IEEE Communications Magazine • May 2008102 0163-6804/08/$25.00 © 2008 IEEEINTRODUCTIONMost recent news surrounding IPTV se...
via a broadband modem, as opposed to digitalcable receivers which currently feature integrat-ed coax/RF tuner/receivers.Fo...
• Statmux — Performs statistical time-divisionmultiplexing of real-time video contentfrom multiple sources• Groomer — “Tra...
IEEE Communications Magazine • May 2008 105tion. It was designed as a cost-effective methodto expand program capacity in a...
cable modem termination system (CMTS), and aPacketCable Multimedia (PCMM) [2] policyserver to support IPTV.CMTS AND CMThe ...
IEEE Communications Magazine • May 2008 107Having realized the value of QAM sharing,CableLabs ( s...
IEEE Communications Magazine • May 2008108selected Resource Reservation Protocol (RSVP)[6] for resource reservation.Figure...
IEEE Communications Magazine • May 2008 109ing forward with the M-CMTS architecture,which separates the CMTS MAC and the P...
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IEEE Communications Article - May 2008

  1. 1. IEEE Communications Magazine • May 2008102 0163-6804/08/$25.00 © 2008 IEEEINTRODUCTIONMost recent news surrounding IPTV seems todisproportionately mention the telcos. Thismight seem unfair to cable operators, includingmany who have been providing television servicefor decades, began using fiber to deliver analogtelevision content back in the 1980s, adoptedand deployed digital video delivery technologiesin the 1990s, and were one of the first to exploretelevision interactivity with two-way services suchas video on demand (VOD). Currently thelargest subscription television service provider inthe world is a cable operator.In many regions of the world, cable is theincumbent subscription television service, with along history that can be a two-edged sword:cable operators deployed many innovative tech-nologies, while facing the challenge (as manygreenfield IPTV operators have just learned) ofcontinually enhancing their offerings while main-taining support for first-generation “legacy” ser-vices and devices. To effectively compete in themarketplace and introduce new services cost-effectively, cable companies have focused onleveraging their investment in the existing infra-structure.First-generation cable TV systems were ana-log, allowing only one program per frequency(channel) within the allocated spectrum. A mod-ulated analog signal occupies a channel of either6 or 8 MHz (depending on world region). Theseearly systems started with 450 MHz of band-width on the hybrid fiber-coaxial (HFC) plant,but as subscriber demand for content grew, radiofrequency (RF) integration and innovationallowed that capacity to expand to 550 MHz.Subsequent advances yielded cable plant designsthat could accommodate plant bandwidths of750 MHz, 860 MHz, and now 1 GHz. Further-more, cable operators also leveraged advances inbroadcast-quality video compression that made itpossible to place multiple digital programs in thesame 6- or 8-MHz channel previously occupiedby a single analog channel. This new digitalbroadcast technology enables the hundreds ofchannels currently offered by many cable opera-tors.Until recently, the primary industry competi-tor to cable has been direct broadcast satellite(DBS). One operational advantage of DBS is itsability to deliver signals to a large geographicalfootprint containing millions of subscribers, mostoften nationwide or multinational. Compare thisto a local cable headend, where the reach of theHFC plant may be a few hundred thousandhomes at most. However, satellite signals usedfor contemporary DBS services are unidirection-al, with most DBS receivers relegated to using alow-speed telephone line and modem to signalinformation (e.g., subscriber requests) upstream.Cable operators have upgraded their networks tosupport robust two-way communication, dedicat-ing a portion of the plant spectrum to upstreamtraffic. Cable companies have used this advan-tage to add interactive video, broadband data,and voice services to form a “triple play” bundleto compete against DBS.The initial success of cable operators in usingtriple-play bundling to provide multiple services,particularly voice services, has not gone unno-ticed by the telcos. Today the traditional voice(wireline) companies are starting to build theirown triple-play bundle by deploying video deliv-ery systems that compete with cable, while shar-ing the same two-way connectivity advantageover DBS that the cable systems have. Manywireline companies are building IPTV systemsthat natively support IP receivers (set-top boxes,STBs) that connect to the broadband networkABSTRACTIPTV systems are beginning to be deployedby telcos, to compete with cable and other tele-vision service providers. While cable networkspectral bandwidths are larger than their wire-line equivalents, bandwidth-consuming servicessuch as HDTV are quickly filling this spectrum,prompting cable operators to consider videoswitching options as a means to increase pro-gramming capacity. This article examines thecable video network architecture, and in particu-lar new emerging technologies that can providecable networks with the means to transition toan IPTV architecture.IPTV SYSTEMS, STANDARDS AND ARCHITECTURES:PART IIS. V. Vasudevan, Xiaomei Liu, and Kurt Kollmansberger, Cisco SystemsIPTV Architectures for Cable Systems:An Evolutionary ApproachVASUDEVAN LAYOUT 4/23/08 12:39 PM Page 102
