Re: Details of ppp & modem shut-down - PPP

This is a discussion on Re: Details of ppp & modem shut-down - PPP ; > > eas-lab@absamail.co.za writes: > > ]By using "ATM2D 3407501" to my modem ( M2 for sound on), > > ]I can hear when the carrier stops. Perhaps the CD led would > > ]show this. But carrier TX and ...

+ Reply to Thread
Results 1 to 11 of 11

Thread: Re: Details of ppp & modem shut-down

  1. Re: Details of ppp & modem shut-down

    > > eas-lab@absamail.co.za writes:

    > > ]By using "ATM2D 3407501" to my modem ( M2 for sound on),
    > > ]I can hear when the carrier stops. Perhaps the CD led would
    > > ]show this. But carrier TX and Rx are different.
    > >
    > > ]There is 2 to 3 seconds delay from the 'ppp-off' command until
    > > ]the carrier sound stops.
    > >
    > > ]Apparently the modem must exit the talking-ppp-mode
    > > ]into the command-mode, to drop-out the line-relay ?
    > >
    > > ]Q1. what are the ppp steps/states during exit ?
    > >
    > > ]Q2. what are the *details* of the modem changing from
    > > ]'talking ppp' to command mode ?
    > >
    > > ]Q4. going the other direction: what tells the modem to exit
    > > ]command-mode into talking-ppp-mode ?

    .... snip ..
    ] OK, so it's called "active mode".

    Hooda Gest wrote:
    > The modem is in that mode once it leaves command mode.
    > And it leaves command mode at the end of the dial command
    > unless the dial command is terminated with a semi-colon.


    OK, but HOW/WHAT specifies "the end of the dial command" ?
    eg. if the modem detects BUSY after dialing, it won't enter
    "active mode".
    I'm guessing it depends on detecting carrier ?

    > It will stay in active mode until either DTR is lowered
    > (hardware control) or the escape sequence is sent by the
    > modem's host system.


    So the modem is brought out of active mode by either software
    or hardware.

    > > How does the local machine know that its modem has "trained with a
    > > modem on the far side", so that it can send the 7 char "CONNECT"
    > > string (plus ) ?


    > The CD (Carrier Detect) lead goes high and the modem sends the
    > "CONNECT" report to its host system... if the modem is set to verbose
    > mode. It also may include other data about the connect if those
    > parameters are set.


    I think that's an over simplification:
    Carrier Detect does not mean it has successfully "trained with a
    modem on the far side". I'm guessing that Carrier Detect means
    it can start trying to "train". After some procedure/dialog, the detail
    of which we don't (yet) know are completed, "CONNECT" can be
    sent to the local serial port.
    ================================================== ===========
    OK, let's try to write this down in detail:----

    The modem starts in the command mode.
    { Does it automatically go to command mode, when powered off/on ?? }

    The modem receives the fixed string "AT" at the speed which the
    local serial port is programed to send it (DTE rate).

    By measuring the time of the fixed signal to complete, the modem
    knows the DTE rate.

    Further serial command strings are expected to be at this DTE rate.

    After a successfull dial and 'connect' to the remote modem, a
    carrier signal will be received and be detected by the CD hardware.

    After carrier has been detected, the trained starts with the remote
    modem. If this suceeds, the char-string "CONNECT" is
    sent to the local serial port at the DTE rate determined previously
    (perhaps the TX & RX speeds could be different).

    " Then pppd performs a close() on the serial port device, which makes
    the serial port driver turn the DTR pin off."
    So when DTR is switch off by the serial port, the modem goes to
    active mode ?

    And stays in active mode until the serial port switches DTR on ?

    But this would only happen if the local machine knows that it's
    time to go off line.

    Apparently the local machine could alternatively send "the
    escape sequence" ( "+++" with certain timing ?) ?
    The 2 alternatives could be described as hardware or software
    control ?

    Can the far end signal switch the local modem out of active mode
    without the local machine's confirmation. eg. by sending
    "the escape sequence" to be directly interpreted by the local
    modem ?
    I guess not.

