Computer Networks – What Is a Peer?

networks

From Tanebaum's Computer Network:

A five-layer network is illustrated in Fig. 1-13. The entities comprising the
corresponding layers on different machines are called peers. The peers may be software processes, hardware devices, or even human beings. In other words, it is
the peers that communicate by using the protocol to talk to each other.

I don't understand what a peer is, because the usage of English word "comprising" can be ambiguous ( A whole comprises parts. Parts comprise a whole.)

Can you point/circle out what part in Figure 1-13 is a peer?

Are a peer and a host in the figure the same concept?

Thanks.

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Best Answer

In figure 1-13, there are many peers.

  • Host 1, Layer 5 is a peer to Host 2, Layer 5.
  • Host 1, Layer 4 is a peer to Host 2, Layer 4.
  • Host 1, Layer 3 is a peer to Host 2, Layer 3.
  • Host 1, Layer 2 is a peer to Host 2, Layer 2.
  • Host 1, Layer 1 is a peer to Host 2, Layer 1.

Note that it is commutative, so for example,

  • Host 2, Layer 5 is a peer to Host 1, Layer 5.

But to make sure you don't misunderstand, a peer is not always a layer. If I talk to you on a telephone, you and I are peers in that communication.

In the same telephone call, let's say they are mobile phones. The mobile phones each convert the audio to/from radio waves to a tower. In this case, the mobile phones are peers on the phone network.

However, in the same example, I am not a peer with your mobile phone because we are on different levels.

I hope this clears it up.

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