As there is no direct support of communication at this level, it is through the mechanisms of level 1 and 0 that the transfer will take place with data structure at each level not directly related to the upper level one. For instance the data structure exchanged at level 2 may be a sequential file. At each level, the communication is based on a protocol and the data structures to be exchanged are different. level 2: communication between users processes (subscribers) in different hosts. level 1: communication between hosts connected to the net. We will consider here only three levels: -level 0: communication between nodes of the subnets. In a distributed computers network based on a packet-swltched communication subnet, it is possible to describe the system as a hierarchical structure.
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