James Farmer

End-to-End Performance

The various parts of the broadband distribution system work together to provide a transmission path whose performance allows delivery of adequate quality signals to the end user. This means that the total system performance requirements must be defined and that the allowable signal degradation must be allocated among signal acquisition, headend processing, distribution system, and terminal equipment. This chapter discusses the quality standards and the question of allocation. It addresses the calculation of the cascaded broadband network sections. It provides the analysis of the typical performance of a modern hybrid fiber-coaxial (HFC) distribution system, including factors that are not normally part of the cascaded network calculations, such as group delay and several classes of undesired signals that affect transmission channels. Typical HFC broadband distribution networks provide a broadband thermal noise floor at a level of approximately −45 dBc relative to normal video levels, as measured at the entry point to buildings and referenced to a 4-MHz bandwidth. The noise level in other bandwidths, after set-top terminals, or relative to other signal levels is easily calculated. Various signals may occur adjacent to or within standard 6-MHz channels as a result of network nonlinearities, normal signal loading, and/or attached subscriber receivers.

Architectural Elements and Examples

This chapter focuses on the architectural elements. The architecture of a system determines the services it can deliver. It controls effective bandwidth, reliability, flexibility, and distribution of signal processing. The chapter also describes some of the architectural elements and how each is related to essential network characteristics. The most common small-system architecture in use today is the single star, with nodes connected directly to a single headend. In large regional systems, formerly independent headends are often linked by either digital or 1550-nm analog, fiber-optic links to a large master headend and become hubs. Single coaxial cable lowsplit-band plans are used in the vast majority of systems, with individual nodes serving 400 to 1000 homes. Coaxial amplifier cascades vary from 1 to 6 in typical new upgrades. It is possible to “push” much signal processing out to hubs in order to increase the efficiency of the headend-to-hub links, or to centralize the processing for easier management at the expense of needing more fiber capacity. In the largest systems, both structures may be used in multitiered architectures. In considering architecture, initial cost is certainly a factor. But just as important is the ability to scale to meet market demand and opportunities without “stranding” capital and without causing excessive service interruptions to existing customers because of required reconfiguration. The relationship between architecture, network reliability, and network availability is a major topic in itself.

Linear Microwave Signal Transportation

This chapter shows the basic operation of broadband amplitude-modulated microwave equipment and the essential calculations and methodology required to engineer amplitude-modulated microwave links (AMLs). For most applications, fiber optics is the preferred choice for broadband signal transportation. Microwave links, however, are still a viable and cost-effective way to transport signals where construction costs or physical or regulatory barriers prevent cable construction. Broadband AMLs are simple in concept and straightforward to design. The performance of typical AMLs is comparable in noise and distortion to links constructed using directly modulated distributed feedback laser transmitters, but it suffers from occasional outages due to atmospheric conditions. More important, however, is that fiber-optic links have almost unlimited potential bandwidth, while current U.S. regulations limit 12-GHz AMLs to about 500 MHz of spectral width. The use of higher microwave frequencies, although acceptable for short paths, is limited by the dramatic increase in rainfall and atmospheric attenuation. The analysis of AML performance involves a combination of trigonometry and the use of empirical data on climatic factors, but it is amenable to straightforward computation using simple spreadsheet templates.

Linear Broadband Distribution Systems

This chapter provides an introduction of linear broadband distribution systems. Cable television systems have moved far beyond simple delivery of television programming to include high-speed data services, voice telephony, networking, transactional delivery of digital video under the interactive control of customers, and targeted advertising delivery, to name a few. To manage this complex business, what was formerly known simply as the “headend” has also evolved into a hierarchy of national, regional, and local signal processing centers. Similarly, the subscribers premise has evolved to often include local distribution networks that allow communication among devices as well as with the external network. In the near future, communications will be provided by operators to multiple, diverse end terminals, including wireless devices.

Linear Fiber-Optic Signal Transportation

This chapter provides a brief introduction to some principles of optics that are important to fiber transmission of light. It examines optical fibers in detail, particularly the interaction between the fiber and the signals passing through it. It also focuses on passive optical devices and compares them to their coaxial counterparts. It also examines the total performance of optical links with end-to-end performance calculations of noise and distortion. Linear fiber-optic links are capable of transporting the full spectrum of cable television services over distances exceeding 20 miles without amplification. Amplification, readily available at 1550-nm wavelength, can extend the attainable distance severalfold, with only a minor impact on end-to-end performance. The basic carrier-to-noise (C/N) performance of optical links is limited by transmitter intensity noise, optical shot noise, the interaction between transmitter linewidth and doubly scattered light in the fiber, and receiver postamplifier noise. The interaction between transmitter residual phase noise and the fiber can further reduce the noise under some conditions. Distortion is fundamentally limited by both small-signal nonlinearities in the transmitter and clipping caused by large-signal peaks. Additionally, interaction between incidental transmitter FM and fiber dispersion can cause second-order distortion, as can interaction between peak power levels and the glass material itself. Increasing the optical modulation index per carrier in an optical link will improve C/N at the expense of distortion; increasing operating levels in a coaxial network will have the same effect. Similarly, decreasing the number of carriers will allow higher C/N and lower distortion per channel, as in a coaxial network.