    Is there any reason for the serial port to change speed, when the
    modem switches from command mode to active mode and vice versa ?

    What other parameters of the serial port might need to change ?
    I think none - since parity and word size don't need to change ?
    ============

    How much of that is wrong ?

    -- Chris Glur.


  2. Re: Details of ppp & modem shut-down

    On 02 Jul 2003 18:11:06 GMT, eas-lab@absamail.co.za wrote:

    ~
    ~ OK, let's try to write this down in detail:----
    ~
    ~ The modem starts in the command mode.
    ~ { Does it automatically go to command mode, when powered off/on ?? }

    Yeah, normally a modem will be in AT command mode once powered on.

    ~ The modem receives the fixed string "AT" at the speed which the
    ~ local serial port is programed to send it (DTE rate).

    Right. Actually it will receive `A' (or `a'), then optionally
    some pause (mark bits), then `T' (or `t').

    ~ By measuring the time of the fixed signal to complete, the modem
    ~ knows the DTE rate.

    Basically. If I recall correctly, first the modem is looking for `A' or
    `a', and infers the DTE rate from what it sees. Then it looks for `T' or
    `t' and infers the parity.

    ~ Further serial command strings are expected to be at this DTE rate.

    The rest of the line up to the is expected to be at that DTE rate.
    Once that command line is processed, the autobauding begins anew for
    the next command line.

    ~ After a successfull dial and 'connect' to the remote modem, a
    ~ carrier signal will be received and be detected by the CD hardware.
    ~
    ~ After carrier has been detected, the trained starts with the remote
    ~ modem. If this suceeds, the char-string "CONNECT" is
    ~ sent to the local serial port at the DTE rate determined previously
    ~ (perhaps the TX & RX speeds could be different).

    No, you've got the sequence wrong. It goes like this (in the
    most common case):

    - originate modem's AT parser receives the ATDxxx command.
    - originate modem goes offhook (loop closed on an FXO loopstart line)
    - originate modem listens for dial tone
    - originate modem transmits the digits in DTMF
    - originate modem waits to hear answerback tone.

    On the answer side, here's what happens:

    - answer modem detects ring voltage
    - answer modem toggles RI lead with the ring cadence
    - answer modem outputs "RING" on its Rx lead to its DTE
    - answer modem goes offhook
    - answer modem sends answerback tone (2100 Hz ANSam or whatever)

    Now the originate modem hears the ABT and the modems begin training.
    Once the modems decide that they have trained, each does this:

    - output the string "CONNECT" followed optionally by informative stuff
    (e.g. 28800 ARQ/V.42bis) followed by
    - THEN they raise CD

    Now the applications running in the DTE - either because they
    have seen the CONNECT string, or because they have seen CD go
    high - know that the modem link has reached data mode, and they
    can begin passing application data (PPP or whatever.)

    ~ " Then pppd performs a close() on the serial port device, which makes
    ~ the serial port driver turn the DTR pin off."

    In more detail, the DTE will normally toggle DTR off for a brief
    period (maybe a second or so), then raise it again.

    ~ So when DTR is switch off by the serial port, the modem goes to
    ~ active mode ?

    I don't know whawt "active mode" is.

    If the modem is in data mode (i.e. has carrier), then when it sees
    DTR drop, it will clear the call. While DTR is low, it will NOT
    normally accept commands on the AT interface.

    ~ And stays in active mode until the serial port switches DTR on ?

    When DTR goes back on, the modem will again accept AT commands.

    ~ But this would only happen if the local machine knows that it's
    ~ time to go off line.
    ~
    ~ Apparently the local machine could alternatively send "the
    ~ escape sequence" ( "+++" with certain timing ?) ?

    Modems that use the patented "Hayes '302" method will require
    that the escape sequence be surrounded by time gaps. Other modems
    use the "TIES" (Time Independent Escape Sequence) method.

    ~ The 2 alternatives could be described as hardware or software
    ~ control ?

    Yes, I guess you could use those terms. I would be more inclined
    to use the term "inband" for the "+++" method and "out of band"
    for the DTR toggle method.