  2. 2. via a broadband modem, as opposed to digitalcable receivers which currently feature integrat-ed coax/RF tuner/receivers.For some time, digital broadcast services havebeen deployed by cable operators over IP net-works for regional transport from headend tohub for video, voice, and data. Cable companiesare beginning to look to expand their offeringsto non-RF receivers by considering an IPTV ser-vice over Data Over Cable Service InterfaceSpecification (DOCSIS) [1], the industry stan-dard for providing IP services over cable. IPTVfor cable can also be seen as an extension ofentertainment-grade video services to non-RFclients.This article surveys the existing native digitalvideo delivery architecture deployed by cableoperators. It also examines advances in technol-ogy that support the consideration of an IPTVservice delivered over a cable access network.CABLE VIDEO ARCHITECTURETo aid in the subsequent discussion, this sectiondefines the video services, provides a high-leveloverview of the video transport architecture, anddiscusses the basic video system architecture foreach service. This section ends with a short dis-cussion of how these services and architecturescan easily be integrated into an IPTV service.VIDEO SERVICE DEFINITIONThere are two main categories of digital videoservices that are discussed: broadcast and on-demand.Broadcast Services — A broadcast service isdefined as a service in which one copy of a pro-gram is sent to all subscribers in a one-to-manymodel. An example would be a television net-work such as ESPN, where two different sub-scribers would see the same program andadvertisements.Broadcast services include:• Digital broadcast — Encoded/compressedprograms are sent to a subscriber’s STB,where they are decoded/uncompressed forplayback on a TV. All programs are sent toall subscribers regardless of which programsare actually being watched.• Switched digital video (SDV) — SDV is likedigital broadcast in that all programs areencoded/compressed. SDV is the firstimplementation of a broadcast video servicein which only requested programs are sentto a group of subscribers (the subscribergroup). A subscriber group typically consistsof all of the subscribers served by one ormore HFC nodes, which can range any-where from 125 to 2000 subscribers. SDV isthe first broadcast video service for cablethat requires two-way communication withthe subscriber for program selection.On-Demand Services — An on-demand ser-vice is defined as an interactive service where arequested video program is streamed to one(and only one) subscriber in a one-to-one model.An example would be an on-demand movie,where two different subscribers requesting thesame movie at the same time would see differentcopies. All on-demand content is encoded/com-pressed and stored on servers to be retrieved byeach individual subscriber on request.On-demand services may include the follow-ing:• Video on demand — Content (e.g., a movie)is stored on a server. A subscriber views alist of titles and requests to view a specifictitle. The VOD system authorizes andstreams the requested content to that spe-cific user.• Network-based personal video recorder(nPVR) — This service functions like ahome PVR, except that the storage is partof the service provider network. A sub-scriber uses the STB remote to programspecific content to record, and then watchesthe content at a user-chosen time in muchthe same manner as VOD.VIDEO TRANSPORT ARCHITECTUREAt a minimum, modern digital cable networksconsist of a headend, a regional IP transport net-work, a certain number of hubs, an HFC net-work, and a certain number of subscribers.Figure 1 shows a composite view of the majorcomponents of this architecture for the differentvideo services.• The headend (equivalent to a telco videoheadend office, VHO) is where contentacquisition occurs for both the broadcastand on-demand services. The headend isalso where most of the back office systemsreside. (Back office systems include billingsystems, asset management systems, autho-rization systems, etc.)