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.
DC NATURE OF DATA TRANSMISSION
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.
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 conditions for binary transmission, an open loop (no current
flowing) and a closed loop (current flowing). Keep in mind that we could reverse this,
namely, change the convention and assign marking to a condition of current flowing or
closed loop, and spacing to a condition of no current or an open loop.
Simplified diagram illustrating a dc loop with (a) neutral keying and (b) polar keying.
In fact, this was the older convention, prior to about 1960.
10.6 DC NATURE OF DATA TRANSMISSION 10.6.1 dc Loops Binary data are transmitted on a dc loop. Keep in mind that we could reverse this, namely, change the convention and assign marking to a condition of current flowing or closed loop, and spacing to a condition of no current or an open loop.