A CATV-Based High-Speed
`Packet-Switching Network Design
`
`David Charles Feldmeier
`
`© Massachusetts Institute of Technology 1986
`
`This research was supported in part by the Defense Advanced Research
`Projects Agency of the Department of Defense and monitored by the Office of
`Naval Research under contract number N00014-83-K-0125
`
`Massachusetts Institute of Technology
`
`Laboratory for Computer Science
`
`Cambridge, Massachusetts 02139
`
`Petitioner ARRIS Group, Inc.’s
`
`EXHIBIT
`
`ARRIS883IPRI0001060
`
`

`
`A CATV-Based High-Speed
`Packet-Switching Network Design
`
`by
`David Charles Feldmeier
`
`Submitted to the
`Department of Electrical Engineering and Computer Science
`on April 24, 1986 in partial fulfillment of the requirements
`for the Degree of Master of Science
`
`Abstract
`
`A high-speed packet-switching data network to the home can be built on an existing,
`unmodified,
`residential cable television (CATV) system.
`This thesis considers the
`theoretical and practical
`technical aspects of providing such a service and suggests a
`possible system design. All network data must pass through the CATV hub, so the
`network design is divided into three major parts: upstream transmission, downstream
`transmission, and access scheme.
`
`Upstream transmission is difficult because of the high noise level on the upstream channel
`caused by ingress of shortwave signals and impulse noise. The noise level is increased by
`the CATV system topology that funnels all system noise to the headend. Several noise-
`reduction techniques must be used simultaneously for robust upstream transmission. The
`downstream channel has low noise, but the data signal must be compatible with the
`CATV system, video signals and television receivers. Vestigial sideband data modulation
`“is suggested for total system compatibility. Existing access schemes, such as those for
`local area networks and satellite networks, are unsuitable for a high-speed CATV-based
`network. Modified versions of two satellite access schemes are suggested as possible
`solutions. The best techniques for upstream transmission, downstream transmission and
`access scheme are combined into a single proposed system.
`
`Key Words: cable television, metropolitan area networks, broadband networks, access
`control techniques
`
`Thesis supervisor.’ Jerome H. Saltzer.
`
`ARRIS883IPRI0001061
`
`

`
`Acknowledgments
`
`I would like to thank my thesis advisor Jerry Saltzer for proposing a CATV-based
`
`computer network as a research project and for his thoughts on various aspects of
`
`communication via residential CATV systems.
`
`John Cafarella of Micrilor spent many hours with me, discussing aspects of modulation
`
`and error-correction coding for noisy channels, both of which are important for the design
`
`of the upstream transmission system. His help and expertise are greatly appreciated.
`
`George Papadopoulos of the University of Patras, Greece, spent a few weeks considering
`
`possible access schemes for CATV networks. He and I had several meetings discussing
`the pros and cons of various access schemes.
`
`Stanly Reible of Micrilor helped with measurement and interpretation of noise on some
`
`local CATV systems.
`
`My special thanks to Patrick Mock, Katy Isaacs, and Carine Bickley for their helpful
`
`comments on my thesis.
`
`ARRIS883IPRI0001062
`
`

`
`Table of Contents
`
`Chapter One: Introduction
`
`1.1 Background
`1.2 Goals
`
`1.3 Communication Systems to the Home
`1.4 Previous Work
`
`Chapter Two: An Introduction to Cable Television
`
`2.1 Community Antenna Television
`2.2 Distribution Plant
`2.3 Residential versus Institutional Cable
`
`2.4 Constraints on Network Design
`2.4.1 Central Clock
`2.4.2 Economics
`
`2.4.3 CATV Topology
`2.4.4 Noise
`2.5 Problem Division
`
`2.6 Summary
`
`Chapter Three: Upstream Data. Transmission
`
`3.1 Power Measurements on CATV Systems
`3.2 Phase Distortion
`
`8.2.1 The Cause of Phase Distortion
`
`3.2.2 The Effects of Delay Distortion
`3.2.3 Overcoming Delay Distortion
`3.3 System Characteristics
`
`3.3.1 CATV Topology
`3.3.2 Digital Regenerators
`3.4 Upstream Noise
`3.4.1 White Noise
`3.4.2 Narrow Band Noise
`
`3.4.3 Impulse Noise
`3.5 Effective Channel Utilization
`
`3.5.1 Modulation Techniques
`3.5.1.1 Noise and Signal Power Considerations
`3.5.1.2 Delay and Interference Considerations
`3.5.2 Selective Spectrum Utilization
`3.5.3 Coding
`3.6 System Noise Budget
`
`ARRIS883IPRI0001063
`
`

