Enhanced Distributed Channel Access Protocol Research Articles

Effect of service differentiation on QoS in IEEE 802.11e enhanced distributed channel access: a simulation approach

The enhanced distributed channel access (EDCA) protocol is a supplement to IEEE 802.11 medium access control (MAC), ratified by IEEE 802.11e task group to support quality of service (QoS) requirements of both data and real-time applications. Previous research show that it supports priority scheme for multimedia traffic but strict QoS is not guaranteed. This can be attributed to inappropriate tuning of the medium access parameters. Thus, an in-depth analysis of the EDCA protocol and ways of tuning medium access parameters to improve QoS requirements for multimedia traffic is presented in this work. An EDCA model was developed and simulated using MATLAB to assess the effect of differentiating contention window (CW) and arbitration inter-frame space (AIFS) of different traffic on QoS parameters. The optimal performance, delay, and maximum sustainable throughput for each traffic type were computed under saturation load. Insight shows that traffic with higher priority values acquired most of the available channels and starved traffic with lower priority values. The AIFS has more influence on the QoS of EDCA protocol. It was also observed that small CW values generate higher packet drops and collision rate probability. Thus, EDCA protocol provides mechanism for service differentiation which strongly depends on channel access parameters: CW sizes and AIFS.

Service Differentiation in IEEE 802.11ah WLAN under Restricted Access Window based MAC protocol

The restricted access window (RAW) based channel access scheme proposed for IEEE 802.11ah wireless LAN (WLAN) divides the stations (STAs) in the network into groups and allocates RAW slots for each group of STAs. In this way, the STAs belonging to each group can attempt to access the medium during their designated RAW slot by invoking the enhanced distributed channel access (EDCA) protocol. This paper describes an analytical model to evaluate the saturation throughput of IEEE 802.11ah WLAN under the RAW based scheme, when STAs carry traffic of distinct access categories (ACs) and use the EDCA protocol for medium sharing. However, when groups are formed by randomly selecting the STAs, i.e., if random grouping (RG) scheme is employed, each group will contain STAs carrying traffic of distinct ACs. This will lead to starvation and degraded throughput performance for the low priority class STAs, since they have to contend with high priority class STAs within the same group during the RAW slot assigned. To resolve this issue, we propose an algorithm to realize priority class based grouping (PCG), in which the STAs belonging to the same ACs are grouped together. We also describe an analytical model to evaluate the saturation throughput of the network under the proposed PCG scheme. We establish that saturation throughput of the network and the relative service differentiation ratio of the lower priority class STAs can be significantly improved under the proposed PCG scheme compared to the conventional RG scheme.

Transmission Capacity Characterization in VANETs with Enhanced Distributed Channel Access

The traditional research on the capacity of the Vehicular Ad Hoc Networks (VANETs) mainly lacks realistic models mimicking the behaviors of vehicles and the MAC protocol applied by IEEE 802.11p. To overcome these drawbacks, in this paper, the network transmission capacity analysis for VANETs is carried out from the perspective of the spatial geometric relationship among different vehicles. Specifically, the transmission scheme in this system is set to mimic enhanced distributed channel access (EDCA) protocol, in which the division of priorities is taken into account both the data type and the transmission distance requirement. Meanwhile, the moving pattern of vehicles is described as the classic car-following model according to realistic characteristics of VANET, and the propagation channel is modeled as a combination of large-scale path-loss and small-scale Rayleigh fading. Based on this model, the transmission opportunity under EDCA protocol is quantified and compared with that of CSMA/CA, and then the outage probability is calculated under the worst interfered scenario. Finally, the transmission capacity is thereby calculated and verified by the simulation results.

Enhance the Performance of EDCA Protocol by Adapting Contention Window

The real-time application is the main challenge that faces the wireless network. It must achieve the quality of service (QOS) requirements to success. IEEE802.11 standards use Enhanced Distributed Channel Access (EDCA) protocol and Distributed Coordinate Function (DCF) protocol in its Media Access Control (MAC) layer. EDCA protocol is used to overcome the drawback of supporting QOS in DCF protocol. The mechanism of EDCA protocol is based on dividing the data to four queues; voice, video, best effort and background. The queues have different priorities to access the medium. The voice queue has the highest priority and its data will serve firstly. Using the default parameter values for EDCA protocol will lead to increasing the collisions in the wireless network and decreasing the capacity. In this paper, the limitation of supporting QOS in EDCA protocol was presented by using OPNET simulation and mathematical model. The new mathematical model was designed by using Markov chain mechanism to evaluate the performance of the EDCA protocol under saturation and non-saturation conditions, aimed to separate between uplink and downlink throughput with different data types. Moreover, the new algorithm was proposed to enhance the capacity of the EDCA protocol and increase the number of the active voice users. Through our algorithm, the throughput was increased by 27.72% and the EDCA can support 14 voice users rather than 11 in the default EDCA protocol. The evaluation of our new algorithm was verified by using OPNET simulation and mathematical model.

