es that have limited or no real issue regarding urgency. For example, long accounting files, including payroll, may only require 24- or 48-hour delivery times. In this case why not use the postal service, Federal Express, or UPS? On the other hand, credit card verification has high urgency requirements. A customer is waiting (probably impatiently) to have her/his credit verified during the process of buying some item. Most transaction data messages are highly urgent. Certain data protocols involve more latency than others. One reason for implementing frame relay is its low latency. Let's call latency the time it takes to complete a data message transaction. There are four causes for an increase in transaction time: 1. Propagation delay. 2. The number of message exchanges required to complete a transaction (e.g., hand- shakes, circuit setup, ARQ exchanges, and SO on). 3. Processing time and processing requirements. For example, every effort has been made to reduce processing requirements in frame relay. There are virtually no message exchanges, such as ACK and NACK, with frame relay. 4. Secondarily, we must consider the quality of a circuit. If the circuit is noisy, many ARQ exchanges will occur increasing latency dramatically. 12.1.1.3 The Recipient Must Be Prepared to Receive and "Understand" a Data Message. This is simply a question of compatibility. The data message receiver must be compatible with the far-end transmitter and intermediate nodes. We cannot have one ENTERPRISE NETWORKS II: WIDE AREA NETWORKS transmitting the ASCII code and the companion receiver only able to receive EBCDIC. Such compatibility MAY extend through all seven OSI layers. For example, frame relay is based ONLY on OSI layers 1 and 2. It is the responsibility of the frame relay user to provide the necessary compatibility of the upper OSI layers. 12.2 THE CONCEPT OF PACKET DATA COMMUNICATIONS The concept of a packet-switched network is based on the idea that the network switching nodes will have multiple choices for routing of data packe