• The IP network transports IP services fromthe headend to a number of hubs, aggregat-ing a number of services (e.g., video, high-speed data [HSD], and voice over IP) whileproviding appropriate quality of service(QoS) or priority for each service class.• The hub (equivalent to a telco video servingoffice, VSO) is where the control of band-width-intensive applications (such as on-demand streaming) and HFC-connectedcomponents reside.• The HFC network is the fiber-coax accessnetwork for distribution to the subscriber.• The subscriber site is a business or residen-tial location where coax receivers (e.g.,STBs and cable modems) are located.VIDEO SYSTEM ARCHITECTURESBy combining the service definitions and over-laying them onto the video transport architec-ture previously defined, we have a clearer viewof the video system architecture for each service.Digital broadcast, on demand, and switched digi-tal video architectures are considered as sepa-rate cases. It will be seen that SDV can providea convenient transition to cable IPTV.Digital Broadcast Architecture — The digitalbroadcast architecture begins with the acquisi-tion of content in the headend from satellite andterrestrial sources. Acquired content is generallyforwarded to a multifunction media processingdevice performing the following functions:IEEE Communications Magazine • May 2008 103An on-demandservice is defined asan interactive servicewhere a requestedvideo program isstreamed to one(and only one)subscriber, in a“one-to-one” model.An example wouldbe an on-demandmovie.VASUDEVAN LAYOUT 4/23/08 12:39 PM Page 103
  3. 3. • Statmux — Performs statistical time-divisionmultiplexing of real-time video contentfrom multiple sources• Groomer — “Transrates” content into amaximum bandwidth, limiting the rate ofvideo bursts• Splicer — Inserts ads into programmingbreaks as needed for a geographical marketThe content is then forwarded across the IPtransport network to an IP-enabled quadratureamplitude modulator (QAM), where the contentis encrypted (using an on-board scrambler) andforwarded to the STB for playback. Alternative-ly, many cable companies are looking to offloadencryption from the QAM and move that func-tion to a separate device.An out-of-band (OOB) channel provides theSTB with basic tuning information (e.g., a chan-nel map), decryption keys, software upgrades,and other communications.On-Demand Architecture — The on-demandarchitecture is an as-requested one-to-one ser-vice using (as in the case of an nPVR service)the same acquisition of content in the headendfrom satellite and terrestrial sources. Acquiredcontent is forwarded to a groomer and subse-quently ingested into the VOD server. Pre-authored content such as VOD assets can alsobe ingested directly into the VOD server throughinterfaces such as FTP. Once requested, contentis streamed across the IP transport network to asession-based encryptor. After being encrypted,the content is forwarded to an IP-enabled QAMand subsequently to the STB for playback.Because only “requested” content is sent to asubscriber using pooled resources, the conceptof a “session” is introduced into the on-demandarchitecture. A session is a temporary identifierthat threads the request and subsequent mes-sages that support the delivery of content to aclient. This is accomplished by software withinthe STB that communicates with a session man-ager (SM). The SM in turn coordinates with theVOD backoffice appliances (such as a VODapplication server, an application managementsystem, etc.), which authorize access to therequested content and identify a streamer forthe content. The SM sets up the encryption andQAM resources needed for the session, and thenreplies to the STB’s request with tuning anddecryption information to allow the STB toreceive, decode, and play back the content.An OOB channel provides the STB with theIP address of the SM, software upgrades, andother communications.Switched Digital Video Architecture —Cable and wireline delivery networks have somefundamental physical differences. The HFC net-work is a shared medium, where groups of sub-scribers are connected on a common branch ofcoax cable. Groups of subscribers share access tothe same downstream frequencies, and arbitratefor access to shared upstream frequencies. Wire-line networks, on the other hand, are traditional-ly considered point to point, from a centraloffice directly to a subscriber. Therefore, withsufficient switching capacity placed at the centraloffice, the amount of content that can be deliv-ered to a single household is potentially limit-less. The ability to offer such an amount of videoprogramming in a telco network could beexploited as a competitive advantage. Switcheddigital video (SDV) is a cable technology thatprovides a similar switched video access func-IEEE Communications Magazine • May 2008104I Figure 1. Cable video architecture.Headend IP Hub HFC CustomerAcronymsVOD:STB:DRM:OOB:QAM:Video on demandSet-top boxDigital rights managementOut-of-bandQuadrature amplitude modulationEncoded contentBusinessmanagementsystemAssetmanagementsystemVODapplicationserverVideo sessionmanager/DRMSTBVODand adserverEncryptorVODstreamer/splicerVODQAMBroadcastQAM w/encryptionStatMuxgroomersplicerSTB config andchannel map/DRM OOBAddecisionsystemEncoded contentIPnetworkHFCnetworkVOD clientDRM/decryptDecodeLegendData pathControl pathVASUDEVAN LAYOUT 4/23/08 12:39 PM Page 104
