Note: Descriptions are shown in the official language in which they were submitted.
CA 02216036 1997-09-17
SERIAL TELEPHONE ADAPTER
Field of the Invention
This invention relates to a telephone adapter for use
between a telephony terminal (telephone) and a client such
as a personal computer and more specifically to a serial
telephone adapter (STA) which facilitates transmission of
voice over a computer-based network such as the Internet or
an Intranet.
Background
Communications networks such as the public Internet and
private Intranets are becoming increasingly more accessible
to individuals in both the home and office environments.
Concomitant with the growth in Internet/Intranet services
has been an increase in attempts to provide voice access
between subscribers to the Internet as well as other
computer-based networks.
In the context of full duplex voice communications over
Internet/Intranet networks a multimedia personal computer
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equipped with a full duplexed sound card is required. In
this configuration a sound blaster-compatible card, or
equivalent, with an Internet telephone application running
on the PC is normally used. This system does represent a
new paradigm for voice communications but it does have
several undesirable shortcomings. Since it relies on a
sound-blaster compatible card in the personal computer it
excludes the traditional telephone instrument. Because of
the speaker-phone-like environment of the personal computer
configuration a user of the system does not enjoy privacy in
either transmission or receipt of voice messages. The
system is also prone to inherent latency. There is also no
fall back position should the Internet connection be lost or
if the computers at either end of the conversation shut
down.
The telephone adapter according to the present
invention overcomes the aforementioned shortcomings in that
it enables full duplex voice communication over
Internet/Intranet networks using traditional telephone
instruments. This, of course, provides improved privacy to
the user. Additionally, the adapter interfaces with the
public switched telephone network which allows the telephone
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to be used in its normal mode when the computer is turned
off or otherwise disabled.
Briefly, therefore, the present invention enables
traditional telephone instruments to be used for voice
communications over IP networks. Users are not threatened
with computer interfaces. Latencies due to operating system
interfaces to sound cards are eliminated. Once the computer
is turned off a path to the traditional telephone network is
created.
The invention is applicable to the following
technologies: a) voice over Internet/Intranet; b) telephony
over the Internet/Intranet; c)PABX systems for
Internet/Intranet; d) multimedia personal computers
including Windows 95 and WindowsNT.
Therefore, in accordance with a first aspect of the
present invention there is provided a system for use with a
telephony terminal and a client to provide telephone access
to a computer-based network via the client. The system
comprises: means in the client to execute an operating
system program; an adapter serially connected between the
terminal and the client, the adapter having means to receive
and transmit respectively telephony communications from and
to the terminal; means to convert the telephony
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communications from analog to digital and digital to analog;
and means to control transfer of converted telephone
communications between the client and the adapter; and a
system transfer protocol to coordinate transfer of telephony
communications between the client and the adapter.
In accordance with a second aspect of the present
invention there is provided an adapter for serial connection
between a telephone terminal and a personal computer to
provide telephony access to a communications network via the
personal computer. The adapter has means to receive and
digitize analog signals from the telephone terminal; means
to receive digitized signals from the computer and to
convert the digitized signals to analog form and means to
transfer the signals between the computer and the telephone
terminal.
In accordance with a third aspect of the present
invention there is provided a method of transmitting voice
communications over the Internet comprising: providing the
voice communications via a telephone terminal to a serially-
connected telephone adapter; digitizing the analog voicecommunications in the adapter; transferring the digitized
signals to a serially-connected personal computer having
access to the Internet; packetizing the digitized message
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and transporting the digitized signal to a selected
destination in accordance with an operating program
installed in the computer.
In accordance with a preferred embodiment of the method
aspect, the voice message is received at the destination by
a second computer wherein the signal in digital form is
depacketized, transferred to the adapter whereat the digital
signal is converted to analog form and subsequently provided
to a second telephone terminal.
Brief Description of the Drawings
The invention will now be described in greater detail
with reference to the attached drawings wherein:
Figure 1 represents a prior art voice over Internet system;
Figure 2 is a block diagram of the system according to the
present invention;
Figure 3 is a block diagram showing the components of the
system;
Figure 4 illustrates the format of the adapter reset
message; and
Figure 5 shows the adapter to PC message encoding.
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Detailed Description of the Invention
Figure 1 illustrates a known voice over Internet
architecture. Briefly, client or personal computers 12 and
14 are connected to the Internet 16 via a service provider.
Computers 12 and 14 are provided with a sound blaster-
compatible card or equivalent and the usual internal or
external microphone and speakers. Such operation can be
likened to the use of a conventional telephone in speaker
mode which does not provide any user privacy.