    Note that +++ just moves the local DTE from data mode to AT
    command mode *without* hanging up the call. To hang up the call,
    the DTE would send "ATH". This is as opposed to a DTR toggle
    which hangs up the call.

    ~ Can the far end signal switch the local modem out of active mode
    ~ without the local machine's confirmation. eg. by sending
    ~ "the escape sequence" to be directly interpreted by the local
    ~ modem ?
    ~ I guess not.

    If you're talking about, can one modem cause the other modem to hang
    up the link - then yes, of course this can be done, in several ways.
    One would be to send a LAPM DISCONNECT message. Another would be
    to simply stop transmitting carrier. Another would be to go onhook.
    Or at the PPP layer, you could send an LCP TERMREQ (which wouldn't
    NECESSARILY cause the remote modem to hang up - but in practice
    normally will.)

    If you're talking about - can one modem tell the other modem to go
    out of data mode and into command mode, WITHOUT hanging up ... then
    the answer is, HOPEFULLY not -- but with TIES, this can in fact
    happen. (An amusing relatively recent posting on the issue
    can be seen at http://archives.neohapsis.com/archiv...1-q2/0155.html .)

    ~ Is there any reason for the serial port to change speed, when the
    ~ modem switches from command mode to active mode and vice versa ?

    To me, there's no GOOD reason for such behavior, although in
    practice some modems with certain configurations will behave this way.

    In ancient days, before CTS/RTS full duplex flow control, the DTE
    rate would always follow the DCE rate. E.g. you might have modems
    that supported Bell103 at 300bps, Bell212A at 1200bps and V.22bis
    at 2400bps. So on the answer side (not on the originate side), the
    modem would set its DTE rate to the DCE rate at which it trained.
    In order for the modem link then to communicate with the answer DTE,
    it would have to autobaud (typically by sending several 's) to
    the DTE.

    However, DTE's CAN'T autobaud at rates above 38400 (at least I know
    of none that can), so with modern highspeed rates, this approach
    won't work. The better approach is for the answer-side DTE link to
    be locked at a high rate (e.g. 115200bps) on both the DCE and DTE port.

    There's never been any good reason for the ORIGINATE modem to change
    its DTE rate upon trainup.

    ~ What other parameters of the serial port might need to change ?
    ~ I think none - since parity and word size don't need to change ?

    Not sure what you mean here. PPP requires a link with 8 databits
    and no parity, if that's what you're asking.

    Aaron

  3. Re: Details of ppp & modem shut-down


    wrote in message
    news:3f03203a$0$236@hades.is.co.za...
    > > > eas-lab@absamail.co.za writes:

    >
    > > > ]By using "ATM2D 3407501" to my modem ( M2 for sound on),
    > > > ]I can hear when the carrier stops. Perhaps the CD led would
    > > > ]show this. But carrier TX and Rx are different.
    > > >
    > > > ]There is 2 to 3 seconds delay from the 'ppp-off' command until
    > > > ]the carrier sound stops.
    > > >
    > > > ]Apparently the modem must exit the talking-ppp-mode
    > > > ]into the command-mode, to drop-out the line-relay ?
    > > >
    > > > ]Q1. what are the ppp steps/states during exit ?
    > > >
    > > > ]Q2. what are the *details* of the modem changing from
    > > > ]'talking ppp' to command mode ?
    > > >
    > > > ]Q4. going the other direction: what tells the modem to exit
    > > > ]command-mode into talking-ppp-mode ?

    > ... snip ..
    > ] OK, so it's called "active mode".
    >
    > Hooda Gest wrote:
    > > The modem is in that mode once it leaves command mode.
    > > And it leaves command mode at the end of the dial command
    > > unless the dial command is terminated with a semi-colon.

    >
    > OK, but HOW/WHAT specifies "the end of the dial command" ?
    > eg. if the modem detects BUSY after dialing, it won't enter
    > "active mode".


    After reporting "BUSY", it returns to command mode.

    > I'm guessing it depends on detecting carrier ?


