video services under IEEE 802.11 ... - IEEE Xplore

2014 9th International Symposium on Communication Systems, Networks & Digital Sign (CSNDSP)

QoS Support of Voice/video Services Under IEEE 802.11n WLANs Emna Charfi LETI-ENIS: National School of Engineers, SFAX University Route Soukra Km 4, 3038 Sfax, Tunisia [email protected]

Lamia Chaari Fourati LETI-ENIS: National School of Engineers, SFAX University Route Soukra Km 4, 3038 Sfax, Tunisia [email protected]

Abstract—IEEE 802.11n WLAN was mainly developed to support a high data transmission rate toward 600Mbps based on the aggregation scheme that accumulates several sub-frames to transmit them into a larger frame. This concept reduces overheads and increases efficiency and throughput. Nevertheless, it cannot provide QoS satisfaction for delay sensitive application since it badly affects the delay. To outperform this inefficiency, we have proposed an admission control mechanism named Adaptation of Frame Aggregation AFA-CAC. In this paper, we further investigate the performance of our proposed QoS mechanism on supporting real time applications particularly on audio and video services. Keywords—Aggregation; WLAN; 802.11n; admission control; delay; throughput

I.

INTRODUCTION

Recently, the Wireless Local Area Network (WLAN) has experienced tremendous growth with the proliferation of IEEE 802.11 devices [1][2][3][4][5]. Basically, new WLANs generations aim to satisfy Quality of Service (QoS) requirements for real time applications such as audio and video flows since they have strict requirements in term of rate, delay, and loss. In fact, supporting real time services over WLANs environment requires realizing guaranteed features and mechanisms that are not provided by the original IEEE 802.11 standard. All recent WLANs devices are derived from the original standard IEEE 802.11 [1] that was mainly designed for data applications without considering traffic differentiation as well as QoS requirements. The QoS is basically introduced by 802.11e by providing service differentiation at the MAC layer based on Enhanced Distributed Channel Access (EDCA). It can thus deliver time-critical multimedia traffic in adding to traditional data packets. However, EDCA mechanism was not able to guarantee QoS for applications having strict QoS requirements such as real time services [6]. Concurrently, there is diversity of multimedia applications such as voice, video telephony, video conferencing and high-definition television (HDTV) that have to be transmitted with high data rates. In this aim, IEEE 802.11n [4] was appeared for next generation WLAN to provide high throughput at the MAC layer achieving up to 300 Mbps [7][8]. This high throughput has been achieved via many enhancements at both physical and MAC layers. A key MAC enhancement is the frame aggregation mechanism [9] which is mainly based on increasing the payload size by transmitting multiple frames into a single frame. Mainly, IEEE 802.11n defines two types of aggregation mechanism: Aggregation

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Lotfi Kamoun LETI-ENIS: National School of Engineers, SFAX University Route Soukra Km 4, 3038 Sfax, Tunisia [email protected].

MAC Service Data Unit (A-MSDU) and MAC Protocol Data Unit (A-MPDU). The principle of A-MSDU is to allow multiple MSDUs from the same sources to be sent to the same receiver concatenated in a single MPDU. The principle of AMPDU is to join multiple MPDUs to be sent to the same receiver with a single PHY header. Yet, this scheme causes new aggregation headers which are introduced and become parts of the transmitted frame. The presence of such headers has a negative effect in the case of delay sensitive multimedia applications, particularly when aggregating frames of small payloads [10]. Thus, the aggregation mechanism can badly affect the delay for multimedia applications especially in a differentiated service network [11]. As a consequence, an efficient QoS mechanism such as admission control is a key to promise the QoS required by realtime and multimedia services. Indeed, the main purpose of an admission control scheme is to limit the amount of traffic admitted into a particular service class in a way to maintain the QoS of the existing flows, while simultaneously the medium resources can be maximally utilized. Numerous proposals of call admission control algorithm exist in the literature [12-13], but in the best of our knowledge there is a little few proposals for the IEEE 802.11n. To outperform this limitation, we have proposed a model-based admission control algorithm called Adaptation of Frame Aggregation AFA-CAC in a previous work [14]. This later was developed with two main goals :1) to support the 802.11n aggregation mechanism and the 802.11e prioritized access mechanism EDCA at the MAC layer, and 2) to provide good flow fairness in term of throughput and delay for voice and video applications. The analytical model we developed predicts the achieved throughput and the average delay per AC (Access Category) for an EDCA stations in the saturated condition. The remainder of this paper is organized as follows: In Section II, we give a brief background about the IEEE 802.11n aggregation scheme. In Section III, we discuss some of the relevant research works in the field. Section IV presents the design of the proposed AFA-CAC scheme. Section V concentrates on the performance evaluation of the above QoS mechanism for multimedia traffics. Section VI concludes the paper by summarizing this article’s findings. II.

