Networks - Intro to Computer ScienceYouTube↗  IA↗(long shot)
Education  5K views 5 likes 0 dislikes   23 Feb 2015
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まず一般的なネットワークの話をしたいと思います
First I'm going to talk about networks in general. The
インターネットも1つの特別なネットワークです
internet is just a special kind of network. There are lots
ネットワークを定義するには様々な方法があります
of different ways to define a network. We are going to
ここでは厳密な定義を使うことにします
use quite a precise definition. So we are going to define
ネットワークを エンティティのグループとして定義しましょう
a network as a group of entities. And entities could be
エンティティは人間かもしれませんし コンピュータかもしれません あるいは組織や政府機関かもしれません
people, they could be computers, they could be organizations, they could
ネットワークでは 各エンティティが直接つながっていなくても コミュニケーションが取れます
be governments, that can communicate, even though they're not all directly
この定義の意味が分かるような例を挙げましょう
connected. So here's an example to show what that definition means.
アリスがいます　そしてボブがいます アリスとボブは知り合いであり 会話を交わすことができます
So we have Alice. And we have Bob. And Alice and Bob know each other, and can
彼らが話す方法のことをチャネルと呼びます
talk to each other. The way they talk, we can
それは直接会って話すものかもしれませんし
call the channel. That could be just talking in
電話で話すものかもしれません
person. That could be talking over a telephone. We're going to
これだけではネットワークと呼ぶには不十分でしょう
say that just this is not enough to be
単に2人の人間が会話しているだけだからです
a network. This is just two people having a conversation.
それでは2番目の設定として アリス、ボブ、チャーリーがいるとしましょう アリスはボブとコミュニケーションが取れます
If we had the second scenario, we'll say we have Alice, Bob and Charlie. And Alice can communicate with Bob.
ボブはチャーリーとコミュニケーションが取れます チャーリーはアリスとコミュニケーションが取れます
Bob can communicate with Charlie. And Charlie can communicate
このコミュニケーションは すべて直接行うことができます
with Alice. But they can do all this directly. They're
全員が直接つながっています
all directly connected. If Alice wants to talk to
アリスがチャーリーと話したい場合は チャーリーと直接つながっているので
Charlie, well, she has a direct link to Charlie. She
ボブを通す必要はありません
doesn't need to use Bob. There's no way to
この直接のつながり以外に コミュニケーションの方法はありません
communicate other than across these direct links. So even in
2番目のケースでは 3人がそれぞれコミュニケーションを取れますが
this case where there are three people who can all
これもやはりネットワークとは言えません
communicate we're still going to say that's not a network.
この両方をネットワークと呼ぶ人も多いでしょう
Many people would call both of these networks. We're going
ここではもう少し厳密な定義を使うため
to use a somewhat more precise definition where neither of these
この両方ともネットワークとは見なしません ネットワークと見なせるのはこの3番目の例です ここでもアリス、ボブ、チャーリーという 3つのノードがあるとしましょう
will be considered a network. What will be considered a network is this third example and in the third example we have three nodes. We have Alice, Bob, and Charlie let's
3人の人間がいますが 今はアリスはチャーリーと直接つながっていません
say. They're the three people but now Alice is not directly
ですがアリスには チャーリーにメッセージを送る方法があります
connected to Charlie. But there's a way that Alice can
アリスがメッセージを送れるのは
send a message to Charlie. And she can send the message,
ボブがアリスのためにメッセージを転送できるからです
because Bob will forward the message for
メッセージが2ホップで送られることで
her. Once the message goes through two hops,
2人は直接つながっていなくても コミュニケーションが取れます
to make it so two people are not directly connected, can communicate. And maybe Charlie can
チャーリーも アリスにメッセージを送り返すことができるでしょう 私たちの定義では これをネットワークと呼ぶことにします
also send messages back to Alice. Then we call it a network. So that's going to
少なくとも3つのエンティティがなくてはいけません
be our definition. There has to be at least three entities and there has to be
そして直接つながっていないエンティティが
a way that entities that are not
コミュニケーションを取れる方法がなくてはいけません
directly connected can still communicate. So to see
このネットワークの定義が理解できたかを 確かめるため
that you understand that definition of a
また世界の歴史を少し知るためにも 小テストを行います
network and maybe also know a little bit of world history we're going to have a
ネットワークの概念が生まれたのはいつでしょうか？
quiz. So here are the possible answers. Ten
選択肢は 10年前、30年前、100年前 1，000年前、3，000年以上前です
years old, 30 years old, 100 years old, 1000 years old, or over 3000 years old.
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