Note: Descriptions are shown in the official language in which they were submitted.
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ENHANCED LINE CARD AND PACKETIZING CPE FOR LIFELINE
PACKET VOICE TELEPHONE
CROSS-REFERENCES TO RELATED APPLICATIONS
This application claims priority from Canadian Patent Application Serial
Number 2, 281,356, filed September 1, 1999.
BACKGROUND OF THE INVENTION
The present invention relates generally to the field of Digital Subscriber
Line (DSL) systems, and particularly to packetized voice access systems.
Conventionally, telephone service is provided by the Public Switched
Telephone Network (PSTN), which consists of a multiplicity of telephone
switches
interconnected by various transport media and formats (the PSTN transport
network), and
connected to telephone terminals at subscriber's premises by twisted pair loop
wires.
Figure 1 illustrates a conventional circuit switched telephone network,
represented generally by the numeral 10. In this example, the PSTN transport
network 12
interconnects telephone switches 14. The switches, in turn, interface by means
of POTS
line cards 16 to individual twisted pair loops 18 and telephone terminals 20.
In the case
of typical residential telephone service, the POTS line card 16 provides a
number of
functions required to operate a telephone terminal 20 connected at the
subscriber's
residence. These functions are often collectively referred to as BORSCHT and
usually
include, but are not limited to:
Battery - supplying power;
Overvoltage - protecting the line card against environmentally caused
overvoltages;
Ringing - supplying a signal which energizes the telephone terminal
receiver;
Supervision - detecting whether a subscriber telephone receiver is on hook
or off hook;
Coding - converting the analog loop signal into digital representation;
Hybrid - separating the received from the transmitted signals; and
Termination - terminating the loop with a required standard electrical
impedance at the line card.
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Although implementation details differ, the B, O, R, S, and T functions
typically reside in the Analog Front End 22 and the C, H, and T functions in
the Voice
Engine 24. The line card 16 supplies power to the telephone terminal 20 from
an
uninterruptible power supply 26. The power supply 26 is physically situated in
the
telephone central office and is referred to as the Central Office (CO)
Battery. The CO
Battery plant is engineered and sized to supply power to conventional
telephone terminals
20 even in the event of a general power outage. Therefore, telephone service
is
characterized by a high degree of availability, a grade of service referred to
as "lifeline"
POTS. Typically, a line card 16 is capable of supplying up to 30 mA of current
at 48 V to
a twisted pair loop 18 and telephone 20. However, the exact value of current
depends on
the combined electrical resistance of the twisted pair loop 18 and the
telephone terminal
20. The exact current limit value is implementation dependent and may range
from 20 to
3 5 mA.
With the explosive growth in data communications, a Multi Service Data
Network (MSDN) has evolved which now has more aggregate bandwidth and is
growing
more rapidly than the PSTN transport network. Data communications networks are
developing that more easily support protocols and network elements which
enable the
routing and switching of self contained collections of bits known as packets
or cells.
Essentially, each packet/cell can be independently routed or switched by
devices known
as routers to different destinations based on a destination address contained
within each
packet or cell. This is known as packet switched networking. If a packet
stream contains
digital representations of a voice conversation, routing of these packets
represents a
function analogous to conventional circuit switched telephony switching. It
should be
noted that the term voice is used generically and its definition may be
extended to include
other analog transmissions such as fax, conventional modems, and the like. In
the
following descriptions, packets and cells are understood to be analogous
entities and,
unless specifically indicated, may be used interchangeably and individually
without
restricting the generality of the description. The terms VoIP (Voice over IP)
and VoATM
(Voice over ATM) are to be treated in a similar fashion.
