f multiple (N)- connections to a single (N-1)-connection. This configuration enables bandwidth sharing of the connection to support multiple communications. The (N+1) LAYER demultiplexing is done in the respective peer entity in the destina- tion system. Splitting involves the mapping of a single (N)- connection to multiple (N-1)-connections. This configuration enables expansion of the bandwidth to support a communication that requires a higher throughput of data than can be supported by the bandwidth of a single (N-1)-connection. Splitting can also apply to high surviva- bility situations where the loss of a sole (N-1)-connection would be seriously disruptive to an operation, whereas the loss of one of many connections in a split configuration would only reduce the band- (N) LAYER width. (N-1) LAYER 12.4.2 Data transfer There are two basic types of data transfer: normal and expedited. The unit of transfer by a layer is its protocol data unit (PDU). Every layer has the function for transfer of data in support of the instance of communication. Figure 12.9 Data units 12.4.2.1 Connection-mode The combination of the SDU and PCI becomes the protocol data In the connection-mode, normal data transfer is accomplished by a unit (PDU) for transfer to the peer (N)-entity via the (N-1)-connec- layer through an (N-1)-connection that has been established with the tion or connectionless path, depending upon the mode of operation. destination (N)-entity. Some of the layers will also employ the the (N)-PDU becomes the (N-1)-SDU, which is them processed in following additional functions: a similar manner with the addition of a (N-1)-PCI to produce the (N-1)-PDU for the next layer below. Recursively, this process Flow control regulates the rate of data transfer, SO that PDUs do occurs in each descending layer until the data leaves the OSIE in transmission on the physical media. not arrive at the destination (N)-entity faster than they can be At the destination system, the PDU enters a layer as a parameter processed. Peer-to-peer pr