The nature of the different services expected of the different layers results into particular services expected from the implementation environment. The lower layers (Transport, Network, Data Link and Physical) are responsible for transport of unstructured data. The upper layers (Application, Presentation and Session) deal with the structure, syntax and semantics of the communication.
The upper layers expect more services form their implementation environment than the lower layers. The implementation environment expected by the upper layers can be considered as an extension of the lower layers implementation environment.
It is recognized that at this time only the bottom five layers have matured. This book's primary focus is to create an implementation environment for the lower layers.