[Leapfrog: First Look]
Chip Sets Hurdle QoS Issues To Deliver Wireless Video
By improving transmission and reception quality, two companies developed chip sets that handle standard and HDTV video data streams over wireless networks.
Standard 802.11b networks can only handle one standard video data stream. Yet attempts to replace them with the higher-speed 802.11a wireless connections, which can manage multiple standard video streams or two HDTV streams, have hit roadblocks.
Atmospheric conditions, objects in the signals' path, and antenna angles cause fading and dropouts. Taking different approaches, Magis Networks Inc. and ViXS Systems have developed chips that eliminate these quality-of-service (QoS) issues to deliver high-quality A/V streams over wireless links.
Magis' Air5 system provides wireless connections at data rates of up to 54 Mbits/s over a time-domain multiple-access (TDMA) proprietary link that uses the clear 5-GHz frequency band. It permits multiple streams of DVD, cable and satellite digital video, audio, and data to be delivered over the wireless links without degrading quality.
The tack ViXS takes in its XCode II chip set is independent of the communications link. It encodes and performs transrate conversions of multiple streams of analog and digital content in real time. By adjusting the data bandwidth, multiple high-quality streams can be transmitted on a single channel. This also reduces the amount of memory needed to store the content if the data stream is being written to a hard drive or other storage medium.
The Air5 approach comprises two chip sets. The MGS6200 targets high-end systems that would handle multiple HDTV data streams at data rates of up to 40 Mbits/s. The MGS5200 aims at entry-level systems that handle standard video data streams and maximum data rates of 16 Mbits/s. Each chip set consists of a baseband controller housed in a 324-contact LBGA package and an RF front end in a 97-contact land-grid-array package (Fig. 1).
The MGS6228 RF front-end chip is the same for either set, though the baseband processor is different for each. The MSG6218 handles the 40-Mbit data streams as part of the MSG6200 chip set. The MSG5218, the other half of the MSG5200 chip set, only handles data rates up to 16 Mbits.
Aside from the differences in the data rates and thus the number of data streams the baseband chips can handle, they're functionally similar. One additional difference, though, is that the MSG5218 can be used to connect two pieces of A/V equipment together, while the MGS6218 can connect multiple pieces of A/V equipment.
SECURITY SOLVED Both baseband chips contain a tripleDES encryption/decryption engine that secures sent content, avoiding troublesome theft-of-service issues for content providers. Moreover, it prevents accidental viewing by another receiver since it locks the data stream to a particular receiver. Additional network entry, authentication, and authorization is possible through the Air5 application programming interface.
The media-access controller on the baseband chip is isochronous. It uses pre-assigned time slots to organize and orchestrate access to the network, eliminating time overheads associated with packet routing in TCP/IP wireless networks. The transmitter has knowledge of each receiver, and it can transmit at different bit rates and with different content protection/security on each channel.
The key to the improved QoS is in the MGS6228 RF front end. The novel RF transceiver employs a Crawford spatial wavefront receiver that uses five antennas and two full receive channels to eliminate multipath (ghost) signals. It does this by using the five-antenna array to capture the RF signals and then select the best two of five signals.
This approach actually takes advantage of the multipath signals as opposed to schemes that try to eliminate the multipath signals. It samples the signals every millisecond to continually optimize system reception. After the two signals are selected, they're fed into separate independent receive channels that amplify, filter, frequency-convert, and eventually feed them to the baseband processor. The baseband chip converts the two analog signals into digital streams and then, using DSP techniques, combines them into one robust data stream.
In addition, the baseband chips use dynamic channel selection to further improve signal quality when multiple systems operate in close proximity. When a system is set up, it scans the available channels for one that's not in use by either an Air5 or 802.11 wireless local-area network (LAN). The chip then continuously monitors all channels for possible interference. When potential interference is detected, the chip looks for another unused channel.
Rather than use a proprietary wireless link, the ViXS XCode II can use standard wireless LAN interfaces such as the Intersil Indigo 5-GHz wireless LAN chip set. To leverage the standard interface without data and QoS losses, the XCode II chip efficiently processes content ranging from analog to high-definition digital video and transfers multiple streams to any display device or storage system over the wireless link.
The XCode II is the first chip that can encode and transrate multiple streams of analog and digital content in real time (for example, transrate from high-definition to standard definition). It also can measure and monitor the network bandwidth to ensure QoS. An on-chip encryption/decryption engine lets it maintain content security and provide date encryption/decryption of clear data streams.
To handle all the functions, designers at ViXS combined a MIPS RISC processor, video encoders, transcoding logic, adaptive filtering, and transport demultiplexers and remultiplexers. Also in the mix are a double-data-rate memory controller, a 32-bit/33-MHz PCI bus, an I2C controller, an 8/10-bit CCIR 656 video input, and transport stream output (Fig. 2). All this comes in a 385-contact plastic ball-grid-array package.
Reprints
