On-Demand Network Transport Architecture Louis D. Williamson Sr. Project Engineer, Advanced Engineering Time Warner Cable Abstract each of these services is optimized to satisfy The paper explores the architectural their delivery requirements. All must be layered upon the existing HFC plant that has requirements to support an on-demand interactive digital network within the hierarchy been optimized for the delivery of broadcast analog television channels. This paper will of a hybrid fiber coax network. A traditional discuss how the on-demand interactive entertainment network is fundamentally a entertainment signals are added to an existing broadcast network. As new services, such as HFC plant. telephony, high speed modems and interactive televisions are introduced, can the same GOALS physical transport support them? What are The traditional cable television system the differences in their requirements? What has been a broadcast network. The headend are the behavioral differences of a broadcast was the origination point for all services network versus an on-demand network? The delivered throughout system. All of the paper reviews the current distribution hub­ services flowed from this point to all master hub based HFC network and discusses subscribers. Occasionally there maybe some available options to accommodate new digital local signals that were inserted at different switched services. areas to fulfill franchise requirements, but for the most part all customers receive that same INTRODUCTION channels. In addition, most of the services All HFC networks are not created delivered were analog. Lately, digital services equal. Time Warner Cable's Master Headend, such as video game download channels, and Transport Hub, Distribution Hub digital music services have been deployed, but architectures were designed to accommodate they remain as broadcast service. The primary various new businesses proposed by the delivery mechanism for these services has been industry. These new services can be amplitude modulated fiber-optic links and categorized into three businesses; analog coaxial cables optimized for delivery of Entertainment, PC Data, and Telephony. broadcast services. They all have specific requirements that On-demand interactive entertainment necessitate different implementation strategies services bring major changes to this method of on the Hybrid Fiber Coaxial (HFC) network. delivery. No longer can you deliver the same Entertainment services require a high channel mix to all the customers since a bandwidth, constant bit rate, highly customer in an area may request a specific asymmetrical communication path to deliver program at any time. An on-demand MPEG-2 programs to the television. PC data interactive entertainment system must be requires a high bandwidth, bursty, variable bit designed to handle these on-demand requests rate, communication path to delivery data to simultaneously. The number of simultaneous the personal computer. Telephony requires a requests that the network can service is low bandwidth, symmetrical, isochronous referred to as the 'peak load' of the network. communication path for delivery of voice to the telephone. Equipment developed to deliver 1996 NCTA Technical Papers -375- Designing for peak load is common for 3.Fit into existing broadcast telephone companies, and they have a wealth infrastructure. of historical data to base their design upon. 4.Cost effective deployment. For an on-demand interactive network, peak loading behavior is not very well known. No Let review the components necessary one really knows how many customers will be to support an on-demand interactive requesting the new release of a blockbuster hit entertainment system and TWC's Hybrid the first Saturday night it is available on the Fiber Coaxial design to understand how to network, or what happens to a movie's implement a network that meets these goals. demand over time. The network must be INTERACTIVE COMPONENTS designed to be able to deal with a variety of situations that may require that more channels The required components for an on­ be available in one area than another. This demand interactive television network are: focusing or narrowcasting of channels is a new media server, A TM switch, cable gateway, concept that cable television engineers must QAM modulators, and a data channel understand to be able to design cost effective gateway. These components and their on-demand networks. interfaces will be briefly discussed to determine the requirements that each places on It is imperative that an on-demand the HFC network. interactive network be integrated into our existing broadcast based system, if only to Set-top terminal preserve the investment to date. On-demand The set-top terminal is the consumer interactive services by nature are sent to a interface device. The set-top terminal is specific customer in a specific area. The designed to receive both analog channels and network must now support the routing of a MPEG-2 digital channels. The