By Azimuth Systems
Simple connectivity has helped make IEEE 802.11-based wi-fi the network technology of choice where data cabling is scarce. No longer tethered to a desk, you can browse the Net or collaborate with colleagues from anywhere there is a wi-fi signal. Could the next step in corporate conferencing be broadband video over wi-fi?
To ensure success of video over wi-fi, the IEEE — the international standards body for all things electronic — is developing new standards for performance and performance verification; 802.11n will specify faster transmission sufficient for high-resolution video and 802.11t will specify test metrics and ensure that network performance is adequate. Success of video over wireless will depend on how well the emerging IEEE 802.11 standards will address the requirements of video.
Streaming video and videocon-ferencing are the key digital video applications.
Streaming video includes Broadcast TV, DVD and HDTV video distribution, web-based video and even handheld TV broadcasting based on the emerging DVB-H protocol. Streaming video requires sufficient data throughput with a low rate of packet loss.
While packet loss is virtually unnoticeable to data applications such as web browsing or e-mail, it can significantly degrade video quality. Packet-loss rate can translate into six fold video-frame loss rate, since video frames span multiple data packets.
Videoconferencing applications, being real-time and bi-directional, introduce the added requirement of low latency and jitter on the network. These requirements are similar for voice applications with the limits of about 150 milliseconds (ms) for latency and 50 ms for jitter.
HDTV distribution is the most bandwidth-hungry application, requiring 20 megabits per second (Mbps). In facilities with multiple HD video units, several HDTV streams propagate simultaneously thus requiring more bandwidth than the current 802.11 a/g technology, which tops out at about 20 Mbps.
In comes the new 802.11n transmission technology based on MIMO (multiple inputs/multiple outputs) that guarantees at least 100 Mbps but can deliver up to 600 Mbps depending on the complexity of the 802.11n radio and on the environment.
MIMO is a very significant innovative advancement in wireless data transmission. It turns the longtime nemesis of wireless — multipath — into a friend. Multipath is a common occurrence indoors, where the wireless signal reflects from surfaces, thus creating multiple signals that add together in the air. While today’s 802.11 a/b/g radios struggle to separate the original signal from this muddle, the MIMO radio actually takes advantage of multipath to send multiple data streams via the available paths.
Multipath creates multiple versions of the signal by virtue of reflections from walls, floors, ceilings, furniture and people. The reflections add together in the air presenting a challenge to the receiver of separating out the original signal. Until now multipath was a problem that limited operating range. Now MIMO radios actually use multipath to achieve gains in operating range.
Two techniques employed in MIMO are spatial multiplexing and beamforming. Spatial multiplexing sends more than one data stream simultaneously to improve throughput. Beamforming sends multiple versions of same data stream to improving reception. Spatial multiplexing can yield higher throughput but may not work in some environments.
To speed up the work of the IEEE 802.11n committee, the Enhanced Wireless Consortium (EWC) has organized to work in parallel with the IEEE to help vendors get consensus on the MIMO coding scheme. EWC membership includes 27 wi-fi industry heavyweights such as Cisco Systems Inc., Broadcom Corp., Atheros Communications Inc., Intel Corp. (and test equipment companies such as Azimuth Systems Inc.)
To ensure success of video over wi-fi, the industry needs well-defined test metrics. To that end, the IEEE 802.11t task group was formed in July 2004. The efforts of 802.11t are driven by the industry giants with a vested interest in voice and video over wi-fi — companies such as Dell Inc., Microsoft Corp., Intel and Broadcom. Azimuth Systems was the founding member of 802.11t.
Corporations and consumers want to run video over wi-fi, taking advantage of both high-quality videoconferencing and the next-generation TV sets with digital tuners and integrated wi-fi radios. Service providers offering video over wi-fi understand that testing is key to success and that IEEE 802.11t will help get the new wireless technology ready for its appearance in the big time.
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