|عنوان فارسی مقاله:||MARS: انتخاب سرعت لینک–لایه برای انتقال چندرسانه ای درشبکه های بی سیم مش|
|عنوان انگلیسی مقاله:||MARS: Link-layer rate selection for multicast transmissions in wireless mesh networks|
|رشته های مرتبط:||مهندسی کامپیوتر و فناوری اطلاعات، شبکه های کامپیوتری، سامانه های شبکه ای و سیستم های چند رسانه ای|
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|نشریه||الزویر – Elsevier|
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IEEE 802.11 devices dynamically choose among different modulation schemes and bitrates for frame transmissions. This rate adaptation, however, is restricted only to unicast frames. Multicast (and broadcast) frames are constrained to use a fixed low bit-rate modulation, resulting in low throughput for multicast streams. Availability of high bandwidth and efficient use of the medium is crucial to support multimedia multicast streaming applications such as IPTV, especially in multihop mesh networks. To address this problem, we propose a rate adaptation algorithm for multicast transmissions in these networks. The proposed algorithm, MARS, is distributed in nature, and relies on local network measurements to select a transmission bit-rate for a given multicast group. The algorithm also facilitates the joint use of bit-rate selection and link-layer mechanisms such as acknowledgements and retransmissions to improve reliability of high throughput multicast streams. Based on implementation and evaluation on a testbed, the algorithm provides up to 600% gain in throughput compared to traditional 802.11 networks in some scenarios. Additionally, the algorithm can support multicast streams while consuming a small fraction (20%) of the resources compared to the basic 802.11 operation.
Wireless mesh networks (WMNs) are touted to provide the next generation of ubiquitous network connectivity. IEEE 802.11-based WMNs enable service providers to extend connectivity to a wide geographical region with minimal cost by avoiding installation of cables and expensive base-stations. Numerous towns and cities around the world have deployed or plan to deploy IEEE 802.11-based WMNs to blanket the region with WiFi access. A parallel trend in network usage is the increased popularity of multimedia applications such as voice and video. The explosive growth in the usage of websites such as youtube.com is a testament to this trend. As the usage of these applications increases, it becomes necessary to support these applications on WMNs as well. A key characteristic of several multimedia streaming applications is their use of multicast transmissions for transportation of content. Multicast streams are particularly important when several clients wish to receive the same audio/video stream, e.g., a live stream of a football match, IP-based radio station, IPTV, webcast of a live TV (SkyPlayer from Sky Networks, UK). The ITU focus group for IPTV standardization recommends the use of IP multicast for the live video mode of operation of IPTV . Multicast streams are also supported by popular multimedia streaming servers such as Windows Media Services, SHOUTCast and VLC. These multicast streams are characterized by their requirement of high bandwidth and low latency. In addition, these streams cannot tolerate high packet loss rates. In this context, we examine the ability of 802.11 WMNs to support high bandwidth, low latency, low loss multicast streams. In a multi-rate 802.11 network, devices are capable of transmitting packets using different modulation schemes and bit-rates varying from 1 Mbps to 54 Mbps. These devices dynamically select an appropriate transmission bit-rate using a rate adaptation algorithm to obtain high bandwidth. However, as per the 802.11 standard, the use of rate adaptation algorithms is currently restricted to unicast frames only. Multicast and broadcast transmissions are forced to use a fixed lower modulation bit-rate, usually the lowest bit-rate of 1 Mbps in a 802.11b/g network.1 This restriction has two effects related to multicast streams. First, it inherently limits the capacity available for a multicast stream, and thereby the maximum achievable bandwidth. Second, a stream that uses a bit-rate of 1 Mbps consumes a disproportionate fraction of airtime for transmission of its packets, and therefore may adversely impact other existing flows in the network, possibly even causing network congestion. Further, 802.11 multicast frames, unlike unicast frames, are transmitted without requiring an acknowledgement (ACK frame) from the receiver(s). This absence of feedback from the receiver inhibits the automatic error recovery through link-level retransmissions, and directly impacts the reliability of the multicast application streams. For these reasons, multicast-based streaming applications have generally been considered unsuitable for 802.11-based WMNs. Therefore, in order to support high throughput multicast streams, there is a need for a method to transmit multicast frames with low cost to the network. In addition, it is desirable to increase the degree of link-level reliability for multicast packets, similar to that provided by retransmissions for unicast frames. To this end, we propose Multicast Auto Rate Selection (MARS), wherein multicast transmissions in a wireless mesh network utilize higher modulation rates in order to increase the throughput of multicast flows. Our motivation for this approach is driven by the insight that mesh routers in most networks are deployed with careful planning, and in such networks, links among neighboring mesh routers are frequently of high quality to support higher modulation rates. Further, the mesh backhaul nodes are stationary, and link quality among neighbors can be measured quite accurately. MARS is a measurement driven scheme that actively tracks the quality of the links between a mesh router and its neighbors. Using this measurement, MARS selects the best transmission rate for each multicast group based on link quality to the members of that group. We equip MARS with the ability to retransmit multicast frames to increase link-level redundancy and thereby improve reliability. This flavor of the protocol is called MARS-R. The transmitter node selects one of the receiver nodes to respond with an ACK on successful packet reception. These schemes address the challenge of maximizing end-end throughput using a distributed solution, given the diverse set of link qualities throughout the network. Our contributions in this work are as follows: We motivate the case for rate adaptation of multicast frame transmissions in 802.11-based WMNs, via an analysis of link qualities in real networks. We propose MARS, a distributed measurement-driven rate adaptation approach for multicast transmissions. Further, the MARS-R enhancement enables retransmissions for such frames. We provide a practical implementation of MARS. Performance evaluation on a testbed shows that we are able to obtain about 600% increase in maximum endend throughput for some scenarios. MARS provides the same throughput performance as traditional 802.11 but requires only 20% of the transmission time. We find that the use of retransmissions and ACKs in MARS-R significantly increases the packet delivery ratio, and the associated additional overhead does not adversely affect performance. The remainder of the paper is organized as follows. Section 2 surveys related work. We motivate the case for multicast rate adaptation in Section 3. The details of the design and implementation of MARS are presented in Sections 4 and 5, respectively. We evaluate the performance of the algorithm in Section 6. Section 7 concludes the paper. We note that throughout the paper the term bit-rate refers to the physical modulation rate used to encode the 802.11 frames, frame refers to a MAC layer entity, and packet refers to a network/application-layer entity. 2.