ADSL tweaks for efficient video transmission

For all the hype over fiber being the preferred last-mile technology for delivering triple-play IPTV, the vast majority of the world's 15.4 million IPTV subscribers - just over 12 million, in fact - are connected via DSL, according to Point Topic. Little surprise, given that Point Topic also reports that as of Q1 2008, 65% of the world's broadband subscribers are using xDSL. Fiber subscriber numbers are growing faster than DSL these days, but still accounted for just 11% of the global broadband pie.

Perhaps more revealing, however, is that DSL-based IPTV is mostly running on ADSL2plus rather than VDSL2. While VDSL2 promises much faster bandwidth speeds than ADSL2plus, it also has a shorter loop range and is reportedly much more expensive to roll out (albeit still cheaper than fiber). That's not stopping VDSL2-based rollouts - especially for telcos deploying it as part of broader fiber-to-the-node rollout strategies - but for the time being, it's not resulting in a lot of ADSL replacement either.

Consequently, broadband service providers are looking for ways to milk more efficiency out of their ADSL lines such as poaching lower frequency bands normally reserved for services like ISDN to boost uplink speeds from 256 kbps to around 1 Mbps, says Erwin Ysewijn, access business marketing VP for Infineon Technologies.

Infineon is also touting a new 'IPTV over DSL' package that includes features such as predictive error decoding (also called erasure decoding) and gamma layer retransmission.

On the downlink side, gamma layer retransmission uses an intelligent traffic shaping and discrimination algorithm between the ATM and TCP/IP layer that allows the CPE and central office gear running Infineon's Amazon-SE and Danube chips to selectively resend dropped packets.

'This creates a layer that can tell the CO and the CPE when a packet coming in is a voice packet or a video packet,' Ysewijn says. 'This is more efficient than alpha layer retransmission technology, which just resends all lost packets regardless of service class.'

That said, Ysewijn concedes that gamma layer retransmission is technically slower than alpha layer retransmission, but argues that blindly resending lost packets regardless of service class won't make real-time services any more robust.

Meanwhile, erasure decoding technology on the CPE side of the link improves impulse noise immunity by adding more intelligence to the modem interleaver, and can achieve up to two times better error protection than a non-erasure decoding capable CPE in real world deployment scenarios, Ysewijn said.

Both technologies are still going through the standardization process at the ITU, and can be applied to any xDSL technology, not just ADSL, Ysewijn said. Either way, the object is to make DSL connections as robust as possible for supporting heavy-duty services like video.

'If you're delivering IPTV to a high-resolution TV screen, even if you lose just two bits, it's going to be noticeable,' Ysewijn says.