Multiplex communications – Duplex – Communication over free space
Reexamination Certificate
2000-10-19
2004-02-10
Pezzlo, John (Department: 2662)
Multiplex communications
Duplex
Communication over free space
C370S311000, C370S466000, C455S450000
Reexamination Certificate
active
06690655
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to communication systems and, in particular, to a communication system in which the system infrastructure at least occasionally provides power to system subscriber devices.
BACKGROUND OF THE INVENTION
Communication systems are known to include a system infrastructure and a plurality of subscriber devices. In most communication systems, the subscriber devices (e.g., cellular telephones or wireline modems) include their own DC power source (e.g., a battery or an AC-to-DC transformer which is plugged into an AC wall outlet). However, in standard telephone systems, the system infrastructure provides the minimal DC power necessary to power the subscriber devices (with the exception of cordless telephones which include their own AC-to-DC transformers). The infrastructure provides the power to the subscriber devices in standard telephone systems to ensure that, in the event of a power outage at a subscriber device location, the subscriber device user will still have telephone service, especially with respect to so-called “lifeline services”, such as “911” and others.
Telephony service has traditionally been delivered to consumers by regional telephone companies via circuit switched technologies. However, cable (community access television (CATV)) operators are beginning to offer telephony services over their cable systems, using standardized modems and a packetized Internet Protocol (IP) technology generally known as “voice-over-IP (VoIP)”. An exemplary prior art two-way cable system is illustrated in block diagram form in FIG.
1
.
The prior art cable system includes headend equipment
101
, a hybrid fiber coaxial (HFC) cable plant
103
, a plurality of cable modems
105
,
106
(two shown), and a corresponding plurality of subscriber communication devices
107
,
108
(two shown) coupled to the cable modems
105
-
106
via corresponding communication links
116
,
117
. The headend equipment
101
includes processors, routers, switches, a broadband downstream transmitter, upstream receivers, splitters, combiners, subscriber databases, network management stations, dynamic host configuration protocol (DHCP) and trivial file transfer protocol (TFTP) servers, call agents, media gateways, and billing systems. The HFC cable plant
103
includes fiber optic cables, coaxial cables, fiber/coax nodes, amplifiers, filters, and taps which support transmissions from the headend equipment
101
to the cable modems
105
,
106
over a shared downstream channel
110
and transmissions from the cable modems
105
,
106
to the headend equipment
101
over a shared upstream channel
112
.
Each channel
110
,
112
utilizes a respective transmission protocol to communicate information over the channel
110
,
112
. Typically, the modulation used to convey information over the downstream channel
110
(e.g., 64-ary quadrature amplitude modulation (QAM)) is of a higher order than the modulation used to convey information over the upstream channel
112
(e.g., differential quaternary phase shift keying (DQPSK) or 16-ary QAM), resulting in higher speed downstream transmissions than upstream transmissions. Cable systems in which upstream transmission speeds are less than downstream transmission speeds are typically referred to as “asymmetric” systems. Cable systems in which upstream transmission speeds are substantially equivalent to downstream transmission speeds are typically referred to as “symmetric” systems.
In addition to the particular type of modulation used on each channel
110
,
112
, the shared nature of each channel
110
,
112
introduces other protocol requirements. For example, since the downstream channel
110
is shared, the downstream protocol includes addressing information and each cable modem
105
,
106
monitors the downstream channel
110
for information packets addressed to it. Only information packets addressed to a particular cable modem
105
,
106
(or the attached communication devices
107
,
108
) or addressed to all cable modems
105
,
106
(or the attached communication devices
107
,
108
) (e.g., broadcast messages) are processed by the cable modem
105
,
106
and forwarded to the associated subscriber communication device
107
,
108
as appropriate (e.g., telephone, personal computer, or other terminating device). Since the upstream channel
112
is shared, an upstream channel access protocol is used to reduce the likelihood of collisions of communicated information emanating from the cable modems
105
,
106
. A number of multiple access protocols exist to define upstream channel access, including well-known protocols such as ALOHA, slotted-ALOHA, code division multiple access (CDMA), time division multiple access (TDMA), TDMA-with collision detect, and carrier sense multiple access (CSMA).
Most two-way cable systems abide by and use the upstream and downstream channel protocols defined in the recently-published Data-Over-Cable System Interface Specification (DOCSIS) Version
1
.
0
, which specification is incorporated by this reference as if fully set forth herein. The upstream protocol defined by the DOCSIS standard is a TDMA approach in which timing is controlled by the headend equipment
101
(referred to as the “cable modem termination station” (CMTS) in the DOCSIS standard) and communicated to the cable modems
105
,
106
via time stamp synchronization messages transmitted over the downstream channel
110
. Thus, in order for upstream communication to occur in an orderly, high quality manner, a time reference in each cable modem
105
,
106
must be substantially synchronized with a similar reference in the headend equipment
101
before the modem
105
,
106
begins transmitting information provided by the subscriber communication device
107
,
108
; otherwise, a transmission from one modem
105
may collide with a transmission from another modem
106
.
The headend equipment
101
is typically coupled via an appropriate communication link
119
, such as a fiber distributed data interface (FDDI) link or a
100
baseT Ethernet link, to an external network
114
, such as the public switched telephone network (PSTN) or a wide area packetized network, such as the Internet. Thus, the two-way cable system provides communication connectivity between the subscriber communication devices
107
,
108
and other similar devices, Internet servers, computer networks, and so forth via the external network
114
.
To support the aforementioned “lifeline services”, cable system operators must provide power (at least temporarily during local power outages) to the cable modems
105
,
106
and their attached subscriber communication devices
107
,
108
from the headend equipment
101
via the cable plant
103
. However, present-day cable modems
105
,
106
consume considerable amounts of power (on the order of 9-12 watts per modem currently with a reduction to 4-6 watts as new technologies become available, under normal operating conditions), with the modem's downstream receiving and processing circuitry playing the most significant role in power consumption. Considering that, in a typical two-way cable system, the headend equipment
101
may service thousands of cable modems
105
,
106
. Supplying power to such modems
105
,
106
, even temporarily during local power outages, creates an overwhelming burden on the cable system operators, likely resulting in increased operating and subscription costs.
One approach to reducing the amount of power consumed by the cable modems
105
,
106
is to reduce the transmission rate of the downstream channel
110
(e.g., through the use of a low order modulation, such as QPSK or frequency shift keying (FSK)) and, thereby, reduce the power required by each modem
105
,
106
. While some of the power savings is realized as a result of reduced processing power required to handle the lower data rate, the bulk of the power savings is realized by the requirement that higher modulation schemes require higher-performing lower-noise RF components, which generally require mor
Cooper Michael Jaimie
Miner Mark Charles
Coker Caroline
Motorola Inc.
Pezzlo John
LandOfFree
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