Pulse or digital communications – Cable systems and components
Reexamination Certificate
2000-06-27
2004-07-27
Tse, Young T. (Department: 2634)
Pulse or digital communications
Cable systems and components
C375S222000, C375S377000, C379S399010
Reexamination Certificate
active
06768777
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for determining the number of distribution nodes necessary to provide Very High Speed Digital Subscriber Line (VDSL) services to subscribers.
2. Related Art
Conventional modulator-demodulators (“modems” ), have traditionally been used in home and small business personal computers (PC's) to connect to the Internet over telephone lines. Such modems modulate the digital signals from a computer into analog signals, more specifically modulated tones in the voice-band (DC to 4 KHz) that can be sent over telephone lines through the Public Switched Telephone Network (PSTN), and demodulate incoming analog signals back into digital signals that the computer can process them. However, due to the bandwidth limitations imposed by bandpass filters and codes at PSTN interface points, traditional modems have, for all practical purposes, reached their speed limit with the introduction of the 56 Kbps models.
The above-mentioned speed limitations of traditional voice-band modems make such modems less than satisfactory for meeting the demand for downloading graphic intensive Internet web pages and other information, and even worse for the type of two-way information transfer required for video conferencing. In recent years, several options have been introduced for providing broadband access at speeds significantly higher than voice-band modems. Among the high speed options available are T
1
and cable lines. However, each of these technologies requires that specialized wiring be installed at the subscribers location, at a cost that may be prohibitive to home and small business users.
Among the newest high-speed alternatives is Digital Subscriber Line (DSL) technology. DSL uses a subscriber's existing copper POTS (“plain old telephone system”) lines to gain access to the Internet and other high-speed data and video services—up to 55 Mbps. Thus, DSL offers the advantage of making use of lines already connected to the subscriber, but with speeds exceeding T
1
and cable lines.
The use of DSL requires the presence of a DSL modem at the subscriber and a counterpart at the service provider, usually the local telephone company, connected by a twisted pair copper telephone line. DSL modems send and receive data, over POTS lines, in a frequency range that is higher than the voice-band, and which permits much higher data rates. And because voice-band modem transmissions and voice calls use only a portion of the available bandwidth of the POTS line, a subscriber can carry on a telephone conversation, or use a voice-band modem, while at the same time operating a DSL modem.
One factor that must be taken into account in applying DSL technology is that the twisted pair subscriber lines have distortion and losses that increase with frequency and line length. Thus, for DSL to work properly, there is a limit to the line length between a subscriber's DSL modem and the phone company's answering DSL modem, the permissible line length decreasing the higher the data rate offered over the line. To account for this limitation, DSL providers must limit the length of the copper line over which the DSL signal is transmitted.
FIG. 1
shows a conventional method for sending a basic telephone signal from the telephone company's central office to the subscriber via twisted pair copper telephone lines. As shown in the figure, some subscribers, such as those having telephones
106
, have twisted pair lines
104
connected directly to the central office (CO)
100
. Other subscribers have their telephones
110
connected to the CO
100
through a cross connect
102
. The use of the cross connect
102
makes it practical to connect a larger number of subscribers to the CO
100
, and most CO's are large enough so that over 80% of the twisted pairs are connected through cross connects.
The telephone network was originally designed to provide voice-band telephone service up to 4 KHz. In order to provide VDSL services, operating typically at a much higher frequency than voice-band signals, the subscriber copper line may have to be less than 2500 feet, which is the typical range of present VDSL modems. Since subscribers may be located over 12,000 feet from the central office or cross connect, the telephone company's VDSL modem is placed within 2500 feet of the subscriber and connected to the CO by means of an optical fiber. Although the figure of 2500 feet will be used throughout the specification as the critical length, that length may vary depending upon the data rate, wire gauge, and other factors.
FIG. 2
shows the basic method for connecting DSL lines to subscribers. Elements in common with
FIG. 1
will be assigned the same reference numerals as in that figure.
As shown in
FIG. 2
, VDSL NODE
1
112
, comprising a VDSL modem present at the CO
100
, is connected directly via twisted pairs
104
, to subscribers using telephones
106
, each of which is less than 2500 feet from the CO
100
. On the other hand, twisted pairs
108
, associated with telephones
110
, each are connected to the CO
100
via cross connect
102
and are at a distance greater than 2500 feet from the CO
100
. To supply VDSL service to these subscribers, an optical line
114
is run to a VDSL NODE
2
116
located within 2500 feet of each of the subscribers. The information to be supplied to the subscribers is sent as an optical signal via the optical line
114
to the VDSL NODE
2
, converted into an electrical signal and supplied to the twisted pair lines
108
for distribution to the subscribers.
FIG. 3
shows a typical distribution of twisted pairs from a cross connect
200
to the subscriber. In the figure, pairs
1
-
26
supply service to Main Street. Pairs
27
-
50
supply service to Grove Street. Pairs
51
-
61
supply service to Joy Street, and pairs
62
-
70
supply service to Alice Street. In a layout such as is shown in
FIG. 3
, the trunk telephone cable would typically run along Main Street, with branch cables being separated from the trunk cable for side streets such as Grove Street, Joy Street and Alice Street.
As can be seen from the distance indications on
FIG. 3
, the layout includes pairs that are more than 2500 feet in line length from the cross connect
200
. Thus, assuming the cross connect itself has a node supplied directly from the CO, additional VDSL nodes would be needed to supply pairs more than 2500 feet from the cross connect.
In the layout of
FIG. 3
, it can be discerned by visual inspection that a node, supplied by optical cable, would have to be installed at the cross connect
200
to supply pairs located at a line length less than 2500 feet from the cross connect
200
. A node placed at a cross connect will be referred to hereinafter as a “first level” node. However, since a number of the pairs shown in the figure are located more than 2500 feet in line length from the cross connect, additional node or nodes, to be referred to hereinafter as “second level” nodes, will be necessary to supply those pairs.
For example, it appears from a cursory examination of
FIG. 3
that a second level node would have to be placed on Main Street at a line length 2500 feet from the cross connect to supply pairs on Main Street located at a line length more than 2500 feet from the cross connect
200
, another second level node placed on Grove Street at a line length 2500 feet from the cross connect to supply pairs on that street that are located at a line length more than 2500 feet from the cross connect, and yet another second level node placed on Alice Street to supply pair
66
, which is located at a line length more than 2500 feet from the cross connect.
FIG. 3
shows a layout of only a small number of streets. In actual street layouts, which for a particular CO may have over 100 cross connects, each having over 1000 pairs, calculation of the number of branches containing pairs having a line length longer than 2500 feet (hereinafter “branches of interest”) can take many weeks. Further, even if it i
Armiento Craig A.
Cooperman Michael
Forcucci Albert M.
Lovell John W.
Fitzpatrick ,Cella, Harper & Scinto
Lugo David B.
Suchtya, Esq. Leonard C.
Tse Young T.
Verizon Corporate Services Group Inc.
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