    That would be one way of putting it. Actually, once dialing is complete, it
    is in Originate mode waiiting for carrier. Once carrier is detected and a
    connection is negotiated , it goes into Online mode.

    >
    > > It will stay in active mode until either DTR is lowered
    > > (hardware control) or the escape sequence is sent by the
    > > modem's host system.

    >
    > So the modem is brought out of active mode by either software
    > or hardware.


    Yes, that's generally true. Most modems can be set to ignore one or the
    other, however. It depends upon how it is configured. Usually, the modem is
    set to pay attention to DTR (&D2) or hardware control.

    >
    > > > How does the local machine know that its modem has "trained with a
    > > > modem on the far side", so that it can send the 7 char "CONNECT"
    > > > string (plus ) ?

    >
    > > The CD (Carrier Detect) lead goes high and the modem sends the
    > > "CONNECT" report to its host system... if the modem is set to verbose
    > > mode. It also may include other data about the connect if those
    > > parameters are set.

    >
    > I think that's an over simplification:
    > Carrier Detect does not mean it has successfully "trained with a
    > modem on the far side". I'm guessing that Carrier Detect means
    > it can start trying to "train". After some procedure/dialog, the detail
    > of which we don't (yet) know are completed, "CONNECT" can be
    > sent to the local serial port.


    I don't think so, I believe the CD is set when carrier (speed and type of
    modulation) between the modems is established. The CONNECT is sent when all
    parameters are established (error correction, parity, data compression).

    > ================================================== ===========
    > OK, let's try to write this down in detail:----
    >
    > The modem starts in the command mode.
    > { Does it automatically go to command mode, when powered off/on ?? }


    Usually. Many modems can be configured to go into Originate or Answer mode
    on powerup (or reset). Useful for leased line operation.

    >
    > The modem receives the fixed string "AT" at the speed which the
    > local serial port is programed to send it (DTE rate).
    >
    > By measuring the time of the fixed signal to complete, the modem
    > knows the DTE rate.


    I am not sure how it determines it, I just know it does. But modems can be
    configured to be variable, meaning it will adjust according to connect rate
    (rarely used), or to use fixed rate (most common).


    > Further serial command strings are expected to be at this DTE rate.


    Exactly. If the serial port rate changes after the modem has been given an
    AT, any other commands given will not be "seen" by the modem. It may be
    possible to send another AT to reset the modem to the new speed but I have
    not tried that nor do I see a reason to do something like that.

    >
    > After a successfull dial and 'connect' to the remote modem, a
    > carrier signal will be received and be detected by the CD hardware.


    Dial, wait for carrier, carrier detect (CD high), additional parameters
    established (CONNECT reported).

    >
    > After carrier has been detected, the trained starts with the remote
    > modem. If this suceeds, the char-string "CONNECT" is
    > sent to the local serial port at the DTE rate determined previously
    > (perhaps the TX & RX speeds could be different).


    Basically. When connecting with a V.90 modem in V.90 mode, the Tx and Rx
    will always be different, of course. But V.34 also allows for different Tx
    and Rx speeds.

    >
    > " Then pppd performs a close() on the serial port device, which makes
    > the serial port driver turn the DTR pin off."
    > So when DTR is switch off by the serial port, the modem goes to
    > active mode ?


    The modem drops carrier (when set to follow normal DTR operation) when DTR
    goes low and then the modem goes into command mode.

    >
    > And stays in active mode until the serial port switches DTR on ?


    DTR means Data Terminal Ready. If DTR is not high, the modem will not accept
    commands from its host. So, DTR is dropped to cause the modem to drop
    carrier and return to command mode then DTR is raised so that the modem
    "knows" the system is ready.

    >
    > But this would only happen if the local machine knows that it's
    > time to go off line.


    The local machine is the device in charge of the serial port, so I would say
    "yes". The local machine would not simply drop DTR without some software
    telling it to do so. That software could be your application (operations are
    complete, files have been transferred, and it is time to drop the
    connection) or it could be manually entered by clicking on a disconnect
    selection of a menu.

    >
    > Apparently the local machine could alternatively send "the
    > escape sequence" ( "+++" with certain timing ?) ?


