ould take a The OSI Reference Model defines many additional layer elements different route to the destination system. The (N)-protocol PCI that further characterise the OSIE. The various elements defined can contains a sequence number that is used for the resequencing apply at some or all layers, depending on the specific service function. requirements for the particular layer. Acknowledgement provides confirmation for delivery or re- ceipt of PDUs by a destination (N)-entity. Generally, the (N)- 12.4.1 Multiplexing and splitting protocol PCI provides the acknowledgement mechanism and sequence numbers to facilitate this function, which generally The layer entities have been shown as mapping on a one-to-one provides for recovery of lost data units. basis from layer to layer. However, additional flexibility is provided 7. Error detection and notification identifies corruption of data through the mapping of multiple entities to provide for more effec- during the transfer process between (N)-entities. Recovery tive use of resources as needed. from such errors can be accomplished by the same mechanism Specific OSI layers 12/9 used for lost data units found by the acknowledgement func- tion. APPLICATION 8. Reset provides for reinitialisation of an (N)-connection without the loss of associations and connections of the upper layers. 9. Routing provides for relaying PDUs across multiple (N)-en- PRESENTATION tities for establishing a connection and transferring data, or for connectionless-mode data transfer. SESSION The connection-mode data transfer also provides for expedited data, which is not intended to be used on a regular basis. Generally, TRANSPORT expedited data is limited in amount of data transferred and its frequency of transfer. It is not subject to the same flow-control constraints, if any, as normal data transfer. The basic rule is that an NETWORK expedited data unit should not arrive at the destination any later than a normal data unit sent after the expedited data unit. In other words, DATA LINK it should n