Share HD video via existing networks

01 November 2007

DHS physical layer (PHY) block diagram
DHS physical layer (PHY) block diagram

Networking the home is perhaps the next great consumer technology challenge, but wireless home networks are not adequate for HD (high definition) video

Home users of music, video and other multimedia content are now starting to want to share this content between devices (PCs, games consoles, hi-fi, media servers) over a high-bandwidth network that offers guaranteed QoS (Quality of Service).

Wi-Fi is suitable for sharing internet connections between computers, where network traffic is light and delivered in bursts. HD video needs higher bandwidth and predictable QoS to provide an acceptable viewing experience. Only a wired medium can offer this.

Why Ethernet-over-powerline?
A user has several options when setting up a wired home network. Ethernet-overpowerline (EoP) is emerging as the most viable alternative to other home networking technologies. Firstly, powerlines are ubiquitous; powerline technology turns each electrical socket in the home into a potential network port. EoP products are plug-and-play devices, and involve no rewiring. Powerline products today offer data rates of up to 200Mbit/sec and multiple QoS levels. This makes them suitable for streaming HD and standard definition video. They also provide two levels of content security. A malicious user has to first break into your home and plug into the power sockets, then overcome the encryption that is built in to EoP. Lastly, the TDMA (time division multiple access) technology on which EoP is based ensures guaranteed bandwidth for video transport.

In EoP systems, copper wires that distribute power inside homes also act as the medium to transmit digital data. The system typically operates by superimposing a modulated carrier frequency on the AC signal carried on a powerline.

New algorithms to overcome noise on the powerline channel have made powerlines a viable high-speed digital content carrier. At the same time, silicon computing power is so cheap that these computationally-intensive algorithms can be implemented on a single chip, affordable to the mass market. An EoP system starts with a transmitter that converts digital data from a PC or any network-connected device to analogue line data, and then overlaps the analogue line data with the powerline. At the receiver end, it converts the analogue line data inputted through the powerline to digital signals and transfers them to the appropriate device.

A version of EoP developed by the Universal Powerline Association (UPA), the Digital Home Standard (DHS) specification, is designed for managed and unmanaged in-home powerline networks. It is based on a master-slave control architecture and uses a peer-to-peer architecture for data transmission.

Communication challenges
Design challenges include voltage spikes. The biggest misconception about EoP technology is that because it uses the powerline as a communication method, its performance is related to the quality of electrical power on the powerline. The DHS PHY, however, operates in the 2MHz to 32MHz frequency range of the powerline channel. AC power in homes is usually distributed either at 50Hz or 60Hz. Using the 2MHz to 32MHz spectrum ensures that digital data signals are less susceptible to voltage spikes or fluctuations at the 50Hz/60Hz frequency bands.

Another problem is noise on the powerline. An EoP product must deliver high-quality HD video even if the user plugs in a blender or hair-dryer into power sockets in their home.

The DHS uses four methods to solve the noise problem. Firstly, the UPA’s technology uses 1,536-carrier OFDM (orthogonal frequency division multiplexing) modulation, with modulation densities from 2bit to 10bit per subcarrier applied independently to each subcarrier.

The noise that the powerline channel is subject to is often in bursts, since it largely comes from home appliances that are used infrequently. The UPA’s version performs frequent channel estimation, exchanging training data between the transmitters and the receivers. Channel estimation provides the EoP devices with information about the parts of the powerline channel that are the biggest contributors of noise. Once noise has been detected, the transmitters perform adaptive bit loading to ensure optimal usage of the powerline spectrum.

Secondly, adaptive bit loading refers to adapting modulation parameters for each pair of transmitters/receivers in real-time depending on channel-quality parameters for each carrier. The signal-to-noise ratio is measured for each carrier and the optimum modulation is chosen, with the objective of achieving the maximum transmission speed while maintaining the desired BER (bit error rate). This minimises interference from other connected devices.

Thirdly, forward error correction methods transmit enough information from the transmitter so that in case of data-loss due to noise, the original transmitted data can be recovered at the receiver without retransmission of the original data. The UPA’s DHS specification uses dynamic Reed- Solomon codes to implement forward error correction.

Finally, broadband powerline communication uses the 2MHz to 32MHz frequencies of the powerline spectrum. These frequencies may be licensed to radio services, including amateur radio. The UPA EoP technology provides programmable ‘spectral notching’ that can be used to avoid frequencies not licensed for use by government regulations. The UPA’s DHS uses windowed-OFDM modulation that provides programmable notches with a negligible loss of performance.

Bandwidth issues
HD TV requires huge amounts of bandwidth. The system must provide smooth video delivery even under difficult conditions, such as intermittent noise, interference from neighbouring powerline networks, or a network saturated with low-priority data.

The DHS uses traffic classification and centralised bandwidth management to achieve this. This technology, known as advanced dynamic time division MAC (ADTDM), is optimised for audio/video-distribution applications in which high performance, stringent bandwidth reservation, strict traffic prioritisation and QoS are of paramount importance. All the nodes in the powerline network are given collision-free access to the channel according to different service priorities. These priorities can be adjusted to suit different applications, including data, VoIP and video-on-demand.

The UPA’s EoP system also uses a master/slave architecture, in which one EoP device on the network is chosen as the master while all other devices are designated as slaves. The master device allocates channel access time to other EoP devices on the network. This is the most effective (and simplest) way to ensure bandwidth allocation to different traffic types on the network. The specification employs 168bit AES encryption to provide secure content distribution.

EoP choices
For distributing HD video content in the home, a data rate of at least 150Mbit/sec is essential. UPA, HD-PLC and HomePlug AV offer 200Mbit/sec throughput. Older powerline technologies such as HomePlug 1.0 and HomePlug 1.0 Turbo offer more modest data rates (14Mbit/sec and 85Mbit/sec respectively).

The UPA’s 200Mbit/sec chipsets have been shipped since 2004, deployed by telecommunications service providers in Europe. HomePlug AV was launched three years after the UPA’s DHS. HomePlug AV 200Mbit/sec chipsets are now sampling.

ASHISH GARG is a strategic marketing manager in the Data Communications Division at Cypress Semiconductor


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