  4. 4. IEEE Communications Magazine • May 2008 105tion. It was designed as a cost-effective methodto expand program capacity in a way differentfrom the previously used methods of plantupgrades and/or video compression enhance-ments.Traditional digital broadcast sends all pro-gramming into the HFC network, along withtable-based metadata information that a receiveruses to discover, locate, and tune to specific pro-grams. A video program is carried on the HFCnetwork whether or not it is being watched.With SDV, as with IPTV, programming termi-nates at the headend (VHO) or hub (VSO) anddoes not explicitly traverse the access networkunless requested. Instead, a receiver signalsupstream to request programming, and a hub-based controller receives the request and enablesthe stream into the HFC network by multiplex-ing the stream into a pool of allocated frequen-cies. SDV leverages a principle used in thetraditional phone network (and made famous byDanish mathematician A. K. Erlang) that thenumber of network resources actually needed inpractice is much lower than the total possibledemand. Similarly, the number of television pro-grams available for viewing with SDV is muchhigher than the number of programs beingwatched by a group of subscribers at any giventime.In an SDV system metadata that describes allbroadcast programming is amended to indicatewhich programs are SDV programs. When anSDV program is selected, tuning software in thereceiver sends an upstream message. An SDVsession manager receives the request and mapsthe program to a frequency within the allocatedpool. This dynamic tuning information isreturned to the receiver. If the program isalready being viewed within the same subscribergroup, the task is as simple as reusing the exist-ing session frequency information.It so happens that the SDV architecture lendsitself conveniently to a transition to cable IPTV.It integrates well with existing services and archi-tectures by working with existing (legacy) STBs.SERVICE/ARCHITECTURE INTEGRATIONOnce video services are migrated to IP, cablesystems obtain many advantages:• Integration of pooling of delivery resources(e.g., acquisition, encryption, and QAM)• Support for different access networks (e.g.,DOCSIS)• Support for additional IP receivers (e.g.,PCs, IP-STB receivers, game consoles)Figure 2 shows an integrated video servicearchitecture with SDV.While SDV now enables cable operators toexpand program capacity (in line with IPTV),what remains is to accommodate receiversbeyond the traditional RF STB. The logicalchoice is to implement an IPTV service offeringby leveraging existing DOCSIS deployments.CABLE IPTV ARCHITECTUREA cable IPTV architecture can evolve from thetraditional cable video architecture with modestchanges. IPTV can be enabled with three newlyadded components: a cable modem (CM), aI Figure 2. Integrated video service architecture.Headend IP Hub HFC CustomerAcronymsVOD:STB:DRM:OOB:QAM:SDV:ERM:Video on demandSet-top boxDigital rights managementOut-of-bandQuadrature amplitude modulationSwitched digital videoEdge resource managerEncoded contentBusinessmanagementsystemAssetmanagementsystemVODapplicationserverSDVencryptorVideo sessionmanager/DRMSTBVODand adserverEncryptorVODstreamer/splicerSDV/VODQAMSDV/VODQAMBroadcastQAM w/encryptionStatMuxgroomersplicerSTB config andchannel map/DRM OOBAddecisionsystemEncoded contentIPnetworkHFCnetworkVOD clientSDV clientDRM/decryptDecodeLegendData pathControl pathERMSDV servicemanagerVASUDEVAN LAYOUT 4/23/08 12:39 PM Page 105
