Embedded Systems Design Europe - October 2007 - (Page 38)

wireless video EDCA is fairly simple to implement, but it can’t guarantee tolerable latency, jitter, or bandwidth levels, and it has no means to handle several applications with the same priority level. HCCA offers signiﬁcant improvements over EDCA but it, too, is inadequate on its own. HCCA relies on a centralized control in the access point (functioning as the HC, or Hybrid Coordinator) that can guarantee the time and duration of the transmission for each of the connected stations. Every station requests access permission from the central AP, accompanied by a trafﬁc speciﬁcation that details the required QoS. The access point then determines if it can support the requested QoS specs and admits or denies the station. Because this process is managed from a central location and predetermined upon registration, access is guaranteed to be contentionfree, and bandwidth, jitter, and latency are all controlled. One problem with HCCA is that it can’t work with a neighbor legacy network. Alternatively, the best approach is a combined solution based on EDCA with the addition of admission control. EDCA already assures that higher priority packets gain access to the medium sooner and, as a consequence, low-priority services don’t hurt the high-priority service’s performance. SUFFICIENT RESOURCES By adding HCCA’s admission control, system resources will always be sufﬁcient for two high-priority services, and high-priority service will never hurt the performance of the existing service with the same priority. For example, admission control will evaluate the system’s resources for simultaneous video and data services, and only allow a second video stream when there are sufﬁcient resources. Beyond 802.1e QoS support, system designers can optimize QoS by using fast link adaptation, which, like legacy rate adaptation, is designed to accommodate the transmitted data (PHY) rate with the channel momentary conditions. Legacy rate adaptation included a proprietary open-loop algorithm, where the transmission station optimized its rate according to MAC counters and sophisticated PHY metrics. In contrast, the fast link adaptation is a closed-loop mechanism – the transmitter deduces the optimized rate based on indications from the receiver. The IEEE 802.11n draft standard deﬁnes the mechanism for exchanging information between the two stations and allows its implementation to be vendor-dependent. By combining fast link adaptation with rate adaptation, it’s possible to achieve a dynamic QoS mechanism that adapts the bit rate based on actual packet-error rate and link conditions. The fast indication can be used by upper layers to take actions and to ensure that the application copes with the available bandwidth. This feature is particularly important in an everchanging home environment. There’s one last QoS approach to consider, in the area of client-to-client communication. This is managed by the AP using a Dynamic Link Setup (DLS), which saves airtime and increases network efﬁciency. In home environments, every device should be able to communicate with any other device within the home. Increased network efﬁciency supports more services while reducing a “hop” through the AP, which improves the performance of delay-sensitive applications. DLS reduces latency because it can support any-to-any device while providing a choice between different connection paths when, for instance, the user changes the channel, rewinds and fast-forwards, or uses gaming commands. Gil Epshtein (GilE@mtlk.com) is the director of product marketing at Metalink. 38 OCTOBER 2007 | embedded systems design europe | www.embedded.com/europe 035-036-037-038_ESDE.indd 38 10/10/07 10:47:57 http://www.techonline.com/virtualab http://www.techonline.com/virtualab http://www.embedded.com/europe

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Embedded Systems Design Europe - October 2007 Contents Linux Set to Dominate Torvalds Updates Linux Kernel ARM Establishes Smart Card Foundry Program Emerson Buys Motorola's Embedded Comms Group LynuxWroks and TTTech to Cooperate on Avionics MontaVista CEO Looks for Acquisitions in Europe Ready: Multiprocessing Technology Provides Opportunity Automotive to Drive MCU Market New Supporters Join COM Express Group Analyst Weighs TI Versus Xilinx Versus PicoChip Cover Feature: Embedded Systems Security Has Moved to the Forefront Trace Exposes the Toughest Real-Time Bugs Employ a Secure Flavor of Linux Use an MCU's Low-Power Modes in Foreground/Background Systems Transporting Video Over Wireless Networks New Products Advertising Contacts

Embedded Systems Design Europe - October 2007

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