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Dialing quality on the way to IPTV

Posted: 01 May 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Tektronix? IPTV? VoIP? XX? XX?

The process of delivering VoIP is fraught with its own set of unique technical challenges. Good video quality is dependent on constant-rate lossless transmission and can raise a lot of quality issues!up to and including loss of video, decoder lockups and failure to locate the correct channel. Some of these problems can come from sources outside the operator's own network. If the sources of potential quality problems are understood well, detection and prevention are possible.

Much of the technology required for a broad-reaching deployment of IPTV is still under development. This technology includes equipment like advanced stbs and algorithms or copy protection within streams. In the next several years, one should expect to see a very competitive environment for IPTV. Many incumbent players face a deeply entrenched pay-per-view model from cable and satellite companies. Maintaining QoS and quickly and cost-effectively deploying new services could very well be the differentiators that successful companies will need to achieve.

Because the IP environment was not originally designed to carry A/V, it is not surprising that new networks will be unlike traditional closed, synchronous cable, satellite or terrestrial systems, which can be easily monitored within a linear transmission and distribution chain.

A large part of the real-world problems operators can expect to deal with!especially traffic-related problems such as burstiness and data drops!are due to mixed voice, multimedia and data traffic running simultaneously over converged networks. What kind of impact can an operator expect from packet loss, out-of-order packets, packet delay and packet-delay variations?

With some new compression systems such as H.264 and SMPTE-VC-1, video is much more highly compressed and fragile than audio. A network might drop a VoIP packet inaudibly, but losing a video packet may result in the loss of more valuable encoded video information. A single video packet drop can result in quality degradation in the form of a single frame drop to the loss of 1s of video, depending on the kind of information lost.

Out-of-order packets can also disturb compression systems. This condition results most often when part of the IP stream is routed over ATM. If the content is an MPEG-2 packet, each packet has a built-in 4bit "continuity counter." In-line test equipment with dual-layer capability!IP and MPEG decode!can easily detect out-of-order packets.

Out-of-order detection
Many protocols used today are affecting this issue negatively. For example, the user-defined protocol does not allow detection of out-of-order packets. Real-time transport protocol (RTP) and RTCP, its control protocol, have a payload field that can be used to identify what is being carried. Yet RTP doesn't resolve all of the problems because it gives a transport layer without strictly specifying how data is organized within the transport.

Work continues in standards groups to help define payload types and minimize resultant problems that can cause bandwidth issues. As this piece of the puzzle evolves, it will likely affect both test-equipment vendors and IP infrastructure equipment makers.

Packet-delay variation and interpacket gaps can stress both routers and STB buffers. This happens when devices are overwhelmed by traffic or when data moves between networks of different delay characteristics. Symptoms can include failure of the decoder buffers to keep up with the stream, causing picture blocking and freezing as packets get lost. More subtle errors include program clock reference (PCR) arrival jitter or drift, which can cause color or video sync loss on some decoders. Ideally, test equipment should be able to MPEG-decode and display PCR inaccuracy, overall jitter, arrival-time jitter and drift so that the source of the problem can be identified and corrected.

The main thing holding back mass deployments of lucrative IPTV is scalability problems involving protocols, forward error correction, digital rights management systems and multicast architectures. Those brave pioneers in IPTV will gain the experience necessary to pressure standards bodies and equipment vendors to produce reliable and interoperable systems. As systems evolve, market forces and the standards work groups will help ensure that the best solutions survive. High availability and QoS should deliver the required return on investment that will allow providers the flexibility and capital they need to ensure a widely satisfied customer base.

- Jon Hammarstrom
Sr. Manager, Global Marketing for Video
Tektronix Inc.

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