Microwave Engineering Europe - November 2007 - (Page 29)

WIRELESS VIDEO 29 QoS. Several enhanced QoS mechanisms must be used on top of the basic QoS foundation to address some key issues. There are multiple QoS strategies to consider. The ﬁrst step is to operate in the low-interference 5-GHz band, with its high channel availability and reduced exposure to interferences from other types of equipment operating in the same frequency range. Next, a number of IEEE QoS standards must be employed. These standards mitigate the problems associated Figure 1: Additional gain can be used to increase throughput (using higher constellation) or to increase the robustness and immunity to interference. 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 Microwave Engineering ● November 2007 ● www.mwee.com 028_029_030_MWEE.indd 29 25/10/07 14:09:18 http://www.spinner.de http://www.spinner.de 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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