Quality of service (QoS), now more commonly referred to under the Quality of Experience (QoE) umbrella, is one ofthree critical ingredients needed for asuccessful IPTV service - along with contentand pricing, QoS/QoE will make or breakthe IPTV business case. The generalconsensus is that IPTV service providersmust at least meet, if not surpass, existingvideo quality expectations of cable andsatellite subscribers.

There is clear movement by theindustry to ensure QoE throughtechnology. As Sandy Fitzpatrick, VP ofanalyst house Canalys points out: “Whilein 2006 most SPs considered access tohigh-quality content to be the mainpriority, they are now shifting their focusto ensuring a reliable technologicalperformance, in order to persuadeconsumers to switch from establishedcable and satellite operators that alreadyoffer premium content.”

The push for standardsThe most visible manifestation of thistrend is the amount of work taking

place at the various industry groupsin attempting to define what

QoS/QoE should entail and howto fulfil the end-user

experience. Efforts on thestandardisation front

include those from theDVB, DSL Forum

and ATIS IIF launching various initiatives.Perhaps most significantly, the ITU

set up the IPTV Focus Group in 2006 to coordinate and promote the developmentof global IPTV standards. “One of theimportant indicators that QoS and QoE arebecoming important issues for serviceproviders is that the Focus Group hasincluded QoS as one of the priorities fromthe beginning of its discussions,” saysFitzpatrick. In addition, the group has recentlydeveloped a definition for QoE, and added it to the list of areas for review.

According to Jean-Christophe Dessange, senior business developmentmanager, broadband & entertainmentsolutions for European markets at Cisco, a typical industry benchmark is to have no more than one video “artifact” orperceived distortion during the viewing of a two-hour movie.

The problem, as Pixelmetrix CEO Danny Wilson observes, comes from the fact that QoE is a subjective concept

from the end-user's point of view. Wilson is pessimistic about the usefulness of existing quasi-standards such as MDI(media delivery index) and MOS (meanopinion score) because video is significantlymore difficult than voice, and analogies areinherently unlikely to bring desired results.

Wilson cites a study undertaken by the ITU-R Video Quality Experts Group (VQEG), which took over 100,000 subjectivetest points - the opinions of real people - andcompared them to almost 4,000 data pointsof machine measurement. The finding wasthat none none of the machine algorithmshad a significant correlation to subjectiveresults. “If you can't approximate MOS by

comparing to actual pictures, it is highlyunlikely there is a reliable

algorithm by measuring far-removed

parameters like packet loss or delay. Might aswell just see if the chassis is warm andcorrelate that to video quality. I think we arestill a way from a practical solution,” he says.

Gino Dion, VP of Business Development,IneoQuest Technologies, adds further foodfor thought, arguing that few standards can fully address the complex nature of the complete video service deliveryplatform. “For example, how would suchmetrics and practices take into account the idiosyncrasy of a particular middlewareplatform?” he asks.

David Price, VP of business developmentat Harmonic, acknowledges that muchdiscussion remains in formulating what QoSmeans in IPTV. “How QoS and QoE worktogether in the video environment is yet to be concluded.”

Nevertheless, as most operators arelooking for the safety of a 'standard' set of parameters and methodologies formonitoring IPTV service performance, thesearch for a single number that indicates

customer satisfaction and QoE continues. According to Osama Aboul-Magd,

project lead for QoS-related standards at Nortel, the metrics discussed by the ATISIIF sub-group, such as packet loss and jitter, are expected to be approved by the end of Q3 2007, after which it will become a US standard and may bebrought to the ITU to become aninternational standard.

The problem with MPEG over IPThe QoE comes down to two elements: the transport network and the applications.The first concerns factors such as networkbandwidth, packet drop, latency and so on.The latter involves encoding, decoding (audioand video alignment), as well as the userinterface and channel change times.

The telco is often very inexperienced in the actual requirement of video delivery,

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There is a great deal of attention being placed onguaranteeing quality of service and experience with IPTV,but the role of the set-top box needs to be more clearlydefined, argues Goran Nastic

The pursuit of quality

Leading IPTV solutions

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where the video has to be continuous andhigh quality. There is a mistaken assumptionthat video is just another set of packets, likeVoIP or data, which are ‘best-efforts’services. “Telcos discovered to theirsubstantial cost that the process of deliveringvideo is in no sense an immediately obviousnatural extension to the delivery of voice ordata services,” says Simon Woodward, CEOof ANT Software.

