How Phones Do

How Phones Do * with applications to computers 1

(I am @litonico_) (I’ve been reading about phones recently and
wanted to talk to you about them) 2

I think telegraphs are easier to think about 3 1.
Phones are weird telegraphs

4 1.1 Telegraphs are pretty much just wires

I don’t, like, *really* know how wires work !5

But it’s something like this: !6

!11 (And then you can send Morse code!)

!12 (you could talk back and forth on one line)

!13 (but it’s easier to have an ‘outgoing message’ and
‘incoming message’ wire so that you don’t talk over each other)

!14 Me You

!15 “talking to each other”

!16 2. I would like to talk to other people
too

!17 2.1 How about: connect everyone?

Talking between ﬁve people

!20 Wire costs money ;-;

- minimize number of wires !21

!22 Only ﬁve wires, everyone chooses who they’d like to
talk to

!23 Is this more readable?

…every person’s telephone would have to have a wire running
through every person’s house !24

- minimize number of wires - minimize length of wires
!25

!26 2.2 I would like to talk to other people
without having a personal wire to their telegraph

!28 “control signals” are things like “what terminal number am
I trying to connect to” “information link” is the literal telegraph bleeps and bloops

!30 We can have many switching machines!

One per area! !31

!32 We’re being so cost eﬀective!

!33 If we want to telegraph Grandma, in Germany:

!34 To Germany Me

!35 BTW, kind of how DNS works, also

!36 3. Wait, how does that box do

!38 3.1 It’s people

!40 Many people

!42 4. A story

!49 5 It’s all computers now (Sadly, they do not
often catch on ﬁre)

!52 5.1 How would you write that program?

!53 (We’re back to phones now, btw. Phones are just
weird telegraphs)

“If one of these breaks, say, on the outskirts of
Murmansk, are you OK with going to ﬁx it?”

- oh god plz don’t crash, ever, plz - thousands
of simultaneous calls - asynchronous - don’t mind if some other exchanges have gotten disconnected - change code remotely, with little downtime

First try: message passing in Smalltalk (reliable memory management w/
garbage collection, concurrent)

“If I sent a ring message to a phone it
was supposed to ring. If the phone A went oﬀ-hook it was supposed to send an (oﬀHook, A) message to the exchange. If the user of phone A dialled some digit D, then a (digit, A, D) message would be sent to the exchange.”

Literally taking coﬀee breaks while garbage collecting

Ok what are we really doing here predicates: idle(N) means
the subscriber N is idle on(N) means subscribed N in on hook ... etc. etc. operators: +t(A, dial_tone) means add a dial tone to A rules: process(A, f) :- on(A), idle(A), +t(A,dial-tone), +d(A, []), -idle(A), +of(A)

Oh, this is a Prolog program

Yep, it’s

Thank you! I’m @litonico_ I write about computers at tinyletter.com/learntocomputer
I work for destroyallsoftware.com (Help me test those code paths)

How Phones Do

How Phones Do

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