hen operated in a packet mode, the packets can arrive at the destination out of order. With a unique segment (packet) numbering plan using a simple numbering sequence, it is a rather simple task for a long data file to be reassembled at the destination it its original order. Flow control. Flow control refers to the management of the data flow from source to destination such that buffer memories do not overflow, but maintain full capacity of all DATA COMMUNICATIONS facility components involved in the data transfer. Flow control must operate at several peer layers of protocols, as will be discussed later. Error control. Error control is a technique that permits recovery of lost or errored packets (frames, blocks). There are four possible functions involved in error control: 1. Numbering of packets (frames, blocks) (e.g., missing packet). 2. Incomplete octets (a bit sequence does not carry the proper number of bits, in this case 8 bits). 3. Error detection. Usually includes error correction (see Section 10.5.3). 4. Acknowledgment of one, several, or a predetermined string of packets (blocks, frames). Acknowledgment may be carried out by returning to the source (or node) the send sequence number as a receive sequence number. 10.10.2 Open Systems Interconnection (OSI) 10.10.2.1 Rationale and Overview of OSI. Data communication systems can be very diverse and complex. These systems involve elaborate software that must run on equipment having ever-increasing processing requirements. Under these conditions, it is desirable to ensure maximum independence among the various software and hardware elements of a system for two reasons: To facilitate intercommunication among disparate elements. To eliminate the "ripple effect" when there is a modification to one software element that may affect all elements. The ISO set about to make this data intercommunication problem more manageable. It developed its famous OSI reference model (Ref. 20). Instead of trying to solve the global dilemma, it decomposed the problem into more manag