The real issue with MPEG over IP is thatone IP packet typically carries around sevenMPEG packets, meaning that losing one IPpacket results in seven dropped MPEGpackets. The additional problem with MPEG-4/H.264 is that in order to capitalise on theefficiency of the code, the GOP (group ofpictures) needs to be stretched because thecoding efficiency increases the further the Iframes are apart. The way encodingalgorithms work is to take one frame, whichis very mildly compressed (more or lesssimilar to the original frame) and containsmost of the data. This i frame serves as amaster or beacon, and the proceedingframes - the b or p frames - are highlycompressed and always refer back to the iframe, using it as a reference.

Winfried Schultz, EMEA video marketingmanager at T&M vendor Tektronix, notes thatpart of the efficiency of MPEG-4 had to begiven up to compensate for the change inGOP structure (ie, making it shorter) in orderto achieve a more robust transmissionstream, something that network operatorsunderestimated at the beginning. This loss ofcoding efficiency meant that more bandwidththan expected was needed with MPEG-4,which affected telcos' business plans.

“This is also why a normal bit error ratemeasurement doesn't work very well,because it doesn't tell you whether you hit something important or less important,”says Schultz.

Alan Delaney, IPTV business developmentdirector at Tandberg, reiterates the message:“If you are very lucky, a dropped IP packetmight be confined to a B frame and onlycause a minor macroblock error, but if it's ani frame the video could be out for 500ms ormore,” he says.

The length of time it would it take toresynchronise a dropped i frame dependson the codec and the set-up, but it's likelyto take at least 500ms. Again, the GOPlength is the most critical factor in recoverytime. Historically, systems have run with aGOP length of 12 frames in 50Hzenvironments and 15 frames in 60Hzenvironments, giving a distance between Iframes of 0.5 seconds. However, doublingthe GOP length to a second can increasethe efficiency of the encode process byover 6%. And in IPTV, setting a longer GOPis becoming a common practice, accordingto Delaney.

Besides robustness, the usual trade off is with channel change time; the longerthe GOP, the slower the channel change.Consumers are getting used to waiting a second on channel changes andbroadcasters are taking advantage tosqueeze out as much efficiency as possible.“A consensus on GOP length of 32 frames isbeginning to emerge, and it's a goodcompromise, delivering an 8% encodeefficiency improvement while increasing the

average channel change time by less thanhalf a second,” says Delaney.

So the exercise is a balancing compromisebetween optimising bandwidth, andoptimising channel change time and QoS.Operators need to decide on depending onthe quality of their network, and whether ornot to add additional overhead for techniquessuch as forward error correction. FEC is one of a number of mechanisms designedto protect the I Frames and overall QoS/QoE.It is achieved by transmitting a small amountof “redundant” data that can be used to reconstruct packets in the event that bit errors cause packets to be dropped. A small amount of additional bandwidth,typically around 5% of the bandwidth of the original video stream, is required for the FEC mechanism. FEC essentially gives the ability to reconstruct a lost packetwithout having to re-transmit it.

RTP packet re-transmission, on the otherhand, tries to get a packet re-transmittedquickly from the network in case of packetloss. In re-transmission, the set-top boxclient uses sequence numbers in RTPpackets to reconstruct the original packetsmaking up the video stream. If any packetsin the sequence are missing, the client canrequest that the missing packets be re-transmitted from the network, wheremultiple seconds of each stream arebuffered by a network appliance. Cisco, for instance, uses FEC and RTP packet re-transmission as techniques for error repair in its Visual Quality Experience (VQE) solution, claiming that this approach offers

maximum flexibility and protectionagainst network issues while

balancing bandwidth availability.ATIS, meanwhile, has specified

Leading IPTV solutions

“A consensus on GOPlength of 32 frames is beginning to emerge, and it's a goodcompromise, delivering an 8% encode efficiencyimprovement whileincreasing the averagechannel change time by less than half a second.”

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that IPTV would always use RTPencapsulation (MPEG over RTP).

If the reason a packet is lost is due tocongestion, then it's possible than re-transmitting will just exacerbate thecongestion issue rather than solve theproblem. In this case, connection admissioncontrol (CAC) can be used. CAC preventsnetwork congestion associated with admittingtoo many video streams by using applicationintelligence to deny a new subscriber's VoDrequest when the network is operating nearfull capacity (most VOD systems today workon this basis). To perform CAC dynamically,policy servers keep track of what'shappening in real-time.

