In this paper we consider transport layer approaches for achieving high rate, low delay communication over edge paths where the bottleneck is a modern WLAN which can aggregate multiple packets into each WLAN frame. We show that regulating send rate so as to maintain a target aggregation level can be used to avoid queue buildup at the WLAN AP. We derive an approximate downlink throughput model for WLANs using aggregation, verify its accuracy using experimental measurements and using this determine a convex low delay rate region. This allows us to derive a low delay proportional fair rate allocation for which we present a prototype transport layer implementation.
Interest in using aggregation in WLANs pre-dates the development of the 802.11n standard in 2009 but has primarily focused on analysis and design for wireless efficiency, managing loss etc. For a recent survey see for example @cite_6 . The literature on throughput modelling of WLANs is extensive but much of it focuses on so-called saturated operation, where transmitters always have a packet to send, see for example @cite_8 for early work on saturated throughput modelling of 802.11n with aggregation. When stations are not saturated (so-called finite-load operation) then for WLANs which use aggregation (802.11n and later) most studies resort to the use of simulations to evaluate performance due to the complex interaction between arrivals, queueing and aggregation with CSMA CA service. Notable exceptions include @cite_17 @cite_11 which adopt a bulk service queueing model that assumes a fixed, constant level of aggregation and @cite_16 which extends the finite load approach of @cite_1 for 802.11a b g but again assumes a fixed level of aggregation.
@cite_8 @cite_1 @cite_17 @cite_6 @cite_16 @cite_11
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In upcoming very high-speed wireless LANs (WLANs), the physical (PHY) layer rate may reach 600 Mbps. To achieve high efficiency at the medium access control (MAC) layer, we identify fundamental properties that must be satisfied by any CSMA- CA-based MAC layers and develop a novel scheme called aggregation with fragment retransmission (AFR) that exhibits these properties. In the AFR scheme, multiple packets are aggregated into and transmitted in a single large frame. If errors happen during the transmission, only the corrupted fragments of the large frame are retransmitted. An analytic model is developed to evaluate the throughput and delay performance of AFR over noisy channels and to compare AFR with similar schemes in the literature. Optimal frame and fragment sizes are calculated using this model. Transmission delays are minimized by using a zero-waiting mechanism where frames are transmitted immediately once the MAC wins a transmission opportunity. We prove that zero-waiting can achieve maximum throughput. As a complement to the theoretical analysis, we investigate the impact of AFR on the performance of realistic application traffic with diverse requirements by simulations. We have implemented the AFR scheme in the NS-2 simulator and present detailed results for TCP, VoIP, and HDTV traffic. The AFR scheme described was developed as part of the IEEE 802.11n working group work. The analysis presented here is general enough to be extended to proposed schemes in the upcoming 802.11n standard. Trends indicated in this paper should extend to any well-designed aggregation schemes. Analysis of the 802.11 CSMA CA mechanism has received considerable attention recently. Bianchi presented an analytic model under a saturated traffic assumption. Bianchi's model is accurate, but typical network conditions are nonsaturated and heterogeneous. We present an extension of his model to a nonsaturated environment. The model's predictions, validated against simulation, accurately capture many interesting features of nonsaturated operation. For example, the model predicts that peak throughput occurs prior to saturation. Our model allows stations to have different traffic arrival rates, enabling us to address the question of fairness between competing flows. Although we use a specific arrival process, it encompasses a wide range of interesting traffic types including, in particular, VoIP. Frame aggregation is one of the several enhancements proposed by IEEE 802.11 Task Group n to improve channel utilization. In frame aggregation, more than one data frame is encapsulated to form an aggregate, and once an aggregate is formed, a station contends to access the medium to transmit the entire aggregate. We refer to the number of data frames encapsulated within an aggregated frame as aggregate size. We claim that a static assignment of aggregate size leads to the following performance trade-off: a small value might be insufficient to mitigate the transmission overheads, thereby nullifying the whole purpose of frame aggregation; whereas, a large value might affect the quality of service experienced by higher layers due to the extra wait time to build an aggregate. In this paper, we characterize this trade-off by studying the impact of aggregate size on metrics like frame latency and channel utilization. To estimate these metrics, we model the transmission queue of an 802.11n station as a bulk service queuing system. We study the impact of aggregate size over a wide range of operating conditions covering several traffic arrival rates from higher layers, service distribution and data frame sizes. Apart from validating the existence of above-mentioned performance trade-off, our results indicate that the choice of aggregate size not only depends on the traffic arrival rate, but also (more interestingly) on data frame sizes. This calls for a dynamic assignment of aggregate size. Since the inception of IEEE 802.11 wireless local area networks (WLANs) in 1997, wireless networking technologies have tremendously grown in the last few decades. The fundamental IEEE 802.11 physical (PHY) and medium access control (MAC) protocols have continuously been enriched with new technologies to provide the last mile wireless broadband connectivity to end users. Consequently, several new amendments of the basic IEEE 802.11 gradually came up in the forms of IEEE 802.11a, IEEE 802.11b, and IEEE 802.11g. More recently, IEEE 802.11n, IEEE 802.11ac, and IEEE 802.11ad are introduced with enhanced PHY and MAC layers that boost up physical data rates to the order of Gigabit per second. So, these amendments are generally known as high throughput WLANs (HT-WLANs). In HT-WLANs, PHY layer is enhanced with multiple-input multiple-output antenna technologies, channel bonding, short guard intervals, enhanced modulation and coding schemes. The MAC sublayer overhead is reduced by introducing frame aggregation and block acknowledgement technologies. However, several existing studies reveal that, many a time, the aforesaid PHY and MAC enhancements yield negative impact on various upper layer protocols, that is end-to-end transport and application layer protocols. As a consequence, a large number of researchers have focused on improving the coordination among PHY MAC and upper layer protocols. In this survey, we discuss impact of HT-WLAN PHY and MAC layer enhancements on various transport and application layer protocols. This paper also summarizes several research works that use aforesaid enhancements effectively to boost up data rate of end-to-end protocols. We also point out limitations of the existing researches and list down different open challenges that can be meaningfully explored for the development of the next generation HT-WLAN technologies. The IEEE 802.11n standard aims at providing a data transmission rate of up to 600 Mbps. The frame aggregation techniques of IEEE 802.11n MAC layer play an important role in the enhancement of channel utilization. In this paper, we investigate the effect of IEEE 802.11n frame aggregation for two aggregation mechanisms: MAC protocol data unit aggregation (A-MPDU) and MAC service data unit aggregation (A-MSDU). We propose an analytical model based on an enhanced discrete time Markov chain (DTMC) model in order to describe the post- backoff behavior due to frame aggregation, and then evaluate the throughput performance. Analytical results based on the mathematical model are verified through simulation. The throughput performance of the A-MSDU mechanism outperforms the A-MPDU mechanism as the frame aggregation size becomes larger under an error-free environment. In addition, the throughput performance is significantly improved as the frame aggregation size increases. Multi-user MIMO allows the simultaneous transmission of several link-layer frames directed to different users by exploiting the channel's spatial streams, increasing proportionally the effective channel capacity. Nevertheless, the system performance can be further increased if several frames directed to the same user are combined together on a temporal scale (i.e. packet aggregation), taking benefit from both space and time dimensions. In this paper, a novel finitebuffer space-time general-distributed batch-service queueing model for such a system is presented. Results show the flexibility and accuracy of our queuing model, providing also a first insight on the relation between the space and temporal aggregation parameters and how they can be adjusted to maximise the system performance.