Video over Wi-Fi
Wi-Fi is all around us, destined to be as ubiquitous as terrestrial broadcast. Can it eventually replace broadcast? Could it make DTV obsolete in a short time? To find out, we need to understand the elements of the system, and see why it's become so widespread.
First things first. Contrary to widespread belief, the term “Wi-Fi” does not mean “wireless fidelity,” though that phrase has been used to promote the system. Wi-Fi is a name trademarked by the Wi-Fi Alliance, a global, nonprofit organization whose goal is to drive the adoption of a single, worldwide-accepted standard for high-speed wireless LANs.
By various accounts, Wi-Fi is not short for anything, having been developed by a branding consultancy searching for a catchy name. Others put the origin at a contracted variation of “wireless physical layer.”
Naming aside, Wi-Fi operates in accordance with the 802.11 set of IEEE standards, with amendments including nearly every letter of the alphabet — 802.11a, 802.11b and so forth. The standard employs orthogonal frequency-division multiplexing (OFDM) — used in DSL services, DVB and integrated services digital broadcasting (ISDB) — with varying bands, modulation and occupied bandwidths. The most popular variants are listed in Table 1.
In the 2.4GHz spectrum, 802.11b, 802.11g and 802.11n each specify 14 overlapping channels, with center frequencies 5MHz apart. The spectral mask for 802.11b requires that the signal be attenuated by at least 30dB at ±11MHz and at least 50dB at ±22MHz from the center frequency. The masks for 802.11a and 802.11g have related, though somewhat more involved, requirements, with the result that an 802.11a/b/g product occupies the equivalent of about five channels. Thus, in a crowded situation, communications in the 2.4GHz band are limited to channels 1, 6 and 11. An 802.11g network is compatible with both 802.11g and 802.11b devices.
The 802.11n standard, expected to be published September 2008, uses multiple-input multiple-output (MIMO) technology that employs multiple antennas and possibly multiple tuners. The D-Link Super G system uses two 802.11g channels to achieve a typical data rate of 40Mb/s to 60Mb/s.
While the 802.11a and 802.11g standards offer the same data rates, the 5GHz 802.11a/n band has more channels and is less susceptible to interference from common devices, such as 2.4GHz cordless phones, cell phones and microwave ovens. Interference from neighboring wireless networks can be a problem in the systems with fewer channels, a problem exacerbated by the popularity of 802.11b. Thus, the 5GHz solutions can provide a better overall wireless connection. However, the higher frequency band may have slightly less range because of the greater signal attenuation through walls.
While it may be obvious that the difference between these technologies and broadcasting is that Wi-Fi is bidirectional, there is a more subtle component, one that makes the connection much more robust. Not only can one communicate in both directions — a must for Internet traffic — but the wireless LAN operates much like a wired LAN, with handshaking an important element. The channels are constantly being negotiated, and if there is interference that disrupts the communications, then the communicating parties are instructed to retransmit the faulty data. Thus, when the channel gets bad, the data does not get corrupted; the data rate just goes down. This is the beauty of the system — reliable, multirate data communications.
Point-to-point vs. broadcast
One of the great attractions of Wi-Fi is that, with a digital connection, any data can be transmitted, including video. The Microsoft Media Center Extender for Xbox is one such product. With a wireless connection to a PC running Windows Media Center, the Xbox console can display video originating from the PC on a TV connected to the Xbox. (See Figure 1.)
The video engine in this case is the game console. However, there are other implementations of this kind of system, where the receiver is a dedicated adapter that connects the display to the wireless network.
Another point-to-point application merely replaces the wired interface used to connect a video source (A/V center or similar) to a display. In this setup, a Wi-Fi video transmitter and receiver provide an analog television user with installation mobility, allowing display placement without a wired connection to the video sources. Using MPEG or similar compression and 802.11a, units can deliver up to 350ft of range (line-of-sight without obstructions) in a small STB-like form factor. Multiple units can be operated simultaneously, and analog NTSC, PAL and SECAM can be supported. (See Figure 2.)
One such unit operates at an actual payload data rate of 4Mb/s, 8Mb/s or 14Mb/s. The Advanced Encryption Standard (AES) is used to assure privacy, and system latency is specified at less than one-second roundtrip. Composite, S-video or HDMI connections are available, and stereo audio can be provided. An integrated IR receiver and IR blaster can be used as well to provide support for remote functions on the video source. Of course, the entire receiver can be integrated with a display, forming a new type of Wi-Fi television. Similarly, various conference-room video projectors now use 802.11b and 802.11g technology to receive video (often at low refresh rates) from PCs without the need for clumsy cabling.
Support for Wi-Fi HD video is just emerging, including HDMI interface. Amimon's WHDI provides wireless HD video connectivity at a quality equivalent to that achieved with HDMI. It uses the MIMO protocol. A demo last year delivered uncompressed 720p content at an equivalent data rate of 1.3Gb/s, from an HD DVD player to a projector. The chip set supports transmission of video at up to 3Gb/s — possibly using lossless or near-lossless JPEG compression — which can support 1080p content. The technology has been demonstrated at ranges of up to 100ft through walls, and has a latency of less than one millisecond.
The most accessible version of this delivery is already in growing use — Wi-Fi connection of a PC. As such, any PC with a wireless card can receive video over the Internet, but some uses are starting to blur the distinction between Internet and broadcast video. Two years ago, Dartmouth College beefed up its Wi-Fi network in Hanover, NH, to support four channels of educational video. Starting with a little more than 600 access points covering 150 buildings in one square mile, the system today comprises 1300 wireless access points. Certain areas of the campus carry a limited selection of channels from DarTV, a service that delivers broadcast television over Dartmouth's data network. Forty channels of both educational and recreational TV are already available on the wired network.
An interesting spinoff of this infrastructure now promises wireless video delivery to video-centric handheld displays, such as the just-released Archos 604 WiFi.
A new Wi-Fi system called WiMAX promises to enable a connection range of at least a few miles from an access point. However, the size and power consumption of the WiMAX receiver still need improvement to service a handheld class of products and users. The FCC recently approved the first WiMAX wireless broadband interface card for notebook computers.
With numerous metropolitan areas now offering ubiquitous Wi-Fi access, it seems conceivable that the original wireless TV system — good old VHF/UHF television, even in digital form — could get serious competition from Wi-Fi networks in the not too distant future, not only for content, but for actual access footprint.
Nonetheless, when channel interference occurs and video data is retransmitted, the lapse in timing must be accommodated somehow. One solution is to switch to lower data rates in the video, but this is complicated by the need to maintain program continuity and the requirement of a dynamic video compression rate. Don't be too surprised if all of these issues are quickly resolved and yet another viable medium for video delivery emerges.
Aldo Cugnini is a consultant in the digital television industry.
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