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Feature Article
April 2001


Digital Television And The Converged IP Network


Though originally designed to transport data, Internet Protocol's packet-based technology can provide high-quality digital video integrated with interactive digital voice and data services over broadband networks. Transporting multimedia such as digital television (DTV) or video on demand (VoD) over IP has many advantages. It also has its weaknesses. For these advanced services, quality can become an issue in the IP environment. Emerging technology overcomes these quality challenges and enables IP to transport high-end digital video on one converged network. IP allows cable television and telecommunications service providers to transport high-quality interactive digital video consumers want over their existing networks.

IP has become a leading contender for video transport because the popularity of high-speed Internet access has spurred the creation of a broadband network that is reaching into homes everywhere. A connectionless, packet switching protocol, IP provides packet routing, fragmentation, and re-assembly, allowing many users of multiple services to share bandwidth. This new model of fast, multiservice delivery contrasts the traditional community access television (CATV) plant, a branch and tree topology that distributes content from a centralized headend out to customer premises. These traditional CATV headends act as satellite and microwave reception areas, and drive a one-way TV broadcast infrastructure. The new model is bi-directional and therefore enables interactive services such as VoD.

Cable TV operators, as owners of the access broadband network, have tapped into the high-speed residential data delivery business. The old model of cable headends as a system of standalone, unidirectional broadcast facilities have become nodes on a bi-directional or full-duplex wide-area network (WAN). These WANs incorporate packet-based data networking technologies like asynchronous transfer mode (ATM) and synchronous optical network (SONET). Cable modems have evolved into high-speed digital packet-based IP gateways to the home. First generation cable modems were initially designed for IP-based data services only. While IP is evolving to support video services such as VoD and DTV, these new media are fundamentally different from data and bring quality of service (QoS) front and center.

Quality is always a priority in transporting video for any purpose, but especially for entertainment. The de facto compression standard for distributed, entertainment quality video like VoD and DTV has become MPEG-2 technology. The innate time-sensitive nature of video makes video stream clocking management every bit as important as content management. MPEG-2 injects into the stream a free running 27 MHz timing clock called a program clock reference (PCR). MPEG-2 DTV systems require that the encoder PCR time clock and the decoder's clock be kept in close synch. MPEG-2 compressed video is sensitive to propagation delay variance. Lost or corrupted data cause noticeable interruptions. That's why MPEG-2 has stringent QoS parameters.

In this emerging broadband network, IP is the transport protocol for MPEG-2. However, IP was originally designed as a data transport protocol. Data is inherently more tolerant to error than media such as real-time voice or digital video. IP technology breaks a digital information stream into packets and gives each packet a destination. IP-based edge devices find a path through available network links. Packet delay variations, packet loss and bit error rate are secondary considerations. For high priority data, where guaranteed delivery is necessary, protocols like transmission control protocol (TCP) determine whether packets have arrived successfully. If they haven't, the protocol initiates a retransmission. While this is fine for non-real-time applications, such as e-mail, Web browsing, and file transfer, it inhibits real-time digital video. Digital video is sensitive to packet delay variation, packet loss and bit error rate. The high bandwidth demands of a video feed make the problem worse.

Many point-to-point networks, such as those for phone service, don't have this quality control problem. IP does not establish any comparable type of point-to-point connection. However, tagging different services to be handled differently by the network is a function of QoS that IP can accommodate. Services can be assigned classifications based on sensitivity and priority. The network is designed to handle them differently. IP can differentiate classes of service (CoS), and distinguish different levels of service and network handling. This is the essence of QoS.

A number of differential traffic-handling approaches can provision IP-based CoS, which in turn enables QoS. Approaches such as IEEE 802.1p, ToS (type of service) marking, and DiffServ mark packets with a priority that tells the network how to treat the packet. The DiffServ standard allows networks to prioritize packets forwarded from the network edge devices based on defined service codes. Video, for example, would be coded differently than Web traffic. While this does not guarantee QoS, it does establish priority relative to other traffic on the network.

Because a typical IP network itself has no direct knowledge of how to optimize the path for a particular application or user, the IP protocol provides a limited facility (ToS) for upper layer IP protocols to convey what service tradeoffs should be made for the particular packet. The ToS field in the IP header is not considered true QoS, but it is still a way to identify special handling for a packet. Next-generation cable modems could use ToS to drive IP-based multimedia over their hybrid fiber coax (HFC) plants.

A number of resource reservation protocols like resource reservation setup protocol (RSVP) and multi protocol label switching (MPLS) exist for next generation systems to implement differentiated services, including video. MPLS handles traffic management for media with different CoS and injects predictability into the networks, dealing with QoS issues, such as black and white management, latency, jitter, and packet/cell loss. MPLS can, in theory, provision QoS over multiple types of network technologies. The trick is getting MPLS provisioned end-to-end over multiple network types (ATM, IP, SONET, and DWDM). An initiative spun off of MPLS -- called multi-protocol lambda switching (MPlS) -- would provision QoS on the fiber layer of the network. Currently, no standard regulates how MPLS should be provisioned end-to-end in the optical (SONET/SDH), ATM, and IP layers together. One should be set in the next two to four years.

The cable industry has developed the Data Over Cable Service Interface Specification (DOCSIS) to standardize high-speed data services implementation over the cable infrastructure. DOCSIS v1.0 prescribes universal ground rules for reliable IP packet routing between the CMTS and the cable modem. More recent DOCSIS versions have included QoS and security features to add packet-based voice services to the mix. Latest generation DOCSIS standards do this by sub-partitioning bandwidth in the cable plant. Each sub-partition is then assigned a different service type and allows particular high priority, jitter, and time sensitive traffic priority over non-real-time critical information.

The technologies and standards are maturing enough for cable operators to consider IP-based video services in conjunction with data and voice services. The next generation cable system includes standards-based network equipment to ensure interoperability with out-of-system networks.

Collapsing the network layers into a single packet protocol ensures maximum revenue/cost by offering flexibility, scalability, and simplicity. Point-to-multipoint services over last mile infrastructure will exploit IP routing's inherent benefits -- forwarding and multicasting -- while addressing QoS issues. Video-aware network edge devices in conjunction with new generation CMTS systems will give cable operators what they've wanted for decades: a single converged multi-service network that allows dynamic video, voice, and data provisioning. The new cable plant will evolve into a multi-service network over a broadband infrastructure. As IP-based WAN technologies advance in conjunction with DWDM, cable operators will be able to allocate bandwidth not only to specific types of services, but also to individual subscribers. The long-term advantages of such a vision are obvious: a single integrated multimedia infrastructure that is flexible, scalable, easy to maintain, and technologically poised to provision the next generation of interactive video, voice, and data services.

David Pecorella is director of product marketing and Marty Dugan is manager of corporate marketing for Artel Video Systems, Inc. Artel provides video networking systems to cable TV and telecommunications service providers who need to satisfy the consumer's ever growing appetite for broadband digital entertainment. The company's products are transforming the world's networking infrastructures by enabling the transmission of broadcast and interactive TV over standards based multi-service networks. 

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