Figure 2 represents a network configuration for interconnection of a
MSDN to a conventional telephone by means of voice packetization technology,
namely,
a Packet Switched Voice Access Network 30. The Multi Service Data Network 32
interconnects and routes packets/cells among various Digital Subscriber Loop
Access
Multiplexers (DSLAMs) 34, which multiplex data streams from multiple DSL Line
Cards
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36. Each DSL line card 36 interfaces to and communicates over a twisted pair
loop 18 to
DSL Customer Premises Equipment (CPE). The DSL CPE may be self contained and
connected to a source of packet/cell data such as a computer (not shown), may
be a DSL
Network Interface Card (DSL NIC) (not shown) internal to a computer, or may be
a CPE
herein called a Voice Packetizing CPE 38. The DSL line card 36 may communicate
using any of a number of methods that impress a signal representing a digital
bit stream
onto twisted pair loops 18 more conventionally used to carry POTS signals.
These
methods include, but are not limited to, the methods or services generally
known as
ADSL (Asymmetric DSL), SDSL (Symmetric DSL), HDSL (High-rate DSL), VDSL
(Very high-rate DSL), ISDN (Integrated Services Digital Network), MVL
(Multiple
Virtual Lines), or CAP (Carnerless Amplitude/Phase).
The Voice Packetizing CPE 38 transforms the analog signals from a
conventional telephone terminal 20 into voice packets or cells suitable to be
carned in a
DSL signal on the twisted pair loop 18 from the subscriber's premises. In
order to
operate a conventional telephone terminal 20, the Voice Packetizing CPE 38
includes
most of the functions associated with a conventional telephone switch POTS
line card 16
in its Voice Engine 42 and Analog Front End 44. Some specific differences from
the
corresponding POTS line card blocks might include coding to a 16 bit linear
digital
representation rather than ~-law or A-law, the provision of 24 V rather than
48 V
powering of the telephone terminal, reduced robustness to line overvoltages,
and the like.
The differences do not impact on the nature or significance of functions
described below.
A Voice over Internet Protocol (VoIP) or Voice over Asynchronous Transfer Mode
(VoATM) Engine 46 may include circuits and algorithms for reducing the bit
rate of the
digitized voice stream received from the Voice Engine 42. The VoIP /VoATM
Engine 46
also processes the reduced bit rate stream into packets or cells and may also
include an
echo cancellation capability for reducing the subjective audible effects of
the delay
introduced by the coding and packetizing operations. These functions typically
implement a set of protocols which may include an embodiment of ITU
Recommendation
H.323, 6.168 or similar standards and algorithms. The packet/cell stream
generated by
the VoIP/VoATM Engine 46 is transmitted to and received from the DSLAM 34 by
means of the DSL modem 40. The DSL modem 40 may implement any of a variety of
digital subscriber loop standards, such as ITU Recommendation 6.992.1 ("Full
rate
ADSL") or 6.992.2 ("ADSL lite").
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Because the circuits contained within the Voice Packetizing CPE 38
require more power than can typically be provided by a Line Card 36, they are
typically
powered by means of a power supply 48 which derives its power from the local
AC
mains. If a local power outage disrupts AC mains power, communication using
the Voice
Packetizing CPE is not possible until AC mains power is restored. This is
problematic for
subscribers accustomed to telephone service being available despite power
outages, and
especially in case of a life or security threatening emergency situation.
Uninterruptible
AC power supplies (UPS) are commercially available which will provide AC power
for a
short time in the event of a local AC mains power outage. However, the
disadvantages of
local UPSs are the purchase cost, the limited time these devices can supply
power, and
the regular maintenance that is required to ensure they are operational in the
event of a
power outage.
Further, in conventional packet based networks, provision of lifeline
packet voice service requires up to 3 separate pieces of equipment: a DSLAM,
POTS
Access Equipment such as a Digital Loop Concentrator, and a Gateway. The
Gateway is
an item of network equipment that provides a translation and mediation of
voice and
signaling signals and protocols between the PSTN and the MSDN. The need for
three
separate pieces of equipment increases overall network and equipment
complexity and
complicates service provisioning and maintenance.
What is needed is a mechanism to obviate or mitigate at least some of the
above disadvantages.
SUMMARY OF THE INVENTION
According to the invention, a voice and data communication system is
provided whereby a line card digitizes and packetizes voice communications
signals even
upon the failure of Customer Premises Equipment (CPE). The system comprises a
line
card for coupling the CPE with a network which line card includes:
an analog front end for coupling the line card to the CPE;
a digitizer for digitizing the received voice signals;
a packetizer for packetizing the digitized voice signals;
a modem for transmitting the packets; and
a system interface for coupling the line card to at least one network.
