1. A property of a communication system (network) such that the output data stream delivered is the exact bit sequence presented to the input of the system without any restriction or exception.

2. A property of a communication system such that the output data stream delivered provides an output bit sequence functionally equivalent to the input bit sequence, from which the exact input bit sequence can be derived.

The second definition implies that the communication system provides protocol translation between input and output devices whereas the first definition implies that there is a compensating translation. The second definition cannot be implemented in all cases since there are functions that are supported by some terminal devices but have no equivalent in others.