Date: Mon, 12 Oct 92 06:15:44 GMT
From: lars@spectrum.CMC.COM (Lars Poulsen)
Newsgroups: comp.dcom.telecom
Subject: A Small Tutorial on Some Networking Stuff
Organization: CMC Network Systems (Rockwell DCD), Santa Barbara, CA, USA
X-Submissions-To: telecom@eecs.nwu.edu
X-Administrivia-To: telecom-request@eecs.nwu.edu
X-Telecom-Digest: Volume 12, Issue 778, Message 9 of 9

In article <telecom12.768.4@eecs.nwu.edu> JIM.J.MURPHY@gte.sprint.com
writes:

> How exactly are we connected with these gateways, why are some
> addresses so complicated, how can we access the TELECOM archives from
> Telemail with only a dumb terminal, how do you prefer us to address
> submissions to this group, what is X.25, X.400, and the hardest
> question, can you explain this in beginners terms?

Although Jim asked for replies to be sent directly to him, I will give
Pat a chance to put it in the Digest if he wants to. I'm feeling in
the mood to teach/preach today; I had a run-in with GTE 611.

    I was trying to get directory assistance for the next city down
the cost, which I think is a Pac Bell area. And since GTE has often
chastised my for using 411 for non-local directory assistance, I tried
to dial 1-805-555-1212, only to get an intercept that said "It is not
necessary to dial 1 or the area code in front of the number you are
calling". But 555-1212 did not work either: It went to fast busy after
555. The poor, underpaid, undertrained girl at the tech service end of
the menu-monster reached by 611 could not understand why I would think
this was a problem, and she felt abused by my attempts to get her to
record a suggestion to route 555-1212 to directory assistance. "Why
don't you just dial 411 like normal people?".

    Anyway, back to the questions.

WHAT IS X.25 ?

In its simplest form, data communication is about moving databytes
from one computer's memory to another's without loss or errors. In
general, we like to get the costs down, and so we generally want to
have several simultaneous connections share the wire. When we do that,
we need to make sure that one stalled connection doesn't prevent the
others from getting their work done. The technical terms for these
features are error control, multiplexing and flow control.

In the early 1970's, it was demonstrated (by researchers working on a
small government-funded research network called ARPAnet) that it was
quite practical to set up a network with fast, permanent lines, and
share these lines between many users, instead of setting up individual
lines for each user like we do/did with telephone calls. The procedure
of getting the same effect out of the shared network as if you had
dialed a temporary connection was called packet switching, as opposed
to the then normal circuit switching. It was decided that this was a
good thing, and that since people would soon be calling on telephone
companies to provide such services, CCITT should standardize the
protocols and procedures by which such networks might be built and
connected to each other. X.25 is the reference number for a CCITT
document that describes how a computer might connect to such a network
over a synchronous line such as a DDS-1 (56 kbps) link.

It soon turned out, however, that the greatest number of connections
were not between two computers, but between a user at a terminal on
one end, and a computer on the other end. Thus, a specification was
written up for a small computer that might be deployed next to the
X.25 packet switch, which might be reached by people with asynchronous
ascii terminals, and which then connected on behalf of the terminal
user to the computer on the other end. This specification came in
three parts, numbered X.3 (what services to provide), X.28 (the user
commands), and X.29 (what the bits on the wire looked like between the
two computers).  Such an auxiliary machine was called a Packet
Assembler/Disassembler, or PAD for short, and most people that talk
about using X.25 are really talking about using an X.29 PAD.

The first two large commercial X.25 networks were Telenet, (which was
started by some BBN people that had worked on the ARPAnet, but soon
sold out to GTE) and TYMNET, which was owned by a computer service
bureau called TYME-SHARE, and was originally mostly used to access
that service, which was the Compu-Serve of its time. (Today, I think
TymShare is gone, and only the network is left.)

When X.25 first came out, we all thought it was horribly complicated.
Today, we can appreciate the simple elegance by which it provides many
services in a modular manner; it just happened to provide so many more
services than what we had ever seen before.

X.25 is still alive and well, but the way in which it is used today
(except for X.29 PADs) uses very few of those services. Accordingly, a
"leaner and meaner" multiplexing protocol has sprung up, called Frame
Relay, which gives better performance over today's faster links.

WHAT IS X.400 ?

One of the most valuable things that people use networks for, is
electronic mail. Only problem is, many people have done it
differently.  CCITT decided to cut through it and define a new
standard, which would be suitable for a telco-provided central place
that all of these all-different systems could use as a neutral
celaringhouse. The X.400 book of standards describes this system.

The only problems are: 

(1) The world doesn't look like that. There are already multiple,
interconnected, independent networks, and there is no clear place for
a clearinghouse to fit in.

(2) The standard was designed by several committees over several
years, and everybody wanted to put something in that would be easy for
them to do, plus something that would be hard for the competition to
do. The result is a stack of paper about 4 inches high. Any practical
implementation is slow, expensive and hard to set up.

(3) There already was a defacto clearinghouse standard. The ARPAnet
had grown, evolved, and spawned clones, that had interconnected with
each other and with FIDOnets and other bulletin boards, using a
different but much simpler Internet specification called RFC-821 and
RFC-822.

Slowly, but surely, X.400 is collapsing under its own weight. 

RFC-822 says an address has an "@" sign in the middle; to the right of
that is the name of a computer, and to the left of that is a
description of a mailbox that that computer will know how to
interpret.

X.400 says mail is addressed to a person; that person has a
Given_Name, a Surname, sometimes a Common_Name. The person belongs to
an organizational unit, which may be a part of yet another
organizational unit, within an organization, which is of course
located in a country.

While X.400 was supposed to be easier for people to deal with (and
might actually be easy, when and if there is a global, distributed,
interconnected database for both the users and the computers to look
each other up in), RFC-822 deals with the world the way the computers
are already set up.

Usually, an RFC-822 mailbox address is either the person's login name,
or their full name as defined when the account was set up, but
sometimes it will be a pointer to a user on a different system, which
cannot be reached directly. In such cases, it is common to stick a "%"
sign in the middle, and have the name of the next system to the right,
and on the left, whatever that machine can deal with.

HOW DO I GET TO THE ARCHIVES ?

Your mailbox on TeleMail is about as limited as anyone can get. All
you can do is send mail. To really get to the archives, you must have
access to the Internet, and you can then just copy files across. There
are, however a few systems that have "robots" that you can send mail
to, and they will copy the file, and mail it to you. Pat describes
this procedure about once a month. Sine I don't need it, I generally
don't save it.


Lars Poulsen, SMTS Software Engineer	Internet E-mail: lars@CMC.COM
CMC Network Products / Rockwell Int'l	Telephone: +1-805-968-4262	
Santa Barbara, CA 93117-3083		TeleFAX:   +1-805-968-8256