> There ya go, Mike, making my eyes glaze over again <g>.
aw Frank.. you're too kind. ;-)
> Actually, your post made quite a few things clear to me. It's
> only when you start to do a project like this you realise how
> much you actually really know about how the net works.
tell me about it. Einstein once said that if you really understand
something, you can explain it to a six-year-old. having tried to do
almost exactly that with the 'net, i was truly amazed to discover how much
i didn't know as well as i thought i did.
> I figured my presentation would go along the lines of:
>
> 1. brief history of the net. The US military requirements, etc.
there are several good histories of the 'net online, but frankly i'd
suggest you look at the first few pages of RFC791, which defines the
Internet Protocol:
ftp://ftp.isi.edu/in-notes/rfc791.txt
and RFC793, which defines the Transfer Control Protocol:
ftp://ftp.isi.edu/in-notes/rfc793.txt
the technical stuff gets pretty opaque, but they both have extremely good
introductions which lay out the purposes and logic behind the whole 'net.
both documents were written for approval by the DOD, which means that at
least part of them had to be accessible to an audience that didn't have the
faintest idea what the techspeak was all about.
as an aside, both RFCs predate the OSI model, and use a simpler network
model of their own. you may prefer to show that to your people, because
it frankly glosses over several of the more fiddly details which were built
into the (later) OSI model.
> 2. Packet switching and why this met the needs of the military.
again, the RFCs will give you a good idea of how the military looked at it.
to understand packet switching, it helps to start by defining the term
'switching', then go on to compare it with the other way of handling
communication, namely circuit switching. the best way to define circuit
switching is to go back to the old-style telephone systems which gave it
that name.
if you think back to your favorite old movie that has a telephone
switchboard operator as a character, you have an example of circuit
switching in action. the office building has, say, 10 telephone lines
coming in, and there are 100 telephones on various desks around the
building. for every phone line comain in, there's a plug on a retractable
cord. for every phone, there's a socket in the plugboard.
the operator sits at the plugboard listening for incoming calls. when
someone dials in, a bulb lights up above that plug. the operator connects
the plug to her own headset long enough to find out who the caller is
trying to reach, then she plugs it into the appropriate socket. as long
as that plug stays in that socket, the two phones at either end are
connected to each other. nobody else can use either the plug /or/ the
socket as long as the two are hooked together. on the positive side, any
of the 10 lines coming in can connect to any one of the 100 phones in the
building. on the negative side, there can only be (at most) 10 incoming
calls at any time.
the ability to connect any plug to any socket is the 'switching' part.
human operators were eventually replaced by automatic relay matrices called
'switches', which is where we got the name. the fact that you have to
have an unbroken electrical circuit between the phones at the two endpoints
is where we got the term 'circuit'.
the point of a circuit switching network is that it has a predefined number
of channels for communication, and operates by connecting and disconnecting
endpoint systems from each other. it's an all or nothing system, because
you're either connected or you're not.
packet switching is more like the mail room in the same building, but the
metaphor isn't quite as obvious. you have roughly the same setup as with
the switchboard as far as input sources and destinations.. there are a set
number of mail deliveries during the day, and the walls are lined with
pigeonholes for each person in the building. you also have a person who
takes the input (an incoming letter) and routes it to the correct output (a
pigeonhole).
(aside: has everybody seen _The Hudsucker Proxy_? the old guy in the
mailroom, throwing envelopes into pigeonholes from 10 feet away at a rate
of about 2 per second, is simply amazing)
now.. assume that the mail is coming from several different sources. one
set of bags will be from the US post office, another will be fresh off the
boat from overseas, a third will be from a private courier, and the fourth
is internal correspondence from those pneumatic-tube thingies. each
source is roughly equivalent to an incoming telephone line. you only have
a certain number of channels, but this time each channel can carry several
different messages simultaneously.
the handsome executive on the fourth floor (who's toying with the
affections of the switchboard operator, she's far too good for him, and
will eventually end up with the keen-eyed young chap in the mailroom, as
soon as her head's no longer being turned by the exec's smooth city ways)
can be corresponding with a dozen people through each channel (sucking up
to the execs above him, stealing the work of the engineer on six, trysting
with his femme fatale in Paris, and arranging a deal to have the apartment
building where the operator's widowed granny lives torn down.. the usual
stuff) without having a monopoly on any of the channels themselves.
the thing which makes it all work is the fact that every letter comes in an
addressed envelope.
the envelope is a packet. it binds a burst of information together, and
tells the router (the young mailroom hero) how to deliver it without having
to see any of the content. once all the envelopes have been addressed
properly, you can dump them in a bag and let the guys in the mailroom sort
them out. there's no upper limit to the number of simultaneous
connections, like circuit switching has, but the performance degrades as
the load increases.
(the movie resolves itself after the mailroom chap 'accidentally' mixes up
a couple of letters. the president of the company learns about the
apartment being torn down.. and that the operator's grandmother is an old
flame from back in 'ought-four. the femme fatale discovers the engineer,
realizes he's a genius, and her passion for particle physics, with which
she had become disillusioned, is reawakened. she calls to break off with
the exec, and unwittingly shatters the operator's dream-bubble. the
operator dives headlong into the arms of the mailroom chap, and that sneaky
Machiavellian bastard has the good sense to keep his mouth shut and just
look noble as the (former) exec is escorted out of the building..
blackout.. roll credits)
the internet uses a packet-switched delivery system, and provides
reliability through redundancy. when one computer sends a message to
another, it keeps sending copies of the same packet until it gets a
confirmation from the other end saying, in effect, "okay, i got it.. you
can shut up now."
and i'll get to the rest in another couple of days.
mike stone <[EMAIL PROTECTED]>
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