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Fig. 1.15 Bit-stuffing

Layer two ensures that, between two nodes in the network, data can
be transmitted and any errors are detected and corrected by
retransmission. Layer two effectively gives an error-free network, but
does not give any way of carrying several conversations at the same time.
This is a function of layer three.

1.4.3 Layer three

Layer three—the network or packet layer—takes care of individual
conversations in a network. It effectively takes care of what happens
between any two end points in the network, whereas layer two takes
care of what happens between any two nodes. See Fig. 1.16.

The simple network contains eight nodes to which four network users
are connected. A conversation between points A and C will involve
separate layer two links from A to P, P to Q, Q to R, R to S, and S to C.
Each link will carry completely separate frames and each will maintain
different sequence numbers. The links may run different window sizes.
Even though each of the links ensures that data is carried with error
detection and correction between the nodes, it does not ensure that the
data is carried correctly within the network.

Fig. 1.16 Multiple conversations in the network; the conversations are between end points A and C, and B and D, and use the intervening layer two links

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