
The rest of this paper is organized as follows. In Section
The IEEE 802.11 wireless local area network is a shared-medium communication network that transmits information over wireless links for all IEEE 802.11 stations in its transmission range to receive. It is one of the most deployed wireless networks in the world and is likely to play a major role in multimedia home networks and next-generation wireless communications. IEEE 802.11 wireless networks can be configured into two different modes: ad hoc and infrastructure. In ad hoc mode, all wireless stations within the communication range can communicate directly with each other, whereas, in the infrastructure mode, an access point (AP) is needed to connect all stations to a distribution system (DS), and each station can communicate with others through the AP. IEEE 802.11 is composed of both a physical layer (PHY) and MAC specifications for wireless local area networks [
In the IEEE802.11 protocol, the fundamental mechanism to access the medium is called the distributed coordination function (DCF). This is a random access scheme which is based on the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol. The standard also defines an optional point coordination function (PCF) which is based on a polled-response mechanism.
In the DCF, if a station has a frame to transmit, it will monitor the channel. If the channel is busy, the MAC waits until the medium becomes idle and then defers for an extra time interval, called the DCF interframe space (DIFS). After sensing the channel within a DIFS, the STA randomly chooses a backoff interval before transmitting. The backoff counter is decremented in terms of a time slot as long as the channel is sensed as being idle. The counter is stopped when a transmission with other STAs is detected on the channel and reactivated when the channel is sensed as being idle again for more than a DIFS. The station transmits its frame when the backoff counter reaches zero. At each transmission, the backoff time is uniformly chosen in the range
IEEE 802.11 EDCA is a new wireless technology which is an enhanced version of IEEE 802.11 DCF. It defines a new supplement to the existing IEEE 802.11 MAC protocol. The IEEE 802.11 EDCA aims at improving the capabilities and efficiency of the IEEE 802.11 MAC protocol by defining a new mechanism to support the QoS services. EDCA specifies four default access categories (ACs). Each STA contends for the channel access and independently starts its backoff depending on its associated AC. Each AC uses AIFS [AC],
The contention method of EDCA is the same as that in DCF. Each STA having a frame to transmit has to wait for the channel to be idle without interruption for a period AIFS [AC], and then it should start a random backoff process with its own
We assume in the following that, for a given station in the priority
Markov chain model for the priority
The backoff counter freezes when the STA senses that the channel is busy in the priority
The backoff counter decrements when the STA senses the channel is idle in the priority
The STA defers the transmission of a new frame and enters stage 0 of the backoff process if the STA finds a collision has occurred at
The STA chooses a backoff delay of the next stage
Let
Let
In IEEE 802.11, the TXOP is a time period when a particular station (STA) that wins the channel access has the right to initiate transmission along with the EDCA parameters of AIFS[AC],
TXOP frame structure.
The purpose of our analysis is to evaluate the saturation throughput and the delay performances of EDCA with TXOP mechanism under ideal channel and channel error scenarios. Based on the previous description, we can derive the close forms for system performance metrics of saturation throughput and delay.
From the analysis that was depicted in the previous chapter, the transmission probability is related to
From the previous description, the saturation delay in a channel error scenario for the priority
Our simulation model is built on Dedicated Short Range Communications (DSRC). DSRC is a block of spectra in the 5.850 to 5.925 GHz band. It is a short-to-medium range communications service that supports both public safety and private operations in roadside-to-vehicle and vehicle-to-vehicle communication environments. DSRC complements cellular communications by providing very high data transfer rates while minimizing latency in the link of short range [
Saturation throughput of the different priority STAs with the TXOP mechanism under a channel error scenario,
Saturation throughput of the different priority STAs without the TXOP mechanism under a channel error scenario,
The throughput performances with the TXOP mechanism that considers different SNR conditions are shown in Figures
Saturation throughput of the different priority STAs with the TXOP mechanism under a channel error scenario,
Saturation throughput of the different priority STAs with the TXOP mechanism under a channel error scenario,
Saturation throughput of the different priority STAs with the TXOP mechanism under a channel error scenario,
Saturation throughput of the different priority STAs with the TXOP mechanism under a channel error scenario,
Saturation throughput with the TXOP mechanism by differentiating the retry limit.
Saturation throughput with the TXOP mechanism by differentiating the retry limit.
Saturation delay with the TXOP mechanism by differentiating the retry limit.
Saturation delay with the TXOP mechanism by differentiating the retry limit.
In IEEE 1609 WAVE, it supports a variety of safety and nonsafety applications with eight levels of access category supports QoS using EDCA in IEEE 802.11. In this paper, we considered the Markov chain model of IEEE 802.11 EDCA under a non-ideal channel. In the non-ideal channel, the frame may be received unsuccessfully due to a channel error. When the frame is received unsuccessfully, it results in an increase in the saturation delay and a reduction in the saturation throughput. In order to improve the efficiency, we used the TXOP mechanism with the proposed model in a non-ideal channel scenario. The proposed model with the TXOP mechanism under a channel error scenario can achieve a higher performance than that without the TXOP mechanism due to fact that the modified model with the TXOP mechanism is allowed to transmit multiple frames during the TXOP duration.