1. Data collection device for monitoring streams in a data network using a packet transmission mode, the device comprising: an extractor of data contained in packets belonging to a stream defined by a transmitter, a receiver and a protocol; a syntax analyzer arranged for: receiving the data from the extractor in real time; breaking down the data received into elements according to syntactic rules of said protocol, the syntactic rules making it possible to represent said elements as a tree structure; and associating respective tree state indicators with at least some of said elements, where the tree state indicator associated with an element labels said element in the tree structure; and an interface for transmitting the tree state indicators to a stream analyzer, together with elements with which they have been associated.