`
`3.7 Summary
`
`Chapter Four: Downstream Data Transmission
`
`4.1 CATV System Considerations
`4.2 Frequency Shift Keying
`4.3 Vestigial Sideband Modulation
`4.3.1 Transmitter Baseband Processing
`4.3.1.1 Number of Signal Levels
`4.3.1.2 Low-pass Filtering
`4.3.2 Amplitude Modulation
`4.3.3 Transmitter Bandwidth Restriction Filter
`4.3.4 Vestigial Sideband Filtering
`4.3.5 Signal Detection
`4.3.5.1 Coherent Detection
`
`4.3.5.2 Envelope Detection
`4.4 A Sony VSB Data Transmission System
`4.4.1 Transmitter
`
`4.4.1.1 Nyquist Filtering
`4.4.1.2 Modulation
`
`4.4.1.3 Transmitter Bandpass Filter
`4.4.2 Receiver
`
`4.5 Channel Capacity
`4.6 Master Clock Reception
`4.7 Summary
`
`Chapter Five: Network Access Schemes
`
`5.1 Introduction
`
`5.2 CATV Access Schemes Considerations
`
`5.2.1 Packet-Switching Broadcast Networks
`5.2.2 Preliminary Access Scheme Evaluation
`5.2.3 Monitoring of the Upstream Channel
`5.2.4 An Example CATV System
`5.2.5 Expected Traffic
`5.2.6 Traffic Modeling
`5.2.7 The Comparison Metric
`5.3 Deterministic Access Schemes
`
`5.3.1 Polling
`5.3.2 Token Passing
`5.3.3 Fixed Reservation
`5.4 Contention Access Schemes
`5.4.1 Aloha
`
`5.4.1.1 Collision Detection
`5.4.1.2 P-Persistent CSMA
`5.4.2 Contention Reservation
`
`ARRIS883IPRI0001064
`
`

`
`5.5 Propagation Delay Determination
`5.6 Summary
`
`Chapter Six: System Design
`
`6.1 Downstream Transmission
`
`6.2 Upstream Transmission
`6.3 Proposed Access Scheme
`6.4 The Network Center
`6.5 The Network Interface
`
`6.6 Summary
`
`Chapter Seven: Conclusion
`
`7.1 Major Points
`7.2 The CATV System as a Metropolitan Area Network
`7.3 Future Research
`7.4 Conclusion
`
`Appendix A: Filter Power Loss
`
`1 Pre-Modulation Filter Loss
`
`2 Transmitter Bandpass Filter Loss
`3 Determination of VSB Filter Loss
`4 Calculation
`
`4.1 Transmitter Bandpass Filter
`4.2 VSB Calculations
`
`Appendix B: Bit Error Rate of an Envelope Detector
`
`Appendix C: Contention Reservation Slot Calculation
`
`ARRIS883IPRI0001065
`
`