  5. 5. cable modem termination system (CMTS), and aPacketCable Multimedia (PCMM) [2] policyserver to support IPTV.CMTS AND CMThe CMTS and CM provide a two-way IP overDOCSIS transport in the HFC network.Although the CMTS can be integrated withDOCSIS media access control (MAC) andphysical (PHY) protocol layers in a singledevice, the cable industry is moving towards amodular CMTS (M-CMTS) [3] architecturewith separated MAC and PHY layers toimprove the economics and scalability of DOC-SIS transport. The CMTS manages the DOC-SIS QAM resources and provides QoS over theDOCSIS channel. It also manages residentialCMs, and can dynamically load-balance CMsamong DOCSIS downstream channels to opti-mize traffic distribution.STB AND IPTV-CAPABLE DEVICESIP STBs or other IPTV-capable devices must beable to receive video over IP transport andremove network jitter (packet delay variation).IP STBs have on-demand and switched digitalvideo client software components, as do RFSTBs. IP STBs can be either standalone devicesor embedded with CMs.Major cable operators are finding “hybrid”STBs an attractive solution for IPTV. A hybridSTB with an embedded CM can receive videofrom either the IP path or the traditionalQAM/RF transport path, presenting a goodstrategy for migration to IPTV. These newerboxes have multiple tuners (typically three),which can be dynamically provisioned to oper-ate in DOCSIS IP mode or classic QAM/RFmode. This makes good sense, especially as thecost of DOCSIS transport is still much higherthan that of direct QAM/RF transport, whilethe cable industry awaits the proliferation andmass adoption of M-CMTS and DOCSIS 3.0[4] products.PCMM POLICY SERVERPolicy servers in DOCSIS networks define QoSpolicies, while CMTSs enforce QoS policies.CableLabs standardized the policy control inter-faces through the PCMM specification. ForIPTV, session managers can request and reserveDOCSIS bandwidth through PCMM policyservers.IMPROVING BANDWIDTH EFFICIENCYIN THE LAST MILENew applications and service offerings drivedemand for ever-increasing bandwidth, especial-ly in last-mile HFC networks. In this segment ofthe network, bandwidth is a scarce resource,shared by hundreds of subscribers across data,voice, and video applications.There are numerous options for cable opera-tors to expand HFC capacity or improve band-width efficiency: splitting optical nodes to reduceservice group size, applying advanced video com-pression algorithms to reduce a stream’s effec-tive bandwidth “footprint,” reclaiming analogchannels, upgrading the plant to higher frequen-cies like 1 GHz, and so on. However, upgradingthe HFC infrastructure to obtain additionalcapacity can be costly, largely because of theneed to possibly dig up local streets to upgradeHFC nodes and/or physical topology, and is thusa nontrivial consideration on the cable opera-tors’ wish list.Perhaps the biggest recent innovation to opti-mize available bandwidth is SDV, which achievesthis by not wasting HFC spectrum on programsthat are not actively viewed. In IPTV over DOC-SIS, video services use the switched or on-demand model instead of the broadcast model.Additional solutions are becoming more impor-tant as cable IPTV gains traction.QAM AND HFC BANDWIDTH SHARINGQAM sharing holds the promise of improvingbandwidth efficiency without the need to over-haul the infrastructure. The typical HFC spec-trum includes both analog and digital broadcastchannels, which operate on fixed frequenciesand are unsuited for bandwidth sharing. Alterna-tively, SDV, VOD, and DOCSIS channels aregood candidates for dynamic bandwidth sharing.Now that a TV program can be obtainedthrough SDV, VOD, or IPTV over DOCSIS,what is the best way to allocate capacity forthese services? Without bandwidth sharing, eachservice must be planned for peak demand with a“tolerable” blocking factor. This is not only inef-ficient, but also not always achievable. WhenIPTV is offered, calculating the demand forIPTV over DOCSIS vs. traditional MPEG overQAM video and planning the HFC capacityaccordingly can be very difficult.Several factors contribute to the bandwidthefficiency of QAM sharing:• Even if SDV and VOD have the same peakhours, a viewer is likely to receive only oneservice at a time.• The gain of QAM sharing comes from thestatistical advantage of a bigger QAM pool,shared by two or more services.• Differing peak hours of DOCSIS and videoservice contribute additional gains. Forinstance, the spectrum allocated for DOC-SIS commercial services during the day mayby reallocated for video services in theevening.Dynamic QAM sharing is being realizedthrough new technologies and standards for thefollowing reasons:• The DOCSIS M-CMTS architecture, whichseparates the DOCSIS MAC and PHY pro-tocol layers. This allows the PHY layer(specifically the QAMs) to be dynamicallyallocated and de-allocated.• The recent emergence of universal QAMs,which can function as either MPEG videoQAMs or DOCSIS QAMs.• Standardization in edge QAM resourcemanagement.The first level of QAM sharing involves shar-ing among video-only QAMs for VOD and SDV.However, a universal QAM pool allows sharingbetween video and DOCSIS applications, whichbecomes increasingly important as IPTV overDOCSIS is offered.IEEE Communications Magazine • May 2008106Perhaps the biggestrecent innovation tooptimize availablebandwidth is SDV,which achieves thisby not wasting HFCspectrum onprograms that arenot actively viewed.In IPTV over DOCSIS,video services usethe switched oron-demand modelinstead of thebroadcast model.VASUDEVAN LAYOUT 4/23/08 12:39 PM Page 106
