 more complex software and notably more storage on both sides of the link. However, there are no gaps in transmission and no time is wasted waiting for the ACK or NACK. Go-back-n ARQ is a compromise. In this case, the receiver does not have to insert the corrected frame in its proper sequence, thus less storage is required. It works this way: When a frame is received in error, the receiver informs the transmitter to "go-back-n," n being the number of frames back to where the errored frame was. The transmitter then repeats all n frames, from the errored frame forward. Meanwhile, the receiver has thrown DATA COMMUNICATIONS out all frames from the errored frame forward. It replaces this group with the new set of n frames it received, all in proper order. 10.6 DC NATURE OF DATA TRANSMISSION 10.6.1 dc Loops Binary data are transmitted on a dc loop. More correctly, the binary data end instrument delivers to the line and receives from the line one or several dc loops. In its most basic form a dc loop consists of a switch, a dc voltage, and a termination. A pair of wires interconnects the switch and termination. The voltage source in data work is called the battery, although the device is usually electronic, deriving the dc voltage from an ac power line source. The battery is placed in the line to provide voltage(s) consistent with the type of transmission desired. A simplified drawing of a dc loop is shown in Figure 10.3a. 10.6.2 Neutral and Polar dc Transmission Systems Older telegraph and data systems operated in the neutral mode. Nearly all present data transmission systems operate in some form of polar mode. The words "neutral" and "polar" describe the manner in which battery is applied to the dc loop. On a "neutral" loop, following the convention of Table 10.1, battery is applied during spacing (0) conditions and is switched off during marking (1). Current therefore flows in the loop when a space is sent and the loop is closed. Marking is indicated on the loop by a condition of no current. Thus we have two conditio