`
`Table of Figures
`
`Figure 3-1: CATV Network Delay Errors as a Function of Signal Frequency
`and Bandwidth (from [30])
`Figure 3-2: Measurement Method for Linear Delay Distortion (from [28])
`Figure 3-3: Maximum Signal Impairment as a Function of Delay for Phase and
`Frequency Modulation (from [28])
`Figure 3-4: Measured Upstream Noise Spectrum from 0 to 30 MHz with a 20
`dBmV Reference for an Example CATV System (from
`Figure 3-5: Probability of Bit Error as a Function of Eb/No for Several
`Modulation Systems (from
`I
`Figure 3-6: Error Rates for Several Modulation Systems (from [27])
`Figure 3-7: Bit Error Probability as a Function of C/N Ratio for BPSK (from
`l23ll
`Figure 4-1: Frequency Responses for the Receiver’s Vestigial Sideband Filter
`and the Transmitter’s Bandpass Filter (from [25])
`Figure 4-2: Bit Error Rate versus Eb/No for BPSK (from
`Figure 4-3: Frequency Characteristics of the Sony Pre-modulation Bandpass
`Filter (from [18])
`‘Figure 4-4: Bit Error Rate as a Function of C/N Ratio for the Sony Digital
`Transmission System (from [l8])
`Figure 1: Vestigial Sideband Filter Frequency Response (from [25])
`
`ARRIS883IPRI0001066
`
`

`
`Table of Tables
`
`Table 3-1: Table of Concatenated Codes \Vith a Bit Error Rate Below
`10‘19(rrom 112})
`Table 4-1: Table of Error Rate versus Sony System Carrier Power
`Table 5-1: Table of k and Ew versus 3
`
`ARRIS883IPRI0001067
`
`

`
`Chapter One
`
`Introduction
`
`The demand for high-speed communication to the home is increasing as the economy
`
`becomes information-oriented. Although no suitable system exists to meet this demand,
`
`a communication network to the home can be built quickly and inexpensively on an
`
`existing residential cable television system. The goal of this research is to propose the
`
`design of a system suitable for providing high-speed packet-switched communication to
`the home.
`
`1.1 Background
`
`As the United States shifts from a manufacturing economy to an information-oriented
`
`economy, data processing and computers play an increasingly important role in our work.
`
`For people who work with information, it is feasible to work one or more days a week at
`
`an office in the home. Working at home has several advantages, including reducing the
`
`time and cost of commuting. Work during non~business hours becomes convenient, so
`
`that personal schedules may easily be shifted. Work at home also allows one to work
`
`during the day without interruptions.
`
`The cost of duplicating office equipment, such as a personal computer, at the office in the
`
`home becomes economically competitive with commuting as the price of electronics
`
`declines. For work at home to be productive, a worker must have access to the resources
`
`available in the office; for example,
`
`the use of a mainframe computer or shared data-
`
`storage system. The worker at home must not become isolated from the office work
`
`environment.
`
`A modern office environment consists of computers and peripherals connected together
`
`ARRIS883IPRI0001068
`
`

`
`with a local area network. A local area network, or LAN, provides inexpensive, high-
`speed communication over a limited area. Many LANs have a data rate of 10 megabits
`
`per second and reside within a building, although network diameters of over a kilometer
`
`are possible. Resources that are too expensive to provide to each user, such as high-
`quality printers or mass-storage devices, are shared using such a network. The network
`
`allows the user of a personal computer (PC) to access those services that are available on
`a mainframe.
`
`An example of an application that requires high—speed communication is remote disk
`
`access. The disk drives in a PC have a small capacity and a low throughput, and do not
`
`allow data to be accessed by more than a single computer. An alternative is to provide a
`
`computer with a large storage system on the network to act as a remote disk that can be
`
`accessed by other computers on the network. With a high—speed network, data can be
`
`retrieved from a remote disk as quickly as from the local hard disk drive.
`
`If data are
`
`stored on a remote disk, a worker at home can use the data that are available at work
`
`without having to keep an extra copy at home.
`
`Another application for high—speed networks is the transmission of pictures, such as
`
`circuit diagrams. Often, the ability to show something to a person rather than describe
`
`it saves time, but only if a picture can be transmitted reasonably quickly. To transmit
`
`the bitmap display of an engineering workstation] takes a tenth of a second at 10 Mbps.
`If the worker at home is restricted to a conventional 1200 bps modem on a phone line,
`
`the same transmission takes almost 14 minutes. With a high—speed network, screen
`
`displays can be transmitted to a workstation as quickly as turning the pages of a book.
`
`A new generation of computers with faster processors, graphic displays and larger
`
`memories increases the need for high-speed communications.
`
`High—speed,
`
`low-cost
`
`communications must be brought
`
`to the home to support the services provided at the
`
`office by a local area network.
`
`1A Symbolics 3600 workstation has an 1100x900 pixel bitmap display.
`
`10
`
`ARRIS883IPRI0001069
`
`