IEEE 802.11N AGGREGATION SCHEME

IEEE 802.11n adapts two approaches for the aggregation data. The first one is the Aggregated MAC Service Data Unit (A-MSDU), and the second is Aggregated MAC Protocol

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Data Unit (A-MPDU) where a number of frames are transmitted together into aggregated packets. This concept reduces the time of transmitting overheads, and reduces the waiting time caused by random backoff period during successive frame transmissions. On the other side, it improves highly throughput in the network. A. A-MSDU aggregation A-MSDU is designed to tolerate multiple MSDUs having the same source to be transmitted to the same receiver concatenated in a one MPDU. The top MAC layer receives packets from the Link Layer and these buffered packets are then aggregated to form a single A-MSDU. For each MSDU subframe in an A-MSDU frame, the MSDU subframe includes the Subframe Header, the MSDU data payload and the Padding field, as it is shown by Fig.1. The Subframe Header includes three fields: the Destination Address (DA), the Source Address (SA) and Length (L) which indicates the MSDU data payload. The A-MSDU aggregation is only tolerable for packets having the same source and destination. The maximum length A-MSDU that a station can receive is either 3839 bytes or 7935 bytes. A single A-MSDU contains multiple MSDU subframes. A single A-MSDU frame form a PSDU (Physical Service Data Unit) frame after adding the MAC header and the FCS field. B. A-MPDU aggregation The principle of A-MPDU is to send multiple MPDU subframes, intended to be sent to the same destination, with a unique PHY header in the goal to reduce the overhead PHY header. For each A-MPDU, every MPDU subframe includes an MPDU frame, the MPDU delimiter and the padding bytes. Multiple MPDU subframes are concatenated into one larger AMPDU frame. All the MPDU subframes within an A-MPDU should be addressed to the same receiver, but the MPDU

subframe could have different source address. With A-MPDU, is fully formed MAC PDUs are logically aggregated at the bottom of the MAC. A short MPDU delimiter is pretended to each MPDU and the aggregate presented to the PHY as the PSDU for transmission in a single PPDU as it is given by Fig.2. The MPDU delimiter is 32 bits in length and consists of a 4-bit reserved field, a 12-bit MPDU length field, an 8-bit CRC field, and an 8-bit signature field. The advertised maximum length may be one of the following: 8191, 16383, 32767, or 65 535 bytes. C. Two-level aggregation A two-level frame aggregation consists of a mix of A-MSDU and AMPDU over two stages as it shown in Fig.3. The basic process is explained as follows: In the first stage, several MSDUs are accumulated to form an A-MSDU frame based on A-MSDU aggregation concept explained obviously. In the second stage, the A-MPDU concept is involved to construct an A-MPDU frame based on accumulating several MPDU subframes. Note that each MPDU includes in its turn numerous AMSDUs subframe which have the same destination. This concept is not mandatory for IEEE 802.11n, and it must be an optional pre-negotiated on a link. III.

RELATED WORKS AND MOTIVATIONS

Among existing works, there are numerous studies which focus on evaluating the performance of the aggregation mechanism for guarantying QoS fairness for real-time flows in IEEE 802.11n WLAN. On the other side, there are some others works that focused on proposing QoS mechanisms such as admission control to provide QoS satisfaction for real-time applications when using the aggregation mechanism. Authors in [15] handle with the weakness of 802.11 aggregation schemes by providing a detailed analysis of packet delay. They considered the packet delay as the amount of time separating the instant of generating this packet and the instant of successfully receiving it. They proved that larger frames increases delays, while transmitting smaller frames is more appropriate real-time data communication sine it reduces delays. Similarly, authors in [16] focused on the negative impact aggregation on some applications. In fact, this mechanism causes more delays and headers especially when aggregating frames of small payloads. To overcome this limitation, authors proposed a new aggregation scheme called mA-MSDU to reduce

