what serial lines does ppp actually use? - PPP

This is a discussion on what serial lines does ppp actually use? - PPP ; When ppp tries to establish a connection with the LCP, IPCP and CCP layers and you have hardware and software flow control disabled, can it establish a connection using only the TXD and RXD serial lines? Are there other lines ...

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Thread: what serial lines does ppp actually use?

  1. what serial lines does ppp actually use?

    When ppp tries to establish a connection with the LCP, IPCP and CCP
    layers and you have hardware and software flow control disabled, can
    it establish a connection using only the TXD and RXD serial lines?

    Are there other lines that PPP requires?

    Thanks.

    Randy

  2. Re: what serial lines does ppp actually use?

    r_long2001@yahoo.com (Randy) writes:
    > When ppp tries to establish a connection with the LCP, IPCP and CCP
    > layers and you have hardware and software flow control disabled, can
    > it establish a connection using only the TXD and RXD serial lines?


    Sure. Depending on the capabilities of the hardware, you may want to
    enable software (XON/XOFF) flow control in that case. But it's not a
    requirement.

    > Are there other lines that PPP requires?


    PPP requires an 8 bit, bidirectional, full-duplex data path.

    It's very helpful if the link has some sort of "up / down"
    notification. On async serial ports, this is typically provided by
    DTR (output) and DCD (input). If you don't have this, then you may
    need to use LCP Echo-Request to detect failed links, and you may have
    to suffer through delays in detecting failed links and erroneous
    detection of failed links during periods of high utilization or high
    packet error rates.

    For async links, it's somewhat helpful if the link has flow control
    lines, but it's not necessarily required. You'll want to have some
    kind of flow control if one device is able to overrun the input
    buffers of the other.

    --
    James Carlson, IP Systems Group
    Sun Microsystems / 1 Network Drive 71.234W Vox +1 781 442 2084
    MS UBUR02-212 / Burlington MA 01803-2757 42.497N Fax +1 781 442 1677

  3. Re: what serial lines does ppp actually use?

    Randy wrote:
    > When ppp tries to establish a connection with the LCP, IPCP and CCP
    > layers and you have hardware and software flow control disabled, can
    > it establish a connection using only the TXD and RXD serial lines?


    Yes, I think so. I would have thought that the "asyncmap a0000" option
    would be enough, but after scanning the code, perhaps not (I'm not a C
    programmer, although I can read it a little).

    > Are there other lines that PPP requires?


    AIUI, a nullmodem cable without hardware flow-control should only need
    the frame (pin 1) and signal (pin 7) ground lines in addition to the
    TXD and RXD lines - which must be configured crossed-over to one another.

    Drop "asyncmap a0000" and try the option xonxoff in it's place. See man
    pppd for details.

    -- Clifford Kite Email: "echo xvgr_yvahk-ccc@ri1.arg|rot13"
    PPP-Q&A links, downloads: http://ckite.no-ip.net/
    /* The signal-to-noise ratio is too low in many [news] groups to make
    * them good candidates for archiving.
    * --- Mike Moraes, Answers to FAQs about Usenet */

  4. Re: what serial lines does ppp actually use?

    r_long2001@yahoo.com (Randy) writes:

    ]When ppp tries to establish a connection with the LCP, IPCP and CCP
    ]layers and you have hardware and software flow control disabled, can
    ]it establish a connection using only the TXD and RXD serial lines?

    Sure. AT the expense of corrupt packets etc. Flow control is "good
    idea".


    ]Are there other lines that PPP requires?

    No

  5. Re: what serial lines does ppp actually use?

    unruh@string.physics.ubc.ca (Bill Unruh) writes:
    > r_long2001@yahoo.com (Randy) writes:
    >
    > ]When ppp tries to establish a connection with the LCP, IPCP and CCP
    > ]layers and you have hardware and software flow control disabled, can
    > ]it establish a connection using only the TXD and RXD serial lines?
    >
    > Sure. AT the expense of corrupt packets etc. Flow control is "good
    > idea".


    Not necessarily. Flow control is a good idea if:

    - there's some arbitrary restriction on link bandwidth between
    the two peers -- e.g., a code-converting device such as a
    modem

    - one of the peers is unable to keep up with the data rate on
    the link

    If neither of those is true (commonly the case with sync links; most
    often the case with two back-to-back async systems), then flow control
    really isn't needed. The bottleneck is elsewhere.

    Note that you won't get "corrupt packets." PPP's FCS will cause any
    corrupted datagrams to be dropped. If you don't have flow control,
    but your configuration needs it, then you'll get really lousy
    performance, not corruption.

    (Technically, there's *always* a bottleneck in any path. What matters
    here is what is done about it -- whether whole packets are discarded
    [good], random hunks of data are dropped [bad], or individual bytes
    distributed throughout the data are dropped [very bad].)

    --
    James Carlson, IP Systems Group
    Sun Microsystems / 1 Network Drive 71.234W Vox +1 781 442 2084
    MS UBUR02-212 / Burlington MA 01803-2757 42.497N Fax +1 781 442 1677

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