`
`1.2 Goals
`
`The office in the home can be efficient
`
`if the home worker has access to the same
`
`computational resources that are available at work. Because distributed computing is
`
`becoming common and computer prices are decreasing, data communication from the
`
`home to the office increasingly will be among computers, rather than between a computer
`
`and a home terminal. Computer to computer communication is bursty, with a high
`
`peak-to—mean ratio of network use.
`
`Unlike computer
`
`to terminal communication,
`
`intercomputer communication traffic is symmetric, with comparable amounts of data in
`both directions.
`
`The goal of this thesis is to describe a system that provides computer communication to
`
`the home, with performance similar to that of a local area network. As with a LAN, the
`
`system transports packets on a single, hared broadcast channel, rather than using point-
`
`to-point links. This network provides high-speed service on demand to a single user at a
`
`time and any one user occupies the channel for a very short time, which allows many
`
`users to share the same channel. Although a high-speed network is needed to handle the
`
`expected traffic bursts, each individual station on the network uses this capacity only a
`
`small percentage of the time. This type of operation is compatible with computer
`
`communication, and data transmission and reception rates are identical because of the
`expected traffic symmetry.
`
`Computer communication requires high-speed bursts for short periods of time, but what
`
`is the minimum necessary speed for acceptable performance? A network to the home is a
`
`metropolitan area network because of the large area that it covers. The Institute of
`
`Electrical and Electronics Engineers (IEEE) 802.6 committee is developing metropolitan
`
`area network standards and the committee is mainly interested in LAN interconnection,
`
`and high-speed voice, video and data service. The 802.6 committee charter stipulates
`
`that
`
`1 Mbps is the minimum speed for a proposed network standard. Because the
`
`network to the home connects computers to LANs, the traffic should be similar to that of
`
`LAN interconnection, so 1 Mbps to the home is the minimal acceptable data rate.
`
`Although the data rate for the entire network is at least 1 Mbps, another important
`
`11
`
`ARRIS883IPRI0001070
`
`

`
`consideration is the minimum throughput for a single user.
`
`The bursty nature of
`
`computer communication demands a high throughput for a single user because at any
`
`given time, only a few stations have data to transmit. The hard disk drive on a personal
`
`computer has a raw bit rate of about 500 Kbps. Since remote virtual disk access is a
`
`desired application, and one of the most demanding, 500 Kbps should be the network
`
`throughput for a single user. Since the data rate is still 1 Mbps, the minimum network
`
`efficiency is 50%.
`
`Throughput and delay on a network are related,
`
`so although throughput may be
`
`increased at the expense of delay, delay must not be excessive.
`
`Interactive activities,
`
`such as remote login, have a maximum tolerable delay between the queueing of a packet
`
`and its transmission. A decent typist will have bursts of over 100 words per minute, and
`
`remote login requires two packets per character (one of which is an acknowledgment).
`
`This is about twenty packets per second, ten packets in each direction, which implies a
`
`maximum delay of 50 milliseconds between packet queueing and transmission.
`
`The computer in the home will be connected to a local area network at the office, and it
`is simplest if both networks use similar protocols. Specifically, LAN protocols correct
`
`data errors by retransmitting damaged packets, so the network error rate should be low
`
`enough for most packets to be delivered undamaged. Local area network research at
`
`MIT suggests that the correct error rate to control is the frequency of corrupted packets,
`rather
`than the bit error
`rate, because
`the system overhead of detecting and
`
`retransmitting damaged packets dominates the resulting performance. The performance
`
`of automatic repeat request error correction depends critically on the packet error rate of
`
`the channel.
`
`If the error rate is too high,
`
`the retransmission overhead dominates the
`
`system. Experience also suggests a network design target of less than one Corrupted
`
`packet per 1000 for acceptable performance.
`
`A communication network to the home must be affordable if it is to be used, so the
`
`system must be comparable in cost to systems that provide similar services. The two
`
`closest systems are local area networks and dial-up telephone modems, both of which cost
`
`$500 to $700 per interface in 1985 dollars.
`
`ARRIS883IPRI0001071
`
`