set-top channel to an area. But, only a portion of the terminals are usually designed to receive both available channels are on-demand, the majority analog and digital channels with one 6 M Hz of channels are still broadcast to all customers. tuner. The introduction of new on-demand Since it is necessary to be able to interactive entertainment services must communicate with the set-top terminal while consider the existing infrastructure and to the consumer is watching an analog program, work harmoniously with it. most set-top terminals have an out-of-band New services must be introduced data channel. The out-of-band channel is incrementally and cost effectively. Even referred to as the Forward Data Channel though we all believe that on-demand (FDC). interactive entertainment services will be a For on-demand interactive great business someday, it would not be entertainment services, the set-top terminal prudent to design or build a network to handle must have a real-time Reverse Data (RDC) to those anticipated peak loads today. The communicate back to the headend. Both the network must be designed so it can grow as FDC and the RDC use standard IP protocol the demand for services increases. for encapsulating messages. In review, the goals of an on-demand Media Servers interactive entertainment system are: Media servers are large computing !.Capability of handling peak loads. platforms that are optimized for delivering 2.Ability to grow as demand increases MPEG-2 compressed video and audio (scalability). 1996 NCTA Technical Papers -376- streams. Proposed servers can deliver from Cable gateways also perform some 100 to 250 different 3 megabits per second additional functions that are required including: video and audio streams. OC3 and OC 12 PID re-mapping, stream encryption, and video interface have been proposed to connect these and audio re-synchronization. servers to the ATM switch. QAM modulators Media servers are designed to be installed in an environmentally conditioned Quadrature Amplitude Modulation room. They also require much more (QAM) is the preferred modulation method maintenance than typical cable television for transportation over the HFC. The QAM equipment. The maintenance required channels that are created by the modulators are includes: loading and removing assets, referred to as Forward Application Transport replacing disk drives, and cleaning fan filters. (FAT) channels. These channels carry MPEG- 2 video and audio to the set-top. FAT ATM Switch channels can also be used as high speed An ATM switch is required to route download channels to dynamically send new the MPEG-2 streams that originate from the applications to the set-top that the consumer media servers. Although the routing of the request. stream can be done with the media servers, the Data Channel Gateway A TM switch provides for redundancy of media servers and helps tremendously with The Data Channel Gateway (DCG) is load balancing and scalability. The input ports used for sending commands and other on ATM switch are OC3c or OC12c to information to the set-tops terminal. The accommodate the outputs for the media DCG consists of two primary units, the servers. The outputs of the ATM switch are Forward Data Channel (FDC) modulator and OC3c. The output feeds the interactive cable the Reverse Data Channel demodulator and an gateway. IP router. Together these two units send and receive all messages for the set-top using Cable Gateways standard IP protocol. Additionally, the DCG The primary purpose of an Interactive must provide the timing necessary to or Broadcast cable gateways is to create synchronize all ofthe set-tops attached to the 'Funny Cable Rates' (FCR). Today there are 2 coaxial plant serviced by the DCG. FCRs, FCRl that accommodates 64 QAM Together these pieces are the primary and FCR2 that accommodates 256 QAM. communication elements that are needed to These rates are needed because they are the support an on-demand interactive rates that can be put into a 6 MHz channel entertainment system. There are many ways spacing using QAM modulation. FCRl is that these elements can be assembled to create approximately 27 megabits per second FCR2 an on-demand interactive entertainment is approximately 36 megabits per second. The system. Next, we will briefly describe Time input to a cable gateway is a SONET OC3c. Warner Cable's HFC network and how we are The output from the cable gateway is electrical planning on assembling these components to TAXI. form a scaleable, cost effective system that can handle high peak loads. 1996 NCTA Technical Papers -377- AHeadend •Hub • Node -Coaxial Cable Fiber Cables -4 - 24 strands --24 -48 strands 48 - 96 strands Time Warner Cable of Gotham City -96+ strands Figure 1 Headend and hub transportation architecture. TWC'S HFC DESIGN Hubs There are 3 primary parts to the The Hub in a TWC system is the TWC's HFC design, the headend, hubs, and facility that is used to serve approximately HFC plant. (See Figure 1). 20,000 home passed.