The predominant approach to QoS todayinvolves provisioning the network in such a way to avoid contention. The idea is for the system to be near capacity but not oversubscribed, much like the modelemployed by airlines, and the art is to use as much intelligence in the network as possible to adapt intelligently to servicedemand but not overprovision to the pointwhere the CapEx spirals out of control. To make it cost-effective, most vendors arguethat this level of intelligence is bestimplemented in a small number of locationsat the edge of the network. FastWeb,Versatel and other early IPTV networks haveadopted this approach.

While Layer 3 video distribution isgenerally seen as the best way to distributevideo in a broadband network, visibilityabove this layer is also required to avoid the 'garbage in, garbage out' scenario.According to Delaney, Tandberg spendsabout 50% of its effort in pre-processing

(motion estimation, noise reduction etc) in order to get a cleaner signal and picturegoing into the encoder.

Tandberg, Harmonic and a batch of othervideo vendors also support ProMPEG FEC to correct dropped packets and conceal a certain level of network problems. CertainIPTV middleware vendors offer this supportas well in their solutions, and middleware can add a lot of value by adding some QoS functions into their stack. Details here are sparse, however, and vendors in this space aren't as quick to discuss theirproducts' capabilities.

Let's not forget the STBAs with middleware, the set-top box seems to have played only a limited and underutilised role in QoS functions,which largely reside in the domain of thenetwork itself. With the move to QoE, itwould seem logical for added algorithmsand agents to make their way into the STBto provide diagnostic information and morecomplete visibility of end-to-end servicequality. Spirent, a T&M vendor, found that70-80% of the problems that occur withDSL happen in the access network. Thiswould suggest that telcos need a strategyfor testing the access network andgathering data from the home.

Today, most client-side solutions rely on various buffering mechanisms. On IPTV STBs, there is a large input buffer(known as the jitter buffer), which is wherethe MPEG stream arrives. It is used tosmooth out any burst activity on thenetwork and allows disordered packets to be collated and the stream reassembled.Clearly there is a limit to the amount of buffering that can be employed (beforeplayback commences), which comes backto the compromise between QoS andchannel change time. IPTV boxes employsignificantly larger buffers to cable boxesbecause copper networks are subject to'cross-talk' interference, especially once the numbers of subscribers startincreasing, which generates incrementalpacket drop in the last mile between theDSLAM and STB.

IPTV often employs SNMP (SimpleNetwork Management Protocol) agents to gather useful statistics on the behaviourof the STB and other networking elementssuch as routers and home gateways. WhileSNMP is standardised, the data that isavailable differs between STBs. In any case,this is used more for monitoring rather thanany corrective action in terms of QoS onthe network.

More recently, companies such asQoSMetrics have developed mechanisms for measuring quality based around theMoving Picture Quality Metric (MPQM)research. Real time performance informationis calculated using models based upon theincoming IP audio/video data and key STBparameters, such as jitter buffer sizes. Thesoftware necessary to do this is embeddedwithin the STB, and the performance data isthen available for the remote query by theoperator and can be used to dynamicallyalter network characteristics to improve QoSor diagnose problems.

“It's entirely feasible, indeed highlyimportant, to implement these mechanismsin order to accurately determine theperceived quality of the television servicedelivered to the customers' home,” saysSimon Forrest, chief technical engineer forIPTV at Pace.

In any case, the client-side will have toevolve as networks become more intelligentand complex, believes Chris Chalkitis, seniorIPTV specialist at Tilgin. “There are three keyissues here: increased QoS demands relatingto HDTV, operator managed services andconsumer generated content. All of theseneed to be handled appropriately, butrequire differing allocation, prioritisation and QoS mechanisms,” he says. Chalkitisalso believes that home networks will playan increasing part in the whole QoS chain,making the interaction between STB andhome gateway increasingly important.

However, as BigBand's chief architectJohn Reister notes, it is in serviceproviders' interest to try and keep the CPE as simple and low-cost as possible.Upgrading STBs with different capabilitiesmakes the process problematic and difficultto manage, so how all this evolves is somewhat of an open question.

What is certain is that QoS/QoE is becoming a key differentiator because viewers dissatisfied with servicequality have the option of churning to other platforms. CSI

Leading IPTV solutions

QoS - how not to do it

One IPTV operator performed 14 truckrolls when faced with a customerproblem only to find later that thesatellite dish at the headend was filledwith rainwater, cutting the RF signal.

Another telco took the approach ofsending engineers to watch TV everytime customers experienced a problem.

“If you have no clue to what'shappening in the living room, you'reprobably giving yourself a false senseof security by complimenting yourselfon how good your servers are.”