`
`The parameters above define a system to provide high-speed communication to the home.
`
`There are several existing technologies that could be used to build this network, and
`these are discussed in the next section.
`
`1.3 Communication Systems to the Home
`
`To provide a communications network to the home at a reasonable cost, the system must
`
`use either free-space transmission (microwave or light) or previously installed cables.
`
`It is
`
`to install dedicated cables to each home because of cost and right-of-way
`impractical
`considerations.
`
`The telephone system is commonly used for data communication to the home. However,
`telephone technology limits the data rate to about 9600 bps, which is too slow to provide
`acceptable packet switching service to the home.
`Data. communication is also an
`
`inefficient use of telephone company resources. Telephone switching equipment expects
`many calls of short (3 minute) duration, but data communication holds a circuit for
`
`much longer. Also, the bursty nature of computer communication means that the circuit
`
`In addition, the long economic recovery
`will often be idle, so system capacity is wasted.
`time for telephone switching equipment precludes its replacement for many years.
`
`A new service to be offered soon by the telephone company is the Integrated Services
`
`Digital Network (ISDN). This service provides two 64 Kbps circuit-switched channels
`
`and a 16 Kbps packet-switched channel to the home for a total of 144 Kbps. Although
`
`this will be a great improvement over a conventional telephone line, it is still slower than
`
`desired and does not exist yet.
`
`Another method of data transmission to the home is the FM Subsidiary Communications
`
`Authority (SCA) channel. Commercial FM stations can broadcast data on an SCA
`
`channel simultaneously with normal broadcasts. The Boston Community Information
`
`System uses this type of broadcast channel to provide data to the home [13]. The system
`broadcasts data to homes in a metropolitan area and is directed toward residential users.
`
`The system is also distributed — all data is broadcast to all receivers and it is up to each
`
`ARRIS883IPRI0001072
`
`

`
`station to selectively receive
`
`the information of
`
`interest,
`
`store and present
`
`the
`
`information to the user in a suitable manner. A disadvantage of this system is that the
`
`data rate is only 4800 bps, and therefore too slow for the applications that we are
`interested in.
`
`An alternative is a satellite-based network. Satellite equipment prices are falling and
`
`Direct Broadcast Systems (DBS) are designed to provide video entertainment to the home
`
`via satellite to small, inexpensive dish antennas with inexpensive electronics. The major
`
`problem with satellite systems is
`
`that
`
`if
`
`the satellite is
`
`in geosynchronous orbit,
`
`propagation delay to the satellite and back is a quarter of a second. This means at least
`
`half a second passes before a reply can arrive from any other computer on the network,
`
`which is too long. Propagation delay could be reduced by lowering the satellite orbit, but
`
`a low orbit requires more satellites so that one is always within line-of-sight, as each
`
`satellite would be in range only a short time.
`
`In addition, the dish antennas at each
`
`home would require a tracking mechanism, which increases the cost for each station.
`
`DBS also does not offer a simple return path from the home. Communication from the
`
`home requires a powerful transmitter operating in the Gigahertz range. The antenna
`
`beam-width must be narrow and directed well enough to avoid interference with other
`
`satellites. These requirements could make the return path quite expensive.
`
`The most straight-forward way of approaching the problem is to use an existing link to
`
`the home - the residential cable television system. A residential Community Antenna
`
`Television (CATV) system distributes television signals with a network of coaxial cables
`
`branching out from the cable headend to homes in a community. Modern CATV systems
`
`not only provide video channels to the home but also carry signals from the home back
`
`to the CATV headend. The coaxial cable is a high-bandwidth medium that is already
`
`installed to the home, meaning that a communication system could be added at a low
`
`marginal cost.
`
`In addition, CATV has the advantage of a small propagation delay
`
`relative to the area covered by the network and it does not require any of the already
`
`crowded radio spectrum as a microwave system would. Of the possibilities considered
`
`above, a CATV system provides the best basis for the desired data communication
`
`network to the home.
`
`ARRIS883IPRI0001073
`
`

