otocol flow control mechanisms are generally employed to co-ordinate the rate of transfer between of an indication primitive. The layer entity processes the PCI part, executes the functions, and discards the PCI, leaving the SDU to the peer (N)-entities. Flow control may also be applied across the layer interface between an (N+1)-entity and an (N)-entity. pass to the next ascending layer via an indication primitive. Again, each layer in turn recursively processes the data units until the final Segmenting enables a single (N)-SDU to be broken up into data is delivered to the destination AP in a form that is meaningful smaller units for processing into multiple (N)-PDUs. At the destination (N)-entity, the SDU segments are reassembled into for processing in the local environment of the destination system. The layer protocols in the family of OSI standards support the a single (N)-SDU before passing to the (N+1)-layer. An (N)- protocol mechanism is employed to keep track of the segments many functions in the different layers. The protocols define the peer during transfer SO they can be recombined at the destination. entity interactions, along with the syntax and information content of Blocking enables multiple (N)-SDUs to be mapped into a the PCIs that convey the actions between communicating systems. single (N)-PDU. Although this function is specified in the OSI These protocol standards are the specifications that provide the Reference Model, it has not been employed in any of the OSI compatibility and interoperability between systems in the world- protocols or configurations. wide multivendor market-place of products for information technol- 4. Concatenation enables multiple (N)-PDUs to be mapped to a ogy applications. single (N)-SDU in the (N-1)-layer. 5. Sequencing returns data units to the same order at the destina- tion (N)-entity that they were sent in from the originating Additional layer elements (N)-entity. This situation could particularly occur in connec- tionless-mode data transfer where each data unit c