Note: Descriptions are shown in the official language in which they were submitted.
CA 02220829 1997-11-12
METHOD OF OPERATING A MODEM IN
THE PRESENCE OF INTERRUPTED DIAL TONE
FIELD OF THE INVENTION
s
This invention relates in general to analog modems and more specifically to an analog
modem which is capable of dialing out when a dial tone is interrupted, stuttered or
lmint~,rrupted.
10 BACKGROUND OF THE INVENTION
Prior art modems suffer from the disadvantage of requiring steady u~ tell ~ted dial
tone in order to dial out. The presence of at least one unheard message in a voice
mailbox for a subscriber's line, in connection with which a central office Call ~nswer
5 feature has been activated, causes an interrupted or stuttered dial tone on the line. In
order for the modem to dial out, the interrupted or stuttered dial tone must be returned
to its regular u~inl~llupled state, which requires the subscriber of the phone line to
retrieve unheard messages from the voice mailbox. The inability of prior art modems
to dial out in the presence of interrupted dial tone imposes delays in modem activation
20 and frustration for the modem user who wishes to have quick access to on-line features via the modem.
SIJMMARY OF THE rNVENTION
25 The present invention is directed at a solution to the problem of allowing modems to
dial out when the dial tone is interrupted or stuttered.
~ccording to the present invention, a method is provided which allows a modem todial out provided that a dial tone is present on the phone line connected to the modem.
30 Users of the present invention are able to rece*e on-line access faster than users of
the prior art. The method implements a software program within the modem. By
comparing the elapsed length of the dial tone with two preclet~rrnined time values (a
CA 02220829 1997-ll-12
m~xi""l,., and a ~ -il lllllll), the method detects the presence and status of the dial tone
and informs the modem of the status of the dial tone. Provided that a dial tone is
present, whether nnint~rrupted, interrupted or stuttered, the modem proceeds to dial
out.
According to one aspect of the present invention, there is provided a method foroperating a modem, connected to a phone line, in the presence of interrupted dial tone
on the phone line comprising the steps of:
~0
a) requesting and receiving elapsed time since the dial tone was on;
b) comparing the elapsed time to a first predetermined time value and ceasing
use of the modem if the elapsed time is greater than the first predetermined time
value;
C) if the elapsed time is less than the first predetermined time value, then
comparing the elapsed time to a second predetermined time value which is less than
the first pre-let~rmined time value and ceasing use of the modem if the elapsed time is
less than the second preclet.ormined time value; and
d) if the elapsed time is greater than the second predetermined time value,~0 c~n~ing the modem to dial out on the phone line.
BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the invention is described herein below with reference to th25 accompanying drawings, in which:
Figure 1 is a block diagram of a well-known modem setup; and
Figure 2 is a flowchart showing the process by which a modem deduces the presence~0 and status of a dial tone in accordance with the pleft;ll~d embodiment.
CA 02220829 1997-11-12
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Turning to Figure 1, a computer 1 and a telephone 3 are shown connected to an
~xtPrn~l modem 5. Although an ext~rn~l modem is shown, the modem may
5 ~lt~-rn~tively be an intern~l modem within the computer 1. The modem 5 is further
connected to a phone jack 7. The phone jack 7 is connected to a phone line 9 which
m~int~in~ a connection with a central of fice 8 where all telephone features areorigin~te~l
10 To initiate a connection with the telephone central office 8 via phone line 9, TELIN
and TELOUT termin~l.s of the modem 5 are connected to the telephone Tip and Ringtermin~ls (normally located at Pin 3 and Pin 4 of the telephone jack 7). When the
modem 5 goes off hook, the modem detects if the dial tone is ON by executing theprogram CHK_TONE which simply reads the Input/Output address $0B on the
15 modem chip. If bit 7 of this address is high, the tone is acknowledged to be On,
otherwise, the tone is acknowledged to be Off. Upon receiving the dial tone, themodem 5 goes offhook and the modem 5 passes through a series of call progress
states beginning with Wait_DialO and proceeding to Parse_DigitsO and then finally to
ConnectO. When in the connected state, Dual Tone Multi Frequency (DTMF) signals
20 are sent to the central of fice 8 in the usual manner. All these programs will be well
known to those skilled in the art.
However, in accordance with the principles of the present invention, a method isprovided for enabling the modem 5 to dial out while a dial tone is interrupted or
2s stuttered.
The status of the dial tone is det~ rmined by a computer program located within the
modem whose syntax is shown herein below:
?DIALSTABLE: DB 08 ; Mitel changed April 15, 1997
for US
to work
CA 02220829 1997-11-12
; 80 ms
CHK_DIALTONE: ; Examine ON duration
LDRP R4, MSlOCLOCK
; Get time elapsed since tone
wa~ on
CMRPIM R4, DEBOUNCE_TIME
; More than the debounce time?