`
`1.4 Previous Work
`
`Several existing or proposed systems that are built on residential CATV system or use
`
`CATV equipment are mentioned below. Although none of
`
`them provide a high-
`
`performance channel to the home, they show the current state of the art.
`
`The prototypical residential CATV-based communications system is the Warner-Amex
`
`QUBE system [20]. This is the first system to take advantage of the two-way capability
`
`of a CATV system. The QUBE system runs a. single channel at a 256 Kbps rate and uses
`
`an adaptive polling access scheme. A polling system queries each user on the system
`
`from a central point one at a time for data traffic. Although polling is simple and
`
`inexpensive, it is an inefficient access scheme for a network the size of a CATV system
`
`because most of the time, the polling system is waiting for a poll response. Adaptive
`
`polling is a better system that polls active stations more often than inactive ones, but
`
`still the network utilization is low.
`
`A more recent system developed jointly by General Instruments and Sytek is MetroNet,
`
`which is designed to bring Videotex service to the home via existing residential CATV
`
`systems [15]. A MetroNet is frequency divided into 50 channels, each of which has a data
`
`rate of 128 Kbps and uses a Carrier-Sense Multiple Access with Collision Detection
`
`(CSMA/CD) access scheme. This data rate is almost
`
`that provided by ISDN, and
`
`although it
`
`is adequate for computer to terminal communication,
`
`it
`
`is too slow for
`
`demanding tasks such as remote-virtual-disk access.
`
`Rather than a single high-speed channel, Metronet has many slower channels. Videotex
`
`and terminal traffic is less demanding than computer traffic and the narrower bandwidth
`
`of the slower channels allows the system to overcome noise on the cable more easily
`
`The relatively slow channel speed allows MetroNet to efficiently use CSMA/CD as an
`
`access scheme, but efficiency declines if the data rate is increased.
`
`Unlike the two systems described above, the following two systems are incompatible with
`
`a residential CATV system. The Ethernet system is suited to an industrial network
`
`within a building or a company. Homenet is a generalized version of an Ethernet that is
`
`ARRIS883IPRI0001074
`
`

