Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!hellgate.utah.edu!wasatch!haas
From: haas@wasatch.utah.edu (Walt Haas)
Newsgroups: comp.dcom.lans
Subject: Re: What services does X.25 provide?
Message-ID: <3279@wasatch.utah.edu>
Date: 8 Sep 89 00:01:55 GMT
References: <796@maxim.erbe.se>
Organization: University of Utah CS Dept
Lines: 70
Keywords: x.25, services, login, e-mail, file transfer, IPC

In article <796@maxim.erbe.se>, prc@erbe.se (Robert Claeson) writes:
> I'm completely new to X.25, and need to know more about it...

Well it's been several years since I wrote my X.25 implementation...
let me blow the dust off these books... *poof*  KERCHEWWW!! ok, now:

> What... is the difference between X.3, X.28 and X.29...

These three protocols are users of X.25 services.  They are closely related
protocols which provide virtual terminal service.  The human user of this lot
has a dumb terminal connected to a device called a Packet Assembler/
Disassembler (PAD) which collects keystrokes into packets for transmission
to the host, and turns packets from the host back into characters for the
terminal.  The PAD may be either a discrete piece of hardware like a TAC or
a program which runs like a TELNET client.  The PAD executes various
algorithms in determining when it has enough keystrokes to forward a packet,
and so on.  These algorithms are controlled by a collection of parameters
known as "PAD parameters".  X.3 specifies what a PAD does, what its PAD
parameters are and what values they may take on.  X.28 specifies how a
terminal and its human user interact with a PAD.  X.29 specifies how a PAD
talks over an X.25 interface to a remote host about the user's virtual
terminal session.

> and what services an X.25-based network provides.

X.25 is a standard by which a host talks to a network; the actual internals
of the network are not specified, and various networks have substantially
different internals.  But the X.25 standard defines reliable data
transmission over virtual circuits, with a means of addressing, much as
TCP/IP does.

> For example, does X.25 only define raw circuits, or are there multiple
> types of circuits (for, say, e-mail, remote login and file transfer)?

The two types of virtual circuits are Permanent and Switched.  When a
Switched Virtual Circuit is established (by a Call Request packet) one of
several parameters of the call is the protocol to be spoken during the
call.

> I can almost guess that X.400 is the X.25 so-called application-layer
> protocol for e-mail.

Basically you have the right idea, but it would be more correct to state
it that "X.400 is the CCITT protocol for e-mail and one possible user of
X.25 services."

> I believe that the combination X.3/X.28/X.29 does the same for remote login,
> but does it define a virtual terminal like ISO VT, or is is more like
> rlogin over TCP/IP?

I'm not familiar with ISO VT (did they finally produce a standard?) and the
X.3/X.28/X.29 standard has no doubt evolved since I implemented it, but
as I recall it has many of the features of rlogin (not, however, the ability
to pass username, terminal type or window size).

> In other words, do I still need a 3270 emulator for my VT220 to use X.25 to
> connect to an IBM mainframe with an X.25 interface?

Depends on the interface - in many cases, your X.25 connection to the IBM
will actually be a connection to a PAD connected milking-machine fashion to
a 7171 controller, thus:

you - terminal - PAD ========= PAD - 7171 - IBM host
                       X.25

In this case perhaps the 7171 will provide the necessary emulation.

Hope some of this helps... hope it's not too out of date!

Cheers  -- Walt Haas    haas@cs.utah.edu    utah-cs!haas