    That would be software control.

    > The 2 alternatives could be described as hardware or software
    > control ?



    The two methods are:

    Hardware control (via DTR)
    Software control (via ASCII string sequence Escape code)


    > Can the far end signal switch the local modem out of active mode
    > without the local machine's confirmation. eg. by sending
    > "the escape sequence" to be directly interpreted by the local
    > modem ?
    > I guess not.


    Not if the modem is properly configured. The escape sequence should only be
    recognized when it comes in from the serial port (DTE side) and not from the
    line (DCE side).


    > Is there any reason for the serial port to change speed, when the
    > modem switches from command mode to active mode and vice versa ?


    No, not any reason I can think of.

    >
    > What other parameters of the serial port might need to change ?
    > I think none - since parity and word size don't need to change ?


    There would be no reason to change them unless you are re-configuring the
    modem to dial out to another system which required different parameters.

    > ============
    >
    > How much of that is wrong ?


    It looks like you are getting the gist of it (modem operations).


    --
    Hooda Gest
    "In a New York minute, everything can change..."





  4. Re: Details of ppp & modem shut-down

    Aaron Leonard writes:
    >
    >- originate modem waits to hear answerback tone.
    >


    That's "answer" tone, not "answerback". Look it up in V.25.

    -Greg
    --
    Do NOT reply via e-mail.
    Reply in the newsgroup.

  5. Re: Details of ppp & modem shut-down

    eas-lab@absamail.co.za writes:

    ]> The modem is in that mode once it leaves command mode.
    ]> And it leaves command mode at the end of the dial command
    ]> unless the dial command is terminated with a semi-colon.

    ]OK, but HOW/WHAT specifies "the end of the dial command" ?

    The carriage return.

    ]eg. if the modem detects BUSY after dialing, it won't enter
    ]"active mode".

    No.

    ]I'm guessing it depends on detecting carrier ?
    Yes.


    ]> It will stay in active mode until either DTR is lowered
    ]> (hardware control) or the escape sequence is sent by the
    ]> modem's host system.

    Or carrier is dropped.


    ]So the modem is brought out of active mode by either software
    ]or hardware.


    ]I think that's an over simplification:
    ] Carrier Detect does not mean it has successfully "trained with a
    ]modem on the far side". I'm guessing that Carrier Detect means
    ]it can start trying to "train". After some procedure/dialog, the detail

    No, CArrier detect is an old concept. Modern modems do not use a
    carrier. But the end result, that there is a useable path ( and that
    requires training) is called Carrier Detect.

    ]of which we don't (yet) know are completed, "CONNECT" can be
    ]sent to the local serial port.

    No. CONNECT means that the two modems can talk to each other, and the
    system can assume a route exists between the two .


    ]================================================== ===========
    ]OK, let's try to write this down in detail:----

    ]The modem starts in the command mode.
    ]{ Does it automatically go to command mode, when powered off/on ?? }

    Yes.


    ]The modem receives the fixed string "AT" at the speed which the
    ]local serial port is programed to send it (DTE rate).

    ]By measuring the time of the fixed signal to complete, the modem
    ]knows the DTE rate.

    No. By measuring the timing of the individual bits, and especially the
    start and stop bits, the rate is determined.



    ]Further serial command strings are expected to be at this DTE rate.

    Yes. Furtehr stuff coming down the serial line is expected to be at this
    rate. Some modems will continually monitor the line to see if the rate
    changes as well.



    ]After a successfull dial and 'connect' to the remote modem, a
    ]carrier signal will be received and be detected by the CD hardware.

    No. Modern modems do not use carriers. But the "Carrier detect" is a
    holdover terminology to mean that a path exists between the two modems
    to communicate along.


    ]After carrier has been detected, the trained starts with the remote
    ]modem. If this suceeds, the char-string "CONNECT" is

    No. The training is finished. ( or rather it can retrain if it discovers
    too many errors occuring. While retraining it holds the CD line up, and
    only if it cannot retrain will it drop the line).

    ]sent to the local serial port at the DTE rate determined previously
    ](perhaps the TX & RX speeds could be different).

