I kinda understand how PPP works, but... - TCP-IP

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  1. I kinda understand how PPP works, but...

    hello

    I understand ( to a point ) PPP protocol. With it we can establish
    direct connection between two nodes ( I assume the nodes are connected
    with analog line )

    Now why do we need special protocol to handle direct connections?


    1)
    Say for example, that we would like to include certain PC into
    Ethernet LAN network as one of LAN's nodes.

    Only problem is that this PC is too far away and thus only way it can
    connect to this LAN is via telephone cable.

    * Why can't LAN treat this telephone cable as just another one of its
    cables in its network ( I imagine an adapter of some kind would be
    needed ) and thus packets sent through the telephone wire would be
    usual Ethernet packets?

    * Why needs the ethernet datagram ( or any type of LAN datagram ) be
    encapsulated inside PPP? Does it have something to do with telephone
    lines being analog or...?

    2)
    Is PPP needed only when direct links are analog?


    thank you


  2. Re: I kinda understand how PPP works, but...

    In article <1181514709.214134.189500@q69g2000hsb.googlegroups. com>,
    kaja_love160@yahoo.com wrote:

    > hello
    >
    > I understand ( to a point ) PPP protocol. With it we can establish
    > direct connection between two nodes ( I assume the nodes are connected
    > with analog line )
    >
    > Now why do we need special protocol to handle direct connections?


    Network and Transport layer protocols generally expect to use a Link
    Layer protocol that provides a number of functions such as framing and
    network layer identification.

    >
    >
    > 1)
    > Say for example, that we would like to include certain PC into
    > Ethernet LAN network as one of LAN's nodes.
    >
    > Only problem is that this PC is too far away and thus only way it can
    > connect to this LAN is via telephone cable.
    >
    > * Why can't LAN treat this telephone cable as just another one of its
    > cables in its network ( I imagine an adapter of some kind would be
    > needed ) and thus packets sent through the telephone wire would be
    > usual Ethernet packets?


    Ethernet is a multi-access medium, so it has requirements that a
    point-to-point link doesn't. Why would you want to waste 24 bytes on
    addressing when a point-to-point link just connects two devices directly?

    >
    > * Why needs the ethernet datagram ( or any type of LAN datagram ) be
    > encapsulated inside PPP? Does it have something to do with telephone
    > lines being analog or...?


    Ethernet datagrams aren't encapsulated inside PPP. IP datagrams are.
    Just as on an Ethernet, IP datagrams are encapsulated inside Ethernet
    frames. Basically, for every medium, you need a way of encapsulating IP
    datagrams into that medium's framing mechanism.

    >
    > 2)
    > Is PPP needed only when direct links are analog?


    PPP is needed on point-to-point links that don't have their own framing
    and addressing mechanism. This usually means serial links, because
    modems just deal with bytes, not frames.

    --
    Barry Margolin, barmar@alum.mit.edu
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***

  3. Re: I kinda understand how PPP works, but...

    On Jun 11, 1:59 am, Barry Margolin wrote:
    > In article <1181514709.214134.189...@q69g2000hsb.googlegroups. com>,
    >
    > kaja_love...@yahoo.com wrote:
    > > hello

    >
    > > I understand ( to a point ) PPP protocol. With it we can establish
    > > direct connection between two nodes ( I assume the nodes are connected
    > > with analog line )

    >
    > > Now why do we need special protocol to handle direct connections?

    >
    > Network and Transport layer protocols generally expect to use a Link
    > Layer protocol that provides a number of functions such as framing and
    > network layer identification.
    >
    >
    >
    > > 1)
    > > Say for example, that we would like to include certain PC into
    > > Ethernet LAN network as one of LAN's nodes.

    >
    > > Only problem is that this PC is too far away and thus only way it can
    > > connect to this LAN is via telephone cable.

    >
    > > * Why can't LAN treat this telephone cable as just another one of its
    > > cables in its network ( I imagine an adapter of some kind would be
    > > needed ) and thus packets sent through the telephone wire would be
    > > usual Ethernet packets?

    >
    > Ethernet is a multi-access medium, so it has requirements that a
    > point-to-point link doesn't. Why would you want to waste 24 bytes on
    > addressing when a point-to-point link just connects two devices directly?
    >
    >
    >
    > > * Why needs the ethernet datagram ( or any type of LAN datagram ) be
    > > encapsulated inside PPP? Does it have something to do with telephone
    > > lines being analog or...?