Figure 2 illustrates in block diagram form the system
of the present invention. In the manner shown in Figure 1
the system operates via the public Internet or a private
Intranet 16. Computers 20 and 22 are connected to the
Internet via means which are well-known. In the system
according to the invention, however, computers 20 and 22 do
not rely on a sound blaster-compatible card within the
personal computer for voice communications. In Figure 2
serial telephone adapter (STA) units 24 and 26 are connected
to the computer via one of the PC serial communications
ports (com 1 or com 2). STA units 24 and 26 are connected
serially to telephony terminals such as telephones 28 and 30
respectively. Also, as shown in Figure 2 STA units 24 and
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26 are connected to the public switched telephone network 32
so that if the telephone is not being used to communicate
over the Internet/Intranet it can be used in its normal mode
of operation. By incorporating an interface to a
traditional telephone instrument, the serial telephone
adapter enables voice communications from a traditional
telephone instrument to be transported through a personal
computer over Internet/Intranet networks. Furthermore, it
incorporates an interface to an analog telephone line (PABX
or central office) for voice communications in the event the
PC is turned off or non-operational in general.
Briefly, from the telephone instrument, the serial
telephone adapter digitizes the voice and transmits a
digital stream to the PC over the serial port using
asynchronous communication. The voice stream is then
packetized for transmission over the IP network. From the
IP network, voice is depacketized and the digital stream is
received by the serial telephone adapter, converted to
analog and presented to the analog telephone instrument.
As shown in Figure 3, the main components of the serial
telephone adapter software are Win32 Driver executing under
Microsoft Windows 95 or WindowsNT on a personal computer.
This software could be easily ported to any operating system
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on any platform, although implementation on a platform that
supports multi-tasking makes the task easier. The second
component is the serial telephone adapter firmware executing
on a micro-controller within the adapter box. A 6805 micro-
controller may be used. The third component is the protocol
used for transfer of audio and messages between the PC and
the adapter. A universal asynchronous receiver transmitter
(UART) is used in both the PC and the adapter. Both are
configured for transferring audio and messages between the
PC and alerter within the same channel at 115.2 kilobits per
second. Different protocols are used in each direction.
Audio and messages are transferred between the units over a
single RS 232 serial link. The audio message is G.711 in
mulaw where the audio signal is coded as a 64 kilobit per
second stream of byte values. Messages are embedded within
both the upstream and downstream audio byte streams as a
fixed number of bytes prefixed by the escape byte to
indicate the start of a message. The escape byte value is
Ox7F which was chosen since all instances of Ox7F occurring
in the G.711 mulaw audio stream can be replaced by the value
OxFF which represents an equivalent audio signal level. The
format and meaning of the various messages will be discussed
later.
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Only downstream control messages are acknowledged in
order to simplify the alerter firmware design to fit within
the alerter resource limitations. Each control message sent
from the PC to the serial telephone adapter (STA) is
acknowledged and the PC will not send a new control message
until the acknowledgment is received from the adapter for
the current message. The driver will start a timer when a
control message is sent. If the acknowledgment is received
then the timer is canceled and the driver is free to send a
new message, otherwise the timer will expire causing the
driver to resend the control message. Up to three resends
will be attempted until the driver assumes that the adapter
is malfunctioning and the client application or PC is
informed. Audio and message transfer flow control is
performed only in the downstream direction from within the
adapter's UART interrupt service routines (ISR) due to the
limited RAM buffer size in the adapter. The RS232 clear to
send (CTS) signal line is toggled by the adapter as follows:
~ High: Downstream audio and message transfer is
enabled when the number of bytes in the STA audlo receive
buffer drops below a fixed low water mark threshold.
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~ Low: Downstream audio and message transfer is enabled
when the number of bytes in the STA audio receive buffer
rises above a fixed high water mark threshold.
Although the STA has separate buffers for downstream
audio and downstream messages, and messages are sent one at
a time, both downstream audio and downstream message
transfer is suspended when CTS transitions from a low level
to a high level. The CTS signal is handled transparently by
the Win32 communications port driver.
The STA implements an optional software watchdog timer
that, if enabled must be reset at least once every 19
seconds by a watchdog kick message received from the driver.
If the timer is not reset before it expires, then the STA
will reset.
The Win32 driver essentially consists of two threads
that are integrated with the client application namely a
downstream transfer thread and an upstream transfer thread.
These threads use the Win32 communications port API.
The downstream transfer thread is responsible for
receiving downstream bound uncompressed audio and sending
the audio and messages from the PC to the STA via the serial
port.
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A client application controls the downstream transfer
thread through an API to start or stop the audio transfer
and to send command messages to the STA.
The upstream transfer thread is responsible for
receiving uncompressed audio and messages sent by the STA to
the PC via the serial port, and sending the audio upstream.
A client application controls the upstream transfer
thread through an API to start or stop the audio transfer.
Indication messages (such as off-hook) sent upstream from
the STA to the driver are communicated to the client
application through the client application's API.
As shown in Figure 1 the STA firmware essentially
consists of 3 main components:
1) UART (Universal Asynchronous Receiver Transmitter)
interrupt service routine (ISR);
2) Codec interrupt service routine (ISR) and
3) Main program loop.
The UART ISR receives bytes sent from the PC and stores
them in a buffer implemented as a circular queue referred to
as the downstream buffer (if the queue is not full). UART
interrupts occur once for every byte sent by the PC.
11
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The Codec ISR receives audio bytes from the codec at a
rate of 8 Kbytes/second and stores them in a buffer
implemented as a circular queue referred to as the upstream
buffer. If the queue if full then bytes are 'dropped'. In
practice this is a rare occurrence, so upstream audio
quality is not degraded. Audio bytes with value of Ox7F are
replaced with OxFF.