  6. 6. IEEE Communications Magazine • May 2008 107Having realized the value of QAM sharing,CableLabs ( standard-ized the interfaces for dynamic QAM sharingbetween DOCSIS applications and native videoapplications in the M-CMTS Edge ResourceManagement Interface (ERMI) specifications[5]. Figure 3 illustrates dynamic QAM sharingbased on those specifications, showing videoflows as dotted lines and control flows as solidlines.ERMI specifies several interfaces to an edgeresource manager (ERM). The ERM is a plat-form that manages and leases QAM channelbandwidth for all applications:• ERMI-1 is an interface for edge devices toregister QAM channels to the ERM andnotify the ERM of any QAM failures orstate changes.• ERMI-2 is an interface for the ERM tobind the QAM resources from an edgedevice to the resource allocation request.• ERMI-3 is an interface for the M-CMTScore to allocate QAM-channel bandwidthfrom the ERM.The M-CMTS core uses the downstreamexternal PHY interface (DEPI) [5] protocol toestablish a tunnel to remote QAM channelsafter obtaining QAM resources from the ERM.For native video applications, a video sessionmanager (SM), upon request from an STB,requests QAM resources for each individualvideo session though the ERM. The ERM thenselects the best resource for the request and, ifthe resource is successfully acquired, returns thetuning information. For IPTV, however, the SMdoes not request DOCSIS bandwidth from theERM directly. In this case the request goesthrough a PCMM policy server and from thereto the CMTS. So why not use the ERM directlyfor IPTV?There are several complications for DOCSISbandwidth management:• A CMTS can dynamically load-balance CMsamong different MAC domains and QAMs.• Channel bonding (combining narrowbandchannels to achieve wideband ones; seenext section) in DOCSIS 3.0 is dynamic,and CMs may change bonding groups perflow. This makes it difficult for an ERM tosynchronize the bonding groups and DOC-SIS MAC domains with the CMTS.• In DOCSIS networks providing convergedservices, IPTV shares the same pipe withother applications such as data and voice.The ERM is not in the signaling path ofdata and voice applications, and thus isunaware of their bandwidth consumption.In a nutshell, DOCSIS applications includingIPTV can use the PCMM framework to obtainsession-level bandwidth, while the M-CMTS coreobtains resources from the ERM at the granu-larity of full QAM channels.THE POWER OF DOCSIS CHANNEL BONDINGBefore DOCSIS 3.0 and channel bonding, QAMvideo delivery could only provide channels withan individual capacity of 38.8 Mb/s (using theU.S. example of a 6-MHz channel bandwidthwith 256-QAM modulation). A bundle of nar-rowband channels will use bandwidth inefficient-ly when high-definition (HD) streams, which inMPEG-2 typically range from 10 to 19 Mb/s perprogram, are delivered. It is possible that eventhough the total available bandwidth of all theQAMs within a service area is adequate for asingle HD stream, nevertheless no individualQAM channel can support that stream. Becauseof inefficient bit-packing, the higher the QAMutilization, the higher the possibility of blocking(denial of service for insufficient bandwidth).This leads to suboptimal QAM utilization givenan acceptable blocking rate for SDV or VODservice. With IPTV over bonded channels of 4,8, 16, and 24 QAM channels, the bit-packinginefficiency for HD streams is reduced dramati-cally.CABLE IPTV ADMISSION CONTROLIn the cable IPTV architecture, admission con-trol can be applied to both the IP network fromthe video source to the CMTS, and the HFCnetwork between the QAM and the CM, asshown in Fig. 4. In cable networks admissioncontrol has focused primarily on the HFC por-tion, because that is the most common bottle-neck.In a centralized admission control framework,a component such as a policy server has the net-work topology and bandwidth information, andperforms admission control for all bandwidthrequests. The PCMM framework is an exampleof this centralized model.In a distributed framework, by contrast, net-work nodes are used on the data path for admis-sion control. Given the complexity of an IPnetwork and the difficulty of building a central-ized policy server, this model is popular for IPnetwork admission control.Admission control signaling can be catego-rized as off-path (where the control path is dif-ferent from the data path), on-path (where thecontrol path is the same as the data path), or ahybrid combination of the above. While off-pathsignaling applies direct signaling to the policyserver, on-path signaling traverses the data pathfor bandwidth requests that use the commonlyI Figure 3. Dynamic QAM sharing.VideosessionmanagerEdgeresourcemanagerDOCSISQAMVideoQAMSTBDOCSISMP2TDEPIPCMMPCMMERMI-2ERMI-1ERMI-3CablemodemPCMMpolicyserverM-CMTScoreVASUDEVAN LAYOUT 4/23/08 12:39 PM Page 107