    ]" Then pppd performs a close() on the serial port device, which makes
    ]the serial port driver turn the DTR pin off."

    Depends on how it is set up.

    ]So when DTR is switch off by the serial port, the modem goes to
    ]active mode ?

    ?? No Not sure what you mean by "active mode." If there is no connection
    with the far end other modem, the modem goes into command mode. Or if
    you ask it to with +++



    ]And stays in active mode until the serial port switches DTR on ?

    Or it looses contact with the far modem.


    ]But this would only happen if the local machine knows that it's
    ]time to go off line.

    Whatever. It is software controlled.


    ]Apparently the local machine could alternatively send "the
    ]escape sequence" ( "+++" with certain timing ?) ?
    ]The 2 alternatives could be described as hardware or software
    ]control ?

    No. +++ maintains contact for a while at least with the far end modem.
    It just drops the local modem into command mode. If you then send ATH it
    will hang up the connection with the remote modem.



    ]Can the far end signal switch the local modem out of active mode
    ]without the local machine's confirmation. eg. by sending
    ]"the escape sequence" to be directly interpreted by the local
    ]modem ?

    Yes. If the modem receives a +++ with one second guard times (ie no data
    for one sec) befor and after it will go into command mode.
    If the far end modem stops communicating ( a bus tears through the phone
    lines to your home, a bomb blows up the far end modem, or the other side
    hangs up.) then your modem will also hang up and go into command mode.

    ]I guess not.

    ]Is there any reason for the serial port to change speed, when the
    ]modem switches from command mode to active mode and vice versa ?

    No.


    ]What other parameters of the serial port might need to change ?
    ]I think none - since parity and word size don't need to change ?
    ]============

    ]How much of that is wrong ?

    ]-- Chris Glur.


  6. Re: Details of ppp & modem shut-down

    "Hooda Gest" writes:



    ]I don't think so, I believe the CD is set when carrier (speed and type of
    ]modulation) between the modems is established. The CONNECT is sent when all
    ]parameters are established (error correction, parity, data compression).

    On modern modems there is no carrier, and the speed and type of
    connection is not known until all the parameters are set. The connection
    and the negotiations are intertwined.

    Now whether the CD line is set befor or after the training is complete
    may well depend on the modem type.





    ]Hardware control (via DTR)

    Which is controlled by software.

    ]Software control (via ASCII string sequence Escape code)


  7. Re: Details of ppp & modem shut-down


    "Bill Unruh" wrote in message
    news:be4j2c$k36$1@nntp.itservices.ubc.ca...
    > "Hooda Gest" writes:
    >
    >
    >
    > ]I don't think so, I believe the CD is set when carrier (speed and type of
    > ]modulation) between the modems is established. The CONNECT is sent when

    all
    > ]parameters are established (error correction, parity, data compression).
    >
    > On modern modems there is no carrier, and the speed and type of
    > connection is not known until all the parameters are set. The connection
    > and the negotiations are intertwined.


    Perhaps, it is somewhat retro to refer to the symbol rate as "carrier" but
    it's convenient.

    >
    > Now whether the CD line is set befor or after the training is complete
    > may well depend on the modem type.
    >
    >
    >
    >
    >
    > ]Hardware control (via DTR)
    >
    > Which is controlled by software.


    In a computer, all things are controlled by software. The modem, however,
    doesn't care about what causes the DTR to change state. In modems, hardware
    control is control by DTR; software control is commands in the data stream
    sent to the modem via the Tx lead.




    --
    Hooda Gest
    "In a New York minute, everything can change..."



  8. Re: Details of ppp & modem shut-down

    ~ >- originate modem waits to hear answerback tone.
    ~ >
    ~
    ~ That's "answer" tone, not "answerback". Look it up in V.25.
    ~
    ~ -Greg

    Who says that the V.25 "answer" tone is the only tone that the
    originate modem is listening for? It *could* be listening for
    other tones, and who's to say that these might not be called
    "answerback" tones?

    ("answerback tone" is a Cisco-ism; I concede that "answer tone"
    is more elegant.)