`
`designed to cover a large area efficiently.
`
`Digital Equipment Corporation developed
`
`an Ethernet
`
`system with broadband
`
`the system diameter is
`transceivers that operates on CATV equipment However,
`limited to a few kilometers because the system operates at 10 Mbps and uses a
`CSMA/CD access scheme, which is efficient only over a limited area at high data rates.
`The system also requires 18 MHz of bandwidth in each direction, which would be difficult
`
`to fit onto the upstream channel of a residential CATV system because of noise
`considerations.
`
`Homenet is a system proposed by Bell Labs designed to be built with CATV components
`[16]. The system consists of cells of transmitters that contend for the channel using a
`CSMA/CD system. All transmitters within a cell use the same frequency and receivers
`shift to the frequency of the transmitter’s cell. This system allows the total system to be
`very large, but since all contention is within a cell of a kilometer in diameter, the access
`scheme remains efficient.
`
`Without modification, the structure of a residential CATV system could accommodate
`only one cell, because each cell needs a frequency translator and only the headend could
`have a translator. As a consequence, the access scheme would be inefficient and Homenet
`would not work well on a residential CATV system.
`
`Little research has been done concerning high-speed communications to the home on a.
`
`residential CATV system. Some sytems are low speed for providing videotex services;
`some are high-speed systems that require extensive modification to the CATV system.
`The thesis of D. L. Estrin discusses the technical and regulatory issues surrounding the
`use of a residential cable television system for data communications
`
`This thesis is a study of the technical and system issues of providing high-speed, packet-
`switched computer communication to the home on an existing residential CATV system.
`The purpose of this thesis is to review the type of communications necessary for work at
`home and to describe a system that provides this communication. This thesis outlines a.
`
`system that provides LAN-like communications to the home, and is specifically designed
`
`16
`
`ARRIS883IPRI0001075
`
`

`
`for peer communication among computers on an unmodified residential CATV system.
`
`Chapter 2 contains a tutorial on residential cable television.
`
`It describes the design of a
`
`CATV system and points how it might be used for communication to the home. An
`
`analysis of the system leads to the problem being broken into three parts: access scheme,
`
`upstream transmission, and downstream transmission.
`
`Chapter 3 discusses transmission from the home to the CATV headend. The tree-shaped
`
`CATV system acts as a noise funnel that channels the amplified sum of the noise over
`
`the entire system to the headend. A system is proposed for high-speed transmission in
`
`this adverse environment that includes a robust modulation scheme, selective spectrum
`
`utilization, and error-correction coding.
`
`Chapter 4 covers communication from the CATV headend to the home. Because the
`
`CATV system is designed for communication in this direction,
`
`the design is relatively
`
`simple. The main considerations are compatibility with the current CATV system and
`
`equipment including commercial television receivers, and subscriber receiver cost.
`
`Chapter 5 discusses access schemes for a CATV-based data network.
`
`It begins with the
`
`CATV system constraints and the network expectations, and explores possible access
`
`schemes. Suggestions for improved access protocols include a modification of a satellite
`
`reservation scheme to accept variable instead of fixed slot
`
`lengths and a method of
`
`increasing network efficiency by reducing contention on the system.
`
`Chapter 6 proposes the design of a complete system for providing high-speed packet-
`
`switched communication to the home. The results of chapters 3, 4 and 5 are used to
`
`provide the pieces of the system. The chapter also describes network controller design
`
`and subscriber interface design.
`
`Chapter 7 concludes the thesis by considering the strengths and shortcomings of the
`
`presented analysis.
`
`It also suggests directions for the continuation of this research.
`
`ARRIS883IPRI0001076
`
`

`
`Chapter Two
`
`An Introduction to Cable Television
`
`Since the high-speed data network to the home is to be built on a CATV system, the
`design of a CATV system must be understood. The technical characteristics of a CATV
`
`system will determine how a high—speed data network can be built on the system.
`
`2.1 Community Antenna Television
`
`A Community Antenna Television system is a tree-shaped structure of coaxial cables and
`
`amplifiers designed to distribute television signals to homes in a region. At the base of
`this tree is the system headend, from which the video signals originate. Although
`distribution of video entertainment signals from the system headend to the subscriber is
`the predominant design consideration, most modern CATV systems are two-way systems
`that carry signals both from the headend to the subscriber (downstream), and from the
`subscriber to the headend (upstream). This two-way capability of the cable system
`allows it to be the basis of a data communication network to the home.
`
`A CATV system is frequency-divided into adjacent 6 MHz channels. The most common,
`single-cable design used for
`residential areas divides frequencies into three bands:
`
`downstream channels, upstream channels and the guard band. Downstream channels
`
`carry signals, typically entertainment video, from the cable system headend to the home.
`
`Downstream (or forward path) channels begin at 54 Ml-Iz and continue up to the highest
`frequencies carried by the system - as high as 450 Ml-Iz, depending on the system design
`and technology employed. The upstream (or return path) channels occupy 5.75 to 29.75
`
`Ml-Iz and carry signals from the home to the cable headend. The frequencies from 29.75
`
`to 54 MHz form a guard band between the upstream and downstream channels and no
`
`signals are transmitted at
`
`these frequencies. The cable spectrum below 54 MHz is
`
`ARRIS883IPRI0001077
`
`