  7. 7. IEEE Communications Magazine • May 2008108selected Resource Reservation Protocol (RSVP)[6] for resource reservation.Figure 4 illustrates the admission control sig-naling flows. For a unicast IPTV service, such asVOD or network PVR, the STB sends requeststo the SM, which then selects a video source.Using PCMM, the SM reserves DOCSIS band-width for the video service. The video sourcemay also reserve IP network bandwidth on theCMTS through RSVP, with the result fed backto the SM. In an alternative hybrid approach,after being selected by the SM, the video sourceuses on-path signaling for the IP network admis-sion control from the video source to the CMTSby means of RSVP. At the demarcation pointbetween the IP network and the HFC network(the CMTS), off-path signaling is sent to the pol-icy server for the HFC network admission con-trol.Multicast admission control requires specialconsiderations. Switched multicast video ser-vice incurs high signaling rates because ofchannel change requests. When all the channelchange requests trigger messages to the policyserver, the policy server becomes the controlsignaling bottleneck. What can be optimizedfor multicast is that admission control is need-ed only for new multicast sources in a DOC-SIS MAC domain. In Fig. 4 Internet GroupManagement Protocol (IGMP) is used to trig-ger PCMM requests to the policy server. Whenthe CMTS receives an IGMP request from theSTB, it can decide whether a PCMM requestis needed.SCALING FOR TOMORROW’SENTERTAINMENTFuture video entertainment is about time shift-ing, location shifting, and device shifting. It isalso about content personalization. No longer isbroadcast the main method of video delivery.The evolutionary path is from broadcast to mul-ticast, and eventually to unicast.The new cable video delivery paradigm placestremendous emphasis on the scalability of thecable network. Innovations in cable IPTV archi-tecture attempt to tackle scalability from multi-ple fronts.SCALING VOD SERVERSTraditional VOD servers are single-node storagesystems designed to operate in a centralizedlocation and ingest video assets in non-real time.However, even the largest centralized servercannot scale to the large number of subscribersdemanded by personalized video delivery. Scal-ing centralized VOD servers can lead to unnec-essary hardware duplication for storage.A recent innovation in VOD server architec-ture is the total separation of storage fromstreaming. Storage needs scale linearly with thenumber of hours of on-demand content offeredby service providers, while streaming needs scalelinearly with the number of subscribers watchingthe on-demand content. As mentioned earlier,video sources are called streamers, which play outstored content in real time; storage componentsare called vaults. Both are illustrated in Fig. 5.In this distributed architecture, vaults arelocated in cable headends and optimized for stor-age. Streamers are located at hub sites, and opti-mized for networking throughput. The mostpopular content information is cached at the edgeof the network in streamers, whereas less popularcontent is transferred from a vault “just in time”to maintain streaming timing integrity in the caseof a cache miss (the failure to acquire asset dataon the first try from a streamer’s local cache).SCALING THE IP TRANSPORT NETWORKIn regional IP transport networks, the dominantvideo traffic is unicast. When linear content isprovided with personalization by means of aswitched unicast and network PVR, unicast traf-fic grows substantially. With a distributed VODserver architecture, only content not availablefrom the streams is obtained through regional IPtransport networks. With a typical cache hit rateof 95 percent on streamers, the bandwidthrequirement for the regional transport networkis now an order of magnitude lower than in anondistributed architecture.Cable IPTV requires scalable DOCSIS net-works. Traditional DOCSIS CMTSs have fixedupstream, downstream, and MAC mappings.This prevents the downstream channels fromscaling economically with the demand of cableIPTV. Consequently, the cable industry is mov-I Figure 4. Cable IPTV admission control.VideosourceCablemodemSTBIGMPVideosessionmanagerPCMMDOCSISCMTSPCMMRSVPPCMMpolicyserverIPnetworkHFCnetworkDriven by consumerexpectations andfierce competitionfrom other IPTVproviders, cable IPTVmust reach the nextlevel of sophisticationand scalability tosustain prominence.Solutions such asSDV can provide astepping stone tocable IPTV.VASUDEVAN LAYOUT 4/23/08 12:39 PM Page 108