    >
    > Ethernet datagrams aren't encapsulated inside PPP. IP datagrams are.
    > Just as on an Ethernet, IP datagrams are encapsulated inside Ethernet
    > frames. Basically, for every medium, you need a way of encapsulating IP
    > datagrams into that medium's framing mechanism.
    >
    >
    >
    > > 2)
    > > Is PPP needed only when direct links are analog?

    >
    > PPP is needed on point-to-point links that don't have their own framing
    > and addressing mechanism. This usually means serial links, because
    > modems just deal with bytes, not frames.
    >
    > --
    > Barry Margolin, bar...@alum.mit.edu
    > Arlington, MA
    > *** PLEASE post questions in newsgroups, not directly to me ***
    > *** PLEASE don't copy me on replies, I'll read them in the group ***


    It makes sense now

    thank you


  4. Re: I kinda understand how PPP works, but...

    Barry Margolin writes:
    > In article <1181514709.214134.189500@q69g2000hsb.googlegroups. com>,
    > kaja_love160@yahoo.com wrote:
    > > * Why can't LAN treat this telephone cable as just another one of its
    > > cables in its network ( I imagine an adapter of some kind would be
    > > needed ) and thus packets sent through the telephone wire would be
    > > usual Ethernet packets?

    >
    > Ethernet is a multi-access medium, so it has requirements that a
    > point-to-point link doesn't. Why would you want to waste 24 bytes on
    > addressing when a point-to-point link just connects two devices directly?


    In addition, Ethernet protocol itself requires a set of bitwise
    signaling that would be either impossible or just impractical on
    modems. With a typical modem doing V.42, how would anyone create
    Ethernet-like frames? What would an end of frame delimiter look like?
    (How do you know it's done?)

    > > 2)
    > > Is PPP needed only when direct links are analog?

    >
    > PPP is needed on point-to-point links that don't have their own framing
    > and addressing mechanism. This usually means serial links, because
    > modems just deal with bytes, not frames.


    But certainly not "only," nor does it have anything to do with
    "analog" (whatever that might mean in this context -- perhaps "async"
    was wanted).

    PPP is used, as the name implies, for point-to-point links. Besides
    modems and async serial links, this also includes synchronous links
    such as ISDN B channels, T1 lines, SONET, and a range of others.

    Has the original poster read RFC 1661? If not, then I'd suggest doing
    so. It explains the operation of PPP. (RFC 1547 might also be
    helpful for some historical context.)

    One bit of possible confusion here is that modern Ethernet also uses
    point-to-point links, in the form of twisted-pair wires. However, the
    behavior of the signaling on those wires is quite different from the
    sorts of links that PPP uses. They behave instead as a shared medium.

    --
    James Carlson, Solaris Networking
    Sun Microsystems / 1 Network Drive 71.232W Vox +1 781 442 2084
    MS UBUR02-212 / Burlington MA 01803-2757 42.496N Fax +1 781 442 1677

  5. Re: I kinda understand how PPP works, but...

    kaja_love160@yahoo.com writes:

    > * Why can't LAN treat this telephone cable as just another one of its
    > cables in its network ( I imagine an adapter of some kind would be
    > needed ) and thus packets sent through the telephone wire would be
    > usual Ethernet packets?


    Well, HomePNA is this. Ethernet over telephone cable.

    I have some time used HomePNA-Ethernet adapter. It is just
    as you described :-) Look from your nearest market.


    / Kari Hurtta


  6. Re: I kinda understand how PPP works, but...

    Kari Hurtta writes:

    > kaja_love160@yahoo.com writes:
    >
    > > * Why can't LAN treat this telephone cable as just another one of its
    > > cables in its network ( I imagine an adapter of some kind would be
    > > needed ) and thus packets sent through the telephone wire would be
    > > usual Ethernet packets?

    >
    > Well, HomePNA is this. Ethernet over telephone cable.


    Not exactly. HPNA does *not* run Ethernet framing over a telephone
    line, and is certainly not end-to-end over a telephone call (like PPP
    is). It's more like DSL than like anything else.

    Some links:

    http://en.wikipedia.org/wiki/HomePNA
    http://www.homepna.org/en/index.asp

    --
    James Carlson, Solaris Networking
    Sun Microsystems / 1 Network Drive 71.232W Vox +1 781 442 2084
    MS UBUR02-212 / Burlington MA 01803-2757 42.496N Fax +1 781 442 1677

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