`
`referred to as the subband, and a cable system that allocates that spectrum to upstream
`
`channels is referred to as a. subsplit system; residential cable systems are typical subsplit
`systems.
`
`Each downstream channel normally carries a single vestigial sideband (VSB) video signal.
`VSB is the standard modulation for broadcast video signals in the United States, and it is
`
`an amplitude modulation system followed by a. vestigial sideband filter that eliminates
`
`most of the lower sideband. The audio carrier is processed separately and combined with
`
`the video signal before transmission. Video carriers on modern cable systems are phase-
`locked to a master oscillator so that intermodulation distortion products of the carriers
`
`are phase-coherent with the carriers themselves. These coherent products are removed
`
`along with the carriers during demodulation at
`
`the television receiver, but sideband
`
`intermodulation products limit the subjective picture improvement to 4 - 6 dB [30].
`
`2.2 Distribution Plant
`
`The distribution plant of a CATV system is an array of coaxial cables and amplifiers
`
`that carry signals from the headend to the home. The amplifiers on such a system are
`
`adjusted for unity gain across a cable segment, which means that the gain of an amplifier
`
`is exactly enough to overcome the loss in the previous cable segment. Amplifiers
`
`maintain unity gain with Automatic Gain Control (AGC)2, which allows the amplifier to
`
`adjust its gain automatically so that the signals leaving the amplifier are at the correct
`
`signal level. This dynamic adjustment allows the system to operate correctly as the loss
`
`characteristics of the cable change with time and temperature, but AGC alone is
`
`insufficient because signal
`
`loss is also a function of frequency. Signal loss is roughly
`
`proportional to the square-root of frequency, a characteristic called slope. Amplifiers
`
`have Automatic Slope Control (ASC)
`
`to correct for system slope by increasing the
`
`amplifier gain at higher frequencies. The control voltages for both of these amplitude
`
`controls are derived from either two special pilot tones or two designated video carriers.
`
`With two signals at different
`
`frequencies,
`
`the amplifier has enough information to
`
`2Referred to in some literature as Automatic Level Control
`
`19
`
`ARRIS883IPRI0001078
`
`

`
`determine the proper gain and slope.
`
`CATV amplifiers are unidirectional, but two-way systems need amplification in both
`
`directions. Two-way amplifiers are really two amplifiers in a single box, one for each
`
`direction. Since both upstream and downstream signals travel on the same cable,
`
`the
`
`upstream and downstream channels are separated at each amplifier by duplezing filters3.
`
`The CATV headend feeds into the trunk cable of the distribution plant, and the
`
`amplifiers along the trunk cable maintain the proper signal level throughout the system.
`
`Trunk cables span a city but cannot go everywhere - the trunk cable is too expensive and
`
`the cable system itself allows only a limited number of amplifiers in cascade. Feeder
`
`cables branch out from the trunk cable and serve a limited area, such as a neighborhood.
`
`A bridging amplifier amplifies signals from the trunk and drives the feeder cable.
`
`If a
`
`feeder cable is long or serves many homes, additional amplifiers called line extenders are
`
`used. Branching from the feeder cable to the individual home is done with directional
`
`splitters called taps. Taps are directional and couple power predominantly from signals
`
`traveling away from the trunk cable. This is so that mismatch reflections from taps
`
`farther down the cable are attenuated and do not cause interference at the television
`
`receiver. As distance from the trunk increas