Multiple preemptive EDCA for emergency medium access control in distributed WLANs

The increasingly use of wireless local area networks (WLANs) in public safety and emergency network services demands for a strict quality of service (QoS) guarantee especially a large number of users report an emergency for immediate channel access. Unfortunately, the traditional IEEE 802.11e-based enhanced distributed channel access (EDCA) does not support a strict QoS guarantee for life saving emergency traffic under high loads. Previous studies have attempted to enhance the performance of EDCA called the Channel Preemtive EDCA (CP-EDCA) which is a promising idea to support emergency traffic in WLANs. However, CP-EDCA supports a single emergency traffic only (i.e. no emergency service differentiation) with high delays for increased traffic loads. To overcome this problem, we propose a class of EDCA protocol called Multiple Preemption EDCA (MP-EDCA) as a candidate to support multiple emergency traffics under high loads. Each MP-EDCA node can support up to four emergency traffics (life, health, property and environment) with different priorities in addition to support background (non-emergency) traffic. The proposed protocol privileged the high priority life-saving emergency traffic to preempt the services of low priority ones without much starvation in the network to maintain a strict QoS guarantee. The paper evaluates the performance of MP-EDCA through an extensive analysis of simulation outcome. The results obtained show that MP-EDCA outperforms CP-EDCA in achieving lower medium access control and emergency node delays.

Using Markov Chain Model to Evaluate the Performance of EDCA Protocol Under Saturation and Non-Saturation Conditions

Nowadays, there is a significant increment of internet usage in public locations such as airports, restaurants and bus stations. Ease of extension and low costs make wireless networks more suitable than wire networks in these public locations. Providing quality of service (QOS) is the main challenge that wireless networks face. IEEE802.11e is developed to support QOS for real time applications. It uses the Enhanced Distributed Channel Access (EDCA) protocol rather than the Distributed Coordination Function (DCF). EDCA protocol uses four queues in its MAC layer. These queues have different priorities and serve different types of data (video, voice, best effort and background). On the other hand, DCF protocol has only one queue for all types of data. In this paper, a new Markov chain model is proposed for the EDCA protocol, which works in saturation and non-saturation conditions. The Markov chain model covers the difference between the station and the access point (AP). Also, it presents how the AP is considered as the bottleneck of the network, and it hinders the wireless network to serve more voice users with supporting QOS. The throughput of stations and AP are calculated for different data types. In addition to that, an adjusting contention window mechanism is proposed to enhance the capacity of wireless networks to support more voice users.

Evaluating The Performance for DCF Protocol and EDCA Protocol

Nowadays supporting quality of service (QOS) for real time application is the main challenge of the wireless area network. 802.11standards use distributed Coordination Function (DCF) protocol and Enhanced Distributed Channel Access (EDCA) protocol in the MAC layer. DCF protocol has only one queue for different data types, it deals with data depending on the arriving time. There is no priority to serve real time applications faster. However EDCA protocol has four queues and each queue works with specific data type. Voice, video, best effort and background are the different queues in the EDCA protocol. Different parameters and priorities are defined for each queue. The voice queue reserves the highest priority and serves its data first. In this paper QOS parameters are measured for both DCF and EDCA protocol by using OPNET simulation. The QOS parameters must reach the requirements to support QOS. The results show how QOS parameters do not reach the requirements when using DCF protocol. The values of the end to end delay and the packet loss percentage are 0.514second, 19.04% respectively. But, when using EDCA protocol the end to end delay becomes 0.0624 second and the percentage of the packet loss decreases until reach 0.00617%. So the QOS parameters achieve requirements with EDCA protocol and support QOS.

Saturation Performance Assessment of Differentiation Services Supported in IEEE 802.11e

In the emerging standard IEEE 802.11e, the Enhanced Distributed Channel Access (EDCA) is specified to support quality-of-service (QoS) in wireless local area networks (WLAN). This paper aims at providing a thoroughly analytical model that captures the operation of the AIFSs, contention window sizes and retry limits differentiation for different services of the EDCA mechanism under saturation condition based on a three-dimensional Markov Chain model. The analytical model is validated and provides an in-depth understanding and insights into the EDCA protocol, and effectiveness of different parameters on the system performance in terms of system throughput and frame dropping probability for differentiation services traffic is investigated through extensive numerical and simulation results. Moreover, the analytical model can be cooperated into the admission control algorithm with different parameters in terms of AIFS, contention window sizes and retry limits.

Analysis of Enhanced Collision Avoidance Scheme Proposed for IEEE 802.11e-Enhanced Distributed Channel Access Protocol

In enhanced distributed channel access (EDCA) protocol, small contention window (CW) sizes are used for frequent channel access by high-priority traffic (such as voice). But these small CW sizes, which may be suboptimal for a given network scenario, can introduce more packet collisions, and thereby, reduce overall throughput. This paper proposes enhanced collision avoidance (ECA) scheme for AC_VO access category queues present in EDCA protocol. The proposed ECA scheme alleviates intensive collisions between AC_VO queues to improve voice throughput under the same suboptimal yet necessary (small size) CW restrictions. The proposed ECA scheme is studied in detail using Markov chain numerical analysis and simulations carried out in NS-2 network simulator. The performance of ECA scheme is compared with original (legacy) EDCA protocol in both voice and multimedia scenarios. Also mixed scenarios containing legacy EDCA and ECA stations are presented to study their coexistence. Comparisons reveal that ECA scheme improves voice throughput performance without seriously degrading the throughput of other traffic types.