Re: Programming external modems with MSX

[Adriano Camargo Rodrigues da Cunha]

        Laurens,

> > 1. How to start a serial connection with the modem
> Pfff... Use the standard settings: 8N1, and make the speed configurable (you
> can also set it to the max. speed for simplicity's sake. However I do not
> recommend it for a final version).

        Thanks for the info.

> > 2. How to configure the modem (ATwxyz, etc)
> ATZ is the standard thing. However I suggest to use a config-file instead.
> I do, at least...

        The Hayes strings are the easy part...

> > 3. How to make the modem dial and establish a connection
> send init string:
> ATZ
> OK

        This "OK" isn't sent by the modem itself???

> and then dial:
> ATDT0123456789
> CONNECT 115200

        And also this "CONNECT"?

> Now you are connected to the peer.

        Nice. So I just need to send and receive characters
(including modem commands) from the RS232 without worring about anything?

> > 4. How to detect that the server I dialed send some strings
> > ("login:", "password:", etc)
> That's part of the SLIP protocol. DO NOT USE IT!!! It really, really sucks.
> For example, it doesn't support IP address negociation etc

        Hey fudeba... I know SLIP very well, ok?

> not supported by the server. Instead send a PPP Conf_Req in an HDLC frame
> immediately after the CONNECT message to start using PPP.

        Some ISPs still do authentication via SLIP. At least, here in
Brazil.

> The only disadvantage of PPP over HDLC instead of SLIP is that HDLC is more
> secure and has an FCS check over all the outgoing and incoming bytes, which
> is a bit double since the upper level protocols (IP, TCP) also have a
> checksum. You can ignore the FCS when receiving frames, and it is quite
> easily calculated for outgoing frames (in most cases the outgoing traffic is
> lower than the incoming traffic, so it's not really a problem). And using an
> experimental protocol (I don't know how much servers implement it, but my
> guess is most do) you can even disable the FCS check. However in my PPP
> implementation I don't support that yet.

        FCS is very easy to implement using a lookup table (less
computational cost for slow machines). I don't remember the RFC by now,
but it has a C-coded example of a fast FCS calculation using a lookup
table.

> > 5. How to send to the server some strings (the user name and
> > password)
> Use the Password Authentication Protocol (PAP).

        Most ISPs use PAP, but, as I said, some ISPs use SLIP for
authentication.

> > 6. How to send/receive data from modem (authentication finished,
> > so I want to start sending/receiving data!)
> It's all in the PPP protocol

        The PPP protocol I know. I'm just not familiarized with the
interaction of a modem and PPP. I'm just not sure how to "talk" with the
modem and to know when I start the PPP layer.

> It's all done through PPP (in this case using Terminate_Request packets).

        Yeah, I also know that.

> 1. Configuration Phase (LCP, the Link Control Protocol)
> LCP is an integrated part of PPP. Send Conf_Request, Conf_Reject, Conf_Nak
> and Conf_Ack

        I know, I know...

> packets until all options have been negociated. Options include
> Authentication Protocol negociation (PAP or CHAP (yuk!)

        I didn' study CHAP yet, so I can't comment.

> Header Compression (matters a very few
> bytes, not worth the effort)

        It's worth, since this kind of compression is too easy.

> and several other things which are only making
> things complex like Magic Number etc.

        Magic Number is just useful for loopback test. But generating a
random number at the beginning and using it is not problem.

> 4. Established.
> Now you can start sending IP packets to the peer (using PPP protocol #0021).

        Right. I recommend you implementing Van Jacobson compression.
That's what I'm doing right now on UZIX.

> If you need to be helped out you can ask me specific questions (me=PPP
> expert :)).

        Thank you. But me also PPP expert. I'm just not a modem expert. :)

> > Oh, and I don't have an external modem (someone wants to send me
> > an old 2400bps one? :)
> Bit slow, eh???

        Do you think that if I asked for a 33k6 modem someone would send
it to me? That's why I asked for an old 2400 one! :)
        And for tests, it's better than nothing. :)

> I will take some with me to the Bussum fair
> (but I guess you won't be there... :)

        I would like to. But it's too far away and too expensive to get
there.

> I've got two 2400bps modems layin' around, but the mail cost would exeed the
> value of the modem 10 times. It's not worth the money.

        No problem. Maybe some day I get one from a kind person. :P :)

        Thanks again,


Adriano Camargo Rodrigues da Cunha               ([EMAIL PROTECTED])
Engenharia de Computacao - UNICAMP   
http://www.adrpage.cjb.net           http://if.you.dont.like.msx.usuck.com

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