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A better path to video-over-WLAN

Posted: 03 Apr 2006 ?? ?Print Version ?Bookmark and Share

Keywords:gil ephstein? metalink? voice over wlan? wlan? wireless lan?

Multimedia sharing is critical to the convergence of consumer electronics and wireless. As digital video recorders gain popularity, consumers expect to be able to access stored video anywhere in the home.

This calls for a wireless networking technology that can provide high bit rates to support the distribution of video and HDTV streams from a central location, along with total home coverage. Video applications can't tolerate bandwidth fluctuations, so guaranteed bandwidth and QoS are important requirements. Moreover, a wireless network should provide wire-like performance regardless of changing environmental conditions.

The existing IEEE 802.11a/b/g-based wireless products can't meet these and other challenges in the new home environment. Solutions that attempted to use WLAN technology for video distribution failed to meet consumer expectations for link range and picture quality. Thus, new techniques were developed to support high-quality video applications with higher bandwidth, lower jitter, reduced latency and extended coverage.

MIMO merits
Higher throughput is not the remedy for all issues related to video delivery over WLANs, but it is a major step toward a robust solution. Higher throughput provides much better immunity to interference, along with the means to handle degraded link conditions. Excess bandwidth can also be traded for extended reach and lower power consumption. Options!including multiple-input multiple-output (MIMO), channel bonding, the move to 5GHz, advanced forward error correction, improved MAC efficiency and jitter cancellation, and clock recovery!are available to achieve that.

MIMO!On the transmission side, MIMO encodes a single high-rate data stream by splitting and transmitting it across spatially separated antennas. Having two streams instead of one enables either the delivery of twice the throughput by keeping the same rate for each of the streams or extending the reach of the original stream, since each of the lower-rate streams can use lower constellations and require a lower SNR to be recovered. On the receive side, the MIMO receiver uses algorithms to recover the transmitted signals and combine them into a single stream.

The main advantages of MIMO are higher data transmission rate by a factor equal to the number of transmit streams and its ability to establish a wireless connection in multipath environments.

Channel bonding!Shannon's capacity law indicates that the theoretical capacity limit increases linearly with bandwidth. Thus, the simplest solution to increase the rate of any given system is to expand its operating bandwidth. For WLAN systems, this is usually called channel bonding because the extended bandwidth is achieved by bonding two adjacent 20MHz channels into a single 40MHz channel. The bandwidth increase is actually more than double, since the guard band between the two bonded channels can also be removed.

Moving to 5GHz!The 5GHz band offers more than twenty 20MHz channels in most parts of the world, which allows the support of far more users!much higher bandwidth per user and higher immunity from interference.

Advanced forward error correction!Using the optional low-density parity check (LDPC) provides coding gain that is higher by approximately 3dB than the legacy convolutional code of other IEEE 802.11 standards. An LDPC code is a linear block code specified by a very sparse parity-check matrix. LDPC has already been verified and adopted by DVB-S2 satellite broadcast and 10GbE-over-copper system specifications.

Improved MAC efficiency!The 802.11 MAC and PHY layer has fixed overhead regardless of packet size. Reducing overhead is one of the main concerns in enhancing the current 802.11 WLAN standards. MAC efficiency of IEEE 802.11a/b/g is typically about 50 percent under the best conditions. It can be increased to 70 percent by using an aggregation scheme for packets assigned to the same destination, thus eliminating the overhead linked to each packet and replacing it with a common overhead.

The common overhead associated with each multiple MAC protocol data unit (MPDU) transmission of IEEE 802.11a/b/g is now associated with a large number of MPDUs. This proportionally increases the efficient throughput. Up to 32 MPDUs with the same destination address and priority are aggregated into a single, concatenated payload called an aggregated MPDU.

Aggregate exchange sequences are made possible with a protocol that acknowledges aggregated MPDUs with a single block ACK instead of multiple ACK signals. This protocol effectively eliminates the need to initiate a new transfer for every MPDU.

Jitter cancellation and clock recovery!Most video broadcasts use the MPEG-2 transport standard. To be able to correctly interpret the MPEG presentation and timing information, the decoder clock needs to be locked on the encoder clock. Otherwise, overflow and underflow might occur at the decoder buffer, which risks creating packet loss.

Most common decoders can't lock on the encoder clock when jitter exceeds 500ns. When passing through an asynchronous packet network like a wireless network, a much higher jitter might be introduced and a jitter cancellation mechanism is required.

Better solution
HCCA uses another approach to guarantee QoS. Instead of waiting for an idle time for transmission and using a back-off mechanism, it relies on centralized control by the access point that can guarantee time and duration of transmission for each of the connected stations. Every station wanting to join the network must request permission from the central access point. This request includes a traffic specification that details the QoS required by the station.

The access point then determines if it can support the requested QoS specifications and admits or denies a station. The access point maintains a centralized schedule based on the QoS requirements of all of its registered stations. Then, the access point notifies each of the stations about the time it will have access to the wireless medium.

Since this process is managed from a central location, it is guaranteed that the access will be contention-free. Because everything is predetermined upon registration, HCCA can guarantee bandwidth, jitter and latency!which is otherwise a difficult challenge in a mixed-data and multimedia environment.

- Gil Epshtein
Sr. Product Manager
Metalink Corp.

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