Richard MacKenzie

Throughput and Delay Analysis for p-Persistent CSMA with Heterogeneous Traffic

CSMA/CA is a common access mechanism for contention based wireless channels. One of the ways in which quality of service in contention based wireless networks is achieved is by service differentiation. This can be realised by prioritising packets based on their importance while taking into account the network load. p-persistent CSMA is a form of CSMA/CA and in this paper an analytical model for the throughput and delay of a p-persistent CSMA system is developed where the system contains a finite number of non-saturated stations with both heterogeneous loads and priorities. Results based on this new model show how the throughput and delay for the traffic flows on the network are affected by the relative p-persistence and offered load values for all of the stations in the network.

QoS of Video Delivered over 802.11e WLANs

Many home networks now use 802.ll wireless local area networks (WLAN)s. The 802.11e amendment has been designed to improve quality of service (QoS) over these networks allowing for multimedia applications such as IPTV to be better supported. The H.264 video compression standard is suitable for IPTV due to its high compression and error resilience. Video packets of different slice types are of varying importance to the decoded video quality so can be mapped into different priority queues using 802.11e enhanced distributed channel access (EDCA). We investigate several mapping schemes for a variety of video content to see how the quality of the decoded video is affected as the number of concurrent video connections is increased. The different mapping schemes exhibit different loss patterns in the video sequences and their impact on the video quality is content dependent. Subjective tests were therefore carried out which allow us to perform an accurate and valid assessment of the video quality. Packet loss rate is also reported. Our results show that as the number of concurrent videos approaches the network capacity some mapping schemes show a cliff-edge drop in quality while others offer a more acceptable gradual quality degradation. These schemes cause B-frames to be dropped and in effect reduce the video frame rate. These schemes are more successful for videos that have low or medium temporal activity.

Throughput Analysis of a p-Persistent CSMA Protocol with QoS Differentiation for Multiple Traffic Types

With the widespread success of 802.11 wireless local area networks (WLANs) and the need for quality of service (QoS) the 802.11e amendment was created. This amendment specifies the enhanced distributed channel access (EDCA) procedure which provides QoS through service differentiation. The underlying mechanism for the contention based access of 802.11 and 802.11e is carrier sense multiple access with collision avoidance (CSMA/CA). p-persistent carrier sense multiple access (p-persistent CSMA) can be used to model CSMA/CA. This paper modifies the p-persistent CSMA mechanism to provide service differentiation in a saturated network. This is a new general model for providing QoS through contention based access. This p-persistent CSMA model is then converted into a model for an 802.11e network. Simulations and comparison with other 802.11e models validates this model while the benefit of this model is that it can be modified readily to characterise service differentiation in a non-saturated network.

Achieving service differentiation in IEEE 802.11e enhanced distributed channel access systems

The IEEE 802.11e standard allows for contention-based quality of service (QoS) provisioning using enhanced distributed channel access (EDCA). By tuning the contention parameters for EDCA, the system can adapt to changes in the network scenario. The benefits of this adaptation include the ability to maximise the system capacity while also achieving the service differentiation necessary to provide QoS. The use of fixed sized contention windows is one simple and effective way to improve EDCA performance and this can be modelled as p-persistent carrier-sense multiple access (CSMA). In this study a straightforward throughput and delay analysis is provided for p-persistent CSMA that can be easily applied to an EDCA system within the rules of the current 802.11 standard. Several versions of the analysis are provided to show how further parameters of EDCA can also be included and their effects on system performance are shown. These further parameters include transmission opportunities, internal collision resolution, AIFS differentiation and retry limits. Any number and combination of these further parameters can be included at the same time making this new form of analysis extremely flexible as well as accurate to model a wide variety of EDCA system scenarios.

Throughput and Delay Analysis for a Differentiated p-Persistent CSMA Protocol with the Capture Effect

With the extensive deployment of 802.11 wireless local area networks (WLANs) and the need for better quality of service (QoS), the 802.11e MAC with service differentiation was developed. Analysing the effect of fading, shadowing and near-far effect on the performance of 802.11 is a fundamental consideration in practical situations since the wireless channels are errorprone.This paper develops a p-persistent CSMA protocol, which allows for service differentiation by using multiple traffic types, into an accurate and practical model that takes into account the capture effect in a Rayleigh multipath fading environment. A new analytical approach is developed to calculate the throughput and packet delay for multiple traffic types. Results provided for this new model show that the capture effect increases the throughput by 32% in a highly loaded system thereby demonstrating the importance of including the capture effect in wireless protocols

