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Overcoming DSL STB design challenges for IPTV

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

Keywords:EMEA? Texas Instruments Inc.? STB? IPTV? DM642?

Few operators have deployed integrated DSL modem/IP STBs, commonly called DSL STBs, to date, largely because doing so means designing and building their own products from scratch. Until now, OEMs and ODMs have not entered this market due to the difficulty of interfacing key components and other design challenges. However, component manufacturers are beginning to introduce fully designed and documented development boards with interfaces and other design issues resolved.

Cable multiple service operators scored an enormous hit by using their hybrid fiber/coaxial cable infrastructures and leased lines to deliver triple play video, cable-modem based data services and voice services to customers. Telcos are now responding by beefing up DSL infrastructures and using bandwidth-efficient video codecs to add IPTV, a killer application for ubiquitous broadband penetration, to provide their own triple play. IPTV offers the key advantage of supporting multiple video channels, particularly when advanced codecs are used.

IPTV is expected to grow to account for one-third of the DTV market by the end of the decade. Telcos are using ADSL/ADSL2+ or VDSL to transmit IPTV triple-play content to their residential customers. Many operators and service providers announced market trials with residential video-over-ADSL. Recent analyst reports suggest that while 2004 DSL STB worldwide shipments exceeded 1.3 million units, the market will reach over seven million units and more than $700 million in revenue by 2008.

The STB remains a critical challenge for the many operators and service providers that announced trials with residential video-over-ADSL. Consumers expect video quality equal to that of current TV services, which in turn requires sophisticated QoS schemes to prevent, for instance, a large file download from interfering with TV reception. Random noise bursts that appear on copper lines present another challenge. Rapid changes in technology such as new codecs and INP schemes demand the flexibility to quickly implement new technologies.

As they address these challenges, operators demand that STB OEMs and ODMs move beyond current technology that requires the use of two separate boxes on top of the TV set!a DSL modem and an IP STB. The use of two separate boxes means higher prices, increased user complexity and an additional point of failure. Protecting movie content is also easier in a unified box. Operators also want additional functionality!such as WLANs and personal video recorders!to be integrated into the STB.

Most operators prefer to focus on their core competency and delegate equipment design and its related risks to OEMs and ODMs. Yet, to date, OEMs and ODMs have not entered the integrated DSL/IP STB market, most likely because of a reluctance to take on an expensive design challenge for a market that is just beginning to emerge. As a result, trendsetting providers that made the decision to enter the market with an integrated solution are forced to design their own customer premises equipment and work with contract manufacturers.

Development boards
Today, a new generation of development boards is emerging to reduce time, cost and risk involved in delivering an integrated DSL STB. An example is a development board that incorporates AR7 DSL and DM642 video chips, already present in most integrated STBs to date. The DM642 is based on a dsp core and includes instruction-set extensions for accelerating video and imaging applications, providing full software-programmability. The MIPS CPU within the AR7 router on a chip is responsible for managing the DSL link, performing IP routing and transport stream demultiplexing to deliver raw compressed A/V data to the DSP.

Key interfaces
New development boards reduce design time by managing the integration between the DSL and video chips. For example, the external memory interface is normally used to connect a DSP to different types of memory devices such as SRAM, flash RAM and DDR-RAM. On the AR7, the interface is used to transport traffic from the AR7 to an HPI video interface on the DM642, with the AR7 acting as the master and the DM642 as the slave. In bootup, the AR7 comes up first and fires up the DM642. Video is then fed to a bulk queue in the DM642. The AR7 handles flow control into the bulk queue. The video driver in the DM642 handles the movement of video into four queues on the chip. The DM642 then decodes the video stream and renders the image on the screen.

QoS, INP
QoS is critical in DSL STBs, given the need to distinguish between video, data and voice packets moving through the router and assign the appropriate priority to each. The goal is to avoid a situation where someone on the computer begins downloading a large file and the TV picture immediately deteriorates. A typical packet accelerator uses a priority queuing feature to send prioritized TCP ack packets ahead of the other upstream traffic, resulting in up to 3x increase in throughput. Video packets are typically marked with type-of-service bits so that they can be directed to a separate queue delivered ahead of other packets to avoid TV broadcast channels disruption. Another feature provides the ability to receive a multicast stream delivering broadcast channels by sending a join request to the DSLAM in an IGMP packet. Both the STB and the back office must support this feature.

Another complex issue is the ability to lip-sync or decode the audio at the same time as the matching video, so that a person's voice is played through the speaker at the same time he moves his lips. Each of the two independent streams of data has a timestamp on each packet that indicates the time it was created. Since the video packets take substantially longer to decode, an algorithm runs on the DSP to delay the audio packets so that they will be decoded at the same instant as the matching video packets.

Copper lines!such as those used for DSL transmissions!are subject to impulse noise generally caused by EMI. Impulse noise reduces throughput by causing CRC errors and may also require retraining to the DSL line. Operators delivering IPTV services or DSL emphasize on protecting against disruptions to broadcast TV services that can be caused by this type of noise.

The framing parameter is a measure of the amount of redundant information sent to aid in recreating packets that are lost to noise. A framing parameter of one can protect against bursts of 250ms, while a framing parameter of two can protect against bursts of 500ms and so on. Of course, increasing the value of the framing parameter also increases the memory requirements of the DSL processor. INP was introduced in ADSL2.

Design headstart
Integrated development boards provide OEMs and ODMs with a big head start in designing and manufacturing a leading edge integrated DSL STB. Third-party products further reduce the time it takes to go from concept to market and revenue by providing streaming medial solutions for video, imaging and algorithm development, system integration and OS support.

DSL residential gateway reference designs that add VoIP and wireless home networking functionality are available. Manufacturers can use these development boards to quickly develop complete operator-quality product lines that are ready to deploy, after taking advantage of extensive testing performed on the family in broadband labs and the field hardening that has occurred through real-world deployments.

Neil Quarmby, Technical Staff Senior Member and European IP STB Applications Manager
Michael VanBreda, Manager for EMEA
Texas Instruments Inc.




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