Embedded Systems Design Europe - October 2007 - (Page 37)

wireless video be employed. These standards mitigate the problems associated with enabling several applications to simultaneously access the same bandwidth without hindering applications that are intolerant to time delays and bandwidth ﬂuctuations. The existing 802.11n protocols use the distributed coordination function (DCF) access method to address some of these issues, but this isn’t sufﬁcient. The DCF protocol implements a “listen-before-talk” scheme based on Carrier Sense Multiple Access (CSMA). Using this scheme, a station ﬁrst listens to see if the wireless medium is idle. If it’s not, the station starts a timer with a random back-off interval that’s based on a predetermined range deﬁned by the network parameters. Each station determines individually when to access the medium. Each device has equal opportunity to access the wireless medium, which works well in traditional data applications. But in video, gaming, and other bandwidth-sensitive applications, this “fairness-access” mechanism risks problems with latency and jitter. A better – but still not adequate – QoS approach for wireless entertainment is the point coordination function (PCF), which provides a mechanism for the prioritization of access to the wireless medium. Access is coordinated by one central Point Coordinator (PC) entity, usually the access point (AP). Access to the wireless medium using PCF is given higher priority than medium access based on DCF. NO CONTENTION Additionally, PCF deﬁnes a Contention-Free Period (CFP) and a Contention Period (CP) that alternate periodically over time. The PCF scheme is used for accessing the medium during the CFP, and the DCF mechanism is used during the less-critical CP. During the CFP, there’s no contention among stations because stations are polled by the central point coordinator for transmission, and they don’t try to access the medium independently. Although this approach better coordinates access, it’s a complex implementation, and many technical issues remain unresolved. PCF didn’t ﬁnd its way into real products resulting in further development of the QoS standards. As a result of the shortcomings of the DCF and PCF methods, the industry has developed the IEEE 802.11e standard. This standard introduces the hybrid coordination function (HCF) for QoS support. The HCF deﬁnes two medium-access mechanisms. The ﬁrst is contention-based medium access, also known as Enhanced Distributed Channel Access (EDCA). The second is controlled medium access (including polling), also known as HCF Controlled Channel Access (HCCA). Like PCF, 802.11e supports the option of two phases of operation (in other words, CP and CFP) for EDCA and HCCA. EDCA is used in the CP only, while HCCA is used in both phases. From concept to completion, We can nd a solution! We optimize your heatsink designs. Complimentary designs, from CFD simulations to nal product! radian ISO 9001:2000 Certified Custom and standard solutions to solve your thermal needs. web: www.radianheatsinks.com · tel: 001-408-988-6200 · fax: 001-408-988-0683 Radian Heatsinks is a division of Intricast Company Inc. Thermal-overlay-ese.indd 1 www.embedded.com/europe 10/3/2007 11:55:32 AM | embedded systems design europe | OCTOBER 2007 37 035-036-037-038_ESDE.indd 37 10/10/07 10:47:43 http://www.radianheatsinks.com http://www.mpl.ch http://www.mpl.ch http://www.radianheatsinks.com http://www.embedded.com/europe

Table of Contents Feed for the Digital Edition of Embedded Systems Design Europe - October 2007

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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