Multiplex communications – Channel assignment techniques – Carrier sense multiple access
Reexamination Certificate
1999-01-20
2003-09-02
Vu, Huy D. (Department: 2665)
Multiplex communications
Channel assignment techniques
Carrier sense multiple access
C370S395430, C370S449000, C370S461000, C725S116000, C725S131000
Reexamination Certificate
active
06614799
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates generally to methods and apparatus for transmitting digital data in cable television network systems. More specifically, the present invention relates to methods and apparatus for dynamically adjusting modem back-off parameters in a cable modem network.
2. Background
The cable TV industry has been upgrading its signal distribution and transmission infrastructure since the late 1980s. In many cable television markets, the infrastructure and topology of cable systems now include fiber optics as part of its signal transmission component. This has accelerated the pace at which the cable industry has taken advantage of the inherent two-way communication capability of cable systems. The cable industry is now poised to develop reliable and efficient two-way transmission of digital data over its cable lines at speeds orders of magnitude faster than those available through telephone lines, thereby allowing its subscribers to access digital data for uses ranging from Internet access to cablecommuting.
Originally, cable TV lines were exclusively coaxial cable. The system included a cable head end, i.e. a distribution hub, which received analog signals for broadcast from various sources such as satellites, broadcast transmissions, or local TV studios. Coaxial cable from the head end was connected to multiple distribution nodes, each of which could supply many houses or subscribers. From the distribution nodes, trunk lines (linear sections of coaxial cable) extended toward remote sites on the cable network. A typical trunk line is about 10 kilometers. Branching off of these trunk lines were distribution or feeder cables (40% of the system's cable footage) to specific neighborhoods, and drop cables (45% of the system's cable footage) to homes receiving cable television. Amplifiers were provided to maintain signal strength at various locations along the trunk line. For example, broadband amplifiers are required about every 2000 feet depending on the bandwidth of the system. The maximum number of amplifiers that can be placed in a run or cascade is limited by the build-up of noise and distortion. This configuration, known as tree and branch, is still present in older segments of the cable TV market.
With cable television, a TV analog signal received at the head end of a particular cable system is broadcast to all subscribers on that cable system. The subscriber simply needed a television with an appropriate cable receptor to receive the cable television signal. The cable TV signal was broadcast at a radio frequency range of about 60 to 700 MHz. Broadcast signals were sent downstream; that is, from the head end of the cable system across the distribution nodes, over the trunk line, to feeder lines that led to the subscribers. However, the cable system did not have installed the equipment necessary for sending signals from subscribers to the head end, known as return or upstream signal transmission. Not surprisingly, nor were there provisions for digital signal transmission either downstream or upstream.
In the 1980s, cable companies began installing optical fibers between the head end of the cable system and distribution nodes (discussed in greater detail with respect to FIG.
1
). The optical fibers reduced noise, improved speed and bandwidth, and reduced the need for amplification of signals along the cable lines. In many locations, cable companies installed optical fibers for both downstream and upstream signals. The resulting systems are known as hybrid fiber-coaxial (HFC) systems. Upstream signal transmission was made possible through the use of duplex or two-way filters. These filters allow signals of certain frequencies to go in one direction and of other frequencies to go in the opposite direction. This new upstream data transmission capability allowed cable companies to use set-top cable boxes and allowed subscribers pay-per-view functionality, i.e. a service allowing subscribers to send a signal to the cable system indicating that they want to see a certain program.
In addition, cable companies began installing fiber optic lines into the trunk lines of the cable system in the late 1980s. A typical fiber optic trunk line can be upto 80 kilometers, whereas a typical coaxial trunk line is about 10 kilometers, as mentioned above. Prior to the 1990s, cable television systems were not intended to be general-purpose communications mechanisms. Their primary purpose was transmitting a variety of entertainment television signals to subscribers. Thus, they needed to be one-way transmission paths from a central location, known as the head end, to each subscriber's home, delivering essentially the same signals to each subscriber. HFC systems run fiber deep into the cable TV network offering subscribers more neighborhood specific programming by segmenting an existing system into individual serving areas between 500 to 2,000 subscribers. Although networks using exclusively fiber optics would be optimal, presently cable networks equipped with HFC configurations are capable of delivering a variety of high bandwidth, interactive services to homes for significantly lower costs than networks using only fiber optic cables.
FIG. 1
is a block diagram of a two-way hybrid fiber-coaxial (HFC) cable system utilizing a cable modem for data transmission. It shows a head end
102
(essentially a distribution hub) which can typically service about 40,000 subscribers. Head end
102
contains a cable modem termination system (CMTS)
104
that is needed when transmitting and receiving data using cable modems. Block
104
of
FIG. 1
represents a cable modem termination system connected to a fiber node
108
by pairs of optical fibers
106
. The primary functions of the CMTS are (1) receiving signals from external sources
100
and converting the format of those signals, e.g., microwave signals to electrical signals suitable for transmission over the cable system; (2) providing appropriate MAC level packet headers (as specified by the MCNS standard discussed below) for data received by the cable system, (3) modulating and demodulating the data to and from the cable system, and (4) converting the electrical signal in the CMTS to an optical signal for transmission over the optical lines to the fiber nodes.
Head end
102
is connected through pairs of fiber optic lines
106
(one line for each direction) to a series of fiber nodes
108
. Each head end can support normally up to
80
fiber nodes. Pre-HFC cable systems used coaxial cables and conventional distribution nodes. Since a single coaxial cable was capable of transmitting data in both directions, one coaxial cable ran between the head end and each distribution node. In addition, because cable modems were not used, the head end of pre-HFC cable systems did not contain a CMTS. Returning to
FIG. 1
, each of the fiber nodes
108
is connected by a coaxial cable
110
to two-way amplifiers or duplex filters
112
which permit certain frequencies to go in one direction and other frequencies to go in the opposite direction (frequency ranges for upstream and downstream paths are discussed below). Each fiber node
108
can normally service upto 500 subscribers. Fiber node
108
, coaxial cable
110
, two-way amplifiers
112
, plus distribution amplifiers
114
along trunk line
116
, and subscriber taps, i.e. branch lines
118
, make up the coaxial distribution system of an HFC system. Subscriber tap
118
is connected to a cable modem
120
. Cable modem
120
is, in turn, connected to a subscriber computer
122
.
Recently, it has been contemplated that HFC cable systems could be used for two-way transmission of digital data. The data may be Internet data, digital audio, or digital video data, in MPEG format, for example, from one or more external sources
100
. Using two-way HFC cable systems for transmitting digital data is attractive for a number of reasons. Most notably, they provide upto a thousand times faster transmission of digital data than is presently possible over t
Gummalla Ajay
Khaunte Sunil
Beyer Weaver & Thomas LLP
Cisco Technology Inc.
Phan M.
Vu Huy D.
LandOfFree
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