Coaxial Distribution System Design

This chapter focuses on coaxial distribution networks, how they are designed and powered, and the nature of signal degradation through such networks. Cable television coaxial distribution systems consist of a cascade of cable, amplifiers, and passive RF components. The network typically branches in the downstream direction, using both discrete passive RF devices and devices internal to active equipment used to create the legs. The amplifiers are capable of amplifying signals flowing in both directions, with the downstream bandwidth extending from approximately the lowest broadcast television channel to an upper limit determined by the bandwidth needs of the network, but typically 400 to 1000 MHz. The upstream bandwidth typically extends from 5 MHz to somewhere between 30 and 42 MHz. In the downstream direction, distribution networks with multiple, identical amplifiers in cascade are usually designed and operated so that the gain, measured from amplifier output to amplifier output, is unity.

Coaxial RF Technology

This chapter explores the technology behind coaxial distribution systems. It focuses on the coaxial network technology in detail, including cable, amplifiers, passive components, and powering systems. It provides the characteristics of the coaxial cable, amplifiers, passive components, and powering systems. Amplifier stations for cable systems are designed with unique characteristics that complement the characteristics of the interconnecting cable and passive devices. Integrated powering of active devices through the cable avoids the necessity of separately connecting each distribution system amplifier to the local utility, but requires special circuitry in every device to separate the power and signal paths. Before the mid-1980s, cable television systems used primarily coaxial technology to serve their customers. Modern systems generally use linear fiber-optic links in place of the long, repeated coaxial trunks to interconnect small, physically separate coaxial distribution networks with shared headends.

Architectural Requirements and Techniques

This chapter discusses the categories of service-related parameters that are affected by architecture. It also discusses requirements of various specific service types, and scalability and the interaction of bandwidth assigned to each service with the physical granularity of the network. It emphasizes the process of setting standards and alternative means of meeting those standards, rather than on suggesting specific values. Networks can be described by a number of parameters like noise, distortion, and other signal degradation measures, instantaneous RF bandwidth, reliability and availability, and service-specific and geography-specific scalability. Services that may be delivered over these networks place varying demands on the distribution system, i.e., quality of the RF channel occupied by its signals, occupied bandwidth, both forward and reverse, availability of the network to carry signals, and network powering for unique service-related processing equipment.

Wavelength Division Multiplexing

This chapter deals with the technology and performance issues encountered in various linear wavelength division multiplexing applications, including component performance, mutual interaction in fibers, and link design tradeoffs. Wavelength division multiplexing has become standard in the engineering of cable television and similar networks because it facilitates the delivery of switched services to small groups of customers. It does this by allowing the transport of many independent signals over shared fibers and through shared optical amplifiers. Unfortunately, optical signals on separate wavelengths interact as they travel through the fiber, and those interactions, sometimes in conjunction with discrete optical components in the circuit, generate various levels of crosstalk. Depending on the parameters of a given link, these mechanisms can have a serious effect on recovered RF signal quality. The analysis shows the quality of discrete components in general, and of the wavelength demultiplexer in particular, is typically the limiting factor in achieving acceptable low levels of crosstalk interference. The analysis also shows that the achievable quality for 16-wavelength dense wavelength division multiplexing circuits with lengths approaching 30 km will not be acceptable for analog video (assuming those signals differ among the optical signals) but will be adequate for at least 256 quadrature amplitude modulation signals with adequate component quality.

Network Reliability and Availability

This chapter deals with the calculation of network reliability and service availability. It discusses how those parameters vary as a function of the topology of the distribution system, among other factors. It explores the difference between true availability and that experienced by users of a particular service. The estimation of network availability, failure rate, and other reliability-related factors is very straightforward, if somewhat tedious, given a particular topology, known component failure rates, and repair times. Unfortunately, a great deal of effort needed to produce accurate reliability predictions is involved in developing believable component and commercial power failure rates. Once a logical model of a proposed network is constructed and entered into a spreadsheet for analysis, it is simple to test the effect of various component choices and operating practices. The preliminary analysis done in this chapter illustrates that modern hybrid fiber-coaxial (HFC) networks are easily capable of achieving low customer-experienced outage rates and unavailability for video services. HFC networks that are properly designed for wired telephone services are capable of achieving the required availability, whereas lower cost networks may be entirely adequate for various levels of video and data services. It is the function of the network engineer to design the most cost-effective network to carry the desired services.