Fig.1. A-MSDU aggregation

Fig3. Two-level Aggregation

Fig.2. A-MPDU aggregation

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the A-MSDU and A-MPDU header’s overhead and to support applications with small frame size such as VoIP. With mAMSDU aggregation scheme, small headers are introduced and error control is enabled over the aggregated MSDUs. Authors proved by simulation the performance of this scheme mainly for packets with small sizes. Authors in [17] presented a design of a high throughput MAC supporting QoS requirements which combine the 802.11e Hybrid Coordination Function HCF with the 802.11n frame aggregation scheme to provide QoS satisfaction. The proposed design includes some QoS mechanisms such as: admission control, calculating allocated TXOP, and a scheduler. They showed by simulation that the proposed new MAC protocol is efficient since it improves capacity for real time traffic, and enhances channel utilization, and reduces packet delay for best-effort traffic. By the same token, authors in [18] proposed a dynamic scheduler to adjust to frame aggregation size in the aim to outperform the limitation of this scheme especially in term of delay. This scheduler considers the specific QoS requirements for multimedia applications, and adjusts the aggregated frame size based on frame's access category. Within this scheduler packets, which are insensitive to delay such as Background and Best effort ACs, are forced to wait for other packets. Authors in [19] investigate the performance of video transmission over 802.11n with frame aggregation scheme. He demonstrates that the optimal subframe size as well as the channel conditions improve throughput but have little effect on the video quality. He shows that with retransmission policy, the video quality is improved without increasing the end-to-end delay In our previous work [14], we proposed an admission control algorithm (AFA-CAC) which supports the aggregation scheme in the aim to provide QoS constraints for real time applications and multimedia services. AFA-CAC is based on both predicting the QoS constraints of the already active flows and the new

flow, and adjusting the number of aggregated sub-frames of each flow. We illustrated the the performance of our proposed AFA-CAC in term of satisfying QoS throughput and end-toend delay requirements of voice and video traffics. Among this paper, we aim to more evaluate the performance of our proposal in term of delay, throughput, and number of admitted flows. Particularly, we will give more statistical analysis that proves the efficiency of AFA-CAC in different situations. IV.

AFA-CAC DESIGN

Our proposed AFA-CAC mechanism supports the aggregation mechanism of IEEE802.11n with considering the service differentiation EDCA scheme. The decision criteria are the access delay and the achievable throughput. We considered the achieved throughput as the maximum throughput that could be achieved; and the access delay is the required duration to transmit data from the MAC layer of the sender to the MAC layer of the receiver. We choose to use these two parameters to take decision of accepting or rejecting a new flow since providing QoS satisfaction strongly depends on these two metrics. In fact, video applications are very sensitive in term of throughput, while voice applications are sensitive to both delay and throughput. The principle of our proposed AFA-CAC mechanism within the QAP is summarized in Fig.4.From this working flow, the QAP firstly identifies the AC of each new flow and updates the number of active stations, the number of active AC in each QSTA. Then, it predicts the values of the achievable throughput and the average access delay of each AC. More details of these two metrics prediction is given in [14]. After that, the QAP compares them with the required values of all the flows belonging to the same AC. A new flow of an AC[i] is accepted only when the throughput and delay constraints are satisfied for the four ACs.

Fig.4. AFA-CAC design

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•

Throughput constraint: The amount of the required throughputs for all the admitted flows belonging to a given AC should be smaller than the achievable throughput of this entering AC.

•

Delay constraint: The calculated access delay (average

Similarly, we denote by θ the number of MSDUs subframes contained in one MPDU packet. α and θ depend on the maximum size of A-MPDU and A-MSDU respectively, as it is presented in Fig.5. In this work, we consider that the allowed A-MPDU and A-MSDU length are 32767 and 3839 bytes respectively. All possible values of α and θ are listed in Table.II.

delay) for a given AC should be below the maximal tolerated access delay of flows belonging to this AC. When these two constraints are not satisfied, the number of aggregated A-MPDU will be decreased by substituting one MPDU sub-frame which contains in its part multiple subframes of MSDUs. Indeed, one MPDU is composed by one A-MSDU, MPDU header and FCS fields. Thus, substituting one MPDU is equivalent to substituting one A-MSDU which contains numerous MSDU sub-frames. Mainly, decreasing the number of MPDUs leads to decrease the average delay. Once modifying the number of A-MPDU, the QAP updates all parameters and QoS metrics (achievable throughput, average delay) will be re-estimated. If the number of aggregated frame reaches zero after successive substitutions, and the two above constraints still not satisfied, the flow will be rejected. The total number of aggregated frame A-MSDU and A-MPDU are listed in Table.II. We suppose that each AC[i] is transmitting data A-MDPU packets. Since A-MPDU frame in composed by numerous sub-frames MPDUs, and every MPDU contain multiple sub-frames MSDU, we denote by α the number of MPDUs sub-frames encapsulated in one A-MPDU frame of an AC[i]. TABLE.I.

Voice (G.711) Video BE & BK

V.

The used node topology contains one AP and a variable number of stations N, each one contains the four ACs: voice (AC_VO), video (AC_VI), Best-effort (BE), and background (BK). All flow belonging to the same AC has the same QoS requirements. We suppose that every one second, new station contends to the medium. The MAC parameters are the default EDCA/Aggregation parameters. The mean arrival rate and the average payload size of each application are listed in TableII. QoS requirements of different applications are given in table.I. We suppose that there are no QoS constraints for BE and BK data. In our validation procedure, we are looking for checking the WLAN admissibility in different scenario. We are looking also for showing the impact of the co-habitation of different priorities on the AFA-CAC decisions. That's why; we study the results where we have only voice stations in the WLAN, only video, voice and video, and finally voice, video and data stations. The results we obtained from all of these scenarios are summarized in Table.III.