The main loop performs the following functions:
~ Processing control messages received from the PC.
~ Polling of the STA's DTMF tone receiver.
~ Polling of the STA's hook switch and debounce.
~ Forming and sending indication messages via the UART.
~ Sending audio upstream via the UART.
As described earlier, control messages received by the
UART ISR are stored in a message buffer until they are
processed, and once processed they are acknowledged.
The STA hardware implements a DTMF receiver, which is
polled once each pass through the main program loop. If the
receiver indicates that a DTMF tone is present in the
current upstream audio stream, then the DTMF digit is read
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from a register and the digit is encoded in an upstream
indication message sent to the PC.
The STA hardware indicates the state of the hook
switch, which is polled 3 times in succession each pass
through the main program loop in order to denounce the
switch. If a change of state is detected, then the new
state of the hook switch is encoded within an upstream
indication message sent to the PC.
Upstream audio is read from the upstream audio buffer
transferred to the PC continuously until a control message
is received from the PC instructing the STA to stop sending
the audio.
The messaging protocols will now be discussed in
greater detail. Messages sent from the PC to the adapter
consist of a header byte (Ox7F) followed by 3 body bytes and
are transmitted on the same channel as audio bytes. The
header byte acts as an escape character to indicate that
message bytes follow. Therefore the PC substitutes audio
bytes with the byte value of Ox7F with OxFF. As an example,
Figure 4 illustrates the adapter reset message format.
All messages sent from the PC to the adapter are
acknowledged by the adapter. The PC must wait for each
message to be acknowledged by the adapter before attempting
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to send another message. Alternatively, a sufficient period
of time must be allowed to elapse before attempting to send
another message. This time in a preferred embodiment is
500ms as this is sufficient time for the adapter to process
a message, and be ready to receive a new message.
In general, messages can be sent to the adapter at any
time, in any order without causing operational problems
except as noted for each message.
Messages sent from the adapter to the PC consist of a
header byte (Ox7F) followed by 1 body byte and are
transmitted on the same channel as audio bytes. The header
acts as an escape character to indicate that message bytes
follow. Therefore the adapter substitutes audio bytes with
the byte value of Ox7F with OxFF.
A message is sent from the adapter to the PC whenever a
switch state change is detected, or whenever the adapter is
reset, or to acknowledge a message received from the PC.
Messages sent from the adapter to the PC are not
acknowledged by the PC. The adapter is constantly
transmitting audio, therefore each byte received by the PC
must be examined to determine if it is a message header
byte. The following byte is assumed to be the message body
byte.
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Table 1 lists the formats for the body of messages sent
from the PC to the adapter. Some messages take parameters,
while others do not.
___
=___
Null rnessage 0x00 NIA NIA NiA NIA
Reset -0x01 N/A NIA N/A N/A
Codec Control 0x07 Codec R~istcr See codec Codec Register See codec
Number .~ - Value dc~
Analog Loopback 0x08 On or Olf 0x00 e Olf N/A NIA
0x01 ~ On
10 lack Watchdoa 0x09 N/A NiA NIA NIA
Swilch V~ 0x0A On or Off OxOO - Off N/A N/A
0x01 e On
WatchdogTirrleout 0~0B MullipleValue 0x~0to0xFF~0to NIA N/A
Time Multiple 255 ~ 6375 sec.)
Ringer Start 0x0D Cadence Pattem Each bit N/A NIA
. 0.60
sec. If blt is set.
ringer is on hr
0.50 ~ec. Etc.
R;nger Stop 0x0E NIA NIA N/A NIA
DTMF Mute On 0x10 N/A NIA NIA NIA
DTMF Mute Off 0x11 N/A NIA NIA NIA
P8X Mode- 0x12 NIA NlA N/A N/A
PC Modo 0x13 NIA NIA NIA NIA
-
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All adapter to PC messages are encoded within a single
message body byte as illustrated in Figure 5.
Adapter message bit 7 is set to acknowledge a message
previously sent to the adapter.
Adapter message bit 6 is set when the state of the
adapter switches (hook switch and/or DTMF digit buttons) has
changed.
Adapter message bit 5 is set when the receiver of the
phone set attached to the adapter is off-hook. This bit is
cleared when the receiver is on-hook.
Adapter message bit 4 is set when the adapter is reset
due to power-up, or watchdog time-out.
Adapter message bits 0 through 3 represent the current
state of the DTMF digit buttons of the phone set attached to
the adapter. Table 2 maps the digits to the 4 bit code.
Table~ DTMFDlgltEncodlng
Oxl
Z OxZ
3 0~
4 Ox4
Ox5
6 O~
7 Ox7
8 0
8. 0
O OK~
0~
# ' OxC
16
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While a preferred embodiment of the invention has been
described and illustrated it will be apparent to one skilled
in the art that numerous changes may be made without
departing from the basic concept. It is to be understood,
however, that such changes will fall within the.scope of
this invention as defined in the appended claims.
