The interface between the DCE and the communication channel is currently defined only for voice grade analog signalling as typically used with cable and radio channels. Corresponding definitions for operation of optical fiber channels will be added to this recommended practice when suitable standards for such channels become available.
Channel interfaces for other types of channels, e.g., power-line carrier, are not defined, as these are implementationspecific. Voice-grade channel transmission characteristics are required to be at least equal to those specified for unconditioned voice-grade channels.
The DCE (i.e., modem) to voice-grade channel interface should meet the following specifications:
(1) Terminal Arrangement: Two-wire or four-wire, selectable
(2) Termination Impedance: 600 W ± 10%, isolated and balanced
(3) Transmit Level: 0 dBm nominal, adjustable to -12 dBm in at most 4 dB steps
(4) Receive Level Range: 0 to -30 dBm without adjustment
(5) Modulation Polarity: Per CCITT Recommendation V.1 
(6) Signalling Standards: Per CCITT Recommendations:
600 b/s: CCITT Recommendation V.23  Mode 2 of Section 2.
1200 b/s: CCITT Recommendation V.23  Mode 2 of Section 2.
2400 b/s: CCITT Recommendation V.26  Alternative 2 of Section 2.3
4800 b/s: CCITT Recommendation V.27 bis 
All CCITT Recommendation V.26 and V.27 modems connected to any one communication channel shall use identical combinations of the various options permitted by these CCITT Recommendations, e.g., CCITT Recommendation V.26 data coding, CCITT Recommendation V.27 turn-on sequence timing, etc.
Two modems, when interconnected by a communication channel equivalent to an unconditioned voice grade channel, should provide an end-to-end random bit error rate of less than 1 in 104 b.6 This performance, in conjunction with the security code and the limited message length, ensures that the probability of acceptance of messages received in error will be less than 1 in 1010, as specified in the IEEE Std C37.1-1987.