Embedded Systems Design Europe - October 2007 - (Page 36)

wireless video techniques also contribute to QoS, because higher throughput improves immunity to interferences and makes it easier to handle degraded link conditions. In addition, any excess bandwidth can be traded for longer reach and better power efﬁciency – the more bandwidth, the better. NOT ENOUGH THROUGHPUT At the same time, though, raw bandwidth from higher PHY throughput isn’t enough. What’s needed is higher effective bandwidth at satisfactory levels for the given application, and this requires the additional step of substantially increasing media access controller (MAC) efﬁciency. This can be achieved using an aggregation mechanism that eliminates the overhead linked to each packet and replaces it with a common overhead. Aggregate exchange sequences are enabled with a protocol that acknowledges aggregated MAC protocol data units (A-MPDUs). As a result, there’s a single block acknowledgement (Block ACK) instead of multiple ACK signals, and there’s no need to initiate a new transfer for every MPDU. The result is a MAC efﬁciency of 70% as compared with typical 50% MAC efﬁciency ratings for IEEE 802.11a/b/g, as shown in Table 1. Another key consideration is how far the network can reach; the gold standard is whole-house coverage. Although users can tolerate “dead spots” and limited coverage when using home data networks, dead spots and limited coverage aren’t acceptable for wireless entertainment. Today’s centralized multimedia storage devices are expected to serve as the source for all multimedia content, no matter where in the home it’s being viewed or heard. This means that, unlike data networks, the bit rate can’t drop with increased distance from the access point. In addition, the use of forward error correction (FEC) schemes extends the reach that’s possible at any given data rate. For instance, 3 dB of coding gain derived from using the low-density parity check (LDPC) code translates into about 20% improvement in range. Alternatively, the additional gain can be used to increase throughput (using higher constellation) or to increase the robustness and immunity to interference, as shown in Figure 1. Whole-home coverage at optimal video performance is so critical to wireless entertainment that a key new mandatory test for all wireless entertainment networks should be a dropped-packet performance across the full reach of the typical home environment. CONSIDER QOS The ﬁnal consideration for an optimal wireless entertainment experience is 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 IEEE 802.11a achieves a 25-Mbit/s effective throughput at less than 30 ft., which barely covers the living room of a typical home. Bandwidth-enhancing techniques in the 5-GHz band enable a reach of more than 100 feet at the same rate, with full coverage. 802.11n MIMO 2x3 CB Max PHY throughput Typical application throughput Max PHY throughput @ 30 meter Table 1 802.11g/a 54 25 20 802.11b 11 5.5 4 300 150 60 36 OCTOBER 2007 | embedded systems design europe | www.embedded.com/europe 035-036-037-038_ESDE.indd 36 10/10/07 10:47:30 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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