BCC ?DIALT_CHK
SETFLD DEBOUNCE_DONE
: ; Indicate there is no dialtone
SETFLD VALID_CADENCE
RTS
?DIAL_CHK:
CMR R4, DIALSTABLE
; For primary dial tone
BCC ByeCHK_RESULTS
SETFLD FO~ND_DIALONE
; Indicate dial tone detected
RTS
ByeCHK_RESULTS:
RTS
The computer program listed herein above, may be represented by the
algolill, 1ll shown in the flowchart of Figure 2. After the constant ?DIALSTABLE has
been set, at step 10 the CHK_DL~LTONE program is called for ~X~minin~ the ON
duration of the dial tone previously detected using CHK_TONE discussed above. The
modem 5 then requests the elapsed time since the dial tone has been on, shown in step
12.
The elapsed time is stored in the MS 1 OCLOCK variable and loaded into a register,
R4, using the command line LDRP R4, MSl0CLOCK. MS10CLOCK is a software
timer which is simply a byte in memory working like a counter. The value of
MS l OCLOCK increases every 10 ms in an interrupt service routine of the above
program. The following section of code shows how the MS 1 OCLOCK program runs:
CA 02220829 1997-11-12
INC MSlOCLOCK ; Increment 10 ms clock low byte
BNE ?Exit_IRQ ; breal~ if not wrap time
lNC MSlOCLOCK+l , Increment 10 ms clock mid byte
BNE ?Exit_IRQ ; break if not wrap time
rNC MS 1 OCLOCK+2 ; Increment 10 ms clock high byte
From this program, it is clear that MSCLOCK+2 = $FF(256) MSlOCLOCK+l and
MS10CLOCK+1 = $FF(256) MSlOCLOCK where the value $FF is a decimal value
of 256.
To initialize these variables, the following section of code is used:
LDA #OOH
STA MSlOCLOCK
STA MS 1 OCLOCK+l
STA MSlOCLOCK+2
After loading the elapsed time into register R4, the modem compares this value to a
20 predetermined value (step 14). The command CMRPIM R4, DEBOUNCE_TIME
compares the value in register R4 with the predetermined constant
DEBO~CE_TIME.
If the elapsed time (retrieved in step 12 as MSlOCLOCK) is greater than the
2s pre~letermined constant (in the preferred embodiment chosen to be 2 s), the
DEBOUNCE_DONE flag
(step 16) sets and modem use ceases at step 18. The instruction SETFT n
DEBOUNCE_DONE informs the modem that dialing out is not possible. The
instruction RTS returns the computer program back to the main body of the computer
30 code, which does not form part of the present invention but would be well known by
persons skilled in the art. Otherwise, if the elapsed time is less than the preclet~rrnined
constant, the modem 5 checks with the constant DIALSTABLE discussed below (step
CA 02220829 1997-11-12
20). This branch operation is achieved through the instruction BCC ?DIALT_CHK
which causes the program to jump to the ?DIALT_CHK procedure if the value in R4
is less than the value in DEBO~NCE_TIME (step 14).
In step 20, the R4 value is compared to a second pre-letermined time value to ensure
s that the dial tone is stable. This second compare produces a range for which the dial
tone must fall within before the modem may dial out. The instruction CMR R4,
DL~LSTABLE compares the elapsed time found in step 12 with the predetermined
constant DIALSTABLE. If the value in register R4 is found to be less than the value
of 80 ms (the value chosen in the pl~;rt;lled embodiment), the program jumps to the
10 ByeCHK_RESULTS procedure (step 22) and modem use ceases at step 24. This is
achieved by the comm~n~l~ BCC ByeCHK_RESULTS and RTS respectively.
Otherwise, if the value in R4 is greater than DIALSTABLE, then dial tone is deemed
to have been found (step 26) and the modem 5 dials out (step 28). This is achieved
through the instruction SETFLD FOUND_DL~LTONE which informs the modem
15 that dialing out is possible.
Tn summary, the method of the present invention, allows a modem to properly detect
dial tone, whether the tone is uninterrupted, stuttered or int~llu~t~d. According to the
pler~lled embodiment, the modem is able to detect interrupted or stuttered dial tones
20 from 80 to 160 ms +/- 5 ms (i.e. silent intervals up to 160 ms +/- 5 ms are recognized
as interrupted or stuttered dial tone). Although Call Answer is listed as reason for
causing the interrupted or stuttered dial tone, it should be noted that this modem is
able to detect the presence of the dial tone of the connected phone line and is not used
solely to detect interrupted or stuttered dial tones caused by the Call Answer feature.
It will be appreciated that, although a particular embodiment has been described and
illustrated in detail, various changes and modifications may be made. One such
change is in the name of the variables within the computer program. Provided that all
name changes are made globally (i.e. throughout the program), the program will
30 function in the same manner. Also, in the plt;r~llt;d embodiment, Call Answer is
listed as the source of the interrupted or stuttered dial tone. It should be noted that
there may be other reasons as to why a dial tone might become interrupted or stuttered
CA 02220829 1997-11-12
yet remain stable. Another contemplated modification is in the choice of the
pre~letermined time values. Although the values of 2 s and 80 ms have been chosen
for DEBOUNCE_TIME and DIALSTABLE, respectively in the preferred
embodiment of the present invention, they may be changed to accommodate various
5 other features telephone companies may offer. All such changes and modifications
may be made without departing from the sphere and scope of the invention as defined
by the claims appended hereto.