Effectiveness of H.264 Error Resilience Techniques in 802.11e WLANs

802.11 wireless local area networks are now common in the home. The 802.11e amendment allows for quality of service (QoS) provisioning over these networks to help meet the QoS demands of the growing number of multimedia applications on these home networks. The H.264 video coding standard is suitable for most multimedia applications due to its high compression and error resilience. This paper investigates how the quality of H.264 video is affected as the number of concurrent video streams sent over an 802.11e network is increased. We compare two traffic mapping schemes with and without the use of the flexible macroblock ordering (FMO) tool of H.264. We show that the mapping scheme which prioritises video packets based on their frame type is more successful at maintaining the quality of video when congestion occurs. A more gradual degradation in quality can be achieved rather than the cliff-edge drop that otherwise occurs. We also identify that errors caused by congestion tend to occur towards the bottom of each frame and that FMO can reduce this effect.

An evaluation of quality of service for H.264 over 802.11e WLANs

802.11 wireless local area networks are now a common feature in the home. In order to meet the quality of service (QoS) demands for the increasing number of multimedia applications on these home networks the 802.11e amendment was developed. A suitable video coding standard for these multimedia applications is H.264 due to its high compression and error resilience. In this paper we investigate how the quality of H.264 video is affected as the number of concurrent video streams sent over a multi-rate 802.11e network is increased. Several packet mapping schemes are compared. We show that the mapping schemes which differentiate video packets based on their frame type are more successful at maintaining acceptable video quality when congestion occurs, providing a more gradual quality degradation as congestion increases rather than the cliff-edge quality drop that tends to occur with the other mapping schemes. These differentiated schemes are more successful for videos that do not have a high amount of temporal activity. We also identify that impairments caused by congestion tend to occur towards the bottom of each frame when the flexible macroblock ordering (FMO) feature of H.264 is not used but the use of FMO can reduce this effect.

Packet handling strategies to improve video QoS over 802.11e WLANs

In this paper we look at several techniques that can allow for the quality of downloaded video streams to be maintained when an 802.11e MAC becomes congested. These techniques come from two distinct groups: Packet mapping schemes and packet dropping schemes. The packet mapping schemes determine which 802.lle enhanced distributed channel access (EDCA) queue each video packet joins. One mapping scheme treats all packets equally, while the other differentiates the video packets of different slice types. The packet dropping schemes aim to avoid MAC layer congestion by dropping some of the packets before arriving at the MAC. Two packet dropping schemes are used which both differentiate video packets by trying to drop the lower importance packets first. One packet dropping scheme applies an even amount of loss to each video stream while the other applies an even statistic which results in an uneven amount of loss to each video stream. Results show that all of the schemes that differentiate video packets allow for a more gradual video quality degradation than would otherwise occur. The packet dropping schemes also provide the best delay performance. Results for each scheme are content dependant and the packet dropping scheme that results in uneven amounts of packet losses is the only scheme which treats all types of content well. We show that at high loads this scheme can offer a system performance with a mean opinion score that is at least 0.5 greater than any of the other schemes can provide.

Non-saturated throughput analysis for a QoS differentiated p-persistent CSMA protocol with the capture effect

Analysing the performance of WLANs in non-ideal channel conditions is a fundamental consideration in practical situations since wireless channels are error-prone. This paper presents a closed form analytical solution for the non-saturated throughput of a QoS differentiated p-persistent CSMA protocol that takes into account the capture effect in a Rayleigh multipath fading environment. The results show that the throughput performance with the capture effect depends on the absolute and relative p-persistent values, the total system offered load and relative offered loads per traffic type, as well as the thresholds for capture.

The viability of providing mobile broadband with cognitive femtocells

The viability of providing mobile broadband with cognitive femtocells is evaluated by performing a business case analysis for a scenario where a Fixed Broadband Operator (FBO) uses this approach to extend its offering to include a mobile broadband service. The collective coverage provided by randomly located cognitive femtocells is determined and used to estimate how much of the traffic that will go over the cognitive femtocells and therefore generate revenues for the FBO. With the assumptions made, the cognitive femtocell based mobile broadband service is found to be a reasonable business case for the FBO. The investments over the first few years are moderate and the accumulated cash flow becomes positive after four years. The sensitivity analysis shows that the subscription fee and the general Operational Expenditure (OPEX) are the most critical assumptions in the business case analysis, and that the cost of the cognitive femtocells and their coverage range in suburban areas has a significant influence on the profitability.