QOS REQUIREMENTS

Throughput 64kbps

Delay 50ms

220kbps >= 10kbps

150ms None

PERFORMANCE EVALUATION

Voice only: From Fig.6, we can note that the admission control satisfies the required QoS delay with maintaining the same achieved throughput. Moreover, reducing the number of aggregated MPDU is the key to obtain such performance. On the other side, the AFA-CAC improves highly the admissibility of voice stations. As it is listed in Table.III, our proposal allows 54 voice stations to transmit, while only 3 stations are permitted when the AFAF-CAC is not implemented. The reason of the rejection of the first flow following the last admitted one was the bandwidth constraint.

TABLE.II. DIFFERENT TRAFFIC PARAMETERS

Voice Video BestBackground (G.711) (H.263) effort 160 660 1500 1500 bytes Average bytes bytes bytes payload size The considered maximum A-MSDU size is 3839 bytes 23 5 2 2 Number of MSDU θ The considered maximum A-MPDU size is32767 bytes 8 8 8 8 Number of MPDU α

Video only: From Fig.7, we observe that the admission control mechanism leads to high performance achievement. The delay constraint was accomplished by reducing the number of Aggregated MPDUs. By this clause, the number of admitted flows increased considerably as it is illustrated in Table.III. Our proposed AFA-CAC accepts 74 video stations, while 34 stations are allowed without this QoS mechanism. Voice and Video: The result of the implementation of the AFA-CAC algorithm prove that required QoS delay is satisfied for both voice and video applications. As previously, reducing the number of aggregated MPDUs is the key to obtain such performance as it is shown in Fig.8. Furthermore, the number of admitted flows is enhanced. In fact, 23 video and 33 voices request are accepted while only 15 video and 3 voices are admitted without the proposed admission control algorithm.

Fig.5. A-MDPU frame components

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TABLE.III. NUMBER OF ADMITTED REQUESTS

Scenario Voice only Video only Voice and video Voice and video and data

sensitive applications. Mainly, the aggregation scheme badly affects the admissibility of voice flows, so that reducing the size of the aggregated frame is mandatory to outperform the admissibility of these packets. • The introduction of data flows such as background and best-effort has no important impact on the on the admissibility of voice and video.

Number of admitted requests Without AFA-CAC With AFA-CAC 3 54 34 74 15 video 23 video 3 voice 33 voice 15 video 23 video 3 voice 33 voice

VI.

Voice, video, background, and best-effort: In fact, it is

CONCLUSION

In this paper we have investigated the performance of our new admission control mechanism AFA-CAC proposed in [wiley] for IEEE 802.11n WLAN. We presented and explained the design of this algorithm to be implemented in the QAP and which is responsible for accepting or rejecting a new entering flow regarding its QoS requirements. Then, we validated the proposed solution by a set of realistic scenarios. Basically, we were fascinated to study the behavior of the admission control in different situations: with presence/absence of different application types (voice, video and best effort data services). Our purpose was to check if the proposed AFA-CAC mechanism is able to satisfy the QoS requirements for real time services such as voice and video. From the obtained results, we conclude that AFA-CAC succeeds to guaranty the required QoS delay and throughput for voice and video applications. Moreover, it improves greatly the admissibility of these services types in the network. Such performance is obtained with the optimization of the aggregated frame size which has a negative impact particularly for voice applications.

clearly seen in the curves presented in Fig.9 that without admission control, the access delay increases dramatically to reach very high values compared to the tolerated delay, and the achievable throughput decreases considerably. The admission control algorithm detects this functioning and limits the number of active flows to respect the QoS constraints of voice flows which are very sensitive to delay increase. Furthermore, the AFA-CAC improves the admissibility of voice and video stations as it listed in Table.III. From the obtained results, we can draw the subsequent conclusions: • AFA-CAC was succeeded to satisfy the QoS requirements for real time services such as voice and video. Therefore, the main goal of our algorithm is achieved. • AFA-CAC has an important impact on the admissibility of voice and video packets. It improves highly the number of admitted flows in each situation. • The optimization of the number of aggregated MPDU is the key to obtain such performance. Hence, aggregating a big number of MPDU is not appropriate for QoS delay

Fig.6. AFA-CAC performance for voice only

Fig.7. AFA-CAC performance for video only

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Fig.8. AFA-CAC performance for video and video only

Fig.9. AFA-CAC performance for video, voice and best-effort

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