Microwave Engineering Europe - November 2007 - (Page 30)

30 WIRELESS VIDEO 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 ContentionFree 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 mediumaccess mechanisms. The ﬁrst is contentionbased 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. 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 contention-free, 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 openloop 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 ever-changing 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. Company Information MetaLink Ltd. www.mtlk.com MICROWAVE ENGINEERING EUROPE Free subscription at: www.mwee.com/subscribe MWEE newsletter at: www.mwee.com/newsletter Microwave Engineering Europe ● November 2007 ● www.mwee.com 028_029_030_MWEE.indd 30 25/10/07 14:09:37 http://www.mtlk.com http://www.mwee.com/subscribe http://www.mwee.com/newsletter http://www.mwee.com

Table of Contents Feed for the Digital Edition of Microwave Engineering Europe - November 2007

Microwave Engineering Europe - November 2007 Contents News Comment Metamaterials: Metamaterials Tackle Communications Wavelengths Microwave Components — EM tools: Microwave Component Design Easier With New EM and EDA Tools Cover Feature: RF Testing for OFDMA in LTE Base-Stations Startup Eyes Battery-Free Wireless Sensor Nets High-speed ADC Technology Paves the Way for Software Defined Radios Planning a WiMAX network: Maximising the ROI by Using Advanced Optimisation Tools Transporting Video Over Wireless Networks Ultrawideband Under the Gun Specifying the Proper SAW Filter Products Product Feature: RF Test Solution Supports Emerging 4x4 MIMO as Well as Multiple Commercial Standards Calendar

Microwave Engineering Europe - November 2007

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