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By inverting the data signal with HDLC framing on both ends of a link, the HDLC
zero insertion algorithm becomes a ones insertion algorithm. This guarantees that in
any set of seven bits, at least one bit will be a one. Thus, the HDLC data stream meets
the density requirements of North American T1 lines without sacrificing any
bandwidth.
Clock Signal Inversion
The need to invert clock lines is separate from the need to invert data lines. Most
computer, modem, and terminal vendors adhere to an industry standard
specification known as RS-334. This specification defines the relationship between a
data bit and a reference clock on a synchronous serial link. The specification also says
that a device should transmit data with reference to the rising edge of the clock
signal and that data should be received with reference to the falling edge of the clock
signal.
When using long cables or cables not carrying a clock signal, a phase shift may occur
causing a high number of errors. In such cases, inverting the clock signal may correct
the phase shift. You may also need to invert the clock signal when connecting a
SunHSI/P port to equipment not adhering to the RS-334 standard.
Appendix C
T1 Inverted Data and Clock Signals
37
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