  8. 8. IEEE Communications Magazine • May 2008 109ing forward with the M-CMTS architecture,which separates the CMTS MAC and the PHYlayers to allow the PHY layer to scale indepen-dently of the MAC layer.Still, the most challenging scaling problemconcerns the HFC network bandwidth. Cableservice providers must improve bandwidth effi-ciency and mitigate the bandwidth bottleneckswith the schemes described earlier. This allowscapacity to be increased gradually, avoidingoverbuilding and stranded capital expenditures.SCALING STATISTICAL MULTIPLEXINGVariable bit rate (VBR) video provides consider-able bit savings compared to constant bit rate(CBR) video. To benefit from VBR video, MPEGstatistical multiplexing is widely used, saving asmuch as 30 percent of bandwidth. Conversely,MPEG statmuxes face multiple challenges in meet-ing the demands of unicast video delivery. First, itis expensive to scale statmuxes to the network edgebecause of the heavy processing involved. More-over, statmuxes introduce an inherent delay of 0.5to 1 s, making them unattractive for low-latencyapplications such as channel surfing.Cable IPTV presents new opportunities forbetter statistical multiplexing in DOCSIS net-works. With DOCSIS 3.0 channel bonding, a fatpipe is now available to the home, to the advan-tage of statistical multiplexing. The more streamsin the pipe, the more likely their peaks and val-leys are distributed evenly. Avoiding the expenseof MPEG-level processing of traditional stat-muxes makes statistical multiplexing in IPTV atruly scalable solution.CONCLUSIONAs incumbent video service providers, cable oper-ators have millions of subscribers receiving con-tent using traditional MPEG transport over QAM.Cable IPTV will coexist with traditional video inthe near future, but the IPTV architecture forcable requires special consideration to leveragethe existing infrastructure. Cable operators, com-pared with other IPTV service providers, faceunique challenges and opportunities in offeringIPTV. Driven by consumer expectations and fiercecompetition from other IPTV providers, cableIPTV must reach the next level of sophisticationand scalability to sustain prominence. Solutionssuch as SDV can provide a stepping stone to cableIPTV, providing an increase in programmingcapacity using the traditional MPEG paradigm.Standardization of universal QAMs can also allowQAMs purchased for SDV to be “forward com-patible” with future IPTV implementations. Andthe channel bonding capabilities introduced inDOCSIS 3.0 will allow cable systems to continueto maintain parity in the race to provide increasingbroadband speeds for consumers.REFERENCES[1] CableLabs, “Data Over Cable Service Interface Specifica-tion,”[2] CableLabs PacketCable, “PacketCable Multimedia Speci-fication,” 2005.[3] CableLabs, “Modular CMTS Interfaces,”[4] CableLabs, “DOCSIS 3.0 Interface Specification,” 2007.[5] CableLabs, “DOCSIS M-CMTS Downstream External PHYInterface Specification,” 2007.[6] R. Braden et al., “Resource Reservation Protocol (RSVP),”RFC 2205, Sept. 1997.ADDITIONAL READING[1] CableLabs, “DOCSIS M-CMTS Edge Resource Manage-ment Interface Specification,” 2005.BIOGRAPHIESS. V. VASUDEVAN [M] ( is director,Cable Video Architectures at Cisco Systems. He is a princi-pal inventor of switched digital video for cable networks,and also developed the first components and systems toimplement digital television. His research interests includeadvanced interactive television and video systems commu-nications. He is a member of the SCTE. He received a electrical engineering and computer science from theUniversity of California, Berkeley.XIAOMEI LIU is a video network architect at Cisco Systems.She is the primary author of the CableLabs DOCSIS M-CMTS ERMI specification. Her research interests includevideo over DOCSIS, switched video, video network resourcemanagement and advanced video applications. Shereceived B.E. degrees from TsingHua University, China, andM.S. degrees from the University of Akron and Ohio StateUniversity.KURT KOLLMANSBERGER is the lead network architect for CiscoSystems’ next-generation networks for cable service pro-viders. His research interests include network architecturesfor the resilient carriage of voice, video, and data. He is amember of the SCTE and NCTA. He has over 15 years expe-rience in the networking field and holds a B.S. degree fromthe University of Maryland.I Figure 5. Distributed VOD server architecture.Regional IPtransportnetworkHubstreamerSet-top box Set-top boxHubstreamerIngestVault HeadendVASUDEVAN LAYOUT 4/23/08 12:39 PM Page 109