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Multimedia Transmission - from Voice to HDTV (1/8/2005)

By Peter Horsfall, Broadband Carrier

Broadband Internet and home networks that connect multiple computers, games consoles and other devices, are now commonplace. But how do we expand this to include television, audio and telephony to create a coherent multimedia network? IP (Internet Protocol) has become recognised as the best common platform for devices to inter-communicate, so one day each AV separate may simply have an RJ45 connector. For now however, we will simply look at adding VoIP (Voice over IP) audio and IPTV to the home network.

To understand the network elements required to undertake this, and the requirements of the transport medium, we first need to look at what kind of signals are to be transported around.

Signal types

VoIP is now pretty straightforward. Any modern network will cope with VoIP, and a variety of codecs are available that offer a choice between quality and bandwidth. These range from ITU-T G.729 with a bit rate of 8kb/s, up to ITU-T G.711 for a high-quality bit rate of 64kb/s. Most VoIP devices are now well-established and easy to install - just follow the instructions for cheap/free calls. Mp3-compressed audio needs no explanation. It offers high quality with low bandwidth requirements.

Uncompressed video would require in the order of 200Mb/s of dedicated bandwidth, with which available infrastructures for audio networking could not cope. The video must therefore be compressed, leaving us with two excellent current contenders, namely MPEG-2 and MPEG-4, both of which are in use.

MPEG-2 has become the current de-facto standard for DVB (Digital Video Broadcasting), DVDs and the like, and improves upon the previous MPEG-1 by being more suitable for broadcast formats at higher data rates. It also supports a wide range of bit rates as well as multichannel surround sound coding.

MPEG-4 can match the data rates to the demands of the receiving codec. This allows each receiving device to make use of different speeds based on the size and quality of the image to be presented and bandwidth available. It also means that the required bandwidth can be reduced by up to 50% for the same picture quality.

For high quality, each channel of standard definition TV (SDTV), requires about 5Mb/s of bandwidth using MPEG-2 compression, or 1.5 - 3Mb/s bandwidth using MPEG-4 or Microsoft WM9 compression. HDTV currently requires 12 - 19Mb/s of bandwidth per channel with MPEG-2 compression, although when the new compression technologies for HDTV become fine-tuned, it is expected to require 6.5 - 8Mb/s per channel.

What can we currently achieve?

We can set up a network to be capable of supporting the high bit rates required for video streaming, and we can run video client/server applications on computers so that video files, DVDs etc can be served across the network and viewed on any computer or TV. We can bring the TV itself onto the network by adding a set top box that converts IP to Scart and digital audio, and we can do the same with audio, enabling your computer-based mp3 collection and Internet radio stations from around the world, to play with high quality on your hi-fi system. We can even stream a computer-based digital TV receiver output to all computers and TVs on the network.

With any broadband connection, creating a network of computers games consoles etc., primarily to use the Internet and share files and printers, is not much of a technical issue. Any modern network elements such as wired, wireless, and power line, will all be faster than the Internet bottleneck, so the choice may be determined simply by cost and ease of installation.

A novel way of passing mp3s to your hi-fi avoids all of the inter-room wiring issues by using the mains wiring. A data rate of 14Mb/s is more than sufficient for streaming an Internet radio station or mp3 from a computer hard drive. Content that appears on the Ethernet is passed across the mains circuit and decoded to analogue audio for the hi-fi.

Networking Internet radio to the hi-fi via power line.

Streaming video

For streaming video at good quality, we need to look for a network that has a much faster throughput, offers some control and supports multiple HDTV streams if it is to be future ready. A network that supports 100Mb/s would be a good start.

With a mixture of different services clambering for the available bandwidth, QoS (Quality of Service) must be considered - we cannot just let things take their course. While data can be broken into packets, transported and reassembled with little regard to latency - as happens using TCP (Transaction Control Protocol) over the Internet - voice and video are extremely time-sensitive and need an uninterrupted flow. If data loses a packet, it can be resent, but with video, it has missed the bus.

UDP (User Datagram Protocol) is the usual choice for AV streaming. It is a fast, lightweight, connectorless protocol without any re-transmission or data rate management functionality, although the Windows Media Services UDP Resend function will retransmit lost packets if they can be sent to the client in time to get played.

Network types

Wired networks are always a safe bet, and data rates up to 1000Mb/s over short distances can be supported using CAT 5e upwards. The only downsides are the limit in distance, and where to run and hide the cables.

Wireless 802.11g networks are specified at 54Mb/s, and can sustain approximately 20Mb/s. Later versions have been boosted to 125Mb/s, and 200Mb/s are on the horizon. They are obviously wire free and transportable, but somewhat dependant on the environment. You may not be able to receive a strong signal just where you want it, and if the signal weakens, so will the data rate.

For an easy installation, there are a number of perhaps lesser-known but very strong technologies that can carry the network using the building's existing electrical circuit, telephone extensions and TV coax cables.

Lesser-known options

Of these lesser known options are AV network adapters that take advantage of mains or power outlets in a house. These adaptors are currently available with data rates up to 200Mb/s and with a range of approximately 300 metres. They are designed for streaming HDTV and have eight levels of programmable QoS. They also have integral web browsers and allow for customised or automatic setup.

Networking mixed bitrate demands using 200Mb/s powerline.

HPNA (Home Phoneline Networking Alliance) technology allows all of the components of a home network to interact over the home's existing telephone wiring without disturbing the existing voice or fax services. Version 3 HPNA phoneline adaptors can carry the network via the existing telephone extension cables to each room at a data rate of 128Mb/s per connection by using the unused frequency spectrum that the twisted pair cable is capable of carrying.

There are also adapters that can carry the network along the existing TV coax cable at up to 200Mb/s without interfering with the television signal, this time by using the unused lower end of the frequency spectrum of the cable.

The lower end of the cable spectrum being used to carry data.

Conclusion

If your network is to carry video as well as audio, it should be designed to run at 100Mb/s or more. The choice of network elements is straightforward once the strengths and weaknesses of each technology are understood. Often, the best solution is to use a combination of all of them - wired where it is easy, wireless where the signal is good, and power line around the house for high-speed and easy installation. If you do not need video, the network choice is the same, but lower speeds are fine. Indeed units that combine a modem, router, switch, wireless, and power line are available that create a network infrastructure using all of the technologies discussed, and ensuring that you can connect anywhere. There is a solution for every installation - it is just a question of research.

Peter Horsfall is the Managing Director of Broadband Carrier Ltd, distributor of unique products for the easy creation of high speed secure networks.

www.broadbandcarrier.net