What is the transactional nature of communication?

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http://books.google.com/books?id=oyiodkhIvfIC&pg=RA1-PA406&lpg=RA1-PA406&dq=transactional+nature+of+communication&source=web&ots=oJCHsPHBMM&sig=4S2W3PWNnMhVxNbVFtbirj-drNM&hl=en&sa=X&oi=book_result&resnum=2&ct=result