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Furthermore, the invention provides a system that can direct the voice
signals to either or both of a voice network (e.g. the PSTN) and a data
network (e.g. the
MSDN). The system comprises a line card including:
an analog front end for coupling the line card to a telephone;
a digitizer for digitizing the received voice signals;
a packetizer for packetizing the digitized voice signals;
a modem for transmitting the digitized voice signals to the data network;
a system interface for coupling the line card to the voice network and the
data network; and
a controller for controlling the destination of the voice signal.
The invention will be better understood by reference to the following
description of specific embodiment and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a block diagram of a conventional circuit switched telephone
network interface.
Figure 2 is a block diagram of a conventional packetized voice access
system.
Figure 3 is a block diagram of a lifeline packetized voice access system
according to the invention.
DESCRIPTION OF SPECIFIC EMBODIMENTS
For convenience, like structures in the drawing are referred to using like
numerals.
Figure 3 illustrates a Lifeline Packetized Voice Access system according
to the invention which is represented generally by the numeral 60. The system
60 enables
a lifeline grade of service to be offered for packetized voice telephony using
either a
Lifeline Packetizing Voice (LPV) CPE 62, as described below, or a conventional
analog
telephone terminal 20.
The LPV CPE 62 contains functional blocks comprising a conventional
voice packetizing CPE 38 as previously described and illustrated in Figure 2.
Specifically, it comprises a DSL modem 40, VoIP or VoATM Engine 46, Voice
Engine
42, and an Analog Front End 44, as well as the Power Supply 48. Additionally,
the LPV
CPE contains an optional POTS splitter 78, enabling a telephone terminal 20 to
be
connected to the twisted pair 18 through a relay 80. The common contacts of
the relay 80
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are connected to the telephone terminal 20, and the normally closed contacts
are
connected to the POTS Splitter 78. The relay is energized by a DSL Synch
Detect signal
76 from the DSL modem within the voice packetizing CPE 40.
Under normal conditions, that is when the AC mains power to the Lifeline
Packetizing CPE is present and the DSL modem is synchronized to a valid DSL
signal on
the twisted pair loop, the DSL Synch Detect signal is asserted and energizes
the relay 80.
When the relay 80 is energized, the common contacts are connected to the
relay's
normally open contacts. Therefore, the telephone terminal 20 is connected to
the voice
packetizing CPE 38, causing voice communications to occur by means of a
digital
packet/cell stream over a DSL signal through the DSL Modem 35 of the LPV Line
Card
66, and to the MSDN 32.
If, however, local power to the Lifeline Packetizing CPE 62 should fail, or
the DSL Modem 40 component of the voice packetizing CPE 38 does not detect a
valid
DSL signal, the DSL Synch Detect signal 75 is negated and de-energizes the
relay 80.
Therefore, the telephone terminal 20 connects through the normally closed
contacts of the
relay 80 and the POTS Splitter 78 to the twisted pair loop 18. In this state,
the telephone
terminal is directly connected to the twisted pair (through the POTS sputter
78) and
powered from the CO battery through a Broadband Analog Front End (BAFE) 72 on
a
Lifeline Packet Voice Line Card (LPVLC) 68. Voice communication takes place
through
a Voice Engine 24 and a VoIP/VoATM Engine 46 on the line card 68, in a
packetizing
voice line card fashion.
A number of Lifeline Packet Voice Line Cards (LPVLC) 66 are contained
within appropriate access equipment 68. The equipment 68 could be a Digital
Loop
Carrier system, a voice-capable DSLAM, a data-capable digital telephony
switch, or the
like.
The Lifeline Packet Voice Line Card 66 contains circuitry for interfacing
to both an analog telephone terminal and a DSL-based CPE. A system interface
70
contains circuitry to interface both to the Public Switched Telephone Network
(PSTN) 12
and the Multi Service Data Network (MSDN) 32. The interfaces described here
are
conceptually identical to their respective interfaces shown in Figure 1 and
Figure 2. The
Voice Engine 24 and DSL Modem 35 may of the type illustrated in the respective
blocks
in Figure 1 and Figure 2. The Broadband Analog Front End 72 combines the
functionality of the voice only Analog Front End 22 of the POTS Line Card in
Figure 1
and the DSL Front End 37 of the DSL Line Card in Figure 2. The Broadband Front
End
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72 provides the B, O, R, S, and T functions over the full bandwidth required
to support
both voice and DSL signals, and it satisfies the distinct requirements of both
DSL and
POTS interface types with regard to termination impedance, drive levels,
linearity, and
loop powering.
The VoIP or VoATM Engine 46 performs the function of the type
previously described and depicted in the Voice Packetizing CPE 38 of Figure 2.
The
Engine 46 takes the digitized voice from the voice engine 24, performs
compression,
packetization, and, optionally, echo cancellation. The resulting voice packets
are sent to
the system interface 70, where they may be merged with the packet/cell stream
from the
DSL modem block 35. This merging function may be implemented in the LPVLC or
elsewhere in the Access Equipment 68. The merging is controlled by the merge
controller 74, which produces a merge signal. The merge signal 75 is sent to
the Voice
Engine 24, the VoIP Engine 46, and the System Interface 70 blocks. The merge
controller
74 may be configured to operate in several modes described as follows. If the
merge
1 S control signal 75 is continuously asserted, the system is in Packetizing
Voice mode, and
the line card 66 will packetize the voice signals and direct the resulting
packet stream to
the MSDN, while powering a conventional telephone terminal 20. If the merge
control
signal 75 is continuously negated, the system is in POTS only or Packetizing
CPE only
mode and the line card 66 will not packetize the voice signals. However, the
merge
control signal 75 may also be responsive to a DSL Synch Detect signal 76
produced by
the DSL Modem 35 whenever a valid DSL signal originating from the subscriber
CPE is
detected. This mode is referred to as Lifeline Packetizing CPE mode. According
to the
invention, when the CPE is not fully functional , perhaps due to a mains power
outage,
the line card packetizes the voice signals and directs the resulting packet
stream to the
MSDN, while powering a conventional telephone terminal.
The Lifeline Packet Voice Line Card 66 of the invention is therefore
capable of performing the function of a conventional POTS line card as shown
in Figure
1, or of a DSL line card as shown in Figure 2. When operating as a DSL line
card 36, the
Lifeline Packet Voice Line Card 66 may be part of a conventionally defined
VoIP or
VoATM system, where the voice packetization is performed at the CPE. The
access
equipment 68 in such a system does not process or alter the content of the
packets or cells
in the VoIP or VoATM stream but merely provides an appropriate transport for
the VoIP
or VoATM stream from the subscriber to the MSDN 32.
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The Lifeline Packet Voice Line Card 66 is also capable of operating as a
packetizing voice line card. In this mode, the LPV Line Card 66 provides an
interface for
a conventional telephone terminal 20, through the BAFE 72 and the Voice Engine
24.
Therefore, if the power has failed at a subscriber premises, the telephone
terminal is
powered conventionally through the BAFE 72 from the Central Office (CO)
battery (not
shown) and can still transmit a voice signal to the line card 66. The Voice
Engine 24 on
the line card 66 digitizes the voice signal which is then packetized by the
VoIP/VoATM
Engine 46. The resulting packet/cell stream is delivered through the system
interface 70
to the MSDN 32. Since the telephone terminal is powered from the CO, and the
voice
signal is delivered as a packet/cell stream to the MSDN 32, the net result is
a packet voice
communications capability with a lifeline grade of service.
To this point, the system has only been described as if powering and
communicating with a telephone terminal and voice packetization are
necessarily a
function mutually exclusive with DSL communication. However, the system
definition
may also be extended to provide DSL communication on the same loop and
simultaneous
with a POTS interface. Simultaneous DSL and POTS capability on a single loop
is
defined in the ITU Recommendations for ADSL, for example, but is not
necessarily
possible for any other type of DSL modem or modulation scheme.
Refernng once again to Figure 3, a telephone terminal 20 and DSL modem
82 both communicate on the same loop 18 to the access equipment 68 through a
POTS
splitter 84. In some circumstances the sputter 84 may be optional. Typically,
data from
the DSL CPE 82 and voice signals from the telephone 20 would communicate on
the
same loop 18. The signals are filtered by the BAFE 72 after which the voice
signal is
directed via the Voice Engine 24 and system interface 70 to the PSTN 12. The
data
signal is directed to and demodulated by the DSL modem 35 and the resulting
data
packet/cell stream is directed via the system interface 70 to the MSDN 32.
Both voice
and data processing are, therefore, simultaneously possible on a single line
card.
Alternatively, once the Voice Engine 24 has digitized the voice signal, the
VoIP/VoATM Engine 46 may be used to packetize the digitized voice signal and
the
system interface 70 may merge the voice packets with the data packets from the
DSL
modem 35. The merged packet stream would be sent over the MSDN. In order to
provide simultaneous Lifeline Packet Voice and data communications capability
over the
DSL to the MSDN, the system interface 70 includes a packet or cell
multiplexing
capability. The multiplexer enables the merging of the voice packet or cell
stream from
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the VoIP/VoATM Engine 46 with the packet or cell stream from the DSL Modem 35.
As
previously mentioned, the merging function may be implemented in the LPVLC or
elsewhere in the Access Equipment 68. The multiplexer also enables the
diverging of the
voice packet or cell stream from the MSDN so that the appropriate streams are
sent to the
VoIP/VoATM Engine 46 and DSL Modem 35 respectively.
Therefore it can be seen that the Lifeline Packet Voice Line Card 66 has
the capability to direct the output of the Voice Engine 24 to either the PSTN
12 or, via the
VoIPNoATM Engine 46, to the MSDN 32, or simultaneously to both. The decision
as to
where to direct the voice packet stream can be made on a call-to-call basis.
The
capability to redirect the call exists at any time and for numerous reasons,
for example
during a power failure or a network connectivity failure. The line card 66,
therefore, has
the capability to rapidly alter the path a voice connection takes through
either the PSTN
or the MSDN.
As a result of the architecture used, the system may be able to provide
higher quality audio encoding, referred to as Enhanced Packet Voice. In
conventional
system architectures, it is not always possible to provide Enhanced Packet
Voice due to
the difficulty in providing increased transport bandwidth between the voice
coder and
packetizer, which may be in physically distinct and geographically dispersed
equipment.
Since the architecture in this embodiment of the invention utilizes a voice
coder 24 and a
packetizer 46 on the same card, Enhanced Packet Voice may be implemented.
Enhanced Packet voice is achieved by providing an alternate, higher
quality coding function in the Voice Engine 24 and in the VoIPNoATM Engine 46,
optionally coupled with a capability to code audio frequencies outside the
conventional
300 to 3,400 Hz bandwidth of toll quality voice. Alternatively, it may also be
implemented in the corresponding components at the CPE 62.
Enhanced Packet Voice is useful in applications such as audio
conferencing and echo cancellation, where the non-linearity of conventional ~-
law or A-
law code results in a reduced effectiveness of those algorithms processing the
coded voice
or voice band signal. Examples of alternate coders for the Voice Engine are 16-
bit linear
coders, coders with a higher than usual sampling rate, and coders that are
both 16-bit
linear coders and have a higher sampling rate. The reduction of effectiveness
is typically,
although not exclusively, due to nonlinear coding yielding a reduced quality
of coding of
a desired signal when a stronger, undesired signal is present.
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Examples of alternate coders for the VoIP/VoATM Engine include
perceptual coders defined for studio or CD quality audio. An alternate, higher
quality
coder generally results in a higher packet/cell rate from the VoIPNoATM
Engine.
Although the invention has been described with reference to certain
specific embodiments, various modifications thereof will be apparent to those
skilled in
the art without departing from the spirit and scope of the invention as
outlined in the
claims appended hereto.