    Aaron

  9. Re: Details of ppp & modem shut-down

    unruh@string.physics.ubc.ca (Bill Unruh) writes:
    >eas-lab@absamail.co.za writes:
    >
    >]The modem receives the fixed string "AT" at the speed which the
    >]local serial port is programed to send it (DTE rate).
    >
    >]By measuring the time of the fixed signal to complete, the modem
    >]knows the DTE rate.
    >
    >No. By measuring the timing of the individual bits, and especially the
    >start and stop bits, the rate is determined.
    >


    Measuring the individual bits has not been done for the past 14
    years or more. The modem's USART is clocked at a high speed so
    the incoming "AT" will generate patterns of 1s and 0s. The patterns
    will be different for different speeds and parity settings. The
    modem simply compares the incoming stream of 1s and 0s to the
    patterns stored in ROM. When the pattern matches, the match tells
    the modem which speed is being used.

    >
    >No. Modern modems do not use carriers. But the "Carrier detect" is a
    >holdover terminology to mean that a path exists between the two modems
    >to communicate along.
    >


    Again, this is not correct. Modern PSTN modems do transmit a carrier
    frequency in each direction, which is phase and amplitude modulated
    to convey the data.


    -Greg
    --
    Do NOT reply via e-mail.
    Reply in the newsgroup.

  10. Re: Details of ppp & modem shut-down

    ~ >~ >- originate modem waits to hear answerback tone.
    ~ >~ >
    ~ >~
    ~ >~ That's "answer" tone, not "answerback". Look it up in V.25.
    ~ >~
    ~ >~ -Greg
    ~ >
    ~ >Who says that the V.25 "answer" tone is the only tone that the
    ~ >originate modem is listening for?
    ~ >
    ~
    ~ Are you suggesting that the originating modem might be listening
    ~ for answer tones that do not follow ITU standards? The standard
    ~ answer tones that I'm aware of are all based on V.25.

    In addition to the V.25 (2100Hz ANS/ANSam/ANSpcm) variants, here
    are some other tones that a typical defaultly configured modern
    modem might be listening for when in originate mode:

    - V.21 answer mark tone (1650 Hz)
    - Bell103 answer mark tone (2225 Hz)
    - Bell212A answer ("answerback"? ;-) tone (2225 Hz)
    - V.8bis CRe

    This is off the top of my head; no doubt there are others.

    OK, since you appear to be a bit of a stickler, I just now
    grabbed my copy of the Rockwell "RCVDL56DPFL/SP Modem Data
    Pump Designer's Guide" (1997) off the shelf. And it indicates
    that, in addition to the 2100Hz variants and the ones that
    I listed above, there's also 1300Hz from a V.23 answerer,
    for all you Minitel fans. (See Fig. 6-2. Host Flowchart -
    Originating Automode.)

    ~ >It *could* be listening for other tones, and who's to say that
    ~ >these might not be called "answerback" tones?
    ~ >
    ~
    ~ It *could* be listening for a razzberry, too. Are we discussing
    ~ imaginary modems, or real ones?
    ~
    ~
    ~ -Greg

    Oh, very real. For example, sometimes you'll see a modem
    accidentally train in V.21 since it's latched onto some tone
    or echo as apparently being V.21 FSK tone.

    (I admit that my post was a bit facetious though.)

    Aaron

  11. Re: Details of ppp & modem shut-down

    On Mon, 14 Jul 2003 18:32:03 +0000 (UTC), gerg@panix.com (Greg Andrews) wrote:

    ~ >No. Modern modems do not use carriers. But the "Carrier detect" is a
    ~ >holdover terminology to mean that a path exists between the two modems
    ~ >to communicate along.
    ~ >
    ~
    ~ Again, this is not correct. Modern PSTN modems do transmit a carrier
    ~ frequency in each direction, which is phase and amplitude modulated
    ~ to convey the data.
    ~
    ~
    ~ -Greg

    Modern analog modulations have a carrier frequency; PCM modulations
    (K56Flex, X